IF Filter for MF band

Discussion in 'General Electronics Chat' started by KCHARROIS, Nov 1, 2012.

  1. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    1
    Hello

    So I need to design a bandpass filter at 455KHz with a bandwidth of 14 Khz at 3db circuit for an am receiver. It seems to be pretty hard to find a circuit online to do this or to find a component thats affordable. Is there a circuit out there that I dont know of? Or is there a better IF signal that I could use where components are ready available? I have looked at other threads some do address my problem but not entirely.

    Thanks
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    AM radios have a narrow bandwidth of about 5kHz. If your iF amplifier has lots of extra gain then stagger-tune two IF transformers, one tuned a little low and the other tuned a little high for a greater bandwidth.
     
  3. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
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    I dont understand what your trying to say audioguru?
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    Stagger-tuning is to tune one IF transformer for a peak at 450kHz and tune a second IF transformer for a peak at 460kHz. Then the bandwidth is 15kHz instead of 5kHz but the gain will be reduced.
     
  5. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    Very interesting I never new 455KHz IF transformers existed plus the Q value is very high. Great I guess I have some more reading to do.

    Thanks Audioguru
     
  6. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
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    So heres a circuit containing an IF transformer good for 455KHz. Will this work as a filter?

    Thanks
     
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  7. Audioguru

    New Member

    Dec 20, 2007
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    Cheap AM radios have 455kHz IF transformers. Good AM radios today have crystal filters at 455kHz. Crystal filters cannot be tuned.
     
  8. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
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    Well I've looked at circuits such as a crystal ladder circuit but finding and buying 455khz crystals are expensive and hard to get now.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    People do not make their own AM radios anymore so parts are hard to find.
    Buy an old AM radio and use its parts.

    Why are you trying to make a wideband AM radio? AM radio sounds awful.
     
  10. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
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    The reason why I'm trying tp build an AM radio is to get a base on how heterodyne radios work and then to build and test one. Then I can move on to FM receivers. The rule I gave myself is to build something discrete then move on to IC's I know its gonna sound like crap.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    An AM radio produces sounds from about 50Hz to only about 2.5kHz mainly due to the narrow bandwidth of its IF transformers. Many AM radio stations boost high frequency sounds (pre-emphasis) up to 10kHz (then sharply cut higher frequencies) so that ordinary AM radios have better high frequency response. High quality AM radios have a wide IF bandwidth plus de-emphasis for better sound, reduced hiss and reduced adjacent channel interference.

    The cheap IF transformers each has a bandwidth of 10kHz then two IFs in series produce a total bandwidth of 5kHz and three produce a bandwith of 2.5kHz.

    Here is a typical pre-emphasis curve for an AM radio station:
     
  12. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
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    So something like this...
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    There are hundreds of AM radio schematics in Google.
    Some use IF transformers that are double-tuned like the one you showed and others use single-tuned transformers like this:
     
  14. vk6zgo

    Active Member

    Jul 21, 2012
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    85

    I spent a lot of time working at MF AM radio stations in Australia,& we tested to the full specifications of the Licencing authority.

    Most of the locally made stuff well exceeded the specs.

    Normally,the response was around +-1.2 dB w.r.t 440Hz from 50Hz up to 10kHz ,where it fell to -1.5 dB.
    The response fell off fairly sharply but smoothly, from there.

    When they decided to cram some more stations into the band,we had a 9kHz start to the roll off using external filters.

    The ABC never used pre-emphasis on AM,but I can't speak for the Commercial stations.

    Back in the day,a lot of work was done by such folks as John Moyle & Neville Williams of the old "Radio & Hobbies" magazine & Fritz Langford-Smith of "Radiotron Designers Handbook" fame,to get decent bandwidths out of MF receivers.including such tricks as stagger-tuning,overcoupling,resistive loading,etc.

    Ordinary Australian mantel sets went out to around 6KHz,but when transistor portable radios appeared,most of the imported ones fell over at about 4kHz.
    The more expensive imports had FM as the main band of interest,with AM as a "poor relation",with really lousy quality.

    When belatedly,FM commenced,most of the audio freaks migrated to that system,so there was no more real incentive to make decent AM MF radios.
     
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