Fred Bartoli wrote:> 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. >And the full wavelength is about one inch. Which makes that long trace from R2 really suspicious ... :-) Even the little stub above R2 can make things resonate and then oscillate. -- Regards, Joerg http://www.analogconsultants.com/
Stabilizing pHEMTs
Started by ●March 9, 2012
Reply by ●March 11, 20122012-03-11
Reply by ●March 11, 20122012-03-11
On Sun, 11 Mar 2012 21:13:39 +0100, Fred Bartoli <" "> wrote:>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.Yup, so it's hard to ground the sources of the phemt with just a couple of vias. I suggest a topside copper pour, with a lot of vias, and maybe extend the topside pour to pick up other nearby grounds, and maybe a cap or RC from drain to source copper. I like to make the ground plane layer 2, to keep the ground vias short. -- 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
Fred Bartoli wrote:> > 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.Those pHEMTs and SiGe:C NPNs are hotter than a two-dollar pistol. I wish they had better 1/f behaviour, though. 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 Sunday, March 11, 2012 4:52:37 PM UTC-4, John Larkin wrote:> On Sun, 11 Mar 2012 21:13:39 +0100, Fred Bartoli <" "> wrote: > >Phil Hobbs a =E9crit : > >> Fred Bartoli wrote:> >>> I've not followed all Joerg's prose, but one thing that might also he=lp> >>> 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. > >>=20 > >> Thanks, Fred. The Dremelled hand-made proto oscillated at about 12 GH=z,> >> which I measured by watching the amplitude go up and down as I moved m=y> >> hand within an inch or two of the surface. It went from peak to valle=y> >> in about 1/4 inch, ergo, roughly 12 GHz.=20 > >>=20 > > > >He he, nice! > > > >12GHz is pretty high. Just think that a 1nH via is 75R at that frequency==20> >for example. >=20 > Yup, so it's hard to ground the sources of the phemt with just a > couple of vias. I suggest a topside copper pour, with a lot of vias, > and maybe extend the topside pour to pick up other nearby grounds, and > maybe a cap or RC from drain to source copper.ISTM the pHEMT source is effectively "not grounded" at 12GHz. And, feedthrus might even couple and feed back (though I'm not saying they are). 1uF ceramic C1 won't be stiff at UHF either, not the ones I scouted anyhow. An old r.f. rule was that inductive source + inductive load an oscillator makes. My first 1GHz amplifier breadboards much preferred oscillating at 5GHz to amplifying at 1GHz :-). Phil, even a frequency counter would give you really good insight about if/how layout contributes to the oscillation. If a wire passed through Q1's source via(s) tunes f.osc up, for example... Fred's finger-probe is a good idea. Fred needs to send Phil his finger. --=20 Cheers, James Arthur
Reply by ●March 11, 20122012-03-11
dagmargoodboat@yahoo.com wrote:> On Sunday, March 11, 2012 4:52:37 PM UTC-4, John Larkin wrote: >> On Sun, 11 Mar 2012 21:13:39 +0100, Fred Bartoli <" "> wrote: >>> Phil Hobbs a �crit : >>>> Fred Bartoli wrote: > >>>>> 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. >> Yup, so it's hard to ground the sources of the phemt with just a >> couple of vias. I suggest a topside copper pour, with a lot of vias, >> and maybe extend the topside pour to pick up other nearby grounds, and >> maybe a cap or RC from drain to source copper. > > ISTM the pHEMT source is effectively "not grounded" at 12GHz. > And, feedthrus might even couple and feed back (though I'm > not saying they are). > > 1uF ceramic C1 won't be stiff at UHF either, not the ones I > scouted anyhow. >They actually do, provided they are small. Size is what mostly makes a modern MLCC suitable or not. 0805 is not so cool here, 0402 or smaller would be better. At least 0603. Trace lengths should essentially be zero and often it can't hurt to make the side with the DC voltage on it a small sub-plane. Haven't tried 1uF myself since I use mostly 0.1uF but this one looks good: http://search.digikey.com/us/en/products/C0603X5R0J105M/445-8008-2-ND/2792227 If Phil has more volts on there maybe this one: http://search.digikey.com/us/en/products/C1005X5R1C105M/445-4979-2-ND/2093247> An old r.f. rule was that inductive source + inductive load > an oscillator makes. My first 1GHz amplifier breadboards > much preferred oscillating at 5GHz to amplifying at 1GHz :-). >I always thought Murphy is causing oscillations :-)> Phil, even a frequency counter would give you really good insight > about if/how layout contributes to the oscillation. If a wire > passed through Q1's source via(s) tunes f.osc up, for example... >Usually anything getting close to the area tunes f-osc.> Fred's finger-probe is a good idea. Fred needs to send Phil his > finger. >For proper maintenance that should come with a box of Cote-du-Rhone, but only the good stuff. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●March 11, 20122012-03-11
