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

David Kirkby david.kirkby at onetel.net
Sun Apr 17 07:43:43 EDT 2005

Alberto di Bene wrote:
> David Kirkby wrote:
>>> 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
> Why an odd multiple of a quarter wave ? Wouldn't an even multiple be 
> more adequate, so that
> the input impedance is reflected more or less unchanged at the output ? 
> Am I missing something ?
> 73  Alberto  I2PHD

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.

I was more concerned about what would be the output from the mixer used 
to convert the oscillator down to DC.

Over a short time scale, the noise might be considered sinusoidal at a 
frequency f. Since the mixer does a multiplication of its inputs. If the 
first input is sin(w t), then the other input you will have the same 
amplitude and frequency, but delayed by whatever you set the delay line 
to be. Perhaps this does not matter.

I know lock-in's use internally 90 degree delays, but thinking about it, 
they do it for a different reason. Using my proposed idea, with a 
commerical dual-phase lock-in amplifier, would put 3 mixers into the 
system. This would need some careful analysis

I will perhaps do some calculations/simulations of this, to determine if 
there is any way to recover the noise on the oscillator.

It is obviously not a 'well known' way of doing this, but it might be 

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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