[time-nuts] Re: Phase noise with a lock-in amplifier.

John Miles jmiles at pop.net
Sun Apr 17 19:20:41 EDT 2005

> The point of others about the delay line needing to be huge to get
> uncorrelated noise close in is possibly valid.
> However, there is nothing stopping that mixer being two A/D's, with
> their outputs fed to a CPU that does a simple multiplication. Although I
> suggested an analogue RF mixer, it could be done in the digital domain.
> (Conincedently, DSP based lock-in amplifiers implement the multipliers
> digitally).

Well, there's a gedanken experiment for you.  Why do you need two A/Ds?  How
would their inputs differ, if you didn't have a delay line?  Why not sample
the stream only once, then buffer (delay) it digitally and mix it with the
delayed copy?

Answer: because it won't work.  You'll end up with a comb-filter response if
you do this, which isn't of much value in phase-noise measurement.
Fundamentally, you don't have any information that differentiates the noise
you're trying to measure from the signal you're trying to ignore.  That is
why PN measurements need a clean(er) reference of some sort, whether it's a
spectrum-analyzer LO or a separate signal source used to downmix to

-- john KE5FX

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