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  • SAW filter and DC bypass

    Hello,

    I have following setup for receiving Flarm signals (868.3MHz):
    Antenna -> amplifier -> 10m coax cable -> dongle from rtl-sdr.com with bias tee -> Raspberry

    Now I want to build a SAW filter between the coax cable and the dongle. So I need a DC bypass for the SAW filter. Does anyone know how to build a circuit with a SAW filter and a DC bypass?

    Viele Gre
    Konstantin

  • #2
    Originally posted by Konstantin View Post
    Hello,

    I have following setup for receiving Flarm signals (868.3MHz):
    Antenna -> amplifier -> 10m coax cable -> dongle from rtl-sdr.com with bias tee -> Raspberry

    Now I want to build a SAW filter between the coax cable and the dongle. So I need a DC bypass for the SAW filter. Does anyone know how to build a circuit with a SAW filter and a DC bypass?

    Viele Gre
    Konstantin
    Have a look for bias-t circuits, basically you need one on each side of the saw filter (one to extract the DC and block the power from the SAW and one on the other side of the SAW to re-inject it) or one on the amplifier side and feed the DC in there .
    Last edited by Oblivian; 2017-05-03, 20:40. Reason: fix quotes
    FR24 F-EGLF1, Blitzortung station 878, OGN Aldersht2, PilotAware PWAldersht, PlanePlotter M7.

    Comment


    • #3
      Originally posted by Konstantin View Post
      I have following setup for receiving Flarm signals (868.3MHz):
      Antenna -> amplifier -> 10m coax cable -> dongle from rtl-sdr.com with bias tee -> Raspberry
      Now I want to build a SAW filter between the coax cable and the dongle. So I need a DC bypass for the SAW filter. Does anyone know how to build a circuit with a SAW filter and a DC bypass?
      It's probably better to put the filter between antenna and amplifier, where it should be...

      /M
      F-ESDF1, F-ESGG1, F-ESGP1, F-ESNK1, F-ESNV2, F-ESNV3 F-ESSL4, F-LFMN3
      P-ESGR, P-ESIA, P-ESIB, P-ESGF
      mrmac (a) fastest.cc

      Comment


      • #4
        Originally posted by F-EGLF1 View Post
        Have a look for bias-t circuits, basically you need one on each side of the saw filter (one to extract the DC and block the power from the SAW and one on the other side of the SAW to re-inject it)
        The problem is (or might be):
        If I make a dc bypass with two bias-t circuit on each side of a saw filter, then the unwanted frequencies pass through the bypass.

        Best regards
        Konstantin

        Comment


        • #5
          Originally posted by Konstantin View Post
          If I make a dc bypass with two bias-t circuit on each side of a saw filter, then the unwanted frequencies pass through the bypass.
          No, a properly designed Bias-T should have inductors to block HF from the DC path. But it is still not a good idea.

          /M
          F-ESDF1, F-ESGG1, F-ESGP1, F-ESNK1, F-ESNV2, F-ESNV3 F-ESSL4, F-LFMN3
          P-ESGR, P-ESIA, P-ESIB, P-ESGF
          mrmac (a) fastest.cc

          Comment


          • #6
            Originally posted by MrMac View Post
            No, a properly designed Bias-T should have inductors to block HF from the DC path.
            Hi,

            do you have any links to such a design? I googled without success...

            Best regards
            Konstantin

            Comment


            • #7
              Originally posted by Konstantin View Post
              Hi,

              do you have any links to such a design? I googled without success...

              Best regards
              Konstantin
              There have been some designs posted in the best antenna thread, I think abcd567 has previously posted a basic design, any bias-t should block RF from the DC path, that is one of the basic functions of it as it would not work otherwise, they are simple devices consisting of a capacitor between the RF input and output to let the RF pass and an inductor in the DC line to block RF from the DC path, nothing more, nothing less.image.jpeg
              Last edited by F-EGLF1; 2017-05-06, 21:29. Reason: Added sketch
              FR24 F-EGLF1, Blitzortung station 878, OGN Aldersht2, PilotAware PWAldersht, PlanePlotter M7.

              Comment


              • #8
                Originally posted by Konstantin View Post
                do you have any links to such a design? I googled without success...

                ??

                How about starting with the main wiki page...

                https://en.wikipedia.org/wiki/Bias_tee

                /M
                F-ESDF1, F-ESGG1, F-ESGP1, F-ESNK1, F-ESNV2, F-ESNV3 F-ESSL4, F-LFMN3
                P-ESGR, P-ESIA, P-ESIB, P-ESGF
                mrmac (a) fastest.cc

                Comment


                • #9
                  Originally posted by F-EGLF1 View Post
                  .........any bias-t should block RF from the DC path, that is one of the basic functions of it as it would not work otherwise, they are simple devices consisting of a capacitor between the RF input and output to let the RF pass and an inductor in the DC line to block RF from the DC path, nothing more, nothing less.
                  [ATTACH=CONFIG]8646[/ATTACH]
                  Perfect! Fully agree!

                  .

                  Comment


                  • #10
                    Originally posted by F-EGLF1 View Post
                    There have been some designs posted in the best antenna thread, I think abcd567 has previously posted a basic design......
                    Here it is...




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                    • #11
                      Wow! Thank you very much abcd567 and F-ELGF1! Your replies helped me a lot. Will try this design.

                      Best regards
                      Konstantin

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