Re: Microwave Noise
Chris Bartram G4DGU
Hello again, Andy,
I can certainly see that off-air HF noise could include artefacts
of the non-linear nature of the propagation medium - I'm old
enough to remember the 'Luxembourg Effect' on MW! IMD2 does
nowadays seem to be taken more-or-less seriously, particularly
since the advent of direct sampling receivers. But much VHF and
up equipment still seems to ignore it.
The use of FFT techniques could easily lead to measurement
inaccuracies/uncertainties in a situation where the 'noise'
spectrum was non-stochastic. I may be a bit of a dinosaur, but I
still use a zero-bias diode detector with proven square-law
response, a thermal wattmeter, or alternatively a wideband true
RMS voltmeter for making noise power measurements. True RMS
voltmeters like the Racal-Dana 9300 or 9303 are available surplus
at quite low prices, and with a simple downconverter can be used
as a back-end for noise ratio measurements. However that's getting
a bit off-topic.