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Re: Bandpass filter smilter

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  • joe price
    Kees, When I first received my SR40 I replaced the input transformer with a wide band transformer made by RELCOM. I was using a DDS board to supply the Xtal
    Message 1 of 4 , Feb 1, 2006
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      Kees,

      When I first received my SR40 I replaced the input transformer
      with a wide band transformer made by RELCOM. I was using a DDS board
      to supply the Xtal input with the idea of just using bandpass or
      lowpass filters ahead of rx. to cover a large frequency. It seened to
      work ok at hf frequencies but when i tried it a LF 200-450khx I
      received a lot of broadcast stations that i didn't have with my
      superhet SD40 as sugested by wa6ufq. Further testing revealed the
      following problems.

      Strong spurious response at odd multiples of the receive fqequency
      3x,5x 7x as a example when tuned to 300khz receiver would respond
      to 900,1500,2100khz ect. now you can see why i was hearing strong
      broadcast stations at LW.

      The input transformer I used i rated to 50khz, maybe my receiver has
      a problem but phase and balance look good.
      I also checked another SR40 at 10mhz it also could hear 30mhz.

      Another problem the shows up is automatic phase and balance changes
      when changing bands such as going from 40 to 20 mtrs. Not a concern
      if this data can be stored for each band.

      Can you please verify the above on your receiver?

      Thanks Joe wa9cgz
    • KY1K
      Good observation Joe, you discovered the Achilles Heal of the softrock type detectors! Although it s not a serious problem, you have to be aware of it and
      Message 2 of 4 , Feb 1, 2006
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        Good observation Joe, you discovered the Achilles Heal of the
        softrock type detectors! Although it's not a serious problem, you
        have to be aware of it and limit front end filters response to
        appropriate values.

        The ratio between the lowest frequency passed by the front end filter
        and the highest frequency passed cannot be more than two to one-for
        exactly the reason you describe.

        It responds to harmonics.

        Several have thought of using the softrock for VLF and ULF receivers,
        say from 2 KHz to 80 KHz. And, of course they run into the same problem.

        2 KHz to 4 KHz is ok, 4 KHz to 8 KHz is ok.......but don't try 2 KHz
        to 8 KHz without separate filters for eash 'band'.

        GL to all.

        Art

        At 11:46 AM 2/1/2006, you wrote:

        >Kees,
        >
        > When I first received my SR40 I replaced the input transformer
        >with a wide band transformer made by RELCOM. I was using a DDS board
        >to supply the Xtal input with the idea of just using bandpass or
        >lowpass filters ahead of rx. to cover a large frequency. It seened to
        >work ok at hf frequencies but when i tried it a LF 200-450khx I
        >received a lot of broadcast stations that i didn't have with my
        >superhet SD40 as sugested by wa6ufq. Further testing revealed the
        >following problems.
        >
        >Strong spurious response at odd multiples of the receive fqequency
        >3x,5x 7x as a example when tuned to 300khz receiver would respond
        >to 900,1500,2100khz ect. now you can see why i was hearing strong
        >broadcast stations at LW.
        >
        >The input transformer I used i rated to 50khz, maybe my receiver has
        >a problem but phase and balance look good.
        >I also checked another SR40 at 10mhz it also could hear 30mhz.
        >
        >Another problem the shows up is automatic phase and balance changes
        >when changing bands such as going from 40 to 20 mtrs. Not a concern
        >if this data can be stored for each band.
        >
        >Can you please verify the above on your receiver?
        >
        >Thanks Joe wa9cgz
        >
        >
        >
        >
        >
        >
        >
        >
        >
        >Yahoo! Groups Links
        >
        >
        >
        >
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