Excite DUT with white noise, convert output down to audio band using XO 
and use spectrum analyzer running on PC audio card to visualise. 
Schematic supplied. The nice part is that given a good mixer and LO this 
whould work way up into microwave. Finally an affordable solution for 
filter and amplifier design for homebrew UHF and microwave ?

http://www.qsl.net/7n3wvm/Fil_Meas.html

Peter
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