Today I began working on a TF measurement based on the delay line frequency discriminator setup in elog 4254 using a single mixer (without the 'I' and 'Q' readout).

For this, I re-organised the setup for the PLL measurement of the transfer function (elog 15148), increasing the HEPA for the initial changes while the PSL door was open, and then reverting it back to ~30%:

• I removed the 20dB coupler and connected the splitter directly after the amplifer to split the beat note signal into two coaxial cables one of which was ~1.5m longer than the other
• The recombined signals were combined in a mixer outside the PSL enclosure. I also replaced the 1.9 MHz LPF with a 5 MHz LPF.
• I used an SR 560 to amplify the signal after the LPF.

With the above setup the power that was seen at each channel of the delay line was <1dBm, which is not ideal for the any of the available mixers.

After the group meeting, I changed the amplifer to ZHL-3A (that is near the beat mouth) instead of a ZFL-500HLN because it had a higher gain (~28dB as opposed to ~19dB of the latter). The power seen at each of the delay line channels is over 5.5 dBm. This is consistent with the estimation 0 dBm beat -> -20 dBm after 20dB coupler -> 8 dBm after amplifier -> 5 dBm after splitter with insertion loss of 3 dB.

Is this sufficient enough for the mixer to work? In Attachment 1: A shows the mixer output (point B in Attachment 2) when the IMC is locked, in B the IMC is unlocked at the middle of the spectrum, and each of the dips show the DC voltage being sent to the PSL temperature servo being decreased by 0.01 V.

Gautam pointed me to the location of a few other RF amplifiers (ZHL-32A+, ZHL-1A) which don't possess a higher gain but can be used without disrupting the ALS related work (I was told).

For shorter duration changes that I made later, I opened and closed the PSL enclosure doors without changing the HEPA.

Attachment 2 shows the current setup as is, but I might add a PSL servo tomorrow to stabilise its frequency corresponding to a null mixer output without driving anything else.