Dear Jim LF Group,
Looking at the sidebands on the G7NKS WSPR signals, mains hum or other noise by itself is not a sufficient explanation of what is seen. Simply adding 50Hz, 150Hz, etc. noise into the PC audio channel, then feeding the result into an SSB TX would give unwanted CW at frequencies offset from the SSB carrier frequency by +/-50Hz, 150Hz and so on, i.e. with the carrier "dial frequency" at 502.4kHz, unmodulated carriers would appear at 502.45kHz, 502.55kHz and so on, in addition to the WSPR signal at around 503.9kHz. However, these would probably largely be removed by the SSB filter in the rig. Instead, we see unwanted sidebands offset from the wanted WSPR signal frequency by +/- 50Hz, 150Hz, with identical modulation to the wanted signal. This requires some sort of non-linear process to cause the intermodulation between the presumed mains noise and the WSPR signal somewhere in the TX chain.
One possibility
is that intermodulation occurs in the PA. Jim's description suggests that he is persuading the HF PA in the IC735 to produce output at 500k. At this frequency, the PA linearity may well be poor due to the impedance matching, coupling, decoupling and feedback components in the PA being wrong for the frequency, and high flux densities occuring in ferrite components due to the low operating frequency. But this wouldn't explain why Jim also gets multiple received signals from a strong station with a clean signal, which implies some identical noise source and distortion in the receive path. It would also mean the mains noise getting through the filtering in the rig somehow.
A possibility that would explain the unwanted sidebands appearing on both TX and RX signals is if one of the oscillators in the system has mains noise sidebands. The sound card clock seems unlikely, since this is just a simple crystal oscillator. I see the IC735 has some sort of
multi-loop PLL synthesiser, which would certainly be prone to this type of spurious signal, since any kind of mains noise getting in would modulate the VCO frequencies, and would probably be the same on transmit and receive. This could be checked by receiving a clean carrier somewhere around 500kHz, and examining the audio output using Spec Lab or Argo or similar to see if 50Hz and 150Hz sidebands are present on the received audio tone.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message ----- From: "James Cowburn" <
[email protected]>
To: <
[email protected]>
Sent: Monday, November 16, 2009 8:26 PM
Subject: LF: rule #2 - assumption is the brother of all foul
ups
> LF
>
>
>
> Sidebands are back! I think its caused by my TX getting hot and bothered by
> struggling at 500. From cold it txs 50 watts but this soon drops back to
> around 35, and the sidebands appear. My sigs are better so having the txfr
> outside the shack and at the antenna is a vast improvement, but I now need
> to work on the cooling.
