"...i am pretty sure that synth phase noise and the recent change in
topic regarding magnetic interferance are slightly different."
==========
I'm sure this is true. I have been able to induce modulation of the carrier
by a magnetic field, but I have also noted that the K2 carrier is wider than
other rigs tested, regardless of whether an adjacent valve linear is
connected or switched on.
Unfortunately none of the recent posts have suggested a cure.
Bob G3VVT did mention that some time back L30 was put forward as the weak
point in terms of magnetic susceptability. Has anyone attempted to
magneticallly screen it, and if so with what success please?
I had a QSO with Colum EI5CP some weeks back. He suggested putting a low
ESR capacitor across C67, which seemed a good idea. We both tried that but
it had little noticeable effect, (in my case a 100uF 6V tant with measured
low ESR).
Another possibility discussed was reducing the tuning range of the VCO
varacters so that just the ham bands are covered, without the largish
overlap either end. I'm not sure if that would give enough improvement to
warrant the hassle involved?
Another thought; some of the newer LDO regulators have a much lower noise
output than the 78L05, so it may be worth swapping one in for U8. Again I
don't know if U8's noise contribution to the 5V rail (5B) is a significant
factor, at least it's an easy thing to try. Less important, but is the 8V
rail (8B) clean enough?
Probably the main factor is screening of the VCO/PLL. I'm not sure how
feasible that would be?
Geoffrey GM4SID suspects the sidetone modulates the VCO, and if so what
other internal signals can effect it? He also suggests D8 and D13 as
possible noise sources - is there a feasible lower noise alternative?
I welcome any comments and/or further ideas. I won't be able to do much on
this myself for the next couple of weeks.
73 Fraser G4BJM
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