Fred Bartoli a �crit :> Phil Hobbs a �crit : >> Joerg wrote: >>> Phil Hobbs wrote: >>>> Joerg wrote: >>>>> Phil Hobbs wrote: >>>>>> On 03/10/2012 03:41 PM, Phil Hobbs wrote: >>>>>>> On 03/10/2012 03:21 PM, Joerg wrote: >>>>>>>> Phil Hobbs wrote: >>>>>>>>> On 03/09/2012 05:34 PM, Joerg wrote: >>>>>>>>>> Phil Hobbs wrote: >>>>>>>>>>> So, JL's thread about level shifting a pHEMT switch brings up >>>>>>>>>>> another >>>>>>>>>>> issue: stabilizing the silly things. >>>>>>>>>>> >>>>>>>>>>> I have this little SKY65050 preamp that works very well--about a >>>>>>>>>>> nanoamp >>>>>>>>>>> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 >>>>>>>>>>> pF Cin, >>>>>>>>>>> flat from DC to about 120 MHz where the THS3091 second stage >>>>>>>>>>> craps >>>>>>>>>>> out. >>>>>>>>>>> A very nice amplifier, but for the intended use, it could >>>>>>>>>>> stand a bit >>>>>>>>>>> less noise and a bit less input capacitance. So I tried it >>>>>>>>>>> with an >>>>>>>>>>> NE3508 instead, but I couldn't get it to stop oscillating. >>>>>>>>>>> >>>>>>>>>>> The pHEMT source is grounded, and its drain goes to the >>>>>>>>>>> emitter of a >>>>>>>>>>> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its >>>>>>>>>>> base lead. >>>>>>>>>>> (This bead has high impedance way out beyond 1 GHz--otherwise >>>>>>>>>>> the NPN >>>>>>>>>>> wants to oscillate at around 8 GHz.) >>>>>>>>>>> >>>>>>>>>>> In actual use, the gate will be driven from some high impedance, >>>>>>>>>>> so it >>>>>>>>>>> isn't instantly clear that a bead will help there. Any >>>>>>>>>>> experience >>>>>>>>>>> with >>>>>>>>>>> stabilizing such a device? >>>>>>>>>>> >>>>>>>>>> Question: Did you run the trace to the gate at the other side >>>>>>>>>> of the >>>>>>>>>> ground/supply planes or sandwiched in? >>>>>>>>>> >>>>>>>>>> A photo and maybe the layout would help. >>>>>>>>>> >>>>>>>>> See http://electrooptical.net/www/sed/pHEMT_probe/pHEMT_probe.html >>>>>>>>> >>>>>>>>> There are a few tweaks: The zero-ohm resistor is actually a 1 >>>>>>>>> uF cap, >>>>>>>>> replaced with 1 pF for input capacitance measurements. the >>>>>>>>> pHEMT is a >>>>>>>>> SKY65050 or NE3508, and the collector load is 200 ohms. >>>>>>>>> Supplies are >>>>>>>>> about +-10 V. >>>>>>>>> >>>>>>>> The photo is a bit fuzzy in the upper left but it looks like the >>>>>>>> input >>>>>>>> and output traces are all on the top layer. Input and output >>>>>>>> should be >>>>>>>> on opposite sides of a plane. This doesn't have to be a ground >>>>>>>> plane but >>>>>>>> should not be a split one. I can see some sort of split (faint dark >>>>>>>> lines). >>>>>>>> >>>>>>>> Can't see C5, looks like it may be a bit far away. Which is a >>>>>>>> concern. >>>>>>>> Generally, I'd always have a 0.1uF 0603 (or smaller size) for >>>>>>>> bypassing. >>>>>>>> And smack dab at the top of R2. Anyhow, that trace from Q2 to R2 >>>>>>>> and >>>>>>>> then on to U1 is a bit long and too close to the input trace (R11). >>>>>>>> Moving U1 north by about 0.500" would clean that up quite well. >>>>>>>> >>>>>>>> The via for C1 is too far from it, looks like more than 0.100". >>>>>>>> >>>>>>>> A little ferrite rock in front of Q1's gate would help, right at >>>>>>>> Q1. But >>>>>>>> it must be on the other side. Looks like you can afford a >>>>>>>> roll-off since >>>>>>>> the THS3091 isn't really a rocket, rolling off around 200MHz. >>>>>>>> The bypass >>>>>>>> cap C4 is too far away from it. >>>>>>>> >>>>>>>> Please don't take this as dissing, just some ideas how to make the >>>>>>>> layout a little better :-) >>>>>>>> >>>>>>> Thanks. >>>>>>> >>>>>>> No worries about dissing me about the layout--If it's wrong, Mother >>>>>>> Nature will point it out a lot less gently than you. >>>>>>> >>>>>>> The via near C1 isn't the ground side, it's the R9 side. C1, C5, >>>>>>> and Q1 >>>>>>> all have ground vias right inside their pads. There's a ground >>>>>>> cutout >>>>>>> under the pHEMT to reduce the pad capacitances, with the source >>>>>>> leads >>>>>>> grounded on either corner of the cutout. (There's a zipfile with the >>>>>>> board layout in PDF and Gerber.) >>>>>>> >>>>> I looked in the Gerbers: C5 is way out there in Podunk. So in essence >>>>> you possibly have R2 connected to a large loop, a.k.a. antenna :-) >>>> Hmm. I was sort of relying on the 1k resistor for some isolation, but >>>> that might be the wrong answer. >>> C5 must be right at the other end of R2. Any sort of trace results not >>> only in an inductance which causes undesired gain peaking but also an >>> antenna that will do who knows what. >>> >>>>> The Gerbers imported with errors so I can't see where the via north of >>>>> R2 goes to, but that trace in itself is an inductor as well. If >>>>> that via >>>>> doesn't go anywhere useful, could you trip another cap to GND right at >>>>> R2? Maybe over to the via at C6. >>>>> >>>>>>> I'm not sure that it's input-output related, because it's lovely and >>>>>>> stable with a SKY65050, which theoretically has about the same >>>>>>> f_max as >>>>>>> the NE3508, but I could certainly be mistaken about that. >>>>>>> >>>>> Old Muntz would have said "Then use the Skyworks transistor" :-) >>>> Yup, and if I could live with 0.8 pF C_in and 0.5 nV noise, I would. >>>> Unfortunately there's that 62-electron signal to deal with, and I >>>> really >>>> need 0.3 pF C_in for the wire-bonded pHEMT die. (I realize that I get >>>> hit with ~0.15-0.2 pF per pad even with 0603s.) The issue as usual is >>>> input capacitance differentiating the input noise of the front end amp. >>> I am sure you'll get it stable. Even if it takes some dirty tricks like >>> series resonant circuits. A network analyzer style plot goes a long ways >>> here. That's why having at least a spectrum analyzer with tracking >>> generator is a good thing. >>> >>>>>>> C4 is only about 0.1 inch from the THS3091 (see the C-grid header >>>>>>> for >>>>>>> scale). Is that really too far for a 200 MHz amp? It should be only >>>>>>> about 2 nH or so. >>>>>>> >>>>>>> Cheers >>>>>>> >>>>>>> Phil Hobbs >>>>>>> >>>>>> Oh, and those dark lines aren't splits in the ground plane, they're >>>>>> traces on level 2--the stackup is 1&2 signals, 3 ground, 4 signals, >>>>>> chosen in order to reduce the pad capacitances. >>>>>> >>>>> Ah, ok. Just don't do that on larger boards because it can result in >>>>> warpage and assembler unhappiness. #2 should be some sort of plane, >>>>> ideally the ground plane. Then #3 for power planes which can be split. >>>>> >>>>> -- >>>>> Regards, Joerg >>>>> >>>>> http://www.analogconsultants.com/ >>>> Thanks, Joerg, that's a great help. The Gerbers were generated by >>>> Sunstone from my Eagle files, and they view fine in gerbv, which is >>>> part >>>> of gEDA. >>>> >>> If you generate your own with the Eagle CAM processor you can go to >>> places where it costs less. Then use the difference to take your wife >>> out to dinner :-) >>> >>> A good test is to see if your file load into a software commonly used by >>> PCB houses, such as GC-Preview. There's a free version with nag screen. >>> >>> -- >>> Regards, Joerg >>> >>> http://www.analogconsultants.com/ >> >> Thanks. That was actually the first PCB I ever laid out in my life, >> despite having been a professional designer since 1981--I've always had >> draughtsmen available for the job. (I also usually stick with parts >> slower than 20 GHz f_max.) >> >> Cheers >> >> Phil Hobbs >> >> > > I've not followed all Joerg's prose, but one thing that might also help > is to add a small series RC damping network at the BFP650 emitter. >Obviously meant between the drain/emitter node and ground...> Did you try to probe the oscillation with a small loop and your spec > analyzer to see at which frequency (frequencies) it screams? > > Putting one's finger everywhere on the board might also help to find a > solution (damping). Mine have been pretty good at this and yours > probably work fine too :-) > >-- Thanks, Fred.
