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Ceramic IF filters

Discussion in 'Electronic Design' started by John Wilkinson, Oct 8, 2005.

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  1. Hi,
    I am trying to design a 455KHz IF stage. I would like to use a 455KHz 7KHz
    BW ceramic filter. Looking at the Murata data, they suggest that an
    additional IFT is used to reduce the spurious responses.
    Has anyone used such a scheme?
    Does it mean connecting the IFT directly to the output of the filter?

    Do I really need to worry about the spurious resonses?
    Collins mechanical filters seem to have the same problem, but do these also
    usually have an IFT as well?

  2. I read in that John Wilkinson
    <>) about 'Ceramic IF filters',
    Yes, it's quite normal.
    Murata used to supply the transformer or give you a manufacturer's part
    number and a diagram of how to connect it.
    That depends on what you are making, doesn't it.
  3. Thanks for the reply John.
    Do you have an example of such a setup, with the filter connected to an
    IFT? I would just like to get an idea.
  4. Hi,
    As this is the second IF, the first having a 10KHz BW using a crystal
    filter at 45MHz. Will this aliviate the need for an additional IFT after
    the ceramic filter?

  5. I read in that John Wilkinson
    Murphy rules, I'm afraid. I found one receiver circuit, but I don't
    believe it; it has two coils in series between the signal and ...
    nothing. And I can't see that it's just a printing error; it was drawn

    The other source is a Toko catalogue, which shows the filters and
    transformer(s) all in one can, but no internal circuit details.
  6. I read in that John Wilkinson
    It's got to be a definite 'maybe'. The ceramic filters have a host of
    resonances and any one of those responses could give you a problem.

    These filters are usually quite low impedance. You can get more IF gain
    by matching the impedance, using IFTs, to something more appropriate for
    whatever you are using to feed it and to follow it.
  7. LR

    LR Guest

    Pretty much, though it's better to put the IFT before the filter than after. One
    of the biggest problems with the ceramic filters is stray coupling to/from
    surrounding circuitry. The plastic cased filters are really bad about this. If
    you can afford the metal cased models, the spurious responses are much lower. If
    not, then try adding your own shielding around the filters. If you're cascading
    ceramic filters, orientation is very important too. Don't put them side-by-side,
    but rather in a straight line.

    I don't know about Collins, but the plastic case ceramic filter spurs may be
    only 15dB down from the center of the bandpass, or less depending on circuit
    design and parts layout. If your receiver specs and operating envireonment can
    tolerate this, then you can do without the IFT. Midland had a pocket scanner
    that used a 10.7 MHz xtal filter into a mixer then straight into a CFU-455F,
    resulting in relatively poor spurious rejection, about 40db. But the product was
    relatively complaint free.
  8. Thanks again John for looking into this.
    I will probably end up using an IFT to step down the impedance from around
    2K to 100 Ohms.

    Best regards,
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