Joe Subich, W4TV wrote:
<snip>
With an increasing number of DSP based receivers, we need a test
(rating) system that attempt to measure the "total (peak) receive
signal" necessary to generate a IMD products which exceed the
receiver noise floor. This is different than either two tone IMD3
measurements at 2/5/10/20/100 KHz or a blocking measurement and is
the only way to absolutely compare receivers with different design
philosophies and different IMD "windows."
73,
... Joe, W4TV
Perhaps a variant of the old analog microwave measurement protocol could
be made to work -- feed a broadband noise signal into the receiver's
front end. The noise is notched, so that a specific frequency it is
reduced many dB. Then tune the receiver to the notched frequency and
observe the signal level. Since the IMD in this case is the product of
broadband noise, the IMD will have noise characteristics. Measure the
noise level in the notch and it tells you a great deal about the
performance of the system in the real world.
Analog microwave systems had hundreds or even thousands of SSB signals,
spaced 4 KHz so linearity and low IMD performance was important.
However, the signals were more or less of the same peak amplitude, so
you did not have the case of several S9+60 dB signals and a few very
weak signals. Still, the concept could have some merit in the receiver
context. Lots of details to work out, of course.
Jack K8ZOA
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