[hpsdr] ISD for SDR

[Ahti Aintila]

Hi all,

After a lot of private mailing and experiments with the Active
Integrating Quadrature Sampling Detector concept, I think it is the
time to put my results under the public criticism. Unfortunately, my
previous sending bounced back due to too big attachments, so you
should see the links:
<http://kotisivu.dnainternet.net/ahti/sdr-1000/AIQSDb.pdf>
<http://kotisivu.dnainternet.net/ahti/sdr-1000/isdbrd.jpg>.

Against the common practice and opinions I have used 50% wide I and Q
sampling pulses instead the 25% pulses, because those wider pulses are
readily available without any extra decoding in proper form to the
selected two fast SPDT sampling switches.

Please, understand that my circuit is not necessarily best optimized
for the purpose, but I feel that it is a good starting point for other
experimenters. Already now I can suggest few dB more internal gain for
the ISD so that no post amplifier would be needed before the sound
card (provided that the sound card has differential inputs).

73, Ahti OH2RZ