Satgen354  Going Digital Pt2  by GM4IHJ                          6 Jan 96

Readers of last weeks Satgen353 will if they have tried Dove on AFSK 1200
bps , have found that it went off again, shortly after 353 was posted,
hopefully it will return soon. Meanwhile the best satellite vehicle for
the next step up in digital operating (Fo20 ) is no longer operating in
mode JD. However this is no reason why potential digisat users should
relax, JAS-2 , Fo20s successor should be aloft this year 

Uplink transmissions to the Fo12/Fo20/JAS-2 series of satellites ( and
incidentally to the microsats Ao16,Ao18,and Ao19), use digital FM signals
in the 2m band. Up to 4 frequencies are available for each satellite , and
doppler is not a problem because of the low frequency used, and the wide
bandwidth of the satellite receivers. On the downlink we meet a new kind
of digital modulation called Phase Shift Keying (PSK).  PSK signals
consist of a CW carrier whose phase is switched through 180 degrees when
ever a change in binary level is signalled ( +90 to -90 degrees).
Reception is best achieved in CW mode bandwidth 2.4 Khz approx, but USB
reception is also possible if CW operation is not available. With the
downlink frequency in the 70cm band ( 435 to 437 MHz) , doppler shift on
JAS-2 will be fierce, going from +8 to -8 Khz on some passes , and
reaching a peak doppler rate of 40 Hz/sec in the middle of the pass. So
the standard "home brew" PSK modem designed by G3RUH, incorporates
hardware Auto Frequency Control which does the receiver tuning for you ,
by operating the UP/DOWN circuits of most of the commonly used ICOM,
TRIO/KENWOOD, and YAESU receivers.

You need both a PSK modem of the G3RUH type and your original packet radio
terrestrial type Terminal Node Controller (TNC). The G3RUH PSK modem is
connected as a replacement for the existing modem in your TNC. Your TNCs
Bell 202 FSK modulator demodulator must be disconnected for PSK usage and
the G3RUH PSK board connected in its place. Most TNCs permit this, and the
literature supplied with the PCB on which you build the G3RUH modem, tells
you how to do this for your particular type of TNC. Some radio amateurs
combine a TNC kit and the G3RUH board in a single case and use it
exclusively for PSK. The G3RUH literature also shows you how to make your
TNC/PSK combination switchable between terrestral packet FM operation or
satellite PSK operation.

For F012 and Fo20 reception, GM4IHJ used a 20 element Az rotatable
horizontal yagi antenna or , for close range passes a vertical colinear
antenna, feeding an ICOM 451 receiver wired up to the G3RUH boards auto
frequency control circuit. Transmission of the uplink on 2m was on an 18
year old Yaesu FT 221R which put 10 watts into an azimuth rotatable 5
element XY yagi with switchable polarisation. The station computer ran
ordinary packet communication software program "PACKCOM" connected to a
PAC-comm TNC320 with one port wired through the G3RUH modem which was
mounted in its own box. The PSK modem was wired through plugs and sockets
to the receiver speaker line, the transmitter mike line, the push to talk
line, and to a common ground line. Please note that this use of a simple
communications protocol software package in the computer is only possible
where the operation is through Fo12 and Fo20 ( and probably JAS-2) . For
PSK operation on A016, Ao18 and Ao19 you must use either the complex PB PG
suite of communications protocol software or, the attractive alternative
Windows based WiSP software.