Hal Communications ST-6000, Инструкция по эксплуатации

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5.1.4 Discriminator filters (Figure 5.6)
Separate mark and space discriminator filters are constructed of two high-Q active filters, one
filter for each tone. These filters are specifically designed for broad band-width, linear phase, re-
sponse to assure optimum signal recovery in the face of noise and to minimize phase distortion of
the signal. The space filter center frequency and the bandwidth and gain of both mark and space
filters are controlled by the shift switches and the alignment potentiometers. As in the input filter
section, user adjustment of these filters is NOT recommended.
5.1.5 Discriminator Detectors (Figure 5.1)
The signals from the mark and space discriminator filters are detected in separate active detec-
tor circuits that use a MC 1458 (I13) integrated circuit. This circuit avoids the normal non-lineari-
ties and offset voltages of simple diodes and gives precision rectification of the AC signal over a
wide dynamic range of input signal amplitudes. These circuits further reduce signal distortion when
using the "FM" mode of reception (limiter ON). When combined with the input AGC circuit, they
provide exceptional performance when using the "AM" mode (limiter OFF) . The mark and space
detectors are set-up to generate opposite polarity DC voltages which are then combined to pro-
duce the output data. When combined with the broad-bandwidth discriminator filters, a very linear,
high-performance discriminator is formed. The polarity of the detected voltage is changed in both
filters with the front panel SENSE switch.
5.1.6 Meter Amplifier and Detectors (Figure 5.7)
A portion of both the mark and space signals are also rectified with 1N270 germanium diodes,
filtered, and amplified in a type 741 operational amplifier (I15). Since the polarity of the two di-
odes is the same, the output of the amplifier is positive for either mark or space signals, the ampli-
tude being proportional to the match in frequency between the input tone and the discriminator
mark or space filter. This is the "plus-plus" voltage previously discussed. This voltage is used to
drive the tuning meter (except when the oscilloscope option is installed) and the autostart circuit.
5.1.7 Active Low-pass Filter (Figure 5.7)
Both sections of a MC 1458 IC are used in a three-pole active low-pass filter that follows the
discriminator stage. The cut-off frequency of this filter is set to approximately 82 Hz to minimize
noise on the signal while still allowing use of the ST-6000 at data rates up to 100 baud.
5.1.8 ATC Circuit (Figure 5.7)
When using the ST-6000 in the "AM" mode (limiter OFF), selective fading that reduces the am-
plitude of one of the tones but not the other can give rise to bias distortion. The ATC (Automatic
Threshold Control) circuit acts as a "DC-restorer" circuit that rebiases the input to the slicer to
maintain voltage symmetry for mark and space pulses. This circuit is of most benefit in "AM" mode
when a continuous stream of RTTY data is being received (such as a message transmitted by tape
or from a semiconductor storage device). In "FM" mode (limiter ON), the ATC circuit may be useful
for very deep selective fades but will, in general have little effect since both mark and space tones
will undergo heavy limiting before the discriminator. Since the ATC level restoration depends upon
RC time constants, chosen for a continuous data stream, it may cause distortion when slowly
hand-typed data is received and should therefore be turned OFF when receiving this type of signal.