Marc, Michael and Yuhang

After yesterday's investigation, we found although RF signal is not very clean, but it doesn't have large noise. So we were thinking the 20kHz noise should be just at low frequency and it goes to many other places. For example, elog2330 shows this noise from DDS filter-out even when there is no signal. Besides, elog2331 shows this noise from power supply of DDS board. An important fact was ~20kHz noise is only related with the connection of DDS board.

So we tried to connect individual voltage to DDS board one by one. Then we found out that ~20kHz noise shows up only when voltage is provided to USB. So we could infer that ~20kHz noise comes from USB voltage supply. Although we still don't quite understand what is the exact reason of introducing this noise, we could avoid having this noise by disconnecting USB voltage supply. This is also feasible because we don't need to connect USB so often.

Then we found a solution. We decide to use a switch, which decide whether the voltage will be provided to USB or not. But note that don't connect all USB if the DDS software is open. This is also good because this makes it easier to operate DDS boards. Before this modification, we need to change USB connection by hand if we want to control different DDS board. Now it becomes easier, we use switch to decide which DDS board to be controlled. If we don't need control DDS, we need to switch USB voltage off for avoiding noise.

After applying switches to DDS boards, we did comparison with USB voltage off and on. Figure 1 shows this comparison. We could see that noise frequency is changed. But anyway, if USB voltage is off, we will have 'clean' SHG error signal.