Stabilizing pHEMTs
Started by ●March 9, 2012
Reply by ●March 11, 20122012-03-11
Reply by ●March 11, 20122012-03-11
Fred Bartoli wrote:> > Phil Hobbs a �crit : > > Joerg wrote: > >> Phil Hobbs wrote: > >>> Joerg wrote: > >>>> Phil Hobbs wrote: > >>>>> On 03/10/2012 03:41 PM, Phil Hobbs wrote: > >>>>>> On 03/10/2012 03:21 PM, Joerg wrote: > >>>>>>> Phil Hobbs wrote: > >>>>>>>> On 03/09/2012 05:34 PM, Joerg wrote: > >>>>>>>>> Phil Hobbs wrote: > >>>>>>>>>> So, JL's thread about level shifting a pHEMT switch brings up another > >>>>>>>>>> issue: stabilizing the silly things. > >>>>>>>>>> > >>>>>>>>>> I have this little SKY65050 preamp that works very well--about a > >>>>>>>>>> nanoamp > >>>>>>>>>> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin, > >>>>>>>>>> flat from DC to about 120 MHz where the THS3091 second stage craps > >>>>>>>>>> out. > >>>>>>>>>> A very nice amplifier, but for the intended use, it could stand a bit > >>>>>>>>>> less noise and a bit less input capacitance. So I tried it with an > >>>>>>>>>> NE3508 instead, but I couldn't get it to stop oscillating. > >>>>>>>>>> > >>>>>>>>>> The pHEMT source is grounded, and its drain goes to the emitter of a > >>>>>>>>>> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead. > >>>>>>>>>> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN > >>>>>>>>>> wants to oscillate at around 8 GHz.) > >>>>>>>>>> > >>>>>>>>>> In actual use, the gate will be driven from some high impedance, > >>>>>>>>>> so it > >>>>>>>>>> isn't instantly clear that a bead will help there. Any experience > >>>>>>>>>> with > >>>>>>>>>> stabilizing such a device? > >>>>>>>>>> > >>>>>>>>> Question: Did you run the trace to the gate at the other side of the > >>>>>>>>> ground/supply planes or sandwiched in? > >>>>>>>>> > >>>>>>>>> A photo and maybe the layout would help. > >>>>>>>>> > >>>>>>>> See http://electrooptical.net/www/sed/pHEMT_probe/pHEMT_probe.html > >>>>>>>> > >>>>>>>> There are a few tweaks: The zero-ohm resistor is actually a 1 uF cap, > >>>>>>>> replaced with 1 pF for input capacitance measurements. the pHEMT is a > >>>>>>>> SKY65050 or NE3508, and the collector load is 200 ohms. Supplies are > >>>>>>>> about +-10 V. > >>>>>>>> > >>>>>>> The photo is a bit fuzzy in the upper left but it looks like the input > >>>>>>> and output traces are all on the top layer. Input and output should be > >>>>>>> on opposite sides of a plane. This doesn't have to be a ground plane but > >>>>>>> should not be a split one. I can see some sort of split (faint dark > >>>>>>> lines). > >>>>>>> > >>>>>>> Can't see C5, looks like it may be a bit far away. Which is a concern. > >>>>>>> Generally, I'd always have a 0.1uF 0603 (or smaller size) for bypassing. > >>>>>>> And smack dab at the top of R2. Anyhow, that trace from Q2 to R2 and > >>>>>>> then on to U1 is a bit long and too close to the input trace (R11). > >>>>>>> Moving U1 north by about 0.500" would clean that up quite well. > >>>>>>> > >>>>>>> The via for C1 is too far from it, looks like more than 0.100". > >>>>>>> > >>>>>>> A little ferrite rock in front of Q1's gate would help, right at Q1. But > >>>>>>> it must be on the other side. Looks like you can afford a roll-off since > >>>>>>> the THS3091 isn't really a rocket, rolling off around 200MHz. The bypass > >>>>>>> cap C4 is too far away from it. > >>>>>>> > >>>>>>> Please don't take this as dissing, just some ideas how to make the > >>>>>>> layout a little better :-) > >>>>>>> > >>>>>> Thanks. > >>>>>> > >>>>>> No worries about dissing me about the layout--If it's wrong, Mother > >>>>>> Nature will point it out a lot less gently than you. > >>>>>> > >>>>>> The via near C1 isn't the ground side, it's the R9 side. C1, C5, and Q1 > >>>>>> all have ground vias right inside their pads. There's a ground cutout > >>>>>> under the pHEMT to reduce the pad capacitances, with the source leads > >>>>>> grounded on either corner of the cutout. (There's a zipfile with the > >>>>>> board layout in PDF and Gerber.) > >>>>>> > >>>> I looked in the Gerbers: C5 is way out there in Podunk. So in essence > >>>> you possibly have R2 connected to a large loop, a.k.a. antenna :-) > >>> Hmm. I was sort of relying on the 1k resistor for some isolation, but > >>> that might be the wrong answer. > >> C5 must be right at the other end of R2. Any sort of trace results not > >> only in an inductance which causes undesired gain peaking but also an > >> antenna that will do who knows what. > >> > >>>> The Gerbers imported with errors so I can't see where the via north of > >>>> R2 goes to, but that trace in itself is an inductor as well. If that via > >>>> doesn't go anywhere useful, could you trip another cap to GND right at > >>>> R2? Maybe over to the via at C6. > >>>> > >>>>>> I'm not sure that it's input-output related, because it's lovely and > >>>>>> stable with a SKY65050, which theoretically has about the same f_max as > >>>>>> the NE3508, but I could certainly be mistaken about that. > >>>>>> > >>>> Old Muntz would have said "Then use the Skyworks transistor" :-) > >>> Yup, and if I could live with 0.8 pF C_in and 0.5 nV noise, I would. > >>> Unfortunately there's that 62-electron signal to deal with, and I really > >>> need 0.3 pF C_in for the wire-bonded pHEMT die. (I realize that I get > >>> hit with ~0.15-0.2 pF per pad even with 0603s.) The issue as usual is > >>> input capacitance differentiating the input noise of the front end amp. > >> I am sure you'll get it stable. Even if it takes some dirty tricks like > >> series resonant circuits. A network analyzer style plot goes a long ways > >> here. That's why having at least a spectrum analyzer with tracking > >> generator is a good thing. > >> > >>>>>> C4 is only about 0.1 inch from the THS3091 (see the C-grid header for > >>>>>> scale). Is that really too far for a 200 MHz amp? It should be only > >>>>>> about 2 nH or so. > >>>>>> > >>>>>> Cheers > >>>>>> > >>>>>> Phil Hobbs > >>>>>> > >>>>> Oh, and those dark lines aren't splits in the ground plane, they're > >>>>> traces on level 2--the stackup is 1&2 signals, 3 ground, 4 signals, > >>>>> chosen in order to reduce the pad capacitances. > >>>>> > >>>> Ah, ok. Just don't do that on larger boards because it can result in > >>>> warpage and assembler unhappiness. #2 should be some sort of plane, > >>>> ideally the ground plane. Then #3 for power planes which can be split. > >>>> > >>>> -- > >>>> Regards, Joerg > >>>> > >>>> http://www.analogconsultants.com/ > >>> Thanks, Joerg, that's a great help. The Gerbers were generated by > >>> Sunstone from my Eagle files, and they view fine in gerbv, which is part > >>> of gEDA. > >>> > >> If you generate your own with the Eagle CAM processor you can go to > >> places where it costs less. Then use the difference to take your wife > >> out to dinner :-) > >> > >> A good test is to see if your file load into a software commonly used by > >> PCB houses, such as GC-Preview. There's a free version with nag screen. > >> > >> -- > >> Regards, Joerg > >> > >> http://www.analogconsultants.com/ > > > > Thanks. That was actually the first PCB I ever laid out in my life, > > despite having been a professional designer since 1981--I've always had > > draughtsmen available for the job. (I also usually stick with parts > > slower than 20 GHz f_max.) > > > > Cheers > > > > Phil Hobbs > > > > > > I've not followed all Joerg's prose, but one thing that might also help > is to add a small series RC damping network at the BFP650 emitter. > > Did you try to probe the oscillation with a small loop and your spec > analyzer to see at which frequency (frequencies) it screams? > > Putting one's finger everywhere on the board might also help to find a > solution (damping). Mine have been pretty good at this and yours > probably work fine too :-) > > -- > Thanks, > Fred.Thanks, Fred. The Dremelled hand-made proto oscillated at about 12 GHz, which I measured by watching the amplitude go up and down as I moved my hand within an inch or two of the surface. It went from peak to valley in about 1/4 inch, ergo, roughly 12 GHz. I have a Tek 11802 scope that's fast enough to see that, but my HP 8568B spectrum analyzer tops out at 1.5 GHz. I might get an 8566B one of these times, but haven't needed it for anything else so far. At IBM, I had an HP 70000 series analyzer with tracking generator that went to 18 GHz. I could probably find one of those on eBay, but you have to mix and match the modules to get it to work right, which is frustrating and expensive. On the other hand, the amount of time I've spent on this one would have bought me one easily. I'll try snubbing the BJT emitter and see if that helps. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●March 11, 20122012-03-11
