Hi Roger. That sounds like a nasty problem. I will try and throw some silly
ideas up in the air, and let you shoot them down.
1. What kind of power supply do you have on the 735/740. Are these internal
mains supplies and are the linear or switchers? Do you supply the Datong
from these or an external battery. Low level switcher signals can sometimes
be 'parametrically amplified' by transistors working in a totally different
frequenct range. Have you tried feeding the converter from a battery, as
even bench stabilised supplies do funny things particularly at low loading.
I'm thinking of something modulating the crystal oscillator just a tiny bit.
35kHz is a fairly common typew of frequency for older switchers....but they
are not stable and will drift around (probably killed that one myself)
2. Dave Tong still had an advert in the Jan 1999 Radcom, but a quick search
does not suggest he has a web page, so maybe he can be reached at that
phone/fax number still....I'm sure he would be interested. (0113 274 4822)
3. Is it 'rock' stable or does it move. If its rock stable it probably won't
be a spurious oscillation somewhere. If it happens on 2 samples its a design
effect I guess. I have known crystal oscillators 'hop' between a couple of
close in spurious resonances at an audio rate. This is most likely to happen
if the crystal is being strongly driven. Are there possibly decoupling
components that cause a little 'modulation' on the oscillator as it is
driven into limiting. If this effect is present it may be noticable on an
spectrum analyser, at the crystal frequency. (as another thought, it occurs
to me that the rock is probably a 28Mhz 3rd overtone.....if the fundamental
mode was not fully supressed the 3rd harmonic of that would appear about 30
to 40kHz higher , and it would be quite stable)
4. It might be possible if it is an LF signal generated in the converter to
see the signal at the fundamental on the output of the Datong even on a
sensitive scope or a LW BC receiver.
5. I suppose it could be an intermod product of some kind...I dont think
that sounds likely.
On the whole I think I favour 3. ..... but that is probably only because it
sounds a bit wierd.
The ideas are probably no use at all, but they may start you looking in an
area you perhaps hadn't considered. In brainstorms I find 'silly ideas'
break 'tramline' thinking patterns.
Hope you find it, strange signals are a pain when you are digging for weak
stuff.
73 de Alan G3NYK
[email protected]
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