[Skimmertalk] What causes audio images with QS1Rs?

[K1TTT]

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!

-- 

73, Pete N4ZR
Check out the Reverse Beacon Network at
http://reversebeacon.net,
blog at reversebeacon.blogspot.com.
For spots, please go to your favorite
ARC V6 or VE7CC DX cluster node.

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