Hal Communications ST-6000, Instruction Manual

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4-4
although they are not centered. The 425 Hz shift space-tone at 2550 Hz may or may not be de-
tected by the receiver and the 850 Hz shift space tone at 2975 Hz is not detected for all but the
strongest FSK signals. Obviously, the 425 Hz and 850 Hz shift FSK signals are NOT compatible with
the usual SSB receiver if the "standard" demodulator tones are used. There are two solutions com-
monly applied to this problem, both of which are usable with the ST-6000.
One solution is to simply change the receiver BFO frequency so that the audio pass-band of the
receiver is changed to, say 1500 Hz to 3600 Hz, with will pass all of the "standard" demodulator
tones, centering the pass-band on the 850 Hz shift signal. In this case, the BFO frequency should
be changed so that it is FARTHER AWAY FROM THE CENTER OF THE IF PASS-BAND by approxi-
mately 1000 to 1400 Hz. If the receiver BFO is adjustable, this is a simple solution. However, many
modern SSB receivers do NOT have adjustable BFOs; rather, the BFO may be crystal controlled. In
such cases, the BFO crystal should be changed for one of the correct frequency as determined
above. Alternately, the second solution, outlined below, may be used.
The receiver audio pass-band problem can also be solved by changing the tones used by the
demodulator for detection of mark and space. This technique, commonly referred to as "low-
tones" allows direct use of a voice SSB receiver with no internal modifications to the receiver. The
ST-6000 can be furnished with filters designed to accept a mark frequency of 1275 Hz and there-
fore space frequencies of: 1445 Hz (170 Hz shift), 1700 Hz (425 Hz shift) or 2125 Hz (850 Hz
shift). In some cases, this can be the best solution. However, as discussed in the next section, it
can lead to a basic incompatibility when receiving AFSK VHF signals as well as presenting some
transmitter problems. The ST-6000 can also be adjusted for special sets of input tones between
1200 and 3000 Hz on special order.
4.3.2 Receiver for AFSK Reception
The receiver requirements for AFSK reception are not as exacting as those for FSK reception.
Since the teleprinter data is an AFSK modulation of the transmitter, receiver stability is not general-
ly critical. However, since the data is in the form of audio frequency modulation, the frequency of
the tones is determined at the transmitter and cannot be changed at the receiver by simply adjust-
ing a BFO or similar control. The current VHF-AFSK standards in use by radio amateurs in the Unit-
ed States use the higher-frequency "standard" tones of 2125 Hz for mark and 2295, 2550 or 2975
Hz for space, depending upon the shift used. Therefore, a demodulator with "low-tone" input fil-
ters will NOT be compatible with reception of current VHF-AFSK signals. As in the case of the FSK
receiver, a 500 Ohm audio output is certainly desirable, but may not be as important to perform-
ance, particularly if strong-signal VHF-FM signals are used.
4.3.3 Tuning a RTTY Signal
Tuning of the radio receiver for optimum recovery of the teleprinter signal is an operation which
may require some practice. The tuning objective is to adjust the receiver tuning so that the output
audio tones match the center frequencies of the ST-6000 filter circuits, as indicated on the tuning
oscilloscope.
The oscilloscope takes the place of a tuning meter in the ST-6000. The oscilloscope presents
the standard crossed-ellipse display with the mark signal on the horizontal axis and the space sig-
nal on the vertical axis. Because the discriminator filters in the ST-6000 (like those of the ST-5 and
ST-6) are relatively broad-band, the mark and space scope displays are NOT lines, but ellipses.
The ellipses are fairly narrow when 850 Hz shift is received and wider when 170 Hz is selected.
When the RTTY signal is correctly tuned, the ellipses will have maximum length and be essentially
perpendicular. The major axis of the ellipse is ALWAYS the parameter to observe and maximize.