[time-nuts] New Question on HP3048A Phase Noise Test Set

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jan 19 17:11:22 EST 2008

When the input to the phase detector consists of the OCXO signal plus
another frequency offset from the first i.e.

Vi(t) = A1*cos( w1*t) + A2*cos(w2*t)

Where w1 = 2*PI*(OCXO frequency)

the observed signal at the output of the phase detector is:

Vo(t) = Vi(t)*sin(w1*t)
Vo(t) = A1*cos(w1*t)*sin(w1*t) + A2*cos(w2*t)*sin(w1*t)

Vo(t) = (A1/2)*sin(2*w1*t) + (A2/2)*[sin((w1+w2)t) + sin((w1-w2)t)]

After low pass filtering

Vo(t) = (A2/2)*sin((w1-w2)t)

Thus the amplitude of the discrete spur related component in the low
pass filtered phase detector output is 1/2 (6dB) the amplitude of the
discrete spur itself.

This result is independent of any additional calculations that may be 
done to derive L(f) from the observed spectrum.
The conclusion is independent of any arbitrary phase for the spur signal.
It is also independent of the exact phase detector characteristic.
If the phase detector characteristic isnt sinusoidal the the phase
detector output baseband signal is unchanged.


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