[time-nuts] Re: DIY Low offset Phase Noise Analyzer

[ghf at hoffmann-hochfrequenz.de]

Am 2022-07-05 18:04, schrieb Bob kb8tq via time-nuts:

> If you need that sort of isolation, it certainly can be done.
> NIST has papers on very simple / DIY compatible cascode
> amps that will do the trick. ( chain of common base stages
> driven by a common emitter). Some folks on the list have
> gone a lot further in terms of complexity than NIST did.
> 
> Device wise, the cascode amps seem to work pretty well
> with some very humble transistors ( 2N3904 etc ). There
> likely are fancier parts out there, but some of the really
> old stuff appears to be “good enough”.

I have made a new isolation amplifier but I'm absolutely not happy with 
the available transistors. Anything in sot-89 is either to slow ( 
Zetex/Diodes Inc, the 2N3904-alikes)
or is much too hot.

I want at least 200 MHz to have no phase shift at 100. BFQ19s gave me 1 
GHz of BW. The version in the plot is already heavily sandbaged but 
still has quite an S21 overshoot on the high frequency end. The 
input-voltage to cascode current converter is especially problematic in 
that the smallest capacitive load on the emitter tends to make it more 
unstable. That spoils S11, of course. I even took the feedback from a 
tap of the emitter resistor. Backward isolation is 120 dB over most of 
the useful range but changes depending on the damping methods.

Any ideas of more friendly transistors? BFQ31 were quite well-behaved 
but are extinct now. I still have a reel, but stuff from the secret 
drawer is unfair. And it's PNP.

cheers, Gerhard

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