i analyzed  the data taken with the short cable and compared it to the signal from the tuning input to the VCO.
I used the 9.order polynomial fit from yesterday to convert the VCOMON voltage into absolute VCO frequency.
The beat frequency is then twice that frequency.

On the other hand the cable-delay technique was sampled with channel FREQCOUNT.
I've measured the peak voltage of the amplified mixer response to be about 4.52V.
The frequency change which corresponds to peak-to-peak change in output signal (see yesterdays entry) i've used is the smallest value i measured ( 6.42MHz).

Using the sampled data, divided by 4.52V, taking sin^-1 of that and multiplying it by 6.41MHz /2 gives the measured frequency change.
As i don't get an absolute frequency from that measurement i added an offset of 151.5MHz (this information is from the VCO's absolute frequency) in order to compare the fluctuations with the VCO signal.

RESULT:  beat signal fluctuations over a long period of time can be measured using the VCO feedback signal.
                Both signals absolutely agree except for the region where the mixer signal is at it's peak and so the slope is close to zero and so the uncertainty is too large.

Here the plot:

Matlab-code:

t0 = tconvert('02/06/2011 10:10:00');
dur = 3600*24*1;

chans = {...
'C3:PSL-ACAV_VCOMON',...
'C3:PSL-FSS_FREQCOUNT',...
};

y1 = get_trend(chans{1},'minute',t0,dur);
y2 = get_trend(chans{2},'minute',t0,dur);

y1.data = y1.mean;
y2.data = y2.mean;

y1.data = 2*fittedmodel2(y1.data);
y2.data = 151.5-asin(y2.data./4.52).*3.21;

t1 = linspace(0,dur,dur*y1.rate)';
t2 = linspace(0,dur,dur*y2.rate)';

figure(1)
plot(t1,y1.data, 'r',t2,y2.data, 'b')
xlabel({'time [s]'});
ylabel({'beat frequency [MHz]'});
grid