Paul (and others)
A neat trick with audio spectrum analysis software running on a PC is to
use a diode ring mixer, such as an SBL-1 or equivalent, feed a signal
generator into the oscillator port and monitor the IF port on the sound
card. Pi network attenuators on at least the IF port will reduce
spurious signals which could result from the mixer not being properly
terminated with 50 Ohms. While exact details are perhaps a bit much for
this list, it's neither complex nor expensive to produce and another
useful item for your shack drawer.
To give a broad resistive termination for feeding a signal to such
devices, I use Wes Haywards resistive tap from the June 2001 QST (Simple
RF Power Measurement), this is available from the ARRL members part of
their web site. For a signal generator I use an old Marconi 2018.
The free software "Spectrum Laboratory" from DL4YHF
(http://freenet-homepage.de/dl4yhf/spectra1.html), which also includes a
two tone test generator, and can be configured for an orange graticule
and green trace to look like a proper spectrum analyser.
All good fun for less than $10.
73 Dave, G4AON
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