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

Poul-Henning Kamp phk at phk.freebsd.dk
Sun Apr 17 14:28:34 EDT 2005

In message <4262A4F2.6010508 at onetel.net>, David Kirkby writes:

>So if the two inputs are fed to a mixer have different path lengths, the 
>inputs to the mixers will *not* be identical.

Right, but I think there are much easier ways to measure it.  For
instance, send the signal into one of the modern high-speed A/D
converters (65MHz, 14 bits) and do a FFT on the result.

(If you want to try this, check out the "USRP" which Matt Ettus built
for the GNUradio people: http://www.ettus.com/)

>I'm pretty sure if you low-pass filter the output of that mixer, you 
>will see the oscillator phase noise shifted from 10MHz down to near DC.

Mixing highly correlated (or as in this case: identical) signals is
far from a trivial undertaking.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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