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Home-brew narrow-band RF spectrum analyzer (RE: [AMPS] IMD)

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Subject: Home-brew narrow-band RF spectrum analyzer (RE: [AMPS] IMD)
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Date: Wed, 19 Sep 2001 09:42:59 +0300
One of the minor reasons I bought FT-817 was the
following idea....

A poor man's narrow-band RF spectrum analyzer for
quick and dirty check of the near spectrum of the TX:

- FT-817 (or any other QRP CW TX) as local oscillator

- SRA1H (or higher level) DBM mixer (with diplexer and LPF for output)

- attenuator for local

- sampler and/or attenuator for RF

- PC & sound card & FFT-based audio spectrum display program
connected to the IF port of the mixer

With only real signal (instead of complex signal or I and Q)
the "spectrum analyzer" unfortunately is a DSB receiver.
(Or does anybody know freeware FFT program that would
interprete the L and R audio channels as I and Q?)

With about 44 kHz sampling rate you can place the
LO 10 kHz from your center frequency. If you still
have genuine problems with the image response of the "SA" 
you know that you need to do something to your TX also ;-)

The phase noise of the LO TX can limit the resolution
of the setup. Even FT-817 is not as bad as the local
oscillators of the old spectrum analyzers, though.

I have not yet implemented that idea but soon will try

Of course this thing can not compete with a modern
professional RF spectrum analyzer. But the price
tag is more suitable for home laboratory!

One benefit of FFT (IF-) spectrum analysis is the fact that
you can have very narrow resolution bandwidth. Seeing
the (hopefully only) two tones the PSK-31 idle mode
generates (31 Hz apart!) should not be a problem with
this setup. Another new digital mode MT63 (64-tone OFDM
with 15.625 Hz tone spacing) is an interesting
test for the chain from audio input - TX output.
Unfortunately you will need two PC:s and sound cards 
if you want to see the TX near spectrum with digital
modes. Also the quality of both sound cards will affect
the measurement results.

73, Ilkka OH3NJC

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