TAPR modem mod. for FO20

By Kohjin Yamada, JR1EDE
#: 17246 S5/Amateur Satellites
    19-May-90  02:51:20
Sb: TAPR modem mod. for FO20
Fm: Sueo Asato JA6FTL 71701,251
To: All

In previous mail, I announced that TS-790 rear data port connection with TAPR/JAMSAT modem has no problem on real operation.

Here is the detail of my modefication of TAPR modem.

  1. I checked the waveform at TX varactor cathode and discriminator of another monitor receiver at the same time and compared with original TXD from TNC. By Manchester encoding, data bit "1" become transition (+ -> -),and "0" (- -> +). So,conteneous "0" or "1" data stream make 1200Hz square wave. But "1->0" or "0->1" make 600 Hz. square wave. From this point of view,important thing is not the waveform itself but the phase deviation of transition point to time axis.
  2. By original TAPR/JAMSAT modem, the phase error of transition point at output terminal(hot side JP8) is so large especialy at 600Hz. wave form. This is due to the improper timeconstant of output filter.(R7,C8).
  3. Modefication.
    Change.R7(22K ohm) => 2.2K ohm
    R5(22K ohm) => 4.7K ohm
    Only by this modefication, the phase error improved great.
  4. By upper mod. the output audio level reach to 1.5V PEP,and this value is large enough to direct moduration of TS-790 varactor. But there is no need to direct moduration. Feeding to rear data port have no problem in real operation.In this situation, the waveform at varactor is very differ from output but have no phase error.
  5. It need adjusting of output level(VR6) when feeding rear data port. Maximum set is okay as TS-790's audio limiter is very efficient.

    ===== See you on FO-20 board =====

    Click here to read more mods for the TS-790