[Elecraft] P3 generates noise on 144MHz

[Bill K9YEQ]

I would add further "noise" to the noise concerns... during the process of
repairing PC's/Macs/etc. it is very apparent there are a plethora of
pathetic power supplies and converting devices in the shack.  My P3 detects
them very well. The great thing about the P3 is I can disconnect and find
the offenders.  It's nice my antennas are relatively far away but the idea
of shack noise, which does affect all my receiving equipment, it is very
helpful to note the noise floor on the P3...  The power connection to the P3
appears to need coaxial shielding and filtering, IMHO.  I have mine
connected via the K3 power, and it is via a simple power splitter. This
isn't so hot.

73,
Bill
K9YEQ

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Roger Crofts
Sent: Sunday, February 17, 2013 4:55 PM
To: elecraft reflector
Subject: Re: [Elecraft] P3 generates noise on 144MHz

I fitted a commercial EMI filter (intended for 240V AC operation) inside my
P3. It is a fully sealed unit with solder tags. This unit was connected in
the + and - DC supply line. This filter completely removed any trace of
noise from the power cord to the P3.
I used a short coax with a one inch extension of the center wire to act as a
"sniffer" to determine where the noise was coming from. This coax was
connected to the K144XV input. Did this solve the problem? No! not entirely.
I discovered there was now a small amount of noise being radiated from the
RS232 lead and the IF input coax. These cables had been free of noise
before. It seems to me that it is acting a bit like a boiling kettle - Block
the spout and the steam comes out the holes in the lid!
Why was there so much noise generation in the first place? I used my sniffer
inside the P3 box to try and track down the precise source of the noise. But
no luck. The noise was overwhelming everywhere at S9 + 20dB rising to S9 +
40dB near any of the ICs. This was on any frequency in the 2 meter band and
beyond.
At this point I re-visited the earthing between the panels that make up the
P3 box. If you have been following this thread, you will know that I had
previously cleaned all the mating surfaces. I looked again at the front
panel. It is connected by four screws to the corner blocks. You can see
where rectangles of masking tape have been placed to prevent the black paint
from reaching the area which will mate with the corner blocks. The masking
tape has not quite reached the edge of the board, leaving a one mm wide
strip of paint right at both corner edges. It seemed very unlikely that this
was preventing a good connection between the panels. Nevertheless I removed
all trace of the paint in all four corners of both the front and back panels
using a file and sharp knife. Did this have any effect? Well Yes! It reduced
the noise by about 6dB. The noise which had started at S9 + 5dB was now down
to about S3 which was the target value. The carriers at 19KHz spacing were
just, but only
  just, perceptible with the 2 meter antenna connected. It would have been
interesting to know what the noise would now be like without the EMI filter.
Sorry guys (and gals), I did not do that experiment. When you have got
something that works - leave it alone! I was also able to connect my other
accessories to the switched 12V line from the K3 without affecting the
noise. Another effect of my paint scraping exercise was that the position of
the P3 was no longer critical. Before, if I pulled the P3 forward, the noise
went down, but returned with the P3 in its correct position aligned with the
K3.

Roger Crofts, VK4YB
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