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

[Bob kb8tq]

Hi

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”. 

Why cascode ( or just common base) ? You can manage
the gain pretty well. The amps likely are going to be quite
low gain along with low noise. Thats not a great combination. 

There are a lot of OCXO’s out there that don’t need anything
as crazy as the multi stage NIST amps. If you are looking at
a system (as opposed to an oscillator) isolation already has
been addressed in the instrument. 

Part of the “look at the beat note” process is to observe
that it looks like a proper clipped triangle wave. The slopes
on the rising and falling edges should be similar and of a 
reasonable slope. 

If you have injection locking ( and with a typical low frequency
note) the rising and falling edges will begin to distort. The
“lock” process will start to occur as the note crosses zero. 
This is yet another good reason to use a low frequency 
note. It also is good to look at the slope of *both* zero crossings. 

Bob

> On Jul 5, 2022, at 12:16 AM, Leon Pavlovic via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> I've tried to do a simple and low-cost sound-card PN system too back in the
> days. It's fun, when you see the broadband thermal resistor noise in your
> FFT plots :) But when doing experiments with the crystal oscillators, I've
> found out that one of the *MOST* important things to worry about is
> injection locking, especially if you do OCXOs and very low PN oscillators.
> 
> Before feeding the RF and LO ports of the mixer, you should have isolation
> amps on both ports with about or more than 100dB S12 isolation, not to get
> into trouble. Without them, you'll be measuring unreal PN noise...
> 
> Cheers,
> Leon
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