dagmargoodboat@yahoo.com wrote:> > On Sunday, March 11, 2012 4:52:37 PM UTC-4, John Larkin wrote: > > On Sun, 11 Mar 2012 21:13:39 +0100, Fred Bartoli <" "> wrote: > > >Phil Hobbs a �crit : > > >> Fred Bartoli wrote: > > > >>> 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. > > > > Yup, so it's hard to ground the sources of the phemt with just a > > couple of vias. I suggest a topside copper pour, with a lot of vias, > > and maybe extend the topside pour to pick up other nearby grounds, and > > maybe a cap or RC from drain to source copper. > > ISTM the pHEMT source is effectively "not grounded" at 12GHz. > And, feedthrus might even couple and feed back (though I'm > not saying they are). > > 1uF ceramic C1 won't be stiff at UHF either, not the ones I > scouted anyhow. > > An old r.f. rule was that inductive source + inductive load > an oscillator makes. My first 1GHz amplifier breadboards > much preferred oscillating at 5GHz to amplifying at 1GHz :-). > > Phil, even a frequency counter would give you really good insight > about if/how layout contributes to the oscillation. If a wire > passed through Q1's source via(s) tunes f.osc up, for example... > > Fred's finger-probe is a good idea. Fred needs to send Phil his > finger. > > -- > Cheers, > James ArthurOh, I have a pretty well educated finger of my own. Plus another nine if that one fails. ;) The pHEMT has two source pads, both of which are grounded by a via right inside the pad. So the ground inductance is way below 1 nH. Apart from the bypass for R2, which Joerg pointed out, there aren't any actual traces longer than about 0.08 inch in the signal path. The symptom of the oscillation is a DC bias shift that varies depending on where I put my fingers, even when they aren't actually touching the board. The bypass caps are 0603 MLC as well, so I'd expect them to look like ~1 nH inductors above about 10 MHz. What did you measure on yours? I have a 20 GHz scope and an 18 GHz counter, but the oscillation is too small for the counter to see. 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:> dagmargoodboat@yahoo.com wrote:[...]>> Fred's finger-probe is a good idea. Fred needs to send Phil his >> finger. >> >> -- >> Cheers, >> James Arthur > > Oh, I have a pretty well educated finger of my own. Plus another nine > if that one fails. ;) > > The pHEMT has two source pads, both of which are grounded by a via right > inside the pad. So the ground inductance is way below 1 nH. Apart from > the bypass for R2, which Joerg pointed out, there aren't any actual > traces longer than about 0.08 inch in the signal path. >The trace from collector to the input of U1 is a lot longer. Same for the feedback path around U1. Or the path to U3. Some of that is avoidable, some isn't. The big rule in GHz-work is to "break the line of sight". But you have to fix the R2 issue. I don't see any way to make this work reliably without fixing that. C5 just isn't going to be able to do its job where it is right now.> The symptom of the oscillation is a DC bias shift that varies depending > on where I put my fingers, even when they aren't actually touching the > board. The bypass caps are 0603 MLC as well, so I'd expect them to look > like ~1 nH inductors above about 10 MHz. What did you measure on yours? >A DC shift usually suggests a pretty hefty oscillation. A drab-green helicopter circling the building might indicate that it got out :-)> I have a 20 GHz scope and an 18 GHz counter, but the oscillation is too > small for the counter to see. >That's where one of these would come in handy: http://www.triquint.com/products/d/DOC-A-00000921 -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●March 11, 20122012-03-11
On Sunday, March 11, 2012 7:35:53 PM UTC-4, Phil Hobbs wrote:> dagmargood... wrote: > >=20 > > On Sunday, March 11, 2012 4:52:37 PM UTC-4, John Larkin wrote: > > > On Sun, 11 Mar 2012 21:13:39 +0100, Fred Bartoli <" "> wrote: > > > >Phil Hobbs a =E9crit : > > > >> Fred Bartoli wrote: > >=20 > > > >>> I've not followed all Joerg's prose, but one thing that might als=o 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 s=pec> > > >>> analyzer to see at which frequency (frequencies) it screams? > > > >>> > > > >>> Putting one's finger everywhere on the board might also help to f=ind 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 1=2 GHz,> > > >> which I measured by watching the amplitude go up and down as I mov=ed my> > > >> hand within