Phil Hobbs wrote:> 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.pdf > > Yup, that's the old one. Made some really amazing mixers. >The best mixer ever was the Plessey SL6440. Before that ship went under I quickly bought a small stash. It follows the mantra "Nothing can replace a high IP3 rating, except for a higher IP3 rating".>> >> >>> 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." >They should do the right thing and retire old part numbers. Just like it's done with jerseys in football. BTW, I sent you a few PM replies (totally other subject though) but just realized that it was your gmail address and the one above is different. If they didn't make it I can re-send. -- 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 03/12/2012 02:53 AM, John Larkin wrote:> Cool. We are just now starting to use "sideways" caps, like 0306 > types. Looks like we can expect about half the ESL of regular caps.Take a look at X2Y capacitors from Yageo and Johanson. I think they are promising on relatively thick boards because their via distribution makes some vertical currents cancel (for deblocking planes). I have bought a few from DK, but not yet formally tested. regards, Gerhard
Reply by ●March 12, 20122012-03-12
Gerhard Hoffmann wrote:> On 03/12/2012 02:53 AM, John Larkin wrote: > >> Cool. We are just now starting to use "sideways" caps, like 0306 >> types. Looks like we can expect about half the ESL of regular caps. > > Take a look at X2Y capacitors from Yageo and Johanson. I think they > are promising on relatively thick boards because their via distribution > makes some vertical currents cancel (for deblocking planes). > I have bought a few from DK, but not yet formally tested. >But talk to your assemblers first, don't drop those on them as a surprise. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●March 13, 20122012-03-13
On Sun, 11 Mar 2012 15:30:47 -0700 (PDT), dagmargoodboat@yahoo.com wrote:> >> >> 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.=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. >The problem is that the finger is calibrated only attached to Fred.> >--=20 >Cheers, >James Arthur
Reply by ●March 13, 20122012-03-13
On Sun, 11 Mar 2012 17:16:00 -0700, Joerg <invalid@invalid.invalid> = wrote:>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 >>=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 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?>>=20 > >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. >>=20 > >That's where one of these would come in handy: > >http://www.triquint.com/products/d/DOC-A-00000921Sweet. But it chaps my hide to think about how many months income that thing costs. ?-)
Reply by ●March 13, 20122012-03-13
josephkk wrote:> On Sun, 11 Mar 2012 17:16:00 -0700, Joerg <invalid@invalid.invalid> wrote: > >> Phil Hobbs wrote:[...]>>> 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 > > > Sweet. But it chaps my hide to think about how many months income that > thing costs. >A real brazen RF dude rolls his own, of course. Transistors with >50GHz ft are around a Dollar in singles these days. My little LNA has another problem right now, its NiCd batteries are totally dead and small special size :-( -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●March 13, 20122012-03-13
dagmargood wrote: Might need fancy treatment, like a> bootstrapped > shield.I tried a bootstrapped shield the other day, and found it made things worse. Eventually I decided it was (probably) because the signal I was interested in measuring was around a nanoamp, and the BF862 I was using had a similar level of noise ~1nV/rt Hz - so I was adding more than I was cancelling (yeah I'm mixing units, but my point is both were small so the voltage was probably significant). Surely Phil will face the same problem with his 62-electron signal...?
Reply by ●March 13, 20122012-03-13
josephkk a �crit :> On Sun, 11 Mar 2012 15:30:47 -0700 (PDT), dagmargoodboat@yahoo.com wrote: > >>>>> 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. >> > The problem is that the finger is calibrated only attached to Fred. >> -- >> Cheers, >> James ArthurSure, I won't let my, ahem, calibrated tools go that easily... But I still can send 470pF+100R as a transfer standard. -- Thanks, Fred.
Reply by ●March 14, 20122012-03-14
On 03/13/2012 04:26 PM, Nemo wrote:> dagmargood wrote: > > Might need fancy treatment, like a >> bootstrapped >> shield. > > I tried a bootstrapped shield the other day, and found it made things > worse. Eventually I decided it was (probably) because the signal I was > interested in measuring was around a nanoamp, and the BF862 I was using > had a similar level of noise ~1nV/rt Hz - so I was adding more than I > was cancelling (yeah I'm mixing units, but my point is both were small > so the voltage was probably significant). Surely Phil will face the same > problem with his 62-electron signal...?Bootstrapping is really worthwhile if the bootstrap amp is quiet and accurate enough, but it still introduces a noise current equal to its input noise voltage differentiated by the capacitive reactance. This is the same problem faced by TIAs, of course, but interestingly, you win by combining them. They work sort of independently, though not exactly so--in arm-waving terms, the capacitance improvement adds linearly, and the noise only in RMS. It's also initially pretty surprising how small the gain and phase error in a bootstrap has to be, if you want it to make the effective capacitance to go down by a factor of 10 or more. With a BF862, it's very worthwhile using a current source load, and hanging a couple of really fast emitter followers (also with current source loads) on the FET's source. Use one follower to bootstrap the BF862's drain and the other one to drive the capacitance. Spice says that one's good for a factor of 300, if the capacitance to ground from the sensitive node (a photodiode in that case) is below about 0.1 pF. That's the sort of place where a bootstrapped shield makes a lot of sense--there's no other way to get the capacitance to ground that low. 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 15, 20122012-03-15
On Tue, 13 Mar 2012 23:26:47 +0100, Fred Bartoli <" "> wrote:>josephkk a =E9crit : >> On Sun, 11 Mar 2012 15:30:47 -0700 (PDT), dagmargoodboat@yahoo.com =wrote:>>=20 >>>>>> 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.=20 >>>>>> >>>>> He he, nice! >>>>> >>>>> 12GHz is pretty high. Just think that a 1nH via is 75R at that =frequency=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. >>> 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. >>> >> The problem is that the finger is calibrated only attached to Fred. >>> --=20 >>> Cheers, >>> James Arthur > >Sure, I won't let my, ahem, calibrated tools go that easily... > >But I still can send 470pF+100R as a transfer standard.One last thing, how big is that fingertip? In mm by mm please. And if you have it the sheet resistivity in ohms per square. ?-)
