Hal Communications ST-6000, Instruction Manual

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5-3
5.1.9 Tone Keyer (Figure 5.8)
The tone keyer section of the ST-6000 is very similar in design to that used in the HAL XTK-100
AFSK Oscillator. The circuit is made up of five basic sections: oscillators, keyer, divider, D/A con-
verter, and lowpass filter. Five separate crystal-controlled oscillators are used in the tone keyer,
one for each of the following outputs: mark (all shifts) 170 Hz space, 425 Hz space, 850 Hz space,
and CW-ID (100 Hz below mark). The crystal frequencies are 2000 times the desired output fre-
quency for "high-tones" (mark = 2125 Hz) and 3000 times the desired output for "low-tones"
(mark = 1275 Hz). Crystals for special tone requirements are chosen to maintain a fundamental
crystal frequency in the 3000 to 8000 kHz range and the 2000 or 3000 divider ratio adjusted ac-
cordingly. All five crystal oscillators operate continuously. The oscillators are ICs number 21 and
22.
Selection of which of the five oscillator outputs is passed along to the divider is made by the
control signals applied to gates I21, I23 and I24. The control circuitry is arranged so that only one
of the signals appears at pin 10 of I24 (TP12) at any one time. The gated signal is then divided by
either 200 or 300 in circuits I20, I19, and I18. The jumpers labeled "H" and "L" on the schematic
determine whether the divider ratio is 200 (H) or 300 (L). The output of I18 is then divided by 10
again in the I17 ringcounter circuit. The ten outputs of I17 are combined in a resistor network to
give the digital-to-analog (D/A) conversion. The D/A resistor network is chosen to give a ten-step
approximation to the desired sine-wave output. One-half of integrated circuit I16 serves as a sin-
gle-pole lowpass filter to further suppress any harmonics of the output signal. The second half of
I16 is an adjustable gain output amplifier, capable of output signal levels between 0 and –32 dBm
into a 500 Ohm load. As on the ST-6000 input circuit, a 500 Ohm isolation transformer is used on
the tone keyer output to permit driving a 500 Ohm balanced load and to give DC isolation. The on-
ly user adjustment in the tone keyer section is the output level control. The space frequencies of
the tone keyer are automatically changed with the ST-6000 shift switch to permit true transceive
operation. Also, the SENSE (NORM-REV) switch on the ST-6000 also reverses the mark and space
tones of the tone keyer.
5.2 ST-6000 Control Circuit Board
All of the previous circuits described are located on the input (or analog) circuit board. The con-
ditioned data output of this section is now used to drive the essentially digital sections of the ST-
6000, the slicer, DTH circuit, antispace, autostart and output drivers and interfaces. With the ex-
cept ion of the tone keyer section, all of the digital control, interfacing and main power supply cir-
cuits are located on the control circuit board. These circuits are discussed in the following sections.
As before, the appropriate schematic diagram is referenced for each discussion.
5.2.1 Slicer and DTH Circuits (Figure 5.9)
The analog data output from the ATC circuit (or lowpass filter if ATC is OFF) drives a high-gain
limiting amplifier to produce fast rise-time keying pulses for mark and space data conditions. As
discussed previously in section 4.5, RTTY signal timing distortion can be generated in high-power
RTTY communications circuits by multiple path propagation between the transmitter and receiver.
The most serious effect this multi-path interference has on the received signal is to introduce a
time uncertainty when the mark-to-space (or vice-versa) transition occurs. Since the ST-6000 is ba-
sically a hard-limited, "FM" type of demodulator, this is fairly easily compensated for by introducing
hysteresis in the slicer stage, thus requiring a "firm" transition in the data before the slicer output
is permitted to change states. The DTH (Decision Threshold Hysteresis) switch makes this modifi-
cation with DTH ON.