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Max bandwidth of 3.2768 mHz Cohn ladder filter

Discussion in 'Homebrew and Kit Projects' started by KA9P, Nov 19, 2021.

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  1. KA9P

    KA9P Premium Subscriber QRZ Page

    I've been building up some 4 pole Cohn ladder filters from 3.2768 mHz clock crystals (the really stubby ones) and the filters look great but using 47 pF caps, I get a 350 Hz bandwidth, and going down to 10 pF I'm getting a 1.2 kHz bandwidth.

    I'm wondering if this is a frequency-related thing, a crystal-related thing. a topology-related thing, or a constructor-related thing :)

    Any thoughts would be helpful - is there a different topology that is more suited to broader bandwidths maybe (assuming I haven't messed up).

    Thanks Scott ka9p
     
  2. VK2CLF

    VK2CLF QRZ Member

    From a Youtube video I watched recently(*), it is normal for a Cohn filter to show smaller bandwidth with larger capacitors. I expect the filter input/output impedances will change also, and there will be limits on how far you can stretch things. If you want much wider bandwidth I expect you'll need to use higher-frequency crystals (for the same Q) or just different crystals.

    (*)
     
    KA9P likes this.
  3. KA9P

    KA9P Premium Subscriber QRZ Page

    VK2CLF said "If you want much wider bandwidth I expect you'll need to use higher-frequency crystals (for the same Q) or just different crystals."

    Thanks much, yes, I'm afraid that seems to be the case.

    Unfortunately I'm back fitting filters into a 3.2 MHz IF strip from the receiver I wanted to build when I first started out half a century ago when you could get FT-243 xtals and build a half lattice filter. I guess the upside is the CW performance improvement is amazing. I did learn that Wes Hayward apparently had success getting an SSB Cohn filter in that range by paralleling inductors with the crystals, so I'm trying to chase that article up - might get lucky. Thanks, S
     

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