The entire S53MV spectrum analyzer is very well done, but appears to
be a nightmare to build and adjust. The crystal filter, though, is
fairly straightforward. I'm curious how much bandwidth adjustment is
possible with the variable loading of the crystals. It seemed that 150
KHz was the widest bandwidth but I couldn't find the minimum
bandwidth. For anyone checking it out, note that his description of
the source of the crystals is separated from the description of the
circuit itself. He got the crystals by disassembling 35 KHz bandwidth
filters containing four pairs of crystals. To get the necessary four
matched crystals apparently requires taking the corresponding pairs
out of two separate filters, though that was not entirely clear.
The filter response affects the maximum sweep speed. It's not really
true that ordinary filters are "useless" in an SA, just that the sweep
speed must be lower than with the ideal Gaussian filter. The S53MV
filter might be more useful because of its bandwidth selection than
for its response envelope, though the latter is a nice bonus.
--- In firstname.lastname@example.org, "Geoff Pike" <gi0gdp@...> wrote:
> "Spectrum analyzers require somewhat different IF filters than those
> installed in communication receivers. Communication receivers usually
> require filters with a flat passband, to avoid modulation distortion,
> and a very steep increase of the insertion loss immediately outside
> the useful passband, to reject adjacent channels. Such filters are not
> suitable for spectrum analyzers, since their time response is rather
> slow (ringing!) compared to the filter bandwidth.
> A slow filter response and/or ringing is especially harmful at small
> bandwidths, where the time response of the filter defines the sweep
> time and display update period. Commercial crystal and ceramic filters
> are therefore almost useless in spectrum analyzers. A suitable crystal
> filter or set of different filters has to be specially built for a
> spectrum analyzer.
> A spectrum-analyzer IF filter should have a "triangular" frequency
> response with a sharp peak and smoothly and symmetrically increasing
> attenuation outside the passband. In practice this requires
> under-critically-coupled resonators or better a series connection of
> several single-resonator filters and buffer amplifiers, to avoid any
> interaction among the resonators".
> This has been taken from S53MVs website, worth a visit to see this
> xtal filter which uses variable loading to achieve different
> bandwidths just like HPs older Spectrum Analysers and the Marconi 2380
> series analysers,
> Geoff Pike