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Crystal Filter Matching

Discussion in 'Electronic Design' started by David, Jan 15, 2006.

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  1. David

    David Guest

    I am wanting to place a 45MHz, 2-pole crystal filter between the first
    mixer output and 2nd IF input. (Mixer output is a SA602 and IF input is

    The mixer output is around 1500 Ohms and the IF input is around 4500
    Ohms with 2.5pF capacitance parallel.

    The required source and termination for the filter is 800 Ohms par with
    3pF. The insertion loss of the filter is 2dB max.

    Would the impedances be close enough to use resitive pad between mixer
    and filter (680R in series with 1k2) and a small cap to ground across
    the filter input.
    Then a 980R resistor to ground at filter output and no capacitance (use
    SA605 2.5pF and stray to make up the 3pF).

    This would give total loss around 4.7dB.

    Or would I be better to use say a tapped capacitor or L match from Mixer
    to filter with
    added C on filter input.

    Apparently the SA605 has a "swet spot" for noise at around 800R so maybe
    the Res to ground on the input would be ideal ???

    Thanks for any help, I have not used crystal filters before and unsure
    of implications of the approach.


    David Huisman
  2. Andrew Holme

    Andrew Holme Guest

    There are some good Philips application notes about the SA6xx series e.g.
    AN1993, AN1994, AN1996 with filter matching examples; although I don't see
    any for 800 ohms.

    I think your resistor to ground idea at the filter output is the way to go.

    Since the SA602 output is just an internal 1500-ohm resistor from collector
    to rail, and it doesn't need to see a match, if you can live with the
    losses, I think you could get away with something like an external 1k8 in
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