[hpsdr] Noise Problem or Coincident of Generated Noise

[mikea]

On Tue, Jul 07, 2009 at 10:52:36AM -0700, joetorrey at cablelynx.com wrote:
> ***** High Performance Software Defined Radio Discussion List *****
> 
> I made a couple of changes to mySDR 1000 and Janus/Ozy setup. I also have
> a second receiver (SDR IQ) sharing the same antenna as the SDR 1000.
> I installed the filtering described by Ahti regarding the DC-DC converter
> circuitry and when I tested this the results looked great - no issues
> whatsoever.
> I then removed the Janus board and installed the correction regarding the
> errata/rework data; capaitors to ground from the right and left vcomm
> lines on the akm ad/da chip. I tested and have a noise that raised the
> noise floor some; also a noise hump that moves about at times that is
> quite strong.
> I also checked with the SDR IQ receiver and see it there as well; the SDR
> IQ is powered up with power at the same time as the SDR 1000 is powered
> up; they are tied together in the same cabinet.
> The sdr IQ and sdr 1000 have been setup in this fashion for a long time. I
> disconnected the antenna from between the sdr 1000 and the sdr IQ and
> still see the hump but it does move in frequency.
> I did reuse the capacitors that I removed from the Janus board; just
> placed them correctly on the board.
> I find it difficult to believe that the correctly placed Janus capacitors
> could generate a noise and even get to the second receiver (sdr IQ).
> Also, I am 100% sure that when the first change was made (dc-dc
> converter); the test was excellent with no noise.
> I am now thinking that I have an external source that generates this
> noise; as I have had this before. I have ordered new capacitors (220NF)
> for the Janus board just in case the heat from removall caused a short as
> I did not test with an ohm meter.
> 
> Any help or insight regarding this would be much appreciated.

Over the weekend I noticed on my SDR-IQ a series of very strong noise
bands spaced about 88KHz apart, while using a new-to-me Toshiba
PA3048U-1ACA switching power supply with my Toshiba laptop. They went
away and the overall noise floor dropped about 3 S-units as soon as I
pulled the supply loose from AC power. I haven't tried wrapping the
input and output leads around lotsandlots of toroids yet, but that's
definitely in the cards. You may have something similar going on with
one or more of your switching supplies or wall warts. I'm buuilding a 6A
15VDC linear supply to replace that switcher.

Experimentation showed that that switcher was also inducing noise in the
coax (good RG-6 I installed myself) to the cable box in the bedroom.
_Nasty_ noisy little supply. It's going to meet Mr. Sledgehammer as soon
as the linear supply is up and running.

When you put the SDR-IQ in 190 KHz display mode, do you see multiple
noise bands more-or-less evenly spaced? 

-- 
Mike Andrews, W5EGO
mikea at mikea.ath.cx
Tired old sysadmin 

 1246989995.0