On Sun, 11 Mar 2012 15:08:49 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>Joerg wrote: >> >> Phil Hobbs wrote: >> > Joerg wrote: >> >> Phil Hobbs wrote: >> >>> On 03/10/2012 03:41 PM, Phil Hobbs wrote: >> >>>> On 03/10/2012 03:21 PM, Joerg wrote: >> >>>>> Phil Hobbs wrote: >> >>>>>> On 03/09/2012 05:34 PM, Joerg wrote: >> >>>>>>> Phil Hobbs wrote: >> >>>>>>>> So, JL's thread about level shifting a pHEMT switch brings up another >> >>>>>>>> issue: stabilizing the silly things. >> >>>>>>>> >> >>>>>>>> I have this little SKY65050 preamp that works very well--about a >> >>>>>>>> nanoamp >> >>>>>>>> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin, >> >>>>>>>> flat from DC to about 120 MHz where the THS3091 second stage craps >> >>>>>>>> out. >> >>>>>>>> A very nice amplifier, but for the intended use, it could stand a bit >> >>>>>>>> less noise and a bit less input capacitance. So I tried it with an >> >>>>>>>> NE3508 instead, but I couldn't get it to stop oscillating. >> >>>>>>>> >> >>>>>>>> The pHEMT source is grounded, and its drain goes to the emitter of a >> >>>>>>>> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead. >> >>>>>>>> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN >> >>>>>>>> wants to oscillate at around 8 GHz.) >> >>>>>>>> >> >>>>>>>> In actual use, the gate will be driven from some high impedance, >> >>>>>>>> so it >> >>>>>>>> isn't instantly clear that a bead will help there. Any experience >> >>>>>>>> with >> >>>>>>>> stabilizing such a device? >> >>>>>>>> >> >>>>>>> Question: Did you run the trace to the gate at the other side of the >> >>>>>>> ground/supply planes or sandwiched in? >> >>>>>>> >> >>>>>>> A photo and maybe the layout would help. >> >>>>>>> >> >>>>>> See http://electrooptical.net/www/sed/pHEMT_probe/pHEMT_probe.html >> >>>>>> >> >>>>>> There are a few tweaks: The zero-ohm resistor is actually a 1 uF cap, >> >>>>>> replaced with 1 pF for input capacitance measurements. the pHEMT is a >> >>>>>> SKY65050 or NE3508, and the collector load is 200 ohms. Supplies are >> >>>>>> about +-10 V. >> >>>>>> >> >>>>> The photo is a bit fuzzy in the upper left but it looks like the input >> >>>>> and output traces are all on the top layer. Input and output should be >> >>>>> on opposite sides of a plane. This doesn't have to be a ground plane but >> >>>>> should not be a split one. I can see some sort of split (faint dark >> >>>>> lines). >> >>>>> >> >>>>> Can't see C5, looks like it may be a bit far away. Which is a concern. >> >>>>> Generally, I'd always have a 0.1uF 0603 (or smaller size) for bypassing. >> >>>>> And smack dab at the top of R2. Anyhow, that trace from Q2 to R2 and >> >>>>> then on to U1 is a bit long and too close to the input trace (R11). >> >>>>> Moving U1 north by about 0.500" would clean that up quite well. >> >>>>> >> >>>>> The via for C1 is too far from it, looks like more than 0.100". >> >>>>> >> >>>>> A little ferrite rock in front of Q1's gate would help, right at Q1. But >> >>>>> it must be on the other side. Looks like you can afford a roll-off since >> >>>>> the THS3091 isn't really a rocket, rolling off around 200MHz. The bypass >> >>>>> cap C4 is too far away from it. >> >>>>> >> >>>>> Please don't take this as dissing, just some ideas how to make the >> >>>>> layout a little better :-) >> >>>>> >> >>>> Thanks. >> >>>> >> >>>> No worries about dissing me about the layout--If it's wrong, Mother >> >>>> Nature will point it out a lot less gently than you. >> >>>> >> >>>> The via near C1 isn't the ground side, it's the R9 side. C1, C5, and Q1 >> >>>> all have ground vias right inside their pads. There's a ground cutout >> >>>> under the pHEMT to reduce the pad capacitances, with the source leads >> >>>> grounded on either corner of the cutout. (There's a zipfile with the >> >>>> board layout in PDF and Gerber.) >> >>>> >> >> I looked in the Gerbers: C5 is way out there in Podunk. So in essence >> >> you possibly have R2 connected to a large loop, a.k.a. antenna :-) >> > >> > Hmm. I was sort of relying on the 1k resistor for some isolation, but >> > that might be the wrong answer. >> >> C5 must be right at the other end of R2. Any sort of trace results not >> only in an inductance which causes undesired gain peaking but also an >> antenna that will do who knows what. >> >> >> The Gerbers imported with errors so I can't see where the via north of >> >> R2 goes to, but that trace in itself is an inductor as well. If that via >> >> doesn't go anywhere useful, could you trip another cap to GND right at >> >> R2? Maybe over to the via at C6. >> >> >> >>>> I'm not sure that it's input-output related, because it's lovely and >> >>>> stable with a SKY65050, which theoretically has about the same f_max as >> >>>> the NE3508, but I could certainly be mistaken about that. >> >>>> >> >> Old Muntz would have said "Then use the Skyworks transistor" :-) >> > >> > Yup, and if I could live with 0.8 pF C_in and 0.5 nV noise, I would. >> > Unfortunately there's that 62-electron signal to deal with, and I really >> > need 0.3 pF C_in for the wire-bonded pHEMT die. (I realize that I get >> > hit with ~0.15-0.2 pF per pad even with 0603s.) The issue as usual is >> > input capacitance differentiating the input noise of the front end amp. >> >> I am sure you'll get it stable. Even if it takes some dirty tricks like >> series resonant circuits. A network analyzer style plot goes a long ways >> here. That's why having at least a spectrum analyzer with tracking >> generator is a good thing. >> >> >>>> C4 is only about 0.1 inch from the THS3091 (see the C-grid header for >> >>>> scale). Is that really too far for a 200 MHz amp? It should be only >> >>>> about 2 nH or so. >> >>>> >> >>>> Cheers >> >>>> >> >>>> Phil Hobbs >> >>>> >> >>> Oh, and those dark lines aren't splits in the ground plane, they're >> >>> traces on level 2--the stackup is 1&2 signals, 3 ground, 4 signals, >> >>> chosen in order to reduce the pad capacitances. >> >>> >> >> Ah, ok. Just don't do that on larger boards because it can result in >> >> warpage and assembler unhappiness. #2 should be some sort of plane, >> >> ideally the ground plane. Then #3 for power planes which can be split. >> >> >> >> -- >> >> Regards, Joerg >> >> >> >> http://www.analogconsultants.com/ >> > >> > Thanks, Joerg, that's a great help. The Gerbers were generated by >> > Sunstone from my Eagle files, and they view fine in gerbv, which is part >> > of gEDA. >> > >> >> If you generate your own with the Eagle CAM processor you can go to >> places where it costs less. Then use the difference to take your wife >> out to dinner :-) >> >> A good test is to see if your file load into a software commonly used by >> PCB houses, such as GC-Preview. There's a free version with nag screen. >> >> -- >> Regards, Joerg >> >> http://www.analogconsultants.com/ > >Thanks. That was actually the first PCB I ever laid out in my life, >despite having been a professional designer since 1981--I've always had >draughtsmen available for the job. (I also usually stick with parts >slower than 20 GHz f_max.) > >Cheers > >Phil HobbsOnce you get down to picoseconds, it's easier to do that part yourself, rather than trying to explain it to a layout person. A lot of what I do is sort of instinct anyhow, hard to explain. And PCB layout is fun, done in moderation. -- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●March 11, 20122012-03-11
