Watkins-Johnson Company WJ-8711A, Инструкция по установке, эксплуатации и обслуживанию

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WJ-8711A DIGITAL HF RECEIVER CIRCUIT DESCRIPTION
6-5
The 1st stage of conversion consists of a high intercept mixer that mixes the RF spectrum with
the 1st LO signal, ranging from 40.455 MHz (0.0000 MHz Tuned Frequency) to 70.455 MHz (30.0000 MHz
Tuned Frequency). The mixer output is amplified and bandpass filtered to select the 40.455 MHz difference
frequency. The 1st IF filter provides a 30 kHz bandpass, centered at 40.455 kHz.
After IF filtering and amplification, the 40.455 kHz IF signal is directed to the second
conversion stage, via a voltage controlled attenuator. This circuit provides control over the amplitude of the
signal to the proper output level, preventing overloading of the analog to digital converter stage in the Digital
Assembly. The RF GAIN input at pin 15 of connector E1 is provided by the control processor in the Digital
Assembly (A2). It is the result of the DSP microprocessor sampling the value of the signal level after
digitization. This voltage ranges from 0 to +7 V, providing approximately 60 dB of gain control. This voltage
is strictly dependent on the signal level and is independent of the receiver's AGC or manual gain setting.
The 2nd conversion stage mixes the signal with the fixed 40 MHz 2nd LO signal, producing
the 455 kHz 2nd IF. After filtering to remove the undesired mixing products, the IF signal is split into two
paths. The first path directs the 455 kHz IF out to the rear panel Signal Monitor Output (SMO) connector
(A3J2). This provides a 50 ohm output at a level of approximately 30 dB greater than the RF Input at the RF
IN connector (A3J1). The Signal Monitor Output bandwidth is approximately 30 kHz wide.
The second signal path for the 2nd IF signal is through the third stage of conversion. The
signal is mixed with the 430 kHz 3rd LO signal, producing the 25 kHz 3rd IF signal. After filtering, the 25
kHz IF is output, via pins 19 and 20 of connector E1, to the Digital Assembly for digitization and further
processing. It is a differential output, having a level approximately 53 dB greater than the RF input. The
bandwidth is approximately 25 kHz.
All control over the operation of the Type 797006 RF Assembly is performed by the Digital
Assembly via pins 7 through 15 of connector A3E1. Connector E1 pin 15 provides a 0 to +7 V level,
controlling the gain of the RF signal path. It is the result of sampling of the IF signal level after digitization.
The remaining pins are used for monitoring the RF Assembly operation and sending control data for tuning
and setting RF front end Gain/Attenuation selection.
The PRE/OPT (pin 12), RF ERR (pin 13), and REF SENSE (pin 14) are all outputs from the
RF assembly to the Digital Assembly. They provide the control microprocessor with the operating status. The
PRE/OPT line (pin 12) indicates when the WJ-8711A/PRE optional Preselector is installed in the receiver.
When the optional preselector assembly is present, it sets this line to logic "1", indicating to the control
microprocessor that it must send preselector control data whenever the receiver is tuned. If the option is not
installed, the PRE/OPT line