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Thu May 23 03:00:24 2019
Frequency noise measurement of 2 micron source using PLL
The delay line length is measured to be 15.7 m.
As suggested by Prof.Rana, I used a DC block after the photodetector and the output of it was then fed into a RF amplifier (ZHL-3 A+). Attachment #1 shows the output of the detector (a) MZI output- we see the beat note at 78.8 MHz and SNR is about 37 dB (b) when light is bloked- we see radiation from RF amplifier with a small amplitude of about -63 dBm at 78.8 MHz (c) when RF is turned off.
It can be seen that, after the RF amplifier, the noise level is also increased with the signal.
The RF level that is now fed into the mixer is about -23 dBm. But the PLL was not getting locked-oscilloscope trace is oscillating. A video is attached, which is captured when the there were no gain in SR 560 (G=1) and actuation slope was 10 kHz/V. Attachment 2 and 3 shows the corresponsing traces from SR 785 in full span mode and short spn mode respectively. It is observed that , the oscillation strength was increasing as I increased the gain value in SR 560. I doubt, this is happening because of the higher noise level at the RF port of the mixer in this case. The trace on SR 785 was more stable and it was not swinging as much as we observed before.
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