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mmic switch with dc-transmission

Discussion in 'Electronic Design' started by Vau, Jul 1, 2004.

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  1. Vau

    Vau Guest

    hi everybody!

    I´m a student in electrotechniques and working on a thesis.

    In this I need to switch up to 100 RF signals (up to 200 Mhz). I found
    a lot of different mmic switches, which would be applicable because
    off small smd packages and limited space on pcb.

    My problem is that all found switches need dc-cut capacitors, but I
    need dc transmission because of measuring logic levels.

    Does anybody know mmic switches which transfer dc or any other
    suggestion to solve this problem?

    thanks in advance

  2. John Larkin

    John Larkin Guest

    Most of the integrated RF switches do pass DC, because capacitors are
    hard to fabricate in-chip. But they tend to work best at weird DC
    offsets; you can get around that be figuring out what their inherent
    offset is, and shifting your power supplies to translate that to
    ground. Of course, your control logic levels have to move, too.

    Depending on your isolation requirements, the little Fairchild cmos
    logic switch/mux things might work... they're just analog transmission

    The Hittite HMC253 is a nice 8:1 mux, very fast, and the price just
    dropped from $22 to $5. It conducts DC in its transmission path but,
    interestingly, has series caps on its terminations!

    Don't overdrive any of these things or very strange stuff will happen.

    A 100:1 tree will be interesting in any technology.

  3. Vau

    Vau Guest

    I made different tests with Hittite HMC253. I removed the DC-Cut off
    capacitances on their evaluation board. the switch passes DC now, but
    the isolation between the channels doesn´t seem to function properly
    any more.

    I. e. a sinewave on channel 1 distorted a square wave signal on
    channel 8 which should be passed.

    Any suggestions?

  4. John Larkin

    John Larkin Guest

    Too much signal swing will turn on the internal fets; centering the
    swing vs the DC supply levels will optimize things. If your signals
    are high-level, beyond the specs of the Hittite, you'll have to shop
    around for a different technology. Some CMOS analog switches are fast,
    but I don't know your exact requirements for crosstalk isolation,
    swing, attenuation, etc. No simple answers here.

  5. Joerg

    Joerg Guest

    Hi Vau,

    Check out the SD5400 series of quad switches. I have done several
    multiplexers with these that went from DC to well over 100MHz. You do
    need to have some resistive path to ground though so that the FETs don't
    start to float.

    Regards, Joerg
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