Marc, Michael, Yuhang

The issue of SHG error signal happened after installing amplifiers for DDS1 board. Therefore, we suspect that it may be related to DDS1.

To test if issues are really related to DDS1, we kept DDS1 on but used signal generator to provide modulation and demodulation.

Signal in use: 1dBm sent to EOM, 7dBm sent to mixer. The other in-loop components were the same. Then optimized phase and locked SHG loop and measured error signal spectrum. Figure 1 shows this spectrum and we can see that 20kHz noise is still present.

Other checks could be done:

0. try to do the same test with this entry but DDS needs to be off

1. try to use different rack to power up DDS/servo

2. try to see if 20kHz appear in GRMC error signal

3. try to see if 20kHz signal appears in signal beofre demodulation

4. try to change modulation/demodulation frequency a bit

5. try to use SR560 pre-amplifier to lock SHG