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

Bill Janssen billj at ieee.org
Sun Apr 17 13:55:57 EDT 2005

David Kirkby wrote:

> 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 
> feasable.

Ok second try but with bitmap this time :-)

I have attached excerpts from a  paper presented at a NBS Time and
Frequency seminar
from 1987.

Don't remember if an attachment is "legal" on this group. If it doesn't
go through I will
try to format a message with the info "in - line".

Bill K7NOM

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