[Central-Electronics] Modulator Alignment Using FFT

[Radra]

Recent posts reveal there is some interest in the alignment of the Central
Electronics phasing modulators.  Use of an oscilloscope to display lissajou
patterns is an venerable old-time method.  I have done the alignment with a
more modern technique employing an RF spectrum analyzer.  Well now I realize
not every ham has one of these things sitting around their shack.  So maybe
one can be assembled using readily available items.

I propose turning your PC/audio-card combination into an audio spectrum
analyzer by means of appropriate software.  The RF signal you want to
examine can be down-converted into the audio band simply by mixing with
another CW RF signal set to within a few kHz of the RF signal you want to
examine.  To do this task, you will need a mixer and an RF generator to
provide the "local oscillator" signal [Maybe a CW transmitter will serve as
the generator.].  Feed the resulting output of the mixer into your audio
card (probably via LINE IN). The software then performs an FFT (fast fourier
transform) on the input signal to generate the spectrum.  Both phase and
amplitude information is then available which can be displayed verses
frequency.  This display should clearly show the audio sidebands and
residual carrier.  Adjust your CE to minimize the undesired sideband and
residual carrier.

Depending upon your receiver's demodulation characteristics (bandwidth,
linearity, etc.), you might even be able to use it to feed the audio into
your PC's audio card thus eliminating the mixing step.

I did a quick Google search on the terms "audio", "card", and "FFT" and came
up with a number of hits revealing that such software is available as
shareware.  Mixers with connectors are readily available on eBay and at swap
meets for a few dollars.  Most mixers work best with a local-oscillator
input-signal level of about +7 dBm.  So basically what I have proposed is a
"poor mans" RF spectrum analyzer.  And surely there must be a wealth of
information out there on this subject.  Although I haven't tried it, I think
it will work ok and would be fun to try.

Lyman K7LH