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Tektronix 7L12 Spectrum Analyzer Question

Discussion in 'Electronic Equipment' started by chuck, May 18, 2005.

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  1. chuck

    chuck Guest

    I'm in the market for a used spectrum analyzer like the 7L12
    or the HP 141-T. One important use will be to make two-tone
    IMD measurements on HF SSB transmitters. I am concerned that
    with tone separations on the order of one kHz, the 7L12 may
    not have sufficient bandwidth in the 300 Hz mode to resolve
    adjacent IMD products expected to differ in amplitude by 40
    dB or more.

    Has anyone on the group used a 7L12 for this purpose and is
    the 300 Hz RBW sufficiently narrow?

    There seems little doubt that the 141, with 10 Hz or 100 Hz
    RBW, will handle this.

    Many thanks in advance.

  2. John Miles

    John Miles Guest

    It wouldn't be ideal for 1 kHz spacing. If the -6 dB point is 300 Hz,
    and the shape factor of the 7L12's filters is about 8:1 like other Tek
    analyzers I've used, then the -60 dB point is around 2.5 kHz. Figuring
    a linear falloff (in log space), 1 kHz tones couldn't be resolved below
    about 30 dB, give or take 10 dB at the most.

    I've never used a 7L12; maybe the filters are steeper than that. But
    you can't beat an HP 141T / 8553B / 8552B (not -A!) combination for HF
    work at less than 3x the price. If you've got the space, that's the way
    you should go.

    -- jm
  3. chuck

    chuck Guest

    Thank you, John.

    FWIW, Tek claims a 6/60 dB shape factor of "4:1 or less" at
    all bandwidths on the 7L12. They're analog filters and after
    30 years may not live up to their design specs. In the end
    it strikes me as insufficient headroom for an instrument
    requiring that kind of investment. I'll probably do the
    right thing and find an HP 141-T.

    Thanks again, John.

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