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

Bill Hawkins bill at iaxs.net
Sun Apr 17 12:37:51 EDT 2005

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). The cable length, insulation material
and temperature directly determine the transport delay.

Most of our work is done with frequencies matched to 12 places or more.
Such differences are best measured as rate of phase change rather than
frequency, so phase matters. Phase varies with cable length.

David, you set out to determine "phase noise" with a "lock-in amplifier."
Your description of a lock-in amplifier built from a 747 op-amp and two
FETS fits my definition of a phase detector. A phase detector can only
make a relative measurement, as between two sources of the same 
relatively stable frequency.

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.

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

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.

Does anyone know of a way to measure absolute phase noise? I think
that everything that uses a phase detector is relative to a "better"
frequency standard. The measured phase noise is the product of the
unknown and the standard.

Bill Hawkins

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