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For receive frequencies above 20 kHz, the PLL is fixed-programmed by the CPU for a
reference frequency of 66-2/3 kHz and a VCO frequency of 37-11/15 MHz.
3-6 CONVERTER I/O BOARD (81790): The main A/D and D/A converters on this
board provide interface to:
(1.)
the Digital Signal Processor,
(2.)
the timing and multiplexing circuits for separating D/A data into various
audio and IF channels, and
(3.)
the analog reconstruction filters and audio drivers that form the final audio
outputs of the receiver.
This board also contains the mixer and filter used to convert baseband IF signals back to
455 kHz for the DSP- processed IF-output to rear panel J4.
Refer to Figure (10-23). Connectors 4, 5 and 64 carry serial data to and from the DSP.
Word framing signals for the A/D and D/A converters (CVST and LDAC), and timing
signals for the analog switch de-multiplexers (AF, IF, USB, and LSB) are formed by the
dividers and combinational logic circuits U1-U5. Refer to the timing diagram on the
schematic for the timing relationships between the converters and de-multiplexers.
The 16-2/3 kHz 3rd-IF signal at connector 51 is applied to the sampling input of A/D-
converter U7. On command of CVST from U4a, the analog input voltage is converted to
a serial bit stream and transferred to the DSP via connector 4.
Serial data from the DSP is transferred to D/A converter U8 via connectors 5 and 64.
Under control of LDAC (U4d), the output samples are presented as discrete voltages at
V-out (pin 20). Each sample is steered to the proper audio or IF channel by analog
switch de-multiplexers U9 and U10, timed by AF/IF and USB/LSB signals from U4b,c
and U5a,c.
Reconstruction filters U11-U14 attenuate the sample-clock frequencies (66-2/3 or 33-1/3
kHz) and present a smoothed analog voltage to mode switches U9z and U10z.
For the IF channel, smoothed signals are sent to switching mixer U14b-U15x. The 471-
2/3 kHz 3rd- LO signal (from 52) is subtractively mixed with the 16-2/3 kHz baseband IF
signal in U15x and filtered by 16-kHz bandpass filter FL1 to yield a 455-kHz component.
This signal is then buffered by op-amp U18b and routed to the DSP-processed IF Output
(J4).
Based on the mode selected by the CPU/DSP via connector 5, U9z and U10z connect the
appropriate reconstruction filter outputs to audio and line drivers U16 and U17.
Connector 8 carries both audio channels to the front panel ISB level control and to the
front-panel stereo phone jack. Connector 9 supplies transformer-coupled audio to the
600-ohm line connections on rear-chassis J8. All audio outputs and IF outputs can be
muted by a connection to ground at connector 68, which is available at rear panel DB-15
connector J8 on pin-11. A parallel mute line is tied to the 2nd-LO's shutdown pin via
connector 75.
3-7
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