Switch mode power supplies.
 

On May 10, 1:04 am, wrote:
On May 9, 4:33 pm, wrote:

I hate switch mode power supplies.
I hate wall warts.
And I really really hate switch mode wall wart power supplies.
Linear wall warts are bad enough.

Found this while looking for a better (as in active) RFI probe.
http://www.arrl.org/tis/info/cable-modems.html

A linear wall wart means your radio will have less drift and longer
life since the heat is generated outside the radio. You should learn
to like linear wall warts.

I agree that powering a receiver with an external supply is a very
good idea.
I use a linear Lambda 12V (set to 13.69V), 8A supply.

20 years even expensive Si diodes were rather slow by todays
standards.
Modern 1N4004 have about 6 times the turn off speed of those made in
1981.
This super fast turn off means that there is often a pulse of noise at
turn off.

http://home.computer.net/~pritch/shortwav.htm explains one diode
issue.
Cross modulation can be a very bad problem if you live close enough,
like me,
to a MW station. His solution of bypassing the diodes with caps solves
this
problem and the rapid turn off noise.

I have had good success using 0.001uf/1000pF caps. Others use
different
values.

If you want to do the job correctly, http://www.hagtech.com/pdf/
snubber.pdf
shows how to it.

A friend who is into weak signal VHF had to add a ferrite bead at each
diode with
the capacitor being after the bead and across the diode.

My Lambda power supply has the 1000pF across all the bridge diodes and
a heavy
duty RF choke in series with both sides of the input to the bridge and
both sides
of the output. The secondary is bypassed with a .1uF, and the AC input
goes through
a Curtis 2 stage RFI filter.

I use a DX398 with a home made noise probe that I have discribed
before. For quitening
down power supplies and other devices, I use a 100uH axial ferrite
indyuctor incased
in a Sharpie pen. I can "sniff" each device, junction etc and really
stomp out offending
RFI noise at the source.

Terry