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two tone questions

Discussion in 'Electronic Design' started by Ron H, Mar 15, 2006.

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  1. Ron H

    Ron H Guest

    I am confused over the "ideal" frequencies to use for a two tone test of a
    SSB transmitter. Art Collins' book states that using two tones with a 3 to
    5 relationship will make it easier to identify intermod products and
    harmonics and in fact Collins used 3 and 5 KHz as the two tones. Modern SSB
    rigs only have about 2.7 to 3 KHZ audio passbands so 3 and 5 are not much
    help. I see articles using evrything from 400 & 1900 Hz to 1.1 & 2.2 KHz..
    Is there a "best" pair for testing modern rigs, or is this all a lot of hype
    and any two tones inside the passband wil work?

    Ron H.
     
  2. Tim Shoppa

    Tim Shoppa Guest

    You don't have to use 3 and 5 kHz to have the same ratio - you could
    use 1.5 and 2.5 kHz, or 900 Hz and 1.5 kHz.

    I think the "magic" of the 3:5 ratio is that 2nd-order intermod comes
    out at 2 and 8, and neither 2 nor 8 are harmonics of the 3 and 5. This
    helps you test for intermod independently of nonlinearities in the
    audio chain. 1.1&2.2 doesn't have this, but 400&1900 does.

    The small-integer ratios help a lot with identifying frequencies on
    Lissajous figures (something that 400&1900 doesn't have for appropriate
    values of "small integers".) This doesn't seem particularly relevant to
    intermod testing but it may determine what standard fixed freqs you
    have at your test stations.

    For broadcast-quality (AM or FM radio) audio they tend to use one very
    low frequency (like 60Hz) and one very high frequency (like 7kHz) in
    intermod measurements. I don't think these are relevant to SSB comms
    because they both fall way outside the passband. I seem to recall that
    for broadcast intermod measurements they are also interested in
    higher-than-2nd-order intermod (so they are looking for F2-2F1 and
    F2+2F1 as well) and for SSB voice comms we usually aren't that picky.

    Tim.
     
  3. Joseph2k

    Joseph2k Guest

    And as always the intermodulation test signal products should be in the
    passband.
     
  4. Tim Shoppa

    Tim Shoppa Guest

    Is that necessarily true? I mean, intermod made in the drivers or
    finals have no chance of being filtered out by your sideband filters.
    It ends up splattering all over the band, and while that's not "your"
    passband it is everybody else's!

    Tim.
     
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