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Xtal Filters

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  • Geoff Pike
    Spectrum analyzers require somewhat different IF filters than those installed in communication receivers. Communication receivers usually require filters with
    Message 1 of 2 , Oct 12, 2008
      "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,
      regards
      Geoff Pike
      GI0GDP
    • Sam Wetterlin
      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.
      Message 2 of 2 , Oct 12, 2008
        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.

        Sam W.


        --- In spectrumanalyzer@yahoogroups.com, "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,
        > regards
        > Geoff Pike
        > GI0GDP
        >
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