Recently we switched off DDS board many times for the sake of avoiding RF signal cross-talk.

But recently every time we switch on again DDS, we heard a strange sound. From the sound, we guess the frequency seems to be a fixed audio frequency.

Up to now, our solution is

1. switch it off the rack containing the DDS board.

2. take out DDS3 board

3. switch on the rack

4. Put back DDS3 board while the power is on

By following this procedure, we could avoid the sound problem. But we still don't know what is the reason for this sound.