Sean Breheny wrote:
> As a hobby project I have slowly been building a high-performance HF
> receiver over the last few years. I originally used a switching power
> supply in it. Granted, I'm only slightly beyond newbie when it comes
> to switching power supplies, but I have to say that I found it
> impossible to keep the harmonics from the switcher from making it into
> the receiver. I tried shielding and massive filtering on the input and
> output leads to no real avail. I did not try magnetic shielding,
> however the physical separation between the switcher and the rest of
> the receiver was such that I would find it hard to believe that there
> could be direct magnetic coupling.

It sounds like you tried everything except the one thing that should be at
the top of the list, which is proper grounding and attention to where the
high frequency loop currents go.  This is the most important single thing
for keeping any sort of switching noise from going elsewhere.

A few years ago I redesigned a receiver that had to decode the information
from two 434MHz receiver and pass the resulting data along over ethernet.
The original design used a 6 layer board with the digital section in a can.
The bottom layer formed one face of the can, and a separate piece of custom
fabricated metal was soldered over the digital and switching power supply
section.  Needless to say, this was a expensive solution.

My redesigned version replaced the analog data slicers with a dsPIC, used
two switching power supplies instead of one, and only a 4 layer open board
with no sheild.  However, I paid careful attention to where all the high
frequency loop currents were from the digital parts and the switching power
supplies, used one layer for a pervasive ground plane, and used sub ground
planes for noisy sub-sections.  In the end we beat the FCC emissions limit
by almost 15dB, and got a much lower noise floor from the two RF receivers.
The net result was that it could pick up transmissions from over twice as
far and cost 30% less to produce.


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