On Sun, 11 Mar 2012 20:23:13 +0100, Fred Bartoli <" "> wrote:>Phil Hobbs a �crit : >> Joerg wrote: >>> Phil Hobbs wrote: >>>> Joerg wrote: >>>>> Phil Hobbs wrote: >>>>>> On 03/10/2012 03:41 PM, Phil Hobbs wrote: >>>>>>> On 03/10/2012 03:21 PM, Joerg wrote: >>>>>>>> Phil Hobbs wrote: >>>>>>>>> On 03/09/2012 05:34 PM, Joerg wrote: >>>>>>>>>> Phil Hobbs wrote: >>>>>>>>>>> So, JL's thread about level shifting a pHEMT switch brings up another >>>>>>>>>>> issue: stabilizing the silly things. >>>>>>>>>>> >>>>>>>>>>> I have this little SKY65050 preamp that works very well--about a >>>>>>>>>>> nanoamp >>>>>>>>>>> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin, >>>>>>>>>>> flat from DC to about 120 MHz where the THS3091 second stage craps >>>>>>>>>>> out. >>>>>>>>>>> A very nice amplifier, but for the intended use, it could stand a bit >>>>>>>>>>> less noise and a bit less input capacitance. So I tried it with an >>>>>>>>>>> NE3508 instead, but I couldn't get it to stop oscillating. >>>>>>>>>>> >>>>>>>>>>> The pHEMT source is grounded, and its drain goes to the emitter of a >>>>>>>>>>> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead. >>>>>>>>>>> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN >>>>>>>>>>> wants to oscillate at around 8 GHz.) >>>>>>>>>>> >>>>>>>>>>> In actual use, the gate will be driven from some high impedance, >>>>>>>>>>> so it >>>>>>>>>>> isn't instantly clear that a bead will help there. Any experience >>>>>>>>>>> with >>>>>>>>>>> stabilizing such a device? >>>>>>>>>>> >>>>>>>>>> Question: Did you run the trace to the gate at the other side of the >>>>>>>>>> ground/supply planes or sandwiched in? >>>>>>>>>> >>>>>>>>>> A photo and maybe the layout would help. >>>>>>>>>> >>>>>>>>> See http://electrooptical.net/www/sed/pHEMT_probe/pHEMT_probe.html >>>>>>>>> >>>>>>>>> There are a few tweaks: The zero-ohm resistor is actually a 1 uF cap, >>>>>>>>> replaced with 1 pF for input capacitance measurements. the pHEMT is a >>>>>>>>> SKY65050 or NE3508, and the collector load is 200 ohms. Supplies are >>>>>>>>> about +-10 V. >>>>>>>>> >>>>>>>> The photo is a bit fuzzy in the upper left but it looks like the input >>>>>>>> and output traces are all on the top layer. Input and output should be >>>>>>>> on opposite sides of a plane. This doesn't have to be a ground plane but >>>>>>>> should not be a split one. I can see some sort of split (faint dark >>>>>>>> lines). >>>>>>>> >>>>>>>> Can't see C5, looks like it may be a bit far away. Which is a concern. >>>>>>>> Generally, I'd always have a 0.1uF 0603 (or smaller size) for bypassing. >>>>>>>> And smack dab at the top of R2. Anyhow, that trace from Q2 to R2 and >>>>>>>> then on to U1 is a bit long and too close to the input trace (R11). >>>>>>>> Moving U1 north by about 0.500" would clean that up quite well. >>>>>>>> >>>>>>>> The via for C1 is too far from it, looks like more than 0.100". >>>>>>>> >>>>>>>> A little ferrite rock in front of Q1's gate would help, right at Q1. But >>>>>>>> it must be on the other side. Looks like you can afford a roll-off since >>>>>>>> the THS3091 isn't really a rocket, rolling off around 200MHz. The bypass >>>>>>>> cap C4 is too far away from it. >>>>>>>> >>>>>>>> Please don't take this as dissing, just some ideas how to make the >>>>>>>> layout a little better :-) >>>>>>>> >>>>>>> Thanks. >>>>>>> >>>>>>> No worries about dissing me about the layout--If it's wrong, Mother >>>>>>> Nature will point it out a lot less gently than you. >>>>>>> >>>>>>> The via near C1 isn't the ground side, it's the R9 side. C1, C5, and Q1 >>>>>>> all have ground vias right inside their pads. There's a ground cutout >>>>>>> under the pHEMT to reduce the pad capacitances, with the source leads >>>>>>> grounded on either corner of the cutout. (There's a zipfile with the >>>>>>> board layout in PDF and Gerber.) >>>>>>> >>>>> I looked in the Gerbers: C5 is way out there in Podunk. So in essence >>>>> you possibly have R2 connected to a large loop, a.k.a. antenna :-) >>>> Hmm. I was sort of relying on the 1k resistor for some isolation, but >>>> that might be the wrong answer. >>> C5 must be right at the other end of R2. Any sort of trace results not >>> only in an inductance which causes undesired gain peaking but also an >>> antenna that will do who knows what. >>> >>>>> The Gerbers imported with errors so I can't see where the via north of >>>>> R2 goes to, but that trace in itself is an inductor as well. If that via >>>>> doesn't go anywhere useful, could you trip another cap to GND right at >>>>> R2? Maybe over to the via at C6. >>>>> >>>>>>> I'm not sure that it's input-output related, because it's lovely and >>>>>>> stable with a SKY65050, which theoretically has about the same f_max as >>>>>>> the NE3508, but I could certainly be mistaken about that. >>>>>>> >>>>> Old Muntz would have said "Then use the Skyworks transistor" :-) >>>> Yup, and if I could live with 0.8 pF C_in and 0.5 nV noise, I would. >>>> Unfortunately there's that 62-electron signal to deal with, and I really >>>> need 0.3 pF C_in for the wire-bonded pHEMT die. (I realize that I get >>>> hit with ~0.15-0.2 pF per pad even with 0603s.) The issue as usual is >>>> input capacitance differentiating the input noise of the front end amp. >>> I am sure you'll get it stable. Even if it takes some dirty tricks like >>> series resonant circuits. A network analyzer style plot goes a long ways >>> here. That's why having at least a spectrum analyzer with tracking >>> generator is a good thing. >>> >>>>>>> C4 is only about 0.1 inch from the THS3091 (see the C-grid header for >>>>>>> scale). Is that really too far for a 200 MHz amp? It should be only >>>>>>> about 2 nH or so. >>>>>>> >>>>>>> Cheers >>>>>>> >>>>>>> Phil Hobbs >>>>>>> >>>>>> Oh, and those dark lines aren't splits in the ground plane, they're >>>>>> traces on level 2--the stackup is 1&2 signals, 3 ground, 4 signals, >>>>>> chosen in order to reduce the pad capacitances. >>>>>> >>>>> Ah, ok. Just don't do that on larger boards because it can result in >>>>> warpage and assembler unhappiness. #2 should be some sort of plane, >>>>> ideally the ground plane. Then #3 for power planes which can be split. >>>>> >>>>> -- >>>>> Regards, Joerg >>>>> >>>>> http://www.analogconsultants.com/ >>>> Thanks, Joerg, that's a great help. The Gerbers were generated by >>>> Sunstone from my Eagle files, and they view fine in gerbv, which is part >>>> of gEDA. >>>> >>> If you generate your own with the Eagle CAM processor you can go to >>> places where it costs less. Then use the difference to take your wife >>> out to dinner :-) >>> >>> A good test is to see if your file load into a software commonly used by >>> PCB houses, such as GC-Preview. There's a free version with nag screen. >>> >>> -- >>> Regards, Joerg >>> >>> http://www.analogconsultants.com/ >> >> Thanks. That was actually the first PCB I ever laid out in my life, >> despite having been a professional designer since 1981--I've always had >> draughtsmen available for the job. (I also usually stick with parts >> slower than 20 GHz f_max.) >> >> Cheers >> >> Phil Hobbs >> >> > >I've not followed all Joerg's prose, but one thing that might also help >is to add a small series RC damping network at the BFP650 emitter. > >Did you try to probe the oscillation with a small loop and your spec >analyzer to see at which frequency (frequencies) it screams? > >Putting one's finger everywhere on the board might also help to find a >solution (damping). Mine have been pretty good at this and yours >probably work fine too :-)I want a small programmable surface-mount electrical equivalent of a finger. A ferrite bead is the closest, so far. Hey, there might be some seriousness in that. A couple of people make digitally programmable silicon capacitors, so adding some resistors might be feasible. -- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●March 11, 20122012-03-11
John Larkin wrote:> > On Sun, 11 Mar 2012 15:08:49 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > >Joerg wrote: > >> > >> Phil Hobbs wrote: > >> > Joerg wrote: > >> >> Phil Hobbs wrote: > >> >>> On 03/10/2012 03:41 PM, Phil Hobbs wrote: > >> >>>> On 03/10/2012 03:21 PM, Joerg wrote: > >> >>>>> Phil Hobbs wrote: > >> >>>>>> On 03/09/2012 05:34 PM, Joerg wrote: > >> >>>>>>> Phil Hobbs wrote: > >> >>>>>>>> So, JL's thread about level shifting a pHEMT switch brings up another > >> >>>>>>>> issue: stabilizing the silly things. > >> >>>>>>>> > >> >>>>>>>> I have this little SKY65050 preamp that works very well--about a > >> >>>>>>>> nanoamp > >> >>>>>>>> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin, > >> >>>>>>>> flat from DC to about 120 MHz where the THS3091 second stage craps > >> >>>>>>>> out. > >> >>>>>>>> A very nice amplifier, but for the intended use, it could stand a bit > >> >>>>>>>> less noise and a bit less input capacitance. So I tried it with an > >> >>>>>>>> NE3508 instead, but I couldn't get it to stop oscillating. > >> >>>>>>>> > >> >>>>>>>> The pHEMT source is grounded, and its drain goes to the emitter of a > >> >>>>>>>> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead. > >> >>>>>>>> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN > >> >>>>>>>> wants to oscillate at around 8 GHz.) > >> >>>>>>>> > >> >>>>>>>> In actual use, the gate will be driven from some high impedance, > >> >>>>>>>> so it > >> >>>>>>>> isn't instantly clear that a bead will help there. Any experience > >> >>>>>>>> with > >> >>>>>>>> stabilizing such a device? > >> >>>>>>>> > >> >>>>>>> Question: Did