Hal Communications ST-6000, Instruction Manual

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mitting transceive operation with either polarity, lower tone mark OR higher tone mark. The tone
keyer can be driven from separate keyboard data, from a current loop, or other sources as ex-
plained in section 3. The output of the tone keyer can also be turned on or off with a remote con-
trol as explained in section 3.3.6. The tone output amplitude can be adjusted over the range of
0 to –40 dBm (approximately 1.0 V to 10 mV across 500 Ohms) with an internal potentiometer.
The amplitude is adjusted with the TONE LEVEL potentiometer, located on the input circuit board,
approximately two inches directly behind the ATC front panel switch. This adjustment works exact-
ly the reverse of a volume control full CW is minimum output amplitude and full CCW is maximum
output. Although the output impedance of the tone keyer is 500 Ohms, virtually any load impe-
dance from 500 Ohms to several Megohms can be driven by the ST-6000. It is NOT necessary to
terminate the audio output connector in a 500 Ohm load.
4.13 Transmitting Radio Teleprinter Signals
As mentioned previously in section 4.3, radio teleprinter signals are generated by either shifting
the transmitter RF frequency with the data (FSK) or by modulating the transmitter carrier with au-
dio tones whose frequencies are shifted by the data (AFSK). Usually, FSK transmission is used for
radio frequencies lower than 30 MHz and AFSK above 30 MHz. The ST-6000 can be used to receive
AND transmit both FSK and AFSK types of signals.
4.13.1 Transmitting FSK Signals
There are two different techniques that are normally used to generate a FSK teleprinter signal.
The simplest method involves direct shifting of the frequency of an oscillator stage in the transmit-
ter. Typically, the data signal is used to turn on or off a diode switching circuit that effectively in-
creases the oscillator tuned circuit capacitance on space, thus lowering the transmitter frequency
for space condition of the data. A typical diode keyer circuit is shown in Figure 3.3. Note that the
RS-232 or MIL-188 data outputs are ideally conditioned for this application. Since one is the in-
verse of the other, the mark-space sense or polarity can be changed by selecting either RS-232 or
MIL-188 outputs. Further information on this type of circuit can be found in a current edition of the
Radio Amateurs Handbook (ARRL, Newington, Conn.) or in the Radio Handbook (Ore, Howard W.
Sams, Inc., Indianapolis, Ind.), or other text on radio transmitters. The standard radio amateur po-
larity convention is, to make the mark frequency higher in frequency than the space frequency, al-
though a number of exceptions are to be found, particularly in commercial applications.
A second technique to generate FSK uses a SSB type of radio transmitter. The AFSK tone out-
put from the tone keyer is used as the audio modulation or the transmitter. Since a properly ad-
justed SSB transmitter suppress one sideband and the carrier of the AM signal, the RF output for a
single frequency tone input is simply an RF carrier, displaced from the original carrier frequency by
the tone frequency. When the tone frequency changes, the RF output frequency also changes by
the same amount. Historically, the audio tone standard has been to designate mark as the lower
frequency AUDIO tone and space as the higher. Thus, to achieve "normal" FSK RF output with
mark as the higher RF frequency, the LSB (lower sideband) mode is used in the SSB transmitter
(and receiver as explained in section 4.3). This technique is often mistakenly called the "AFSK-SSB"
or simply '"AFSK" method. However, the end result is exactly the same as if the transmitter were
directly frequency shifted by the data and "FSK" is the true description of the RF signal generated.
At first glance, this SSB method is very attractive; it requires no internal modification to the
transmitter and can use readily available SSB transmitters. However, there are a number of precau-
tions that must be traced to the basic fact that SSB transmitters have been specifically designed to
transmit voice signals and the performance and specifications are optimized for voice applications.
The first conflict in specifications is in the duty-cycle rating of the transmitter. The duty-cycle of the