5-1
5.
ST-6000 CIRCUIT DESCRIPTION
Most of the circuitry of the ST-6000 is constructed on two circuit boards, the input board and
the control board. In addition, a rear panel circuit board provides connection and RF by-passing for
the rear panel connectors and a flexible circuit board connects the LED indicators to the control
board. The power supply and amplifiers for the optional oscilloscope are constructed on two
boards that are attached to the scope shield. A complete block diagram of the ST-6000 is shown in
Figure 5.1.
5.1
ST-6000 Analog Processing Circuits - Input Circuit Board
The audio tones from the receiver are processed by a number of analog circuits on the input
board. These circuits are: input AGC circuit, active bandpass filter, limiter amplifier, active discrimi-
nator filters, discriminator detectors, and active low-pass filter. The appropriate schematic diagram
is referenced for each discussion.
5.1.1 Input AGC Circuit (Figure 5.3)
The audio input to the ST-6000 is first coupled through a 500 to 500 Ohm transformer to the in-
put AGC amplifier. The transformer allows connection of the ST-6000 to either a balanced or un-
balanced audio source and provides DC isolation of the input. A MFC 6040 electronic attenuator IC
and a MC 1458 dual operational amplifier are used in a pre-filter AGC circuit. This circuit is particu-
larly intended to prevent overload of the following active filters by very strong input signals. How-
ever, it also provides some gain as well as smoothing of level variations in weaker signals.
5.1.2 Input Bandpass Filters (Figure 5.4)
A total of three multiple-feedback second-order active bandpass filters are used in the ST-6000,
each using ½ of a MC 1458 operational amplifier. The first filter, using IC3a is tuned close to the
mark frequency (2125 Hz for "high-tones", 1275 Hz for "low-tones"). The second filter is tuned
close to the space frequency. The center frequency of this filter is adjusted with the shift switch to
correspond to the space frequency of the selected shift. A third filter, the "post-filter" is tuned mid-
way between the mark and space frequencies. The gain, bandwidth, and center frequency of the
post filter are adjusted with the shift switch and alignment controls to give an over-all flat band-
pass filter response for the three stages while maintaining steep skirt selectivity. It is NOT recom-
mended that field adjustment of these filters be attempted. Stable, high-quality components are
used in this section to assure trouble-free operation. If problems do occur in this section, it is high-
ly recommended that the unit be returned to the factory for repair and realignment.
5.1.3 Limiter Amplifier (Figure 5.5)
After passing through the input bandpass filter, the signal is next processed by the limiter ampli-
fier stage, IC11. The CA 3130 integrated circuit used in the limiter is capable of gains approaching
120 dB and has, in addition, a wide bandwidth and high input impedance. When this IC is used es-
sentially "open-loop" (no feed-back resistor), it becomes a very effective audio limiter stage. The
very large input signal dynamic range of the ST-6000 (greater than 80 dB) is due, in large part, to
the performance of this stage. When the limiter switch is in the OFF position, the gain of the
CA 3130 is reduced to allow it to act as a linear amplifier for "AM" operation of the demodulator.
Summary of Contents for ST-6000
Page 1: ...ST 6000 FSK DEMODULATOR...
Page 14: ...3 3...
Page 15: ...3 4...
Page 17: ...3 6 Figure 3 DS 2000 and ST 6000 System Connection Schematic...
Page 19: ...3 8 Figure 3 2 Loop Connections to the ST 6000...
Page 20: ...3 9 Figure 3 3 RS 232 I O Connections to the ST 6000...
Page 24: ...3 13 Figure 3 5 Alternate Loop Connections...
Page 25: ...3 14 Figure 3 6 Connections for the ST 6000 for Full Duplex Operation...
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