you run the trace to the gate at the other side of the > >> >>>>>>> ground/supply planes or sandwiched in? > >> >>>>>>> > >> >>>>>>> A photo and maybe the layout would help. > >> >>>>>>> > >> >>>>>> See http://electrooptical.net/www/sed/pHEMT_probe/pHEMT_probe.html > >> >>>>>> > >> >>>>>> There are a few tweaks: The zero-ohm resistor is actually a 1 uF cap, > >> >>>>>> replaced with 1 pF for input capacitance measurements. the pHEMT is a > >> >>>>>> SKY65050 or NE3508, and the collector load is 200 ohms. Supplies are > >> >>>>>> about +-10 V. > >> >>>>>> > >> >>>>> The photo is a bit fuzzy in the upper left but it looks like the input > >> >>>>> and output traces are all on the top layer. Input and output should be > >> >>>>> on opposite sides of a plane. This doesn't have to be a ground plane but > >> >>>>> should not be a split one. I can see some sort of split (faint dark > >> >>>>> lines). > >> >>>>> > >> >>>>> Can't see C5, looks like it may be a bit far away. Which is a concern. > >> >>>>> Generally, I'd always have a 0.1uF 0603 (or smaller size) for bypassing. > >> >>>>> And smack dab at the top of R2. Anyhow, that trace from Q2 to R2 and > >> >>>>> then on to U1 is a bit long and too close to the input trace (R11). > >> >>>>> Moving U1 north by about 0.500" would clean that up quite well. > >> >>>>> > >> >>>>> The via for C1 is too far from it, looks like more than 0.100". > >> >>>>> > >> >>>>> A little ferrite rock in front of Q1's gate would help, right at Q1. But > >> >>>>> it must be on the other side. Looks like you can afford a roll-off since > >> >>>>> the THS3091 isn't really a rocket, rolling off around 200MHz. The bypass > >> >>>>> cap C4 is too far away from it. > >> >>>>> > >> >>>>> Please don't take this as dissing, just some ideas how to make the > >> >>>>> layout a little better :-) > >> >>>>> > >> >>>> Thanks. > >> >>>> > >> >>>> No worries about dissing me about the layout--If it's wrong, Mother > >> >>>> Nature will point it out a lot less gently than you. > >> >>>> > >> >>>> The via near C1 isn't the ground side, it's the R9 side. C1, C5, and Q1 > >> >>>> all have ground vias right inside their pads. There's a ground cutout > >> >>>> under the pHEMT to reduce the pad capacitances, with the source leads > >> >>>> grounded on either corner of the cutout. (There's a zipfile with the > >> >>>> board layout in PDF and Gerber.) > >> >>>> > >> >> I looked in the Gerbers: C5 is way out there in Podunk. So in essence > >> >> you possibly have R2 connected to a large loop, a.k.a. antenna :-) > >> > > >> > Hmm. I was sort of relying on the 1k resistor for some isolation, but > >> > that might be the wrong answer. > >> > >> C5 must be right at the other end of R2. Any sort of trace results not > >> only in an inductance which causes undesired gain peaking but also an > >> antenna that will do who knows what. > >> > >> >> The Gerbers imported with errors so I can't see where the via north of > >> >> R2 goes to, but that trace in itself is an inductor as well. If that via > >> >> doesn't go anywhere useful, could you trip another cap to GND right at > >> >> R2? Maybe over to the via at C6. > >> >> > >> >>>> I'm not sure that it's input-output related, because it's lovely and > >> >>>> stable with a SKY65050, which theoretically has about the same f_max as > >> >>>> the NE3508, but I could certainly be mistaken about that. > >> >>>> > >> >> Old Muntz would have said "Then use the Skyworks transistor" :-) > >> > > >> > Yup, and if I could live with 0.8 pF C_in and 0.5 nV noise, I would. > >> > Unfortunately there's that 62-electron signal to deal with, and I really > >> > need 0.3 pF C_in for the wire-bonded pHEMT die. (I realize that I get > >> > hit with ~0.15-0.2 pF per pad even with 0603s.) The issue as usual is > >> > input capacitance differentiating the input noise of the front end amp. > >> > >> I am sure you'll get it stable. Even if it takes some dirty tricks like > >> series resonant circuits. A network analyzer style plot goes a long ways > >> here. That's why having at least a spectrum analyzer with tracking > >> generator is a good thing. > >> > >> >>>> C4 is only about 0.1 inch from the THS3091 (see the C-grid header for > >> >>>> scale). Is that really too far for a 200 MHz amp? It should be only > >> >>>> about 2 nH or so. > >> >>>> > >> >>>> Cheers > >> >>>> > >> >>>> Phil Hobbs > >> >>>> > >> >>> Oh, and those dark lines aren't splits in the ground plane, they're > >> >>> traces on level 2--the stackup is 1&2 signals, 3 ground, 4 signals, > >> >>> chosen in order to reduce the pad capacitances. > >> >>> > >> >> Ah, ok. Just don't do that on larger boards because it can result in > >> >> warpage and assembler unhappiness. #2 should be some sort of plane, > >> >> ideally the ground plane. Then #3 for power planes which can be split. > >> >> > >> >> -- > >> >> Regards, Joerg > >> >> > >> >> http://www.analogconsultants.com/ > >> > > >> > Thanks, Joerg, that's a great help. The Gerbers were generated by > >> > Sunstone from my Eagle files, and they view fine in gerbv, which is part > >> > of gEDA. > >> > > >> > >> If you generate your own with the Eagle CAM processor you can go to > >> places where it costs less. Then use the difference to take your wife > >> out to dinner :-) > >> > >> A good test is to see if your file load into a software commonly used by > >> PCB houses, such as GC-Preview. There's a free version with nag screen. > >> > >> -- > >> Regards, Joerg > >> > >> http://www.analogconsultants.com/ > > > >Thanks. That was actually the first PCB I ever laid out in my life, > >despite having been a professional designer since 1981--I've always had > >draughtsmen available for the job. (I also usually stick with parts > >slower than 20 GHz f_max.) > > > >Cheers > > > >Phil Hobbs > > Once you get down to picoseconds, it's easier to do that part > yourself, rather than trying to explain it to a layout person. A lot > of what I do is sort of instinct anyhow, hard to explain. And PCB > layout is fun, done in moderation.That's partly why I did it. The other part is that I have some proprietary things that I'm trying to get going, and I like to have my arms around all aspects of a project even if I'm not the only one working on it. A 1-year license for the pro version of Eagle was $250, which seemed like the right answer. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●March 11, 20122012-03-11
Phil Hobbs wrote:> Joerg wrote: >> Joerg wrote:[...]>>> C5 must be right at the other end of R2. Any sort of trace results not >>> only in an inductance which causes undesired gain peaking but also an >>> antenna that will do who knows what. >>> >> The other question that remains: Where does that via north of R2 go to? >> It's right next to the emitter which can have scary results. Because >> there is no bypass cap in the area this whole collector node looks like >> a loose cannon :-) >> >> But it can be kludged into compliance with rework. If this is for a >> product you'll have to do a relayout but I'd first get it to work with >> this board, some copper tape and caps. > > It goes to the bottom layer, all alone on the other side of the ground > plane, and connects to C5. I'll try Dremelling down to the ground plane > and putting in another cap locally. >In the Gerbers I see the one immediately to the east go to C% but the one farther up north seem to go nowhere. If it really doesn't go anywhere there's an easy fix: The Dremel ... whirrrr ... gone :-) -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●March 11, 20122012-03-11
