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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|