Posted by
Ron D'Eau Claire-2 on
Mar 17, 2007; 10:53pm
URL: http://elecraft.85.s1.nabble.com/Filters-and-Spectrum-Analyzer-tp444864p444865.html
Spectrogram is also limited by the dynamic range of many sound cards. I've
used it on some computers that had an internal noise floor barely 30 db
down! Also the accuracy of the crystal clock in the sound card affects the
accuracy of frequency measurements.
Even so Spectrogram is an extremely worthwhile tool for the purposes it has
been recommended: align the BFO frequency to center the bandpass on the
receive frequency and to see, clearly and graphically, how changes affect
the bandwidth and position relative to the center frequency. Considering the
price (free) when downloaded from Tom, N0SS's site
(
http://www.n0ss.net/index_k2.html) it's hard to beat the "investment" for
anyone who has a PC running Windows handy.
Ron AC7AC
-----Original Message-----
Hi Brett,
My personal preference when looking at the response of any filter is to use
a RF Spectrum Analyzer and a tracking signal generator. When it comes to
looking at the response of IF filters already built into a receiver the
tracking generator can work at the antenna frequency rather than at IF,
which makes the generator - pad - receiver connection easy to make, provided
that the front end selective circuits have no effect on the measurement of
the IF filter i.e.their working bandwidth is >>> IF filter bandwidth. The
penalty is that the input of the Spectrum Analyzer needs to be connected to
the IF strip at some point before the product detector, and everything after
this point has to be disconnected unless the working bandwidth of this part
which follows is >>> IF filter bandwidth, *or* has no effect on the
measurement. One also has to ensure that the Spectrum Analyzer and its
connecting cable are transparent to the cascade under measurement, a high
impedance probe helps. AGC has to be OFF of course if the AGC system could
be active. Spectrogram does not require this fiddling, AGC must be OFF, but
being in effect a Spectrum Analyzer working at audio it gives a limited
view - about 20kHz IIRC. What I like about using a RF Spectrum Analyzer and
tracking generator is that a cleaner picture is produced without fuzzy noise
at the top, and it is possible to sweep over a wide bandwidth to view the
ears and spurious responses of the filter. It is also usually possible to
see those other unwanted responses caused by radiated feedback or coupling
via pcb traces etc.
It could be argued that Spectrogram is easier to use for routine alignment,
but has limitations for use as a diagnostic tool.
73,
Geoff
GM4ESD
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