Re: USB rig control


Chuck Hill K0MV
 

Colin and Lou are correct.

Well I'm not well known in these circles, but in some circles I am.  And I am an "RF engineer" if that means competent in this area.

3rd order IM distortion goes by 2*f1 - f2 = fout, and 2*f2 - f1=fout.  f1 and f2 are in the SSB transmitted signal.  So 3rd order IM distortion in the amplifier chain produces frequency products near the transmitted signal.  The 3rd order IM products would be both above and below the SSB signal, not just the opposite sideband.

In the analog TV days the US standard (NTSC) transmitted video vestigal sideband which had the USB, the lower frequency portion of the LSB (to improve SNR), and some carrier.  3rd order IM products involving the carrier would regenerate the LSB.  That would QRM the next TV channel down.

A good reference for this is "RF Design Guide" by Peter Vizmuller c1995 Artech House, page 188.  A description of good debugging techniques too.

73,
Chuck K0MV

On 6/13/21 4:30 PM, Colin Smithers wrote:
It can only re-appear if carrier is present for the wanted sideband to mix with in the non-linear amplifier.

In an SSB transmitter with good carrier suppression it is not possible to regenerate the unwanted sideband through poor IMD in the final stages. In my experience the K3 has negligible carrier content. Given that, It would require an inappropriate TX filter or TX filter adjustment for the unwanted sideband to truly be present.

I think in TV transmitters there is still carrier present.

73, Colin G4CWH

-----Original Message-----
From: Elecraft-K3@groups.io <Elecraft-K3@groups.io> On Behalf Of Joe Subich, W4TV via groups.io
Sent: Sunday, June 13, 2021 10:13 PM
To: Elecraft-K3@groups.io
Subject: Re: [Elecraft-K3] USB rig control


On 2021-06-13 4:45 PM, Lou W7HV via groups.io wrote:
Subsequent distortion in the rig or external amplifier can broaden
> that signal but can't create and add back the opposite side band.

Au contraire! IMD in amplifier stages *will* regenerate the missing
sideband. I saw with my own eyes (spectrum analyzer) in the days of
analog UHF TV ... in "high efficiency" Klystrode/IOT amplifiers. It
was necessary to do significant precorrection (early "Pure Signal")
to prevent the lower sideband from exceeding the "spectral mask"
requirements for protecting the next lower channel.

73,

... Joe, W4TV


On 2021-06-13 4:45 PM, Lou W7HV via groups.io wrote:
I am not a well known engineer and certainly not a respected engineer.  It is my understanding that normal superhets produce SSB by mixing audio with the LO up to the IF where the unwanted sideband is stripped off by the same xtal filters used for RX.  This is fixed and doesn't change with TX band or frequency because mixing to the TX frequency occurs after that.  Subsequent distortion in the rig or external amplifier can broaden that signal but can't create and add back the opposite side band.  So, based on that, I would think if someone is hearing the opposite sideband, there's a problem with the rig unrelated to being used outside the ham bands.

Direct conversion rigs are another matter.

Perhaps a well known and respected engineer will weight in.








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