[time-nuts] Re: Phase noise with a lock-in amplifier.
david.kirkby at onetel.net
Sun Apr 17 19:14:48 EDT 2005
Poul-Henning Kamp wrote:
> 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.
Well, putting something together with those sorts of chips is not
trivual, whereas commerical lock-in amps are on eBay quite cheaply.
Also, the resolution of the fast A/Ds is limited. You say 14 bits, and I
think that is about the state of the art at that speed. Compare that to
audio A/D's, which have 24 bits, and cost a fraction of the cost.
> (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.
That might well be a problem.
Sometimes I have these 'barmy' ideas. Sometimes they lead somewhere
useful. Perhaps this one does not. But when I get some time I will hook
up a commerical lockin and a mixer to an oscillator and see what I
measure. I have a Marconi sig gen that goes to 2.2GHz. I assume the
phase noise on that at 2.2GHz will be a lot poorer than the reference
oscillator in that sig gen (10MHz). It will be intersting to see what
differences are observed on a lockin.
I guess some of you must think I am mad.
Dr. David Kirkby,
Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/
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