The ICOM 751A generates FM trash when the internal switching
supply is used. The problem is the VCO coil is next to the
transformer in the switching supply module, and the magnetic
flux changes permeability of the VCO coils.
MFJ QRP radios do the same thing. Get them near a magnetic
field and the VFO shifts. It moves about 20 kHz from the
speaker magnet, and then if you place the radio on steel
desk it shifts some more because the desk will change
speaker flux density in the VFO coil. This means part of the
radio's drift is related to cabinet expansion or even what
is around the cabinet.
Modulation of FM transceivers can be switching supply trash
on the DC leads, or it can be switching supply magnetic
fields modulation sensitive circuits, but you can be sure it
isn't anything to do with radiated RF energy at the
operating frequency of the transmitter. It's all the LF
stuff near the switching frequency and the first several
(mostly odd) harmonics that causes the problems.
Personally, I'd never use a switcher in an amplifier unless
there was a compelling reason, and if there was a compelling
reason I'd be sure it was both very well filtered at all
frequencies and not physically close to magnetic field
sensitive components.
I can't imagine why an amateur would want to use a
switching supply for a filament. What reason is there to do
that?
73 Tom
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