an inch or two of the surface. It went from peak to v=alley> > > >> 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 frequ=ency> > > >for example. > > > > > > Yup, so it's hard to ground the sources of the phemt with just a > > > couple of vias. I suggest a topside copper pour, with a lot of vias, > > > and maybe extend the topside pour to pick up other nearby grounds, an=d> > > maybe a cap or RC from drain to source copper. > >=20 > > ISTM the pHEMT source is effectively "not grounded" at 12GHz. > > And, feedthrus might even couple and feed back (though I'm > > not saying they are). > >=20 > > 1uF ceramic C1 won't be stiff at UHF either, not the ones I > > scouted anyhow. > >=20 > > An old r.f. rule was that inductive source + inductive load > > an oscillator makes. My first 1GHz amplifier breadboards > > much preferred oscillating at 5GHz to amplifying at 1GHz :-). > >=20 > > Phil, even a frequency counter would give you really good insight > > about if/how layout contributes to the oscillation. If a wire > > passed through Q1's source via(s) tunes f.osc up, for example... > >=20 > > Fred's finger-probe is a good idea. Fred needs to send Phil his > > finger. > >=20 >=20 > Oh, I have a pretty well educated finger of my own. Plus another nine > if that one fails. ;) >=20 > The pHEMT has two source pads, both of which are grounded by a via right > inside the pad. So the ground inductance is way below 1 nH. Apart from > the bypass for R2, which Joerg pointed out, there aren't any actual > traces longer than about 0.08 inch in the signal path. >=20 > The symptom of the oscillation is a DC bias shift that varies depending > on where I put my fingers, even when they aren't actually touching the > board. The bypass caps are 0603 MLC as well, so I'd expect them to look > like ~1 nH inductors above about 10 MHz. What did you measure on yours?I didn't actually measure, but went by the datasheet impedance-vs-f curves. I might even have the datasheets in storage somewhere. That said, 1nH for C1 (if it were that high) =3D 75 ohms at 12GHz, plus the feedthru. That's not trivial. This (75-ish ohms) very roughly jives with Fig. 4 in this document: http://www.avx.com/docs/techinfo/parasitc.pdf I used 100pF bypasses at UHF, to avoid self-resonance problems IIRC. (If the transistor's hot enough a suitable cap becomes an LC tank at some frequency, which the transistor will duly find and resonate.) That said, by the AVX report I linked, typical SMD caps are quite a bit better than I thought. 1uF in 0603 may not be any worse ESL-wise than 100nF, or even 1nF or 100pF in 0603.> I have a 20 GHz scope and an 18 GHz counter, but the oscillation is too > small for the counter to see.That's odd. Oscillations usually rail. I burned up a handful of 8GHz transistors as part of learning that! A spectrum analyzer made measuring and detecting oscillations and tuning sensitivities trivial. The finger calibration took a little longer. --=20 Cheers, James Arthur
Reply by ●March 11, 20122012-03-11
On Sunday, March 11, 2012 6:52:11 PM UTC-4, Joerg wrote:> dagmarg... wrote: > > On Sunday, March 11, 2012 4:52:37 PM UTC-4, John Larkin wrote: > >> On Sun, 11 Mar 2012 21:13:39 +0100, Fred Bartoli <" "> wrote: > >>> Phil Hobbs a =E9crit : > >>>> Fred Bartoli wrote: > >=20 > >>>>> 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 spe=c> >>>>> analyzer to see at which frequency (frequencies) it screams? > >>>>> > >>>>> Putting one's finger everywhere on the board might also help to fin=d 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 val=ley> >>>> in about 1/4 inch, ergo, roughly 12 GHz.=20 > >>>> > >>> He he, nice! > >>> > >>> 12GHz is pretty high. Just think that a 1nH via is 75R at that freque=ncy=20> >>> for example. > >> Yup, so it's hard to ground the sources of the phemt with just a > >> couple of vias. I suggest a topside copper pour, with a lot of vias, > >> and maybe extend the topside pour to pick up other nearby grounds, and > >> maybe a cap or RC from drain to source copper. > >=20 > > ISTM the pHEMT source is effectively "not grounded" at 12GHz. > > And, feedthrus might even couple and feed back (though I'm > > not saying they are). > >=20 > > 1uF ceramic C1 won't be stiff at UHF either, not the ones I > > scouted anyhow. > >=20 >=20 > They actually do, provided they are small. Size is what mostly makes a > modern MLCC suitable or not. 0805 is not so cool here, 0402 or smaller > would be better. At least 0603. Trace lengths should essentially be zero > and often it can't hurt to make the side with the DC voltage on it a > small sub-plane.Digging up a few datasheets you're right--it's better than I thought. I did that work over 20 years ago--I'm not sure if capacitors have improved, or if I'm just not remembering something. I remember looking at bypass impedance carefully, and choosing smaller-valued caps for the highest frequencies as a result. Something like 100nF || 1nF || 22pF was handy in certain situations. I think that effectively prevents resonance peaks too, since no two of the L-C tanks will resonate at any given frequency.