Hal Communications ST-6000, Instruction Manual

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4-6
change of signal state before the slicer and therefore loop keyer stages are allowed to switch. This
multi-path distortion phenomena is usually not iced only on signal s from very high-powered trans-
mitters and therefore will NOT be a problem when receiving most radio amateur RTTY signals.
Therefore, unless the ST-6000 is used to receive signals from known very highpowered transmit-
ters, it is recommended that the DTH circuit be left OFF until multi-path distortion is suspected.
4.6 Use of the Limiter Switch FM vs. AM
The ST-6000 has been designed for optimum performance when the input signals are "hard-lim-
ited" (FM mode, LIMITER ON). To this end, a very high-gain limiter stage, wide-dynamic range in-
put filters, and wide bandwidth discriminator filters are used in the demodulator. For MOST appli-
cations, the FM or LIMITER ON mode will provide superior performance due to the quieting and
capture-effect of a "hard-limited" FM receiver circuit. However, a limiter has a minimum threshold,
below which it is captured by the noise rather than the signal and the limiter will also be captured
by the STRONGEST signal within its bandpass. Therefore, there is at least a theoretical advantage
to using a non-limited, "AM" type of detect ion system for very weak signals or in the face of
heavy interference. When the LIMITER switch is turned OFF, the gain of the limiter is reduced so
that it becomes a linear amplifier. In this mode, the pre-filter AGC circuit and the active discrimina-
tor detectors provide a wide dynamic range, almost as great as that achieved when the limiter is
turned on. Although the ST-6000 performance in AM mode exceeds that of the previous model ST-
6 demodulator, the actual improvement in print may or may not be noticed, depending upon the
receiving conditions. It is recommended that the ST-6000 be used with the LIMITER ON for most
applications and the AM mode used only in the face of strong interference or very weak signals.
Use of higher-gain directive receiving antennas will generally improve weak-signal performance
more than could be achieved with limiter-less RTTY demodulation.
4.7 Use of the SENSE Switch
The SENSE switch allows the ST-6000 to receive signals with either the normal polarity of
mark=lower frequency or the reversed sense where mark=higher frequency tone. ALL features of
the ST-6000 function equally well for either signal polarity. This reverse position is convenient if the
signal has been inadvertently tuned-in using USB rather that LSB receiver mode. Unlike the previ-
ous model ST-5, BOTH the discriminator filters AND the tone generator frequencies are controlled
by this switch, assuring true transceive conditions for both polarities.
4.8 Use of the PRINT Switch
There are many occasions in which it is desirable to turn-off the received data to the teleprinter
loop circuit and just use the internal loop supply to operate the RTTY machines at the station. Op-
erations such as local testing of equipment or preparation of punched paper tape are typical exam-
ples. When the PRINT switch is in LINE position, data from the receiver is allowed to key the loop
circuit. When the switch is in LOCAL position, the loop keyer stages are held in mark condition and
received data will NOT drive the loop. This circuit is also operated whenever the KOS circuit is in
transmit mode to prevent feed-back of the transmitted signal back through the demodulator. The
demodulator can also be held in LOCAL position by grounding the REMOTE STANDBY line on pin 2
of the AUXILIARY connector (J3). Note that, as per the discussions of section 3.3, the PRINT
switch or REMOTE STANDBY line actually control the post-autostart data output. When the AUXILI-
ARY jumper plug furnished is used, the Loop 1 circuit is also controlled. If, however, an external
device such as a UART circuit is connected between the pre-autostart data output and a keyer
stage, the loop status is NOT controlled by either the PRINT switch or the REMOTE STANDBY line.