I had a theory that the +/-100Hz spurious outputs ( +/-120Hz observed in the USA) were caused by ripple modulating the transmitted carrier. I used my general purpose signal generator, otherwise known as the FT-817ND, to transmit a low power carrier (CW key down) into an unscreened dummy load (Elecraft DL-1). I repeated this with the transceiver powered from my bench power supply and then on its internal batteries with the power cable removed. The results in the WSJT-X spectrum display window are shown below.
|K3 RX, FT-817 TX on mains supply.|
|K3 RX, FT-817 on battery power|
I recalled an issue a few years ago when someone sending CW using their K3 had reports of spurious signals +/- the sidetone frequency. This turned out to be audio modulation of the synthesizer by the sound of the sidetone from the K3 speaker. Elecraft provided a fix in the form of a stiffener for the synthesizer board. My K3 is an old one and does not have this modification. You can see that the synthesizer is affected by physical vibration looking at the trace produced when I rapped on the K3 case.
|K3 RX showing the effect of vibration (knock on the case)|
To answer that question I repeated the tests using my Elecraft K2 as a receiver, feeding the headphone output at low level into the cheap USB audio dongle I use for computer sound. You can see the result below.
|K2 on RX, FT-817 on mains power.|
|K2 RX, FT-817 on battery power|
I'm not sure what to make of all this. It does appear that the 'harmonics' - which are really sidebands - that accompany a strong JT9-1 signal are caused mainly by AC ripple modulating the transmitted carrier, but that hum on the receive side can produce a decodable signal as well. The WSJT-X software is extremely sensitive and can detect these components even if they are 30dB or more below the fundamental carrier.
I would appreciate hearing of other theories or tests carried out to explain this phenomenon. It seems to be that this issue is going to be almost unavoidable when mains-powered equipment is used to generate signals that are decoded by very sensitive software.