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

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Hi Gerhard,

On 06.07.22 18:28, Gerhard Hoffmann via time-nuts wrote:
> 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.
> 
Well, what kind of parameter are you after? I suspect, you're looking 
for sufficiently high fT and beta. Your mentioning of too hot devices 
seems to imply you'd like to have these at low collector currents, 
probably not more than a few milliamps. Anything else to look for?

Best regards,
Florian