OK Jim and LF
Yes when you reduced power from 45w? down to 7w the side bands assuming they were still there but at the same relative level may possibly be unreadable by me. 45/7=6.4 times reduction 8dB power drop which I think represents -16dB RX strength add that to the say -30dB SB at 45 watts resulting in, if I have understood this correctly, -46dB side band which is probably below threashold of the Software and I would not see it.
I am sure if it was just the fact you had HUM that would not neccessarily result in IMD there has to be some non linearity for modulation to occur. The hot PA has to be a clue.
As far as your signal at this QTH goes Jim it is not an issue here I am not bothered by your two sidebands at all as you say three for the price of one ...hee hee. However I cannot think of a simple way to stop me reporting them to the Database other than me closing down.... which may be the solution ... only joking.
From: James Cowburn <[email protected]>
Subject: RE: LF: Re: G7NKS sidebands
To: [email protected]
Date: Wednesday, 18 November, 2009, 9:41
The Rig only gets overly hot on powers over abt 20 watt output and 20%+ duty cycle (not to be unexpected given the operation at or beyond the limits of design). When I run it around 10 to 15 watts and on 10% duty it stays just lightly warm so as far as I can tell there is no discontinuity or paradox in the ouput/duty/warming matrix!
The sidebands will no doubt be still there, just beyond the limit of detection
Jim
Dr. James Cowburn
I think the issue is intermodulation products caused by over driving the the linear RF stages of the TX into distortion because when Jim reduces drive the sidebands petty well disappear at this range. These observation is based upon test carried out yesterday. There is another issue Jim seems to have which is that his PA grossly over heats (I think I am right) when he reduces drive and that shouldn't happen. For modulation to take place the output of the modulator has to be mixed (hetrodyned) with the RF signal. In this case I am guessing that the carrier is modulated by noise (50c/s say) in the non linear (over driven linear RF stages) in the TX driver or output.
The fact the PA over heats on reduced drive suggests to me the bias on the PA transistors is being altered by changes to the drive anther indication that the TX output may not be entirely linear
--- On Wed, 18/11/09, James Moritz <[email protected]> wrote:
From: James Moritz <[email protected]>
Subject: LF: Re: G7NKS sidebands
To: [email protected]
Date: Wednesday, 18 November, 2009, 1:13
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.
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