Unfortunately, all switching power supplies generate RFI. That's why the League
publishes those measurements. The only thing you can do is try and minimize the
noise coming out of the switching power supply.
To do that, you have to pay attention to the noise that's actually on the DC
lines coming out of the supply, and the noise radiated from the supply through
the case. Both mechanisms will contribute. Any measures you take to do this
will probably involve a combination of filtering and shielding.
There is one manufacturer that says the following in its ads: "Specifically
designed for ALL applications, including HF." You read this, and you think,
"Hey, they must not generate any noise because they're saying it's good for HF,
too!" But, see paragraph #1 above.
Several years ago I was suckered by this ad and went through a devil of a time
quieting down the power supply so that I could use it portable in a hotel room
with a random wire. And yes, the problem is made worse with an unbalanced
antenna system like a random wire. I worked on the supply and reduced the noise
considerably, but I cannot eliminate it entirely.
I think many hams may not realize that some of the noise they are hearing on
their receivers is coming from their switching power supplies. There is both
narrowband and broadband noise. Some of it is easy to hear because it repeats
every 15 or 40 or 60 kHz or whatever the switching frequency is.
You should have a very good reason to go with a switcher and then be prepared
to deal with the consequences. If you're not willing to, save yourself some
grief and look into a good old-fashioned linear power supply. It weighs more,
and it takes up more space on the operating desk, but boy, it sure is quiet.
Al W6LX
> -----Original Message-----
> From: Hsu, Aaron [mailto:aaron.hsu@unistudios.com]
> Sent: Monday, October 21, 2002 2:38 PM
> To: TenTec@contesting.com
> Cc: 'Steve Ellington'; Jeff Maass
> Subject: [TenTec] RE: Switching Power Supplies (was Samlex 1223)
>
>
> Switching supplies came up as a topic on the CQ-Contest
> reflector a few
> months ago. I posted the following message there and here it
> is for the
> TenTec list...
>
> Oh, as an update, I used the Astron SS-30M with the TS-850S (mentioned
> below) during the Collegiate QSO Party a few weeks back. No
> hash heard.
>
> - NN6O
>
> ------------------
>
> QST did a review of several switching power supplies in the
> January 2000
> issue. The SS-30M had the "cleanest" DC trace with less than
> 20mVpp ripple
> and no detectable switching spikes while the Samlex 1223
> showed less than
> 30mVpp ripple but 600mVpp switching spikes. Broadband noise
> is also 10 to
> 25db higher on the Samlex supply. Surprisingly enough, the
> MFJ unit looks
> like it has the lowest spectral noise of all the supplies
> tested! Spectral
> plots and trace displays are in the report. You can download
> it from the
> ARRL site in the members-only section. It's about 670K in
> size because it
> includes other product reviews.
>
> I've confirmed on my TS-850S/AT that you can hear the
> switching noise. At
> Field Day 2001, we used a couple of Samlex 1223 supplies and
> my '850 would
> pick up a 3 S-unit "bump" in the noise floor every few dozen
> kHertz on most
> HF bands. The noise completely disappeared when we turned
> off the switchers
> and used battery power. Although my '850 heard the noise, we
> weren't able
> to detect it on 6 or 2 meters with a TM-255A 2M radio (with
> TenTec 1209
> transverter for 6M). Nor was the noise discernable on Oak
> Hills or NorCal
> QRP HF rigs (the '850 is known to have a VERY sensitive receiver!). I
> recently purchased a Astron SS-30M, but haven't had time to
> test it yet with
> the same '850. If anyone's interested, I'll hook it up
> sometime soon at
> home and post results.
>
> 73,
>
> - Aaron Hsu, NN6O (ex-KD6DAE)
> {nn6o}@arrl.net
> {athsu}@unistudios.com
> No-QRO Int'l #1,000,006
> . -..- - .-. .- ".... . .- ...- -.--"
>
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