dagmargoodboat@yahoo.com wrote:> 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 �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. > > 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. >At my first employer back in 1986 we already did all SMT. But we were early adopters and that wasn't common in those days. With wired caps things are different. A multi-layer SMT is essentially a whole lot of smaller caps stacked in parallel, like a high-rise building with its umpteen floors.> 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. >It can work but iffy. Because in the end the inductance is what limits the effectiveness of bypassing and that strictly goes by size.> >> 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. > > 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. >It's a sure-fire way of telling something oscillates, usually. But amn easier one is to slowly crank up the supply voltage and watch an analog ammeter. There will be a sudden jump.> 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. >I think the main problem is the missing decoupling at R2. There's a lengthy trace in series with it and that forms a resonant structure, peaks the gain at a very high frequency. That's usually where it'll oscillate. -- Regards, Joerg http://www.analogconsultants.com/
Stabilizing pHEMTs
Started by ●March 9, 2012
Reply by ●March 12, 20122012-03-12
Reply by ●March 12, 20122012-03-12
On 03/11/2012 08:47 PM, 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.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!The output stage is a CFB op amp with about 120 MHz BW. I could try using a bit of stripped-back coax as an antenna, but I sort of like my digital wavemeter. ;)> > A spectrum analyzer made measuring and detecting oscillations > and tuning sensitivities trivial. The finger calibration took > a little longer. >Yup. I've got a saved search in for an 8566B at this point. Should have bitten the bullet and got the nitrogen dewar as well. If this project goes to the next stage, I will. 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 12, 20122012-03-12
On 03/11/2012 04:11 PM, Joerg wrote:> 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. >Cool. I remember those--they're pretty similar to the late lamented Si8901 MOSFET ring mixer. 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 12, 20122012-03-12
Phil Hobbs wrote:> On 03/11/2012 04:11 PM, Joerg wrote: >> 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. >> > Cool. I remember those--they're pretty similar to the late lamented > Si8901 MOSFET ring mixer. >Don't forget to connect the substrate when trying them out. As embarrassing as it is, that happened on my first copperclad lash-up. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●March 12, 20122012-03-12
On 03/12/2012 12:40 PM, Joerg wrote:> Phil Hobbs wrote: >> On 03/11/2012 04:11 PM, Joerg wrote: >>> 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. >>> >> Cool. I remember those--they're pretty similar to the late lamented >> Si8901 MOSFET ring mixer. >> > > Don't forget to connect the substrate when trying them out. As > embarrassing as it is, that happened on my first copperclad lash-up. >Roight. I discover that Vishay Siliconix has re-used the Si8901 part number for an unrelated product! I'm looking at the NMOS ring modulator datasheet from the 1989 databook and the dual PMOS datasheet from web site--same P/N. Probably someone wanted to make sure the old part never ever came back. 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 12, 20122012-03-12
Phil Hobbs wrote:> On 03/12/2012 12:40 PM, Joerg wrote: >> Phil Hobbs wrote: >>> On 03/11/2012 04:11 PM, Joerg wrote: >>>> 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. >>>> >>> Cool. I remember those--they're pretty similar to the late lamented >>> Si8901 MOSFET ring mixer. >>> >> >> Don't forget to connect the substrate when trying them out. As >> embarrassing as it is, that happened on my first copperclad lash-up. >> > > Roight. > > I discover that Vishay Siliconix has re-used the Si8901 part number for > an unrelated product! I'm looking at the NMOS ring modulator datasheet > from the 1989 databook and the dual PMOS datasheet from web site--same P/N. >This? http://www.datasheetarchive.com/dl/Datasheets-111/DSAP0030588.pdf> Probably someone wanted to make sure the old part never ever came back. >In the days of the Intenet that's next to impossible. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●March 12, 20122012-03-12
