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Re: T2 demod questions

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  • Tim Smith
    ... some of ... I must be looking at an old schematic (the one in the files area of the tracker2 group). I don t see C2 in the demod section. There is a C2
    Message 1 of 8 , Dec 2, 2007
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      --- In tracker2@yahoogroups.com, Scott Miller <scott@...> wrote:
      >
      > The EQ jumper is in the input filter section, near C2 and C3. It just
      > switches C3 into the audio input path and flattens the filter response.
      > It's pretty close to the circuit used by the KPC-3 and I think
      some of
      > the TNC-2 clones as well. Somewhere around here I've got plots of the
      > frequency response from the Spice simulation.


      I must be looking at an old schematic (the one in the files area of
      the tracker2 group). I don't see C2 in the demod section. There is a
      C2 on the CPU which appears to be a bypass cap for power. There is
      also a couple of pots on the schematic that I don't see on the board.

      I recorded test tones produced by the T2 and played them back. The EQ
      jumper makes no difference in either tone's amplitude when looking at
      the input pin of the demod.

      I am seeing solid copy with a sound card using AGWPE and what the T2
      misses, the sound card is receiving. I hope to be able to record some
      consistent misses by the T2 to get a better look at the waveform.

      Is there an updated T2 schematic?

      Thanks,

      Tim
    • Hank
      ... Tim, Take a look at t2protoc-schematic.png , Schematic for Proto C revision in the files section and you ll see the EQ jumper and C25 (.1) that gets put in
      Message 2 of 8 , Dec 4, 2007
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        > I was looking at the schematic in the YahooGroups file area to try
        > and see what the EQ jumper actually does.

        > I can't find the EQ jumper in the schematic to see what it's
        > supposed to do.

        Tim,

        Take a look at

        t2protoc-schematic.png , Schematic for Proto C revision

        in the files section and you'll see the EQ jumper and C25 (.1) that
        gets put in parallel with C1 (.01).

        > I'm looking at audio on both sides of the EQ jumper with and without
        > the jumper and see no difference in how the mark/space tones are
        > handled.

        That filter is a high pass. Without the jumper, the response
        rises and flattens out around 4.5 KHz.

        With the jumper, it's already flattened by 1 KHz, so both modem
        tones are treated just about equally (at about -5.7 dB average
        referenced to a 1k Ohm source unloaded).

        Without the jumper, 2 KHz is about 4.5 dB stronger than 1 KHz
        (both frequencies still on the skirt of the filter). There is
        also a higher passband loss without the jumper.

        Note that the passband loss of the filter is sensitive to the
        audio source impedance, and an input resistance of much over
        2k Ohms starts to show up in a progressively higher passband
        loss (chiefly due to the shunt 4.7 K Ohm resistor, R8).

        Without assessing the filter properly using a signal generator
        and level sensor (scope or meter), these response differences
        between jumper settings may not be evident.

        Hank
      • Tim Smith
        ... Thanks Hank...that sheds some light on the input circuit, but I m still having problems with the T2 being either a little deaf or not decoding a number of
        Message 3 of 8 , Dec 7, 2007
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          > Take a look at
          > t2protoc-schematic.png , Schematic for Proto C revision
          > in the files section and you'll see the EQ jumper and C25 (.1) that
          > gets put in parallel with C1 (.01).

          Thanks Hank...that sheds some light on the input circuit, but I'm
          still having problems with the T2 being either a little deaf or not
          decoding a number of packets for other reasons. I do not see much of
          a difference in EQ jumper impact, either in decoded packets or what I
          see on the scope in terms of mark/space levels. Both radios I have
          tried are discriminator-tapped (Icom and Alinco)...which are fixed
          level, but also tried full audio chain as well. In all cases, the
          software TNC is hearing much better than the T2.

          Scott...I did some side by side tests when deploying the T2 at our
          digi site. In comparing with two different radios using AGWPE with a
          SBII sound card, I was seeing the expected packets being decoded.
          However, when replacing the sound card TNC with the T2, I'm missing
          about 20% of the packets. I tried a wide range of levels from 50mv to
          2v ptp. There is one station that the T2 will not receive at all
          despite EQ settings, audio levels, etc (which happens to be an
          adjacent wide area digi).

          I began to notice this at home when letting the trial run in prep for
          the wide digi installation.

          Anyone else seeing problems with T2 deafness?

          Thanks,

          Tim
        • Scott Miller
          I ve had a couple of reports, but nothing consistent. You re always welcome to send it back for testing, and I ll run a full decode test against it to see how
          Message 4 of 8 , Dec 7, 2007
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            I've had a couple of reports, but nothing consistent. You're always
            welcome to send it back for testing, and I'll run a full decode test
            against it to see how it compares to the baseline - that'd tell us if
            there's a problem with your specific unit.

            Also, what is your power setup? I've had at least one report of bad
            decode performance that was caused by a noisy power system. Switching
            to battery power made the problem go away.

            I've had at least a couple of people report specifically that their T2
            did BETTER than AGWPE, and while I won't guarantee that'll always be the
            case it ought to be close.

            Scott

            Tim Smith wrote:
            >
            >
            >
            > > Take a look at
            > > t2protoc-schematic.png , Schematic for Proto C revision
            > > in the files section and you'll see the EQ jumper and C25 (.1) that
            > > gets put in parallel with C1 (.01).
            >
            > Thanks Hank...that sheds some light on the input circuit, but I'm
            > still having problems with the T2 being either a little deaf or not
            > decoding a number of packets for other reasons. I do not see much of
            > a difference in EQ jumper impact, either in decoded packets or what I
            > see on the scope in terms of mark/space levels. Both radios I have
            > tried are discriminator-tapped (Icom and Alinco)...which are fixed
            > level, but also tried full audio chain as well. In all cases, the
            > software TNC is hearing much better than the T2.
            >
            > Scott...I did some side by side tests when deploying the T2 at our
            > digi site. In comparing with two different radios using AGWPE with a
            > SBII sound card, I was seeing the expected packets being decoded.
            > However, when replacing the sound card TNC with the T2, I'm missing
            > about 20% of the packets. I tried a wide range of levels from 50mv to
            > 2v ptp. There is one station that the T2 will not receive at all
            > despite EQ settings, audio levels, etc (which happens to be an
            > adjacent wide area digi).
            >
            > I began to notice this at home when letting the trial run in prep for
            > the wide digi installation.
            >
            > Anyone else seeing problems with T2 deafness?
            >
            > Thanks,
            >
            > Tim
            >
            >
          • Tim Smith
            ... Thanks Scott...going to try a couple of more things and if no progress, I ll take you up on your offer. -Tim
            Message 5 of 8 , Dec 9, 2007
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              --- In tracker2@yahoogroups.com, Scott Miller <scott@...> wrote:
              >
              > I've had a couple of reports, but nothing consistent. You're always
              > welcome to send it back for testing, and I'll run a full decode test
              > against it to see how it compares to the baseline - that'd tell us if
              > there's a problem with your specific unit.

              Thanks Scott...going to try a couple of more things and if no
              progress, I'll take you up on your offer. -Tim
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