[Elecraft] Filters and Spectrum Analyzer.

[Geoffrey Mackenzie-Kennedy]

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