Watkins-Johnson Company WJ-8711A, Installation, Operation And Maintenance Manual

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CIRCUIT DESCRIPTION WJ-8711A DIGITAL HF RECEIVER
6-2
•
Determination of Input Signal Strength,
•
Receiver Gain Control,
•
Signal Detection and Demodulation,
•
Noise Blanking.
Upon completion of the signal processing, a serial data stream representing the receiver's
bandpass filtered IF signal, and the demodulated audio extracted from the tuned signal are routed to the
Analog Reconstruction circuitry. The Analog Reconstruction circuitry converts the digital data back to its
analog form, separates the audio and IF signals, provides post filtering, and, after completion of the analog
reconstruction, provides the final audio and post filtered IF outputs. The reconstructed IF signal is converted
up to 455 kHz and is provided at the rear panel IF OUT connector (A2J1) for external use. This 50 ohm
output provides the 455 kHz IF at a level of approximately -20 dBm (AGC active), with its bandwidth
determined by the operator selected IF bandwidth.
The reconstructed audio is separated into left and right channels and is then directed to the
various receiver audio outputs. The phone audio is routed to the front panel PHONES jack (A1J3) via the
Front Panel circuit card assembly (A1A1) . The audio signal is also provided to the rear panel terminal bus
(TB1). The LINE A and LINE B audio outputs are 600 ohm balanced audio outputs, and the SPEAKER
output is an unbalanced 8 ohm output. The final audio output is the DC AUDIO, which is a DC coupled,
unbalanced, 1 kohm output.
The Type 797214 Digital Assembly also contains a microcontroller that provides an interface
between an external controlling device and the receiver circuitry. A serial control interface is provided at the
rear panel to permit receiver control by an RS-232 compatible computer via A2J3.
Operating power is supplied to all of the assemblies in the WJ-8711A Receiver by the Type
766028 Power Supply (PS1). This assembly accepts the incoming line voltage and converts it to the DC
voltages required for proper receiver operation. It accepts an AC input ranging between 90 and 264 VAC and
provides the -12 V, +12 V, GND, and +5 V outputs to the operational assemblies.
6.2
CIRCUIT DESCRIPTIONS
6.2.1
TYPE 797006 RF ASSEMBLY, (A3)
The Type 797006 RF Assembly (A3) functions as the RF tuner for the WJ-8711A Digital HF
Receiver. It receives a 0 to 30.0000 MHz input spectrum from the RF signal source and provides RF tuning to
extract the signal of interest from the input spectrum. The signal of interest is converted to 455 kHz and
25 kHz IF signals that are provided as outputs for further processing. Refer to the Type 797006 RF Assembly
Block Diagram in
Figure 6-2 as a reference for the following module description. For a more detailed
illustration of the RF Assembly circuitry, refer to the Type 797006 RF Assembly schematic diagram, Foldout
FO-5 .
The RF Assembly consists of an RF Input circuit, three Mixing stages for signal conversion, a
highly stable reference generator, and three local oscillators. These module sections interconnect as illustrated
in
Figure 6-2 to produce the required outputs.
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