[Premium-Rx] Noise Power Ratio Testing of Radio Receivers: preselectors

[Adam Farson]

Dear Michael,

During my NPR test run on the Perseus direct-sampling SDR receiver, I
observed that the NPR increased by 3 to 5 dB with the internal preselector
in-line vs. bypassed. Assuming that passive IMD in the preselector is
negligible, the added RF filtering will reduce the bandwidth ratio BWR
(refer to my article for details) and thus raise the NPR.

In addition to the noise generators, I also have a noise receiver (the other
end of the NPR test setup). It is possible to measure the NPR degradation
due to passive IMD, if any, in the tuned-circuit components by connecting
the preselector between the two instruments and running a test with very
high noise loading.

Cheers for now, 73,
Adam VA7OJ/AB4OJ

> His analysis demonstrated the continued value of traditional LC
preselectors, albeit that in his time, and > still now, tracking
preselectors were and are a very costly way of doing things.