I built my triggered SAV-551 Colpitts oscillator, at 130 MHz. You recommeded using a gate resistor, so I used 4.7 ohms 0603 as a placeholder.
Jitter was ghastly; the phemt was clearly oscillating at some high frequency. Gate resistance helped. It looks good at *499 ohms*.
Yikes. I suppose that the gate capacitance of a phemt follower is some small fraction of a pF, so 499 ohms isn't totally insane. That RC is clearly a Q-killer too.
Gotta tune it some more.
Try a Murata BLM18BA050SN1 bead. It's 5 ohms at 100 MHz but goes way up between there and about 5 GHz. A 500-ohm resistor is capacitive above there anyway.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
OK, I'll try the 5 ohm bead. I don't know what frequency it was oscillating at. I might get a 25 ohm part too.
Sources suggest that the shunt capacitance of an 0603 resistor is around 0.04 pF, which is only 20 ps tau. But the 500 ohms is still nasty.
-- John Larkin Highland Technology, Inc The best designs are necessarily accidental.
Fifteen minutes on Murata's SimSurfing website was pretty educational for me. Their BA and BB series are very good indeed.
IIRC Joe Gwinn posted one a year or so ago that was a bit better than the BLM18BB series at VHF, but I know of none that holds up as far into the gigahertz as the BAs, specifically the 5- and 10-ohm ones. The BLM15BB050SN1 peaks at about 120 ohms resistive near 3 GHz. How they do that, I have no idea.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Phil Hobbs wrote: ...
And what happens with DC loading is also unclear...
-- Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt --------- Tel. 06151 1623569 ------- Fax. 06151 1623305 ---------
I am currently working on an obsolete parts issue and have a low noise ampl ifier that is going obsolete in that SOT-343 package. I need the amplifier version in the same package and not the individual transistor. I am looki ng at a Qorvo part (SPF 5043) and it looks good compared to my other part e xcept the gain variation across my receive band (960-1212MHz) is A LOT! It is on the order of 1.5 dB. I have two of them in the receiver front end which makes a gain variation of 3db across the band. I can find a flatter part but the NF is 1 dB higher (the new Qorvo part is lower NF) than origi nal part and I do not want to give the receiver sensitivity but this gain v ariation bugs me. I am trying to do this without a board re-spin , which i n our current environment is probably too much to coordinate right now. I will probably go with the Qorvo part for the front end LNA and then find a flatter part (higher NF) for the second amplifier.
I would have passed by this post but I noticed the SAV-551 part which I was just looking at yesterday. It took me a while to realize that some of the se parts are transistors and some are amplifiers. At the top level "find" tool at Mouser they are all presented as amplifiers.
When DC loading from base current is a problem, you have other problems, too.
Cheers, Gerhard
On the Murata data sheet, the graph of the 5 ohm part just looks flat, on the scale they use. I'll just try a few beads. Anything is better than a 500 ohm resistor.
-- John Larkin Highland Technology, Inc The best designs are necessarily accidental.
The RF boys have some annoying habits. They call a transistor an amplifier, draw it as a triangle, and label the pins RF IN and RF OUT/BIAS and GROUND. They rarely give decent transistor-style DC specs or curves. They often say "adjust this bias pot until it works." They never have Spice models. If the parts do weird things under some conditions, they just quit posting data there to hide the fact.
SAV-551 has better-than average specs. I have a Spice model based on something Phil sent me, but it doesn't include wirebonds and such package parasitics. I need to TDR the parts and upgrade the model.
The Mini-Circuits MMICS are pretty good. I've characterized some for their DC behavior, and got some surprises. We use them in time domain, for pulses, so all those s-params and Smith charts aren't enough. For example, we bias them off-center to increase the output swing for known-polarity short pulses.
-- John Larkin Highland Technology, Inc The best designs are necessarily accidental.
It's phemt too.
The one I have shows a nearly purely resistive peak still rising at 3 GHz:
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
BA and BB series are very good indeed.
This here is a 10 Ohm bead, Murata BLM18BB100SN1D 0603
The sticker says "base stopper bfp640 Hobbs" :-)
<Yes, one can compute Z from s-parameter S11. :-)
If someone wants the s-parameters, just say it.
2001 points from 300 KHz to 8GHz. But they are also in Murata's ADS design kit etc. My fixture is not ideal.Pretty good to 8 GHz. It reads 12 Ohms @100 MHz == Marker 1, nominal value is 10 Ohms.
cheers, Gerhard
Very interesting--I'm surprised it holds up that well. What does the phase look like out there?
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Am 29.01.21 um 10:54 schrieb Phil Hobbs:
<Picture looks like ADS but is QUCS studio, free software. Ties own stuff, octave, Kicad, GHDL, Icarus etc together
<There is English text also.
Am 29.01.21 um 18:03 schrieb Gerhard Hoffmann:
Smith diagram:
The horizontal axis through the circuit equals real impedance. Left side is R=0, middle is 50 Ohms, right side is infinity. The Upper half of the circle is inductive, lower side is capacitive.
The bead is capacitive above abt. 6.2 GHz where the strong resonance is. I have no idea if that is the bead itself, the test board or both together. Above the resonance, the bead may be capacitive, but it is still quite a high impedance.
When the trace leaves the circle, then there comes more energy back from the port than is put into it. That means amplification and implies an active device. That's not the case here.
The VNA was calibrated at the SMA connector plane, so we measure the bead in series with a few mm of 50 Ohm microstrip. That does not change much, just a little bit of phase rotation at high frequencies. cheers, Gerhard
Am 29.01.21 um 18:45 schrieb Gerhard Hoffmann:
oohps. s/circuit/circle/
Wow, it's pretty resistive--Q < 2 for the whole range except for the parallel resonance. Thanks!
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
I don't recall this. Can you point to it?
I do recall some stuff on baluns, where lossy ferrite can be an advantage, but don't recall where that came up.
Joe
My mistake--it was John Miles, back in the "Interesting new pHEMTs" thread from the palmy days of July 2018.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
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