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IF Filters for AM/voice reception

Discussion in 'Homebrew and Kit Projects' started by HEXICPYTH, Mar 27, 2020 at 4:54 AM.

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    I'm trying to make an IF filter for my homebrew superhet but I'm having a hard time making an IF filter with a bandwidth wide enough(somewhere between 9-20kHz-ish) for voice reception. My local oscillator covers between 3.3Mhz-6.1Mhz because those frequencies were easy enough to make a stable oscillator at which didn't swing wildly in amplitude/impedance across the frequency range.

    I would to keep my IF frequency comfortably below the upper limit of my 11Mhz signal generator so if I'm using the difference frequency than I'm sort-of constrained to either 455kHz or a nonstandard frequency like 3.2Mhz(which would give a nice 100kHz-2.9Mhz RF range).

    It's easy enough to find crystals for these frequencies, but in my experiments the crystals seem to have way too low of a bandwidth(tens to hundreds of Hz)/too high of a Q to be useful anything other than CW or maybe SSB. I have found some ceramic resonators between these frequencies, but the datasheets I have found(like this one) don't seem to provide any helpful information that could be used to find the -3dB bandwidth. How exactly would one go about making a wide-enough IF filter for AM reception below 10.7Mhz+ where most modern receivers keep their IFs??
  2. SM0GLD

    SM0GLD Ham Member QRZ Page

    You need to design a lattice filter and to do that you must use several crystals with different frequency.

    Depending on your needs of attenuation and passband ripple you can use 2, 4 or 6 crystals.
    Last edited: Mar 27, 2020 at 8:17 AM
  3. KE0ZU

    KE0ZU Ham Member QRZ Page

    Or, use an easily available ceramic filter. They have fairly good shape factors and are inexpensive. Murata and Kyocera both made them by the millions.
    W1BR likes this.
  4. G4COE

    G4COE Ham Member QRZ Page

    For voice I would have thought 6Khz would be OK.... any wider would let all the noise in, filters need not be elaborate.

  5. W1BR

    W1BR Premium Subscriber QRZ Page

    There are tons of used 10.7 MHz crystal filters for old FM two way radios floating around on eBay that will give you a wide AM BW. If you don't mind a lower IF, there also quite a few odd-ball IF mechanical filters that could work.
    SM0GLD likes this.


    If I decided to go the duel-conversion route, would I need to provide good selectivity at both IFs to get adequate performance? Could I use a high-ish first IF just for image rejection, maybe with a high impedance LC bandpasss IF filter and then downconvert to a lower IF like 455kHz where I could provide sharp selectivity with a commercially-available ceramic filter?
  7. N8YX

    N8YX Ham Member QRZ Page

    I'd use a crystal filter for each, with LC bandpass filtering ahead of the first mixer. There are a lot of published designs which you can use for reference, and a lot of OEM filters which can be pressed into service in a homebrew design.
  8. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    A stagger-tuned 455khz IF can (or two stages thereof) is fine for what you want. Standard practice for AM broadcast reception for decades.


    Won't this make bandpass filtering significantly more difficult(if it tracked the LO) because images will appear in-band with my 2.5Mhz LO tuning range?


    While I was at work I realized that if I really wanted good image rejection, I could upconvert my local oscillator to 10.8-13.3Mhz with a mixer,7.5Mhz crystal oscillator, and a highpass filter. This provides >10Mhz seperation between images(i.e relaxed lowpass filter for image rejection) while also preserving the low RF range(~100kHz-2.6Mhz-ish difference frequency, depending on choice of ~7.5Mhz crystal), allowing me to continue to simulate an RF input with my sub-optimal signal generator.

    Other than adding another mixer, my only concern is that the 7.5Mhz crystal oscillator could radiate significantly, which might make it difficult extend this receiver later on to higherer bands by bulk downconversion because that crystal oscillator would show up as a giant whistle at 7.5Mhz and all of it's harmonics.

    Is this sound logic?

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