Today we compared beat noise measured from phase lock loop(PLL) and cable delay technique (CD.)
The results do not agree well. Eventhough we can get the calibration factor correctly, CD is not working yet.
We want to use CD instead of PLL because CD has no phase noise added to the signal which is the limiting
source for PLL at high frequency.
One drawback is that to get the calibration factor for CD. It has to be done every time with each measurement
as the calibration factor varies with the carrier frequency of the beat, cables used for the measurement, but it's not that hard.
We use CD to measure the phase noise of IFR2023b used for PLL, its phase noise was measured and added in the noise budget.
The result at 100kHz input range from CD agrees well with the previous measurement, we can see the feature of the phase noise nicely.
This ensures that we got the CD calibration [Hz/V] correctly.
Then we switch to a 500 ft cable for better resolution and measure the phase noise of the LO again.
The results from 100 kHz input range are similar.
However, for phase noise at 10kHz input range (with lower phase noise,) CD cannot measure it properly by either short or long cables.
there is no phase noise's feature at high frequency like the previous result measured by SR785, note that
CD's noise floor is still lower than LO phase noise.
With that in mind, we measured beat noise by PLL(10kHz input range) and CD (500ft +2w amplifier).
We also injected a 1.092 kHz peak, generated by a func generator through a speaker on the table, to compare for the calibration.
1)If we use our regular calibrations for PLL and CD, the noise looks like this
2)If we match the 1.092 kHz peaks in both plot (I use a regular calibration for PLL). The result looks like this
And the coherence between PLL and CD is quite low (we have to think about it if the two signal should be coherence or not)
We will figure out what happen with the cable delay tech.
To sum up, we
1) make sure that we get the calibration factor correct for 500 ft cable setup with 2W amplifier,
by measuring ifr2023B phase noise at 100kHz range and compare it to the previous result.
2) are not certain why the LO phase noise and beat results are not similar at 10 kHz range
3) trust signal from PLL more than CD, since the beat noise from CD changed
(while PLL gave the similar result)when we adjusted the equipment, and we could not find out why.