Hal Communications ST-6000, Инструкция по эксплуатации

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4-3
4.3 Reception of Radio Teleprinter Signals
Radio teleprinter signals are usually generated in two forms: direct shifting of the transmitter
carrier frequency by the mark and space teleprinter data (called ''FSK") or by shifting the frequen-
cy of audio tones with the data and using the audio tones as modulation in either an AM or FM
transmitter (called "AFSK"). FSK transmissions are usually used in the HF frequency range, below
30 MHz and AFSK is the normal technique for VHF transmissions, above 30 MHz. A currently popu-
lar method used to transmit FSK signals is to apply the frequency-shifted tones as modulation to a
single-side-band (SSB) transmitter. Since a properly adjusted SSB transmitter signal is the same as
that by direct FSK, a separate RF frequency for mark and space data conditions. Reception of such
signals is therefore exactly the same as if direct FSK, had been used. Use of this technique will be
discussed in greater detail in section 4.13.
4.3.1 Receiver for FSK Reception
The performance of the ST-6000 depends to a great extend upon the characteristics of the re-
ceiver used and the care taken in tuning the FSK signal. Obviously, the better suited the receiver,
the better the quality of the printed displayed output. Some of the desirable receiver characteris-
tics and features are:
1. The receiver should preferably be of the SSB type with selectable sideband. Standard conven-
tion is to use LSB mode for FSK reception.
2. The frequency stability of the receiver should be very good – a small amount of drift when re-
ceiving a 170 Hz shift signal can cause poor print from even very strong signals.
3. The tuning ratio of the main tuning knob should be slow to allow precise adjustment of the
FSK signal into the receiver.
4. The selectivity of the receiver should be slightly greater than the shift to be received: for
850 Hz shift, a bandwidth of 1200 to 2100 Hz is adequate; for 170 Hz shift, a bandwidth as
narrow as 400 Hz can be used. Two limitations should be kept in mind, however; the narrower
the bandwidth, the greater the frequency stability requirements on the receiver, and wider
bandwidths permit more interference and noise to be processed with the FSK signal.
5. A "slow" AGC with fast attack and slow decay is very desirable when receiving FSK signals.
6. As mentioned in section 3.2.1, a 500 Ohm audio output connection or speaker to 500 Ohm
transformer are highly recommended for connection to the ST-6000 input.
7. An adjustable BFO frequency or adjustable pass-band tuning are very desirable features for re-
ception of FSK signals.
Proper positioning of the receiver's BFO with respect to the IF pass-band and the FSK signal is
particularly important. The standard convention when transmitting HF FSK signals is to transmit
mark data at the higher RF frequency and space at the lower. The separate mark and space RF
frequencies are both received within the receiver IF pass-band and mixed in the product detector
with the BFO to produce audio tone beat signals which then drive the ST-6000. Since the audio fil-
ters of the ST-6000 follow the demodulator standard of mark being the lower frequency audio
tone, it follows that the BFO of the receiver should be HIGHER in frequency than the FSK signal.
This corresponds to use of the LSB (lower side-band) mode of the receiver. However, if the signals
is inadvertently tuned-in using USB (upper side-band) mode, the sense can be corrected with the
SENSE switch on the ST-6000 front panel.
The receiver BFO frequency positioning also determines the RANGE of audio frequencies that
can be detected and used to drive the ST-6000. The normal SSB receiver has generally been de-
signed for optimum performance with voice signals with a typical audio frequency pass-band of
300 to 2400 Hz. However, the so-called "standard" demodulator audio tones have been 2125 Hz
for mark with the space tone higher in frequency by the amount of the shift received. For 170 Hz,
both the mark (2125 Hz) and space (2295 Hz) tones fall within the 300 Hz to 2$00 Hz pass-band,