On Mar 12, 9:53=A0am, Joerg <inva...@invalid.invalid> wrote:> dagmargoodb...@yahoo.com wrote:> > Something like 100nF || 1nF || 22pF was handy in certain > > situations. =A0I think that effectively prevents resonance > > peaks too, since no two of the L-C tanks will resonate at > > any given frequency. > > It can work but iffy. Because in the end the inductance is what limits > the effectiveness of bypassing and that strictly goes by size.Right, but at r.f. it's entirely possible the cap will self-resonate. That's what stacking values avoids. For example, 1nH parallel-resonates with 1uF at 5MHz, and with 10nF at 500MHz. Either capacitor alone in a broadband amp runs risk of resonance, but the two in parallel avoids it. [snip]> > Also, the fact that a finger in the area changes anything at > > all is noteworthy--that's a very small amount of coupling. > > It's a sure-fire way of telling something oscillates, usually. But amn > easier one is to slowly crank up the supply voltage and watch an analog > ammeter. There will be a sudden jump. > > > 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. > > I think the main problem is the missing decoupling at R2. There's a > lengthy trace in series with it and that forms a resonant structure, > peaks the gain at a very high frequency. That's usually where it'll > oscillate.That radiates and capacitively couples--certainly not helpful. I wonder about the gate line too. Might need fancy treatment, like a bootstrapped shield. Whew. -- Cheers, James Arthur
Reply by ●March 12, 20122012-03-12
On Mar 11, 8:47=A0pm, dagmargoodb...@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 =E9crit : > > > > >> Fred Bartoli wrote: > > > > > >>> I've not followed all Joerg's prose, but one thing that might a=lso help> > > > >>> is to add a small series RC damping network at the BFP650 emitt=er.> > > > > >>> 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 your=s> > > > >>> probably work fine too :-) > > > > > >>> -- > > > > >>> Thanks, > > > > >>> Fred. > > > > > >> Thanks, Fred. =A0The Dremelled hand-made proto oscillated at abo=ut 12 GHz,> > > > >> which I measured by watching the amplitude go up and down as I m=oved my> > > > >> hand within an inch or two of the surface. =A0It 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 fre=quency> > > > >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. =A0My 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. =A0If a wire > > > passed through Q1's source via(s) tunes f.osc up, for example... > > > > Fred's finger-probe is a good idea. =A0Fred needs to send Phil his > > > finger. > > > Oh, I have a pretty well educated finger of my own. =A0Plus another nin=e> > if that one fails. ;) > > > The pHEMT has two source pads, both of which are grounded by a via righ=t> > inside the pad. =A0So the ground inductance is way below 1 nH. =A0Apart=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. =A0The bypass caps are 0603 MLC as well, so I'd expect them to l=ook> > like ~1 nH inductors above about 10 MHz. =A0What did you measure on you=rs?> > I didn't actually measure, but went by the datasheet impedance-vs-f > curves. =A0I 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. =A0That'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. =A01uF 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. =A0Oscillations usually rail. =A0I burned up a handful > of 8GHz transistors as part of learning that! > > A spectrum analyzer made measuring and detecting oscillations > and tuning sensitivities trivial. =A0The finger calibration took > a little longer. > > -- > Cheers, > James Arthur- Hide quoted text - > > - Show quoted text -Lurking... Just wondering (out loud) if the 'small' oscillations could be pick-up? With Phil's finger providing the antenna or shield.. (depedning on the disposition of his other digits) George H.
Reply by ●March 12, 20122012-03-12
On 03/12/2012 02:15 PM, George Herold wrote:> On Mar 11, 8:47 pm, dagmargoodb...@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.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. >> >> -- >> Cheers, >> James Arthur- Hide quoted text - >> >> - Show quoted text - > > Lurking... Just wondering (out loud) if the 'small' oscillations could > be pick-up? With Phil's finger providing the antenna or shield.. > (depedning on the disposition of his other digits) > > George H.For testing, it runs inside a nickel-plated steel can (used for 70 mm film reels). There are some nice Russian 47-nF feedthrough caps on the power supplies, so it's clean as a whistle. Open the box and it goes nuts. 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 12, 20122012-03-12
On 03/12/2012 01:09 PM, Joerg wrote:> Phil Hobbs wrote: >> On 03/12/2012 12:40 PM, Joerg wrote: >>> Phil Hobbs wrote: >>>> On 03/11/2012 04:11 PM, Joerg wrote: >>>>> 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. >>>>> >>>> Cool. I remember those--they're pretty similar to the late lamented >>>> Si8901 MOSFET ring mixer. >>>> >>> >>> Don't forget to connect the substrate when trying them out. As >>> embarrassing as it is, that happened on my first copperclad lash-up. >>> >> >> Roight. >> >> I discover that Vishay Siliconix has re-used the Si8901 part number for >> an unrelated product! I'm looking at the NMOS ring modulator datasheet >> from the 1989 databook and the dual PMOS datasheet from web site--same P/N. >> > > This? > > http://www.datasheetarchive.com/dl/Datasheets-111/DSAP0030588.pdfYup, that's the old one. Made some really amazing mixers.> > >> Probably someone wanted to make sure the old part never ever came back. >> > > In the days of the Intenet that's next to impossible. >"You can't bring that part back, it'll screw up our documentation system." 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
