Hi Mike,
I have had numerous oscillation problems with the original KK7B design 902
and 1296 MYz transverters. I never did really determine the true causes.
On the 902 transverter, something in my LO chain was oscillating and
causing spurious oscillations that were mixing with the LO crystal
oscillator. I replaced all the MMIC's, changed around gain blocks to
distribute the gain differently, etc. and I think I finally cured it by
putting bypass caps everywhere I could think of that wouldn't attenuate
the LO signal.
On the 1296 transverter my receive chain was oscillating. After swapping
MMIC's again, etc., I finally solved that by putting each gain stage in
its own little RF-tight box. It was a pain to do but seemed to cure it.
I don't know if there are some inherent stability issues in the layouts
of those boards or if I just happened to be unlucky. I've never had the
troubles with the no-tune LO board or the 2304 and 3456 MHz transverter
boards.
73, Zack W9SZ
On Sun, 10 Feb 2008, Mike & Becca Krzystyniak wrote:
> Hi Guys,
>
> I was testing out a Motorola 902 cell amp yesterday. While bench testing
> the unit I noticed on the spectrum analyzer that there was what appeared to
> be a serious spurious problem.
>
> At first I thought it was the PA osciallting but a quick test revealed it
> was coming from my older model DEMI transverter. Seems there is a lot of
> gain peaking short of self oscillation in the 900 MHz TX upconverter. Has
> anyone with an older model DEMI 902/903 transverter see this problem? The
> spurious levels and amplitides are most sensitive (to probes, bypasses, and
> finger farads) at the TX pin switch, but I can get it to fly by simply
> touching center conductor on the TX N connect, where the TX path coax exits
> the box.
>
> Has anyone seen the need to go back and add zebra strips to the
> feedthrough holes? Is this a job for some absorber materials?
>
> It seems there is one gain stage that is marginally stable. Any thoughts
> or suggestions would be appreciated.
>
> Thanks...
>
> 73's Mike K9MK/5
>
>
>
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