Hal Communications ST-6000, Инструкция по эксплуатации

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4-10
typical voice is at best 50 % (less compressors, etc.) while a RTTY transmission has a 100 % duty
cycle. SSB transmitter power amplifier stages are usually designed to take advantage of the re-
duced duty cycle of a voice signal to produce relatively high output powers in small enclosures
with proportionally smaller power supplies. If the same SSB transmitter is operated in RTTY service
at full voice ratings, the output amplifier and/or power supply will usually fail. Therefore, the SSB
transmitter rating must usually be reduced by at least 50 % when 100 % duty-cycle RTTY trans-
mission is used. There are a few commercially available SSB transmitters that will accommodate
RTTY at full power, usually with the addition of a blower or heavy-duty power supply. The user
should carefully check the rating of his SSB transmitter before using it in RTTY service.
Another problem often encountered when the tones are used with a SSB transmitter is very sim-
ilar to the BFO-passband problem previously discussed in section 4.3.1. Since the SSB transmitter
is designed to transmit the voice frequency range of 300 to 2400 Hz, it follows that some of the
standard "high-tones" will NOT be transmitted, particularly the 2975 Hz space tone for 850 Hz
shift. As with the SSB receiver, there are two ways to solve this problem, shift the carrier oscillator
frequency with respect to the filter passband, or use lower frequency tones. The same procedure
used to change the receiver BFO frequency can be used to change the transmitter carrier oscillator
frequency. This may even be less convenient to do in the transmitter than the receive since it usu-
ally involves changing a crystal in the transmitter. Use of the "low-tone" set of keyer frequency is
always the simplest solution. However, use of the "low-tones" should be done with care for the fol-
lowing reasons:
1. SSB transmitters generally have no more than 55 dB of carrier rejection when properly
aligned. If the balanced modulator has not been recently readjusted, the carrier rejection
may well be no more than 25 to 35 dB. Transmission of a small amount of carrier with a
SSB voice signal is not usually objectionable. However, when the SSB transmitter is used to
transmit RTTY signals, the unsuppressed carrier is now a spurious emission which is illegal
and may cause receiving problems. In general, the carrier suppression and adjustment sta-
bility of a PHASING-TYPE of SSB transmitter is even worse and this type of transmitter
should NOT be used to generate RTTY signals.
2. When the "low-tones" are used, the mark frequency is 1275 Hz. If there is any distortion in
the SSB transmitter audio or modulator, the second harmonic may be generated, causing
radiation of still another spurious signal.
3. The present convention for AFSK-VHF transmission of RTTY signals in the United States is
to use the "high-tones", 2125 Hz mark and 2295, 2550, 2975 Hz space tones. A demodula-
tor set-up for "low tones" is NOT compatible with this application.
Conversely, if the transmitter carrier oscillator is shifted so that the standard "high-tones" can
be used, the carrier rejection is greatly increased, audio distortion products generated in the trans-
mitter do NOT fail within the transmitter filter passband and the demodulator is then usable for
BOTH FSK and AFSK applications. Note that 170 Hz shift with "high-tones" can be used without
changing the transmitter carrier oscillator since both 2125 Hz (mark) and 2295 Hz (space) fall with-
in the voice passband. However, the carrier rejection will be no better than it is for a voice trans-
mission.
Alternately, the following. points FAVOR the use of the "low-tones":
1. The current IARU international standards call for use of "low-tones"
2. The "low-tones" have been successfully used in commercial and military applications for
some time with satisfactory results.
3. If the precautions noted above regarding carrier rejection and transmitter audio distortion
are observed, the total system performance using "low-tones" can be just as satisfactory as
a system using "high-tones".