> Haven't tried 1uF myself since I use mostly 0.1uF but this one looks good=:>=20 > http://search.digikey.com/us/en/products/C0603X5R0J105M/445-8008-2-ND/279=2227>=20 > If Phil has more volts on there maybe this one: >=20 > http://search.digikey.com/us/en/products/C1005X5R1C105M/445-4979-2-ND/209=3247>=20 >=20 > > An old r.f. rule was that inductive source + inductive load > > an oscillator makes. My first 1GHz amplifier breadboards > > much preferred oscillating at 5GHz to amplifying at 1GHz :-). > >=20 >=20 > I always thought Murphy is causing oscillations :-) >=20 >=20 > > Phil, even a frequency counter would give you really good insight > > about if/how layout contributes to the oscillation. If a wire > > passed through Q1's source via(s) tunes f.osc up, for example... > >=20 >=20 > Usually anything getting close to the area tunes f-osc.Yes, but even that gives you cues. E.g., if approaching one side tunes the thing higher, and the other side tunes it lower. Also, the fact that a finger in the area changes anything at all is noteworthy--that's a very small amount of coupling. An adjunct to the finger is a grounded piece of shield, placed between this and that to see if / where they're coupling. Of course the coupling could be inside Q1 itself--I haven't scrutinized the specs--but external effects can be swiftly gauged and assessed with the methods we've been discussing. Starting with solid grounded copper foil on top, then removing the absolute minimum sure saves an awful lot of mystery and learning in these situations. =20 --=20 Cheers, James Arthur
Reply by ●March 11, 20122012-03-11
On Sun, 11 Mar 2012 17:47:30 -0700 (PDT), dagmargoodboat@yahoo.com wrote:>On Sunday, March 11, 2012 7:35:53 PM UTC-4, Phil Hobbs wrote: >> dagmargood... wrote: >> > >> > On Sunday, March 11, 2012 4:52:37 PM UTC-4, John Larkin wrote: >> > > On Sun, 11 Mar 2012 21:13:39 +0100, Fred Bartoli <" "> wrote: >> > > >Phil Hobbs a �crit : >> > > >> Fred Bartoli wrote: >> > >> > > >>> 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. >> > > >> > > Yup, so it's hard to ground the sources of the phemt with just a >> > > couple of vias. I suggest a topside copper pour, with a lot of vias, >> > > and maybe extend the topside pour to pick up other nearby grounds, and >> > > maybe a cap or RC from drain to source copper. >> > >> > ISTM the pHEMT source is effectively "not grounded" at 12GHz. >> > And, feedthrus might even couple and feed back (though I'm >> > not saying they are). >> > >> > 1uF ceramic C1 won't be stiff at UHF either, not the ones I >> > scouted anyhow. >> > >> > An old r.f. rule was that inductive source + inductive load >> > an oscillator makes. My first 1GHz amplifier breadboards >> > much preferred oscillating at 5GHz to amplifying at 1GHz :-). >> > >> > Phil, even a frequency counter would give you really good insight >> > about if/how layout contributes to the oscillation. If a wire >> > passed through Q1's source via(s) tunes f.osc up, for example... >> > >> > Fred's finger-probe is a good idea. Fred needs to send Phil his >> > finger. >> > >> >> Oh, I have a pretty well educated finger of my own. Plus another nine >> if that one fails. ;) >> >> The pHEMT has two source pads, both of which are grounded by a via right >> inside the pad. So the ground inductance is way below 1 nH. Apart from >> the bypass for R2, which Joerg pointed out, there aren't any actual >> traces longer than about 0.08 inch in the signal path. >> >> The symptom of the oscillation is a DC bias shift that varies depending >> on where I put my fingers, even when they aren't actually touching the >> board. The bypass caps are 0603 MLC as well, so I'd expect them to look >> like ~1 nH inductors above about 10 MHz. What did you measure on yours? > >I didn't actually measure, but went by the datasheet impedance-vs-f >curves. I might even have the datasheets in storage somewhere. > >That said, 1nH for C1 (if it were that high) = 75 ohms at 12GHz, >plus the feedthru. That's not trivial. > >This (75-ish ohms) very roughly jives with Fig. 4 in this document: >http://www.avx.com/docs/techinfo/parasitc.pdfCool. We are just now starting to use "sideways" caps, like 0306 types. Looks like we can expect about half the ESL of regular caps. -- 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
