[RFI] Modem PS noise

dgsvetan at rockwellcollins.com dgsvetan at rockwellcollins.com
Wed Mar 5 12:29:58 EST 2008

Note to Rick and All,

Placing a bypass cap across the switching transistor of a switch mode 
power supply is NOT the place to put a cap, as Rick found out.  There will 
be a snubber cap already included in the circuit, if the circuit topology 
supports use of snubbers.  Please refer to any of a number of sources on 
switching power supply design before attempting to "improve" them for 
noise performance.

Some of you on this list may recall that the ARRL lab ran tests on several 
commercial switching power supplies intended for amateur radio use.  This 
was back around the 2000 to 2002 time frame, if I recall.  The best (least 
noise) power supplies were those from Astron and MFJ.  I can't speak for 
how MFJ quiets their power suuplies, but I own the SS-35 Astron unit and 
took it apart to see how Astron did the job.  Answer:  lots of ferrites 
and bypassing (in the correct places), as well as good board layout 
techniques.  What they did FAR exceeds FCC requirements for Part 15 
devices and their associated power sources, making for an excellent 
in-shack supply for running most rigs and accessories. 

My point in all of this is that if you have a sleazo cheapo wall wart 
switcher that is powering some device in your shack or house, do the 
second thing that Rick did:  replace it with a linear supply equivalent. 
Otherwise, plan to do some "sniffing" with probes and possibly some major 
surgery to said el cheapo wall wart in order to quiet it down.  One thing 
to keep in mind:  it will probably be easier and faster to get a linear 
equivalent power supply going.  Most devices seem to use voltages of 4.5, 
5.0, 6.0, 9.0, or 12.0 from their wall warts.  Many of us have plenty of 
old linear wall warts laying around that are unregulated, but "clean" in 
the RF sense.  If you have such an old wart gathering dust in the junque 
box, you can easily select one that has an unregulated voltage that is at 
elast 3 to 5 volts more than your device requires and then (gasp and 
horrors!) homebrew a simple series linear regulator to provide just 
exactly the voltage you want.  Check out the LM78-series of regulators. 
They are plentiful and cheap, come in TO-220 packages, and a suitable 
circuit can be assembled on a small piece of circuit card, perf board, or 
even a plain, old-fashioned 3 or 4 lug terminal strip.  Be creative!

73, Dale


rick darwicki <n6pe at yahoo.com> 
Sent by: rfi-bounces at contesting.com
03/05/2008 11:07 AM

rfi at contesting.com

[RFI] Modem PS noise

I finally got a well enough regulated 5V non-switching PS to power up my 2 
Wire 2701 Modem.
The S-9+ noise on 160m is completely gone. 

I tried a little bypass capacitor on the switching transistor which also 
cured the noise, but only because I shorted the transistor and blew the 
supply up hi hi

Rick, N6PE
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