Phil Hobbs wrote:> Fred Bartoli wrote: >> Phil Hobbs a �crit : >>> Joerg wrote: >>>> Phil Hobbs wrote: >>>>> Joerg wrote: >>>>>> Phil Hobbs wrote: >>>>>>> On 03/10/2012 03:41 PM, Phil Hobbs wrote: >>>>>>>> On 03/10/2012 03:21 PM, Joerg wrote: >>>>>>>>> Phil Hobbs wrote: >>>>>>>>>> On 03/09/2012 05:34 PM, Joerg wrote: >>>>>>>>>>> Phil Hobbs wrote: >>>>>>>>>>>> So, JL's thread about level shifting a pHEMT switch brings up another >>>>>>>>>>>> issue: stabilizing the silly things. >>>>>>>>>>>> >>>>>>>>>>>> I have this little SKY65050 preamp that works very well--about a >>>>>>>>>>>> nanoamp >>>>>>>>>>>> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin, >>>>>>>>>>>> flat from DC to about 120 MHz where the THS3091 second stage craps >>>>>>>>>>>> out. >>>>>>>>>>>> A very nice amplifier, but for the intended use, it could stand a bit >>>>>>>>>>>> less noise and a bit less input capacitance. So I tried it with an >>>>>>>>>>>> NE3508 instead, but I couldn't get it to stop oscillating. >>>>>>>>>>>> >>>>>>>>>>>> The pHEMT source is grounded, and its drain goes to the emitter of a >>>>>>>>>>>> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead. >>>>>>>>>>>> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN >>>>>>>>>>>> wants to oscillate at around 8 GHz.) >>>>>>>>>>>> >>>>>>>>>>>> In actual use, the gate will be driven from some high impedance, >>>>>>>>>>>> so it >>>>>>>>>>>> isn't instantly clear that a bead will help there. Any experience >>>>>>>>>>>> with >>>>>>>>>>>> stabilizing such a device? >>>>>>>>>>>> >>>>>>>>>>> Question: Did you run the trace to the gate at the other side of the >>>>>>>>>>> ground/supply planes or sandwiched in? >>>>>>>>>>> >>>>>>>>>>> A photo and maybe the layout would help. >>>>>>>>>>> >>>>>>>>>> See http://electrooptical.net/www/sed/pHEMT_probe/pHEMT_probe.html >>>>>>>>>> >>>>>>>>>> There are a few tweaks: The zero-ohm resistor is actually a 1 uF cap, >>>>>>>>>> replaced with 1 pF for input capacitance measurements. the pHEMT is a >>>>>>>>>> SKY65050 or NE3508, and the collector load is 200 ohms. Supplies are >>>>>>>>>> about +-10 V. >>>>>>>>>> >>>>>>>>> The photo is a bit fuzzy in the upper left but it looks like the input >>>>>>>>> and output traces are all on the top layer. Input and output should be >>>>>>>>> on opposite sides of a plane. This doesn't have to be a ground plane but >>>>>>>>> should not be a split one. I can see some sort of split (faint dark >>>>>>>>> lines). >>>>>>>>> >>>>>>>>> Can't see C5, looks like it may be a bit far away. Which is a concern. >>>>>>>>> Generally, I'd always have a 0.1uF 0603 (or smaller size) for bypassing. >>>>>>>>> And smack dab at the top of R2. Anyhow, that trace from Q2 to R2 and >>>>>>>>> then on to U1 is a bit long and too close to the input trace (R11). >>>>>>>>> Moving U1 north by about 0.500" would clean that up quite well. >>>>>>>>> >>>>>>>>> The via for C1 is too far from it, looks like more than 0.100". >>>>>>>>> >>>>>>>>> A little ferrite rock in front of Q1's gate would help, right at Q1. But >>>>>>>>> it must be on the other side. Looks like you can afford a roll-off since >>>>>>>>> the THS3091 isn't really a rocket, rolling off around 200MHz. The bypass >>>>>>>>> cap C4 is too far away from it. >>>>>>>>> >>>>>>>>> Please don't take this as dissing, just some ideas how to make the >>>>>>>>> layout a little better :-) >>>>>>>>> >>>>>>>> Thanks. >>>>>>>> >>>>>>>> No worries about dissing me about the layout--If it's wrong, Mother >>>>>>>> Nature will point it out a lot less gently than you. >>>>>>>> >>>>>>>> The via near C1 isn't the ground side, it's the R9 side. C1, C5, and Q1 >>>>>>>> all have ground vias right inside their pads. There's a ground cutout >>>>>>>> under the pHEMT to reduce the pad capacitances, with the source leads >>>>>>>> grounded on either corner of the cutout. (There's a zipfile with the >>>>>>>> board layout in PDF and Gerber.) >>>>>>>> >>>>>> I looked in the Gerbers: C5 is way out there in Podunk. So in essence >>>>>> you possibly have R2 connected to a large loop, a.k.a. antenna :-) >>>>> Hmm. I was sort of relying on the 1k resistor for some isolation, but >>>>> that might be the wrong answer. >>>> C5 must be right at the other end of R2. Any sort of trace results not >>>> only in an inductance which causes undesired gain peaking but also an >>>> antenna that will do who knows what. >>>> >>>>>> The Gerbers imported with errors so I can't see where the via north of >>>>>> R2 goes to, but that trace in itself is an inductor as well. If that via >>>>>> doesn't go anywhere useful, could you trip another cap to GND right at >>>>>> R2? Maybe over to the via at C6. >>>>>> >>>>>>>> I'm not sure that it's input-output related, because it's lovely and >>>>>>>> stable with a SKY65050, which theoretically has about the same f_max as >>>>>>>> the NE3508, but I could certainly be mistaken about that. >>>>>>>> >>>>>> Old Muntz would have said "Then use the Skyworks transistor" :-) >>>>> Yup, and if I could live with 0.8 pF C_in and 0.5 nV noise, I would. >>>>> Unfortunately there's that 62-electron signal to deal with, and I really >>>>> need 0.3 pF C_in for the wire-bonded pHEMT die. (I realize that I get >>>>> hit with ~0.15-0.2 pF per pad even with 0603s.) The issue as usual is >>>>> input capacitance differentiating the input noise of the front end amp. >>>> I am sure you'll get it stable. Even if it takes some dirty tricks like >>>> series resonant circuits. A network analyzer style plot goes a long ways >>>> here. That's why having at least a spectrum analyzer with tracking >>>> generator is a good thing. >>>> >>>>>>>> C4 is only about 0.1 inch from the THS3091 (see the C-grid header for >>>>>>>> scale). Is that really too far for a 200 MHz amp? It should be only >>>>>>>> about 2 nH or so. >>>>>>>> >>>>>>>> Cheers >>>>>>>> >>>>>>>> Phil Hobbs >>>>>>>> >>>>>>> Oh, and those dark lines aren't splits in the ground plane, they're >>>>>>> traces on level 2--the stackup is 1&2 signals, 3 ground, 4 signals, >>>>>>> chosen in order to reduce the pad capacitances. >>>>>>> >>>>>> Ah, ok. Just don't do that on larger boards because it can result in >>>>>> warpage and assembler unhappiness. #2 should be some sort of plane, >>>>>> ideally the ground plane. Then #3 for power planes which can be split. >>>>>> >>>>>> -- >>>>>> Regards, Joerg >>>>>> >>>>>> http://www.analogconsultants.com/ >>>>> Thanks, Joerg, that's a great help. The Gerbers were generated by >>>>> Sunstone from my Eagle files, and they view fine in gerbv, which is part >>>>> of gEDA. >>>>> >>>> If you generate your own with the Eagle CAM processor you can go to >>>> places where it costs less. Then use the difference to take your wife >>>> out to dinner :-) >>>> >>>> A good test is to see if your file load into a software commonly used by >>>> PCB houses, such as GC-Preview. There's a free version with nag screen. >>>> >>>> -- >>>> Regards, Joerg >>>> >>>> http://www.analogconsultants.com/ >>> Thanks. That was actually the first PCB I ever laid out in my life, >>> despite having been a professional designer since 1981--I've always had >>> draughtsmen available for the job. (I also usually stick with parts >>> slower than 20 GHz f_max.) >>> >>> Cheers >>> >>> Phil Hobbs >>> >>> >> I've not followed all Joerg's prose, but one thing that might also help >> is to add a small series RC damping network at the BFP650 emitter. >>A snubber is a good idea although tough at such a low impedance node. A bead might be easier but requires a cut. I think if Phil cleans up the layout around R2 that would go a long way. That only requires a bypass cap and a snippet of copper tape.>> Did you try to probe the oscillation with a small loop and your spec >> analyzer to see at which frequency (frequencies) it screams? >> >> Putting one's finger everywhere on the board might also help to find a >> solution (damping). Mine have been pretty good at this and yours >> probably work fine too :-) >> >> -- >> Thanks, >> Fred. > > Thanks, Fred. The Dremelled hand-made proto oscillated at about 12 GHz, > which I measured by watching the amplitude go up and down as I moved my > hand within an inch or two of the surface. It went from peak to valley > in about 1/4 inch, ergo, roughly 12 GHz. > > I have a Tek 11802 scope that's fast enough to see that, but my HP 8568B > spectrum analyzer tops out at 1.5 GHz. I might get an 8566B one of > these times, but haven't needed it for anything else so far. ...Check out their new 12.4GHz analyzer, came out very few weeks after I bought their 4.4GHz version: http://signalhound.com/ Being skeptical at first I must say I am impressed now. Ok, the software is still a bit hokey but this thing has already paid itself in the few months I had it. Did a lengthy EMC debug with it a couple weeks ago and this saved us half a kilobuck alone in rental fees. Plus the rental wouldn't have allowed me to listen in via SSB demodulation which saved time during hunts. I don't think they have a TG for that yet but I have their 4.4GHz TG.> ... At IBM, I > had an HP 70000 series analyzer with tracking generator that went to 18 > GHz. I could probably find one of those on eBay, but you have to mix > and match the modules to get it to work right, which is frustrating and > expensive. On the other hand, the amount of time I've spent on this one > would have bought me one easily. > > I'll try snubbing the BJT emitter and see if that helps. >But clean up the R2 connection. Without that it's all like roulette. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●March 11, 20122012-03-11
