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

David Kirkby david.kirkby at onetel.net
Sun Apr 17 14:03:30 EDT 2005

Bill Hawkins wrote:
> Dr. David Kirkby wrote,
> "I was not thinking of impedance matching at all. If the mixer is 50Ohm 
> input (as most are) and the oscillator has a 50 Ohm output, the cable 
> length would have no effect on this at all."
> The cable length has no effect on frequency (if the output device is
> buffered with an amplifier). 

But what if a 10MHz (for example) oscillator is not perfect?  The output 
is no longer a pure sine wave at 10MHz, but could be considered as an 
infinite number of oscillators, all of different frequencies, with 
amplitudes and phases that are essentially random. Those oscillators 
closer to 10MHz would on average have higher amplitudes, but all 
oscillators will have some amplitude.

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

> As I understand it, a phase detector that has both inputs fed by the
> same frequency is only capable of measuring systematic errors, such as
> may be caused by differences in the FET response times. Of course, it
> will also measure differences in cable length.

But the inputs will not be of the same frequency normally, due to the 
phase noise on the oscillator.

> There was a time when obscure discoveries could be rediscovered when
> the right opportunity arose. These days, with information at the speed
> of the Internet, the process of natural selection works much faster.
> The phase noise question has been around longer than the Internet, so
> it seems to be true that the best solutions have been found.

Well, it never hurts to consider new ideas. That might be silly, fatally 
flawed in some way.

I'm 99.9% sure the mixer, with two different cable lengths from a 10MHz 
oscillator will produce two ranges of frequencies. One will be close to 
DC, but will extend up a few hundred kHz or so. The other will be close 
to 20MHz, but extend a few hundred kHz either side.

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.

I think if you put an audio spectrum analyser on the output of the 
mixer, after the low-pass filter, you would see the phase noise of the 

In fact, would you not hear the oscillator phase noise on a speaker if 
you amplified the low-pass filtered signal from the mixer? (I'm ignoring 
the fact the noise on the audio system might be lower than the 
oscillator phase noise).

I'm less sure, but it it possible a lock-in might be able to see this 
phase noise, by setting the reference to the offset you want.

Question, what would you see on the lock-in if you used its internal 
oscillator as as reference and added to this (op-amp configured as an 
adder) the output from the mixer? The op-amp would measure its own 
reference amplitude, but you would have introduced noise on it from the 
10MHz reference, so the lock-in should see that noise too.

Perhaps I'll sketch out what I mean and put a circuit diagram.

The Standford dual-phase lock-in has functions to measure the noise on 
the signal.

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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