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DBM and constant R band pass filters.

Discussion in 'Electronic Design' started by Fred Bartoli, Apr 18, 2004.

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  1. Fred Bartoli

    Fred Bartoli Guest


    I have to use some DBMs in a measurement instrument and of course have to
    design the following bp filters.

    I have often seen recommended that the DBMs see the resistive load of a
    constant R filter (diplexer) rather that the reactive load of a more
    traditional filter.

    I can think of (model) the DBM mixer to the first order as follows :

    .-- +/-1 at LO frequency
    .------. ZG Zd .-------.
    | | ___ ___ | BPF |
    VIN--+------------>| MULT |---|___|--|___|---+---| H(p) |--- VOUT
    | | | | | |
    | '------' | '-------'
    | ___ .-.
    `-|___|----. | |
    ZG | | | ZL
    .-. '-'
    | | |
    | |Zd |
    | '-' |
    I in| | |
    | | |
    V .-. |
    | | |
    | |ZL GND
    created by Andy´s ASCII-Circuit v1.22.310103 Beta

    ZG being the source impedance,
    Zd the diode dynamic resistance,
    ZL the filter input impedance,
    H(p) the BPF voltage transfer function.
    The multiplier is a high Zin, null Zout device with unity gain.

    I can't see any requirement in this first order model for a constant R load.

    Do I miss something ? Is it needed because of second order effects ?
    Any thought, any pointer to (preferably) detailed analysis is welcomed.

  2. John Stephensen. QEX May-June 2001

  3. Tim Wescott

    Tim Wescott Guest

    The constant-R load is a requirement for diode-ring mixers, not
    double-balanced mixers in general. The diode-ring mixer acts like four
    switches that turn on and off as a function of the LO, RF and IF port
    voltages. You'd like this to just be as a function of the LO, however.

    By it's nature a diode-ring mixer does exactly as good a job of mixing
    from the IF port to the RF port as it does going in the other direction
    (this can be handy for transceiver design, by the way). So when you
    terminate the IF (or any other port) with a non-constant R you get
    different voltage-current relationships at the image frequency than you
    do at the intended IF. These oddball voltages or currents (a) affect
    the mixing, and (b) get dutifully converted into energy that appears
    back at the RF port to get mixed _again_. Taken to extremes people with
    very large brains can even terminate a diode-ring mixer make a
    parametric amplifier from RF to IF.

    My understanding is that you don't need a perfect DC to light constant-R
    termination: you just need to terminate for the expected frequencies out
    the IF port; primarily the IF and it's image (so RF +/- LO), but to be
    thorough you should consider the LO's odd harmonics.
  4. Joerg

    Joerg Guest

    Hello Fred,

    A diplexer is certainly the Rolls Royce method. Elegant and impressive. But
    there are others.

    You could also provide a buffer amp that has a matching input impedance and the
    mixer will be happy. One of my favorites is the AD603 if it fits the dynamic
    range requirements, frequency range and cost limits. That gives you a db-linear
    gain control of 40dB which often comes in real handy. They don't use Gilbert
    cells in there but a patented voltage controlled attenuator so there isn't a
    dynamic range penalty with gain reduction.

    Regards, Joerg.
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