John Larkin a �crit :> On Sun, 11 Mar 2012 20:23:13 +0100, Fred Bartoli <" "> wrote: > >> Phil Hobbs a �crit : >>> Joerg wrote: >>>> Phil Hobbs wrote: >>>>> Joerg wrote: >>>>>> Phil Hobbs wrote: >>>>>>> On 03/10/2012 03:41 PM, Phil Hobbs wrote: >>>>>>>> On 03/10/2012 03:21 PM, Joerg wrote: >>>>>>>>> Phil Hobbs wrote: >>>>>>>>>> On 03/09/2012 05:34 PM, Joerg wrote: >>>>>>>>>>> Phil Hobbs wrote: >>>>>>>>>>>> So, JL's thread about level shifting a pHEMT switch brings up another >>>>>>>>>>>> issue: stabilizing the silly things. >>>>>>>>>>>> >>>>>>>>>>>> I have this little SKY65050 preamp that works very well--about a >>>>>>>>>>>> nanoamp >>>>>>>>>>>> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin, >>>>>>>>>>>> flat from DC to about 120 MHz where the THS3091 second stage craps >>>>>>>>>>>> out. >>>>>>>>>>>> A very nice amplifier, but for the intended use, it could stand a bit >>>>>>>>>>>> less noise and a bit less input capacitance. So I tried it with an >>>>>>>>>>>> NE3508 instead, but I couldn't get it to stop oscillating. >>>>>>>>>>>> >>>>>>>>>>>> The pHEMT source is grounded, and its drain goes to the emitter of a >>>>>>>>>>>> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead. >>>>>>>>>>>> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN >>>>>>>>>>>> wants to oscillate at around 8 GHz.) >>>>>>>>>>>> >>>>>>>>>>>> In actual use, the gate will be driven from some high impedance, >>>>>>>>>>>> so it >>>>>>>>>>>> isn't instantly clear that a bead will help there. Any experience >>>>>>>>>>>> with >>>>>>>>>>>> stabilizing such a device? >>>>>>>>>>>> >>>>>>>>>>> Question: Did you run the trace to the gate at the other side of the >>>>>>>>>>> ground/supply planes or sandwiched in? >>>>>>>>>>> >>>>>>>>>>> A photo and maybe the layout would help. >>>>>>>>>>> >>>>>>>>>> See http://electrooptical.net/www/sed/pHEMT_probe/pHEMT_probe.html >>>>>>>>>> >>>>>>>>>> There are a few tweaks: The zero-ohm resistor is actually a 1 uF cap, >>>>>>>>>> replaced with 1 pF for input capacitance measurements. the pHEMT is a >>>>>>>>>> SKY65050 or NE3508, and the collector load is 200 ohms. Supplies are >>>>>>>>>> about +-10 V. >>>>>>>>>> >>>>>>>>> The photo is a bit fuzzy in the upper left but it looks like the input >>>>>>>>> and output traces are all on the top layer. Input and output should be >>>>>>>>> on opposite sides of a plane. This doesn't have to be a ground plane but >>>>>>>>> should not be a split one. I can see some sort of split (faint dark >>>>>>>>> lines). >>>>>>>>> >>>>>>>>> Can't see C5, looks like it may be a bit far away. Which is a concern. >>>>>>>>> Generally, I'd always have a 0.1uF 0603 (or smaller size) for bypassing. >>>>>>>>> And smack dab at the top of R2. Anyhow, that trace from Q2 to R2 and >>>>>>>>> then on to U1 is a bit long and too close to the input trace (R11). >>>>>>>>> Moving U1 north by about 0.500" would clean that up quite well. >>>>>>>>> >>>>>>>>> The via for C1 is too far from it, looks like more than 0.100". >>>>>>>>> >>>>>>>>> A little ferrite rock in front of Q1's gate would help, right at Q1. But >>>>>>>>> it must be on the other side. Looks like you can afford a roll-off since >>>>>>>>> the THS3091 isn't really a rocket, rolling off around 200MHz. The bypass >>>>>>>>> cap C4 is too far away from it. >>>>>>>>> >>>>>>>>> Please don't take this as dissing, just some ideas how to make the >>>>>>>>> layout a little better :-) >>>>>>>>> >>>>>>>> Thanks. >>>>>>>> >>>>>>>> No worries about dissing me about the layout--If it's wrong, Mother >>>>>>>> Nature will point it out a lot less gently than you. >>>>>>>> >>>>>>>> The via near C1 isn't the ground side, it's the R9 side. C1, C5, and Q1 >>>>>>>> all have ground vias right inside their pads. There's a ground cutout >>>>>>>> under the pHEMT to reduce the pad capacitances, with the source leads >>>>>>>> grounded on either corner of the cutout. (There's a zipfile with the >>>>>>>> board layout in PDF and Gerber.) >>>>>>>> >>>>>> I looked in the Gerbers: C5 is way out there in Podunk. So in essence >>>>>> you possibly have R2 connected to a large loop, a.k.a. antenna :-) >>>>> Hmm. I was sort of relying on the 1k resistor for some isolation, but >>>>> that might be the wrong answer. >>>> C5 must be right at the other end of R2. Any sort of trace results not >>>> only in an inductance which causes undesired gain peaking but also an >>>> antenna that will do who knows what. >>>> >>>>>> The Gerbers imported with errors so I can't see where the via north of >>>>>> R2 goes to, but that trace in itself is an inductor as well. If that via >>>>>> doesn't go anywhere useful, could you trip another cap to GND right at >>>>>> R2? Maybe over to the via at C6. >>>>>> >>>>>>>> I'm not sure that it's input-output related, because it's lovely and >>>>>>>> stable with a SKY65050, which theoretically has about the same f_max as >>>>>>>> the NE3508, but I could certainly be mistaken about that. >>>>>>>> >>>>>> Old Muntz would have said "Then use the Skyworks transistor" :-) >>>>> Yup, and if I could live with 0.8 pF C_in and 0.5 nV noise, I would. >>>>> Unfortunately there's that 62-electron signal to deal with, and I really >>>>> need 0.3 pF C_in for the wire-bonded pHEMT die. (I realize that I get >>>>> hit with ~0.15-0.2 pF per pad even with 0603s.) The issue as usual is >>>>> input capacitance differentiating the input noise of the front end amp. >>>> I am sure you'll get it stable. Even if it takes some dirty tricks like >>>> series resonant circuits. A network analyzer style plot goes a long ways >>>> here. That's why having at least a spectrum analyzer with tracking >>>> generator is a good thing. >>>> >>>>>>>> C4 is only about 0.1 inch from the THS3091 (see the C-grid header for >>>>>>>> scale). Is that really too far for a 200 MHz amp? It should be only >>>>>>>> about 2 nH or so. >>>>>>>> >>>>>>>> Cheers >>>>>>>> >>>>>>>> Phil Hobbs >>>>>>>> >>>>>>> Oh, and those dark lines aren't splits in the ground plane, they're >>>>>>> traces on level 2--the stackup is 1&2 signals, 3 ground, 4 signals, >>>>>>> chosen in order to reduce the pad capacitances. >>>>>>> >>>>>> Ah, ok. Just don't do that on larger boards because it can result in >>>>>> warpage and assembler unhappiness. #2 should be some sort of plane, >>>>>> ideally the ground plane. Then #3 for power planes which can be split. >>>>>> >>>>>> -- >>>>>> Regards, Joerg >>>>>> >>>>>> http://www.analogconsultants.com/ >>>>> Thanks, Joerg, that's a great help. The Gerbers were generated by >>>>> Sunstone from my Eagle files, and they view fine in gerbv, which is part >>>>> of gEDA. >>>>> >>>> If you generate your own with the Eagle CAM processor you can go to >>>> places where it costs less. Then use the difference to take your wife >>>> out to dinner :-) >>>> >>>> A good test is to see if your file load into a software commonly used by >>>> PCB houses, such as GC-Preview. There's a free version with nag screen. >>>> >>>> -- >>>> Regards, Joerg >>>> >>>> http://www.analogconsultants.com/ >>> Thanks. That was actually the first PCB I ever laid out in my life, >>> despite having been a professional designer since 1981--I've always had >>> draughtsmen available for the job. (I also usually stick with parts >>> slower than 20 GHz f_max.) >>> >>> Cheers >>> >>> Phil Hobbs >>> >>> >> I've not followed all Joerg's prose, but one thing that might also help >> is to add a small series RC damping network at the BFP650 emitter. >> >> Did you try to probe the oscillation with a small loop and your spec >> analyzer to see at which frequency (frequencies) it screams? >> >> Putting one's finger everywhere on the board might also help to find a >> solution (damping). Mine have been pretty good at this and yours >> probably work fine too :-) > > I want a small programmable surface-mount electrical equivalent of a > finger. A ferrite bead is the closest, so far. >I've once found it to be more like 100pF+470R series to ground... It was on a complicated multiphase PWM current source board, which was incomprehensibly unstable at low voltage. I have been driven crazy for two days when my magic finger (tm) finally found that the ICs ramp generator wasn't linear in the low duty cycle region and the added snubbfinger linearized it. Ten minutes of cut and try later the finger was declared to be about 100pF+470R (unless it was 100R+470pF) and all was working as it should.> Hey, there might be some seriousness in that. A couple of people make > digitally programmable silicon capacitors, so adding some resistors > might be feasible.-- Thanks, Fred.
