[hpsdr] ISD - more work needed

[Ahti Aintila]

Still far away from the target! Obviously some design changes will be
needed for the Integrating Sampling Detector (ISD). What about a
modified mixer of  SM5BSZ?

Here are some tentative measurement results with a system having no
amplifier before the ISD, only 20 dB amplifier between the ISD and the
sound card:
1.8 MHz: noise floor -133 dBm (11.7 Hz), 500 Hz noise -119.3 dBm
28.5 MHz: noise floor -125 dBm (11.7 Hz), 500 Hz noise -111.0 dBm

The attached picture shows -10 dBm signal on spectrum display using
DSB ±6 kHz filter. Maximum signal the ISD now can take is more than 0
dBm, so more amplification may be used to improve the sensitivity.
However, we want to have a sampling detector that is not clipping
strong signals and still has noise floor somewhere -160 dBm/11.7 Hz.

73, Ahti OH2RZ

P.S. Sorry Bob, no two tone generator yet.
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