[Skimmertalk] What causes audio images with QS1Rs?

Paul Higginson paul_group at greenrover.demon.co.uk
Sun Feb 24 05:10:28 EST 2013

I can't see anything external to the qs1r that could cause this, but 
willing to be convinced otherwise of course.

Do the Skimmer ops in question know of the issue? Has there been any change 
- do they use a different bandwidth during contests ? - I do.

Any changes made are sometimes good clues.

Substitute would be a very quick test

73 de Paul   LA/GW8IZR

------- Original message -------
> From: K1TTT <K1TTT at ARRL.NET>
> To: n4zr at contesting.com, RBN-OPS at yahoogroups.com, 
> Skimmertalk at contesting.com
> Sent: 24.2.'13,  0:01
> Damaged qs1r's?   that was the final determination on mine, lightning 
> damage
> did something to the sampling circuitry that caused some kind of 
> distortion.
> Maybe phil would remember exactly, I sent it to him to see if it was 
> worth
> repairing.  That was the same time I was trying to use the cwsl tee 
> software
> and it seemed to be behaving badly, you could see the images and some 
> other
> kind of intermittent noise when watching the bands using the hdsdr 
> software
> on the tee data stream.  Since replacing the qs1r the cwst tee and hdsdr
> works perfectly, so that may be a way to test.  When I did a simple image
> test a couple weeks ago I could watch on the hdsdr on 10m and run a 
> rather
> loud 10m signal from a radio up and down the band and not generate any
> image, when I did that with the damage there was an obvious image 
> sweeping
> the opposite direction of my tx... basically if it is always the same
> distance on the opposite side of the mid range of the band it almost has 
> to
> be inside the sampling circuitry, anything outside the sampler would most
> likely not be at the sampling image frequency but would be somewhere else
> and half the time would move along with the signal being decoded instead 
> of
> opposite it... hopefully that makes sense, its kind of hard to visualize.
> David Robbins K1TTT
> e-mail: mailto:k1ttt at arrl.net
> web: http://wiki.k1ttt.net
> AR-Cluster node: 145.69MHz or telnet://k1ttt.net
> -----Original Message-----
> From: Pete Smith N4ZR [mailto:n4zr at contesting.com]
> Sent: Saturday, February 23, 2013 21:30
> To: RBN-OPS at yahoogroups.com; Skimmertalk at contesting.com
> Subject: [Skimmertalk] What causes audio images with QS1Rs?
> During the ARRL CW contest, we had two Skimmers using the QS1R/Skimmer
> Server combination that each produced many false spots, with the 
> "signature"
> commonly associated with Skimmers using simple SDRs, ones which rely on a
> computer sound card for A/D conversion. That is, the two spots are spaced
> equidistant from the receiver's center frequency, one is significantly
> stronger than the other, and they arrive at the server typically within 
> the
> same second.
> We're having no luck isolating what is different about those particular
> systems.  So far we've ruled out inadequate power supplies, and the idea 
> of
> excessive gain before the receiver doesn't look promising.  Does anyone 
> have
> any theories we can explore?  I'll be glad to provide the data in Excel
> format for anyone who wants it.
> Thanks in advance!

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