Reply by ●March 11, 20122012-03-11
John Larkin wrote:> On Sun, 11 Mar 2012 20:23:13 +0100, Fred Bartoli <" "> wrote: > >> Phil Hobbs a �crit :[...]>>> >> I've not followed all Joerg's prose, but one thing that might also help >> is to add a small series RC damping network at the BFP650 emitter. >> >> Did you try to probe the oscillation with a small loop and your spec >> analyzer to see at which frequency (frequencies) it screams? >> >> Putting one's finger everywhere on the board might also help to find a >> solution (damping). Mine have been pretty good at this and yours >> probably work fine too :-) > > I want a small programmable surface-mount electrical equivalent of a > finger. A ferrite bead is the closest, so far. > > Hey, there might be some seriousness in that. A couple of people make > digitally programmable silicon capacitors, so adding some resistors > might be feasible. >I've done that in product designs where picosecond inaccuracies had to be auto-calibrated out. Use a SD5400 as variable resistors with one of the FETs in the DC path to servo out the TC. For GHz work one probably needs a different chip of that sort, smaller geometries. http://www.calogic.net/pdf/SD5000_Datasheet_Rev_A.pdf They became expensive for a while but unlike crude oil they came back down nicely. Well under $2. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●March 11, 20122012-03-11
Phil Hobbs a �crit :> Fred Bartoli wrote: >> Phil Hobbs a �crit : >>> Joerg wrote: >>>> Phil Hobbs wrote: >>>>> Joerg wrote: >>>>>> Phil Hobbs wrote: >>>>>>> On 03/10/2012 03:41 PM, Phil Hobbs wrote: >>>>>>>> On 03/10/2012 03:21 PM, Joerg wrote: >>>>>>>>> Phil Hobbs wrote: >>>>>>>>>> On 03/09/2012 05:34 PM, Joerg wrote: >>>>>>>>>>> Phil Hobbs wrote: >>>>>>>>>>>> So, JL's thread about level shifting a pHEMT switch brings up another >>>>>>>>>>>> issue: stabilizing the silly things. >>>>>>>>>>>> >>>>>>>>>>>> I have this little SKY65050 preamp that works very well--about a >>>>>>>>>>>> nanoamp >>>>>>>>>>>> of input current, 0.5 nV/sqrt(Hz) noise in the flatband, 0.8 pF Cin, >>>>>>>>>>>> flat from DC to about 120 MHz where the THS3091 second stage craps >>>>>>>>>>>> out. >>>>>>>>>>>> A very nice amplifier, but for the intended use, it could stand a bit >>>>>>>>>>>> less noise and a bit less input capacitance. So I tried it with an >>>>>>>>>>>> NE3508 instead, but I couldn't get it to stop oscillating. >>>>>>>>>>>> >>>>>>>>>>>> The pHEMT source is grounded, and its drain goes to the emitter of a >>>>>>>>>>>> BFP650 cascode NPN, which has a 10-ohm ferrite bead in its base lead. >>>>>>>>>>>> (This bead has high impedance way out beyond 1 GHz--otherwise the NPN >>>>>>>>>>>> wants to oscillate at around 8 GHz.) >>>>>>>>>>>> >>>>>>>>>>>> In actual use, the gate will be driven from some high impedance, >>>>>>>>>>>> so it >>>>>>>>>>>> isn't instantly clear that a bead will help there. Any experience >>>>>>>>>>>> with >>>>>>>>>>>> stabilizing such a device? >>>>>>>>>>>> >>>>>>>>>>> Question: Did you run the trace to the gate at the other side of the >>>>>>>>>>> ground/supply planes or sandwiched in? >>>>>>>>>>> >>>>>>>>>>> A photo and maybe the layout would help. >>>>>>>>>>> >>>>>>>>>> See http://electrooptical.net/www/sed/pHEMT_probe/pHEMT_probe.html >>>>>>>>>> >>>>>>>>>> There are a few tweaks: The zero-ohm resistor is actually a 1 uF cap, >>>>>>>>>> replaced with 1 pF for input capacitance measurements. the pHEMT is a >>>>>>>>>> SKY65050 or NE3508, and the collector load is 200 ohms. Supplies are >>>>>>>>>> about +-10 V. >>>>>>>>>> >>>>>>>>> The photo is a bit fuzzy in the upper left but it looks like the input >>>>>>>>> and output traces are all on the top layer. Input and output should be >>>>>>>>> on opposite sides of a plane. This doesn't have to be a ground plane but >>>>>>>>> should not be a split one. I can see some sort of split (faint dark >>>>>>>>> lines). >>>>>>>>> >>>>>>>>> Can't see C5, looks like it may be a bit far away. Which is a concern. >>>>>>>>> Generally, I'd always have a 0.1uF 0603 (or smaller size) for bypassing. >>>>>>>>> And smack dab at the top of R2. Anyhow, that trace from Q2 to R2 and >>>>>>>>> then on to U1 is a bit long and too close to the input trace (R11). >>>>>>>>> Moving U1 north by about 0.500" would clean that up quite well. >>>>>>>>> >>>>>>>>> The via for C1 is too far from it, looks like more than 0.100". >>>>>>>>> >>>>>>>>> A little ferrite rock in front of Q1's gate would help, right at Q1. But >>>>>>>>> it must be on the other side. Looks like you can afford a roll-off since >>>>>>>>> the THS3091 isn't really a rocket, rolling off around 200MHz. The bypass >>>>>>>>> cap C4 is too far away from it. >>>>>>>>> >>>>>>>>> Please don't take this as dissing, just some ideas how to make the >>>>>>>>> layout a little better :-) >>>>>>>>> >>>>>>>> Thanks. >>>>>>>> >>>>>>>> No worries about dissing me about the layout--If it's wrong, Mother >>>>>>>> Nature will point it out a lot less gently than you. >>>>>>>> >>>>>>>> The via near C1 isn't the ground side, it's the R9 side. C1, C5, and Q1 >>>>>>>> all have ground vias right inside their pads. There's a ground cutout >>>>>>>> under the pHEMT to reduce the pad capacitances, with the source leads >>>>>>>> grounded on either corner of the cutout. (There's a zipfile with the >>>>>>>> board layout in PDF and Gerber.) >>>>>>>> >>>>>> I looked in the Gerbers: C5 is way out there in Podunk. So in essence >>>>>> you possibly have R2 connected to a large loop, a.k.a. antenna :-) >>>>> Hmm. I was sort of relying on the 1k resistor for some isolation, but >>>>> that might be the wrong answer. >>>> C5 must be right at the other end of R2. Any sort of trace results not >>>> only in an inductance which causes undesired gain peaking but also an >>>> antenna that will do who knows what. >>>> >>>>>> The Gerbers imported with errors so I can't see where the via north of >>>>>> R2 goes to, but that trace in itself is an inductor as well. If that via >>>>>> doesn't go anywhere useful, could you trip another cap to GND right at >>>>>> R2? Maybe over to the via at C6. >>>>>> >>>>>>>> I'm not sure that it's input-output related, because it's lovely and >>>>>>>> stable with a SKY65050, which theoretically has about the same f_max as >>>>>>>> the NE3508, but I could certainly be mistaken about that. >>>>>>>> >>>>>> Old Muntz would have said "Then use the Skyworks transistor" :-) >>>>> Yup, and if I could live with 0.8 pF C_in and 0.5 nV noise, I would. >>>>> Unfortunately there's that 62-electron signal to deal with, and I really >>>>> need 0.3 pF C_in for the wire-bonded pHEMT die. (I realize that I get >>>>> hit with ~0.15-0.2 pF per pad even with 0603s.) The issue as usual is >>>>> input capacitance differentiating the input noise of the front end amp. >>>> I am sure you'll get it stable. Even if it takes some dirty tricks like >>>> series resonant circuits. A network analyzer style plot goes a long ways >>>> here. That's why having at least a spectrum analyzer with tracking >>>> generator is a good thing. >>>> >>>>>>>> C4 is only about 0.1 inch from the THS3091 (see the C-grid header for >>>>>>>> scale). Is that really too far for a 200 MHz amp? It should be only >>>>>>>> about 2 nH or so. >>>>>>>> >>>>>>>> Cheers >>>>>>>> >>>>>>>> Phil Hobbs >>>>>>>> >>>>>>> Oh, and those dark lines aren't splits in the ground plane, they're >>>>>>> traces on level 2--the stackup is 1&2 signals, 3 ground, 4 signals, >>>>>>> chosen in order to reduce the pad capacitances. >>>>>>> >>>>>> Ah, ok. Just don't do that on larger boards because it can result in >>>>>> warpage and assembler unhappiness. #2 should be some sort of plane, >>>>>> ideally the ground plane. Then #3 for power planes which can be split. >>>>>> >>>>>> -- >>>>>> Regards, Joerg >>>>>> >>>>>> http://www.analogconsultants.com/ >>>>> Thanks, Joerg, that's a great help. The Gerbers were generated by >>>>> Sunstone from my Eagle files, and they view fine in gerbv, which is part >>>>> of gEDA. >>>>> >>>> If you generate your own with the Eagle CAM processor you can go to >>>> places where it costs less. Then use the difference to take your wife >>>> out to dinner :-) >>>> >>>> A good test is to see if your file load into a software commonly used by >>>> PCB houses, such as GC-Preview. There's a free version with nag screen. >>>> >>>> -- >>>> Regards, Joerg >>>> >>>> http://www.analogconsultants.com/ >>> Thanks. That was actually the first PCB I ever laid out in my life, >>> despite having been a professional designer since 1981--I've always had >>> draughtsmen available for the job. (I also usually stick with parts >>> slower than 20 GHz f_max.) >>> >>> Cheers >>> >>> Phil Hobbs >>> >>> >> I've not followed all Joerg's prose, but one thing that might also help >> is to add a small series RC damping network at the BFP650 emitter. >> >> Did you try to probe the oscillation with a small loop and your spec >> analyzer to see at which frequency (frequencies) it screams? >> >> Putting one's finger everywhere on the board might also help to find a >> solution (damping). Mine have been pretty good at this and yours >> probably work fine too :-) >> >> -- >> Thanks, >> Fred. > > Thanks, Fred. The Dremelled hand-made proto oscillated at about 12 GHz, > which I measured by watching the amplitude go up and down as I moved my > hand within an inch or two of the surface. It went from peak to valley > in about 1/4 inch, ergo, roughly 12 GHz. >He he, nice! 12GHz is pretty high. Just think that a 1nH via is 75R at that frequency for example.> > I'll try snubbing the BJT emitter and see if that helps. > > Cheers > > Phil Hobbs > > > >-- Thanks, Fred.
