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1987Revision E testing

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  • Scott Miller
    Aug 2, 2007
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      I got the first batch of Proto 'E' boards in, and built up six of them
      for testing. Like Proto 'D', it's the same form factor as the previous
      revisions but the components are all surface mount, with the exception
      of the connectors, crystal, and LEDs.

      Changes from previous revisions include:

      - No tuning pot. In my testing, I found that there wasn't any real
      benefit to being able to adjust the RX tuning, compared to having it set
      with a precision resistor to the calculated optimum value. It seems to
      perform well across the whole temperature range, and it eliminates one
      of the more failure-prone parts.

      - Active RX filter. This takes the place of the passive filter network,
      and seems to squeeze a bit more performance out of the demodulator.
      Proto 'D' had a quad op amp, but Proto 'E' just has a single op amp in
      an SOT-23 package.

      - Ferrites on all off-board signals to reduce RFI.

      - Red and green LEDs in the TX and RX holes instead of pots. A solid
      green LED on receive, separate from the activity LED, makes it easier to
      see what's going on and frees up the ACT LED for other signals.

      - Jumper arrangement changed to match the OT1+, with the exception of an
      EQ jumper in place of A1/A2. This eliminates a few assembly steps and
      takes up less space.

      So far, performance is looking good. I did discover something
      interesting - if you remove the 510k resistor, it actually seems to
      improve RX performance a bit. I haven't confirmed if this works on the
      previous versions, and testing is inconclusive so far on the T2-135.

      I was just looking over my test result archives, and it occurs to me
      that the decode performance is better than any other '2211-based TNC
      I've seen, so maybe it's time to finally declare it 'good enough' and
      get this thing in production!

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