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[TowerTalk] OmniVI heterodyne

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Subject: [TowerTalk] OmniVI heterodyne
From: Steve Hunt <steve@karinya.net>
Date: Sat, 02 Aug 2014 12:31:22 +0100
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Ever since I got my OmniVI I've been annoyed by a low level heterodyne whose pitch is independent of the main tuning but which changes rapidly with the PBT setting. There was some discussion of it on this list back in 2006 when I first purchased the radio, but I never got to the bottom of the problem. Having recently taken the covers off to look at Jim Allen's CW offset issue, this morning I thought I'd take another look at the annoying heterodyne.

It was clearly generated within the PBT board: removing the input to board didn't affect the heterodyne; disconnecting the output removed it. And it was being injected upstream of the IF filters rather than downstream.

The audio pitch of the heterodyne changed frequency at a high rate as the 15.300MHz PBT oscillator was varied - indicating it was probably related to a high value harmonic of that oscillator. I still haven't worked out exactly what was beating with what!

The waveform looked fine leaving VCXO Q4, but was badly distorted and rich in harmonics at the collector of buffer Q3. TT obviously recognised an issue here as there is a LPF ( L4, L5, C12, C13 and C14) between this point and the mixer.

To cut a long story short, tacking a 220pF capacitor between the collector of Q3 and GND cleaned up the waveform significantly without reducing the drive to the mixer. And the heterodyne has disappeared!!!

Anything much less than 220pF didn't fully remove the heterodyne; anything much more than 220pF reduced the drive to the mixer. YMMV!

I don't usually like this type of "patch"; I much prefer to fully understand a problem and arrive at a properly-engineered solution; but in this case I don't really have the time to pursue it further. During the 2006 discussion, several OmniVI owners mentioned they had the same issue, so I'm posting this in case it may help them.

73,
Steve G3TXQ
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