> Sorry Tom this time you may be wrong.
When the switcher is installed in the 751A radio, it couples directly
into the VCO coils and other inductors near the switcher.
The only cure is to move or remove the switcher, or increase the
thickness of the metal shield to several skin depths at the
switching frequency so it indeed becomes a shield.
> I use this IC 751A switcher OUTSIDE the transceiver all the time in my
> portable station with very little problems - I hear only some tiny
> carriers wandering around on 80. I use a windom wire antenna and had some
> problems before, before they rebuild the open air power lines that earlier
> were closer the antenna. These power lines are open air lines and extend
> up to 2 kilometers from the summerhouse and are nowadays below earth close
> to the antenna. When I had noise problems the power lines were pretty
> close to the antenna, say 60 meters.
My point exactly. The switcher, as well shielded and filtered as it
is, still causes problems.
I live in a quiet rural environment, I do so because I want to be able
to hear weak signals on any frequency. The last thing I want is to
have carriers "wandering around" from a switching supply.
It also is annoying that the switcher cause the VCO in the 751A to
FM when the switcher is mounted inside the radio. Although the
carrier looks perfectly flat, a spectrum analyzer will show multiple
sidebands on the carrier of the 751A when the switcher is mounted
inside. You can hear the same effect on receive, despite the output
lines being almost perfectly filtered, because the magnetic field of
the transformer in the supply "leaks through" the shield because it
is not thick enough (or conductive enough, take your pick).
The most simple solution is to throw the switcher in the junk pile,
and use a linear supply or an exceptionally well filtered external
switcher.
73, Tom W8JI
w8ji@contesting.com
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