[Elecraft] Switching P/S Horror Stories

Thu Apr 4 17:56:00 2002

Hi

It is possible to design a quiet power supply. Tektronix had a switching =
power supply in all of their 7000 series scopes from the 1970's. Some of =
these scopes had high gain amplifiers with bandwidths of up to 1 GHz in t=
he same case as the switcher and you could not see any hint of the power =
supply at any frequency. Switchers are noisy because they drive the power=
 transformer with a fast rise time square wave. It is very hard to filter=
 the harmonics of this square wave. The way Tektronix solved the problem =
was to drive the power transformer with a ultrasonic sine wave. That way =
there are no harmonics, only the fundamental frequency making the filter =
design more straight forward. =20

The reason we are stuck with noisy power supplies is these supplies came =
out of the computer industry and there is not a requirement for quiet sup=
plies with computers as many of you know.

There are some designs sold for ham use that use a variation of the Tektr=
onix method. The MFJ switchers are one and I have been happy with them. I=
 can only hear a very low level of hash if I lay the antenna of a general=
 coverage receiver on top of the power supply at any frequency from broad=
cast to 30 MHz. I have not noticed any hash on my K1 or K2 using the 45 a=
mp or 25 amp version. MFJ's adds say no hash and it looks like they are r=
ight.

As usual no interest in MFJ or Tektronix just a customer

Don Brown
KD5NDB  =20
----- Original Message -----
From: Vic Rosenthal
Sent: Thursday, April 04, 2002 3:24 PM
To: [email protected]
Subject: Re: [Elecraft] Switching P/S Horror Stories

lhlousek wrote:
>
> Based on the QST review I bought an Alinco 25A switcher.  It's a very n=
ice supply with adjustable voltage, voltage lock,
> meter, small size, etc.  Works absolutely FB...except for 80m.  It gene=
rates noise and birdies all over the 80m band.

I have had noise problems with switchers as well.  What I want to know is=
 "why
is it so hard to filter/bypass these supplies to get rid of the noise?"

I'm old enough to remember the advent of TVI and the development of TVI
suppression techniques for homebrew and commercial rigs.  Why can't we ta=
ke a
similar approach to switching power supplies?

73,
Vic, K2VCO
Fresno CA

--- StripMime Report -- processed MIME parts ---
multipart/alternative
  text/plain (text body -- kept)
  text/html
---