[Central-Electronics] Modulator Alignment Using FFT

[Jim Candela]

Lyman,

   Welcome back to the reflector. It's been a long
time since we've heard a peep from you. On the topic
of FFT, there is a free program that I'm playing with
that is so capable, and complicated that my head
spins. That said, it is really nice, and FREE. It is
called "Spectrum Lab". Here is the link:

www.qsl.net/dl4yhf

I currently have it connected to my Icom R-75 audio
output to my sound card in my ham shack computer. It
can do stereo, so looking at the two audio channels on
a phasing rig is doable in both time domain, and
frequency domain at the same time using seperate
windows within the same program.

Give it a try, but don't blame me when you stay up all
night playing with it!

Regards,
Jim
WD5JKO

--- Radra <radracom at verizon.net> wrote:

> 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
> 
> 
> 
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