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

David Kirkby david.kirkby at onetel.net
Fri Apr 15 20:30:52 EDT 2005

Does anyone know if you can use a lock-in amplifier, say something like 
the Standford SR830 dual phase unit


to measure phase noise from an oscillator?

I was thinking of something like this:

1) Take an oscillator (say 10MHz)
2) Split the output into 2 with a power splitter.
3) Feed one output from the power splitter into the RF port of a mixer.
4) Feed the other output from the power splitter into the LO port of a 
mixer, via a cable that is a odd integer multiple of a quarter wave

The outputs from the mixer, assuming the oscillator was perfect would be 
  the sum (2f) and difference (DC). But since the oscillator has noise 
on it the noise will not mix to DC since the noise will arrive a

Hence the output from the mixer is the noise on the oscillator mixed 
down to DC.

Use a lockin amp, set its internal reference to some figure (say 1KHz) 
and measure the magnitude on a lock-in amplifier. That gives you an rms 
noise voltage in whatever bandwidth you want - you set the bandwidth 
with the lock-in filters, which on that model can be varied from 10us to 
30ks (i.e.  10us to 8.3 hours).

That's not the usual way of using a lock-in, but it is basically a very 
high Q filter, with a centre frequency set to whatever you use as its 
internal reference. On that particular unit, the reference can be from 
0.001Hz to 102KHz.

I've built a simple lock-in using noting more than a 741 op-amp, a 
couple of FETs and a few resistors. The reference input open/closed the 
FET switches so the op-amp worked with either a gain of +1 or -1. It 
worked quite well. Not as good as the SRS830, but then it cost next to 
nothing to build.

The main reason I ask about the use of a lock-in amp is that we have 
almost as many lock-in amps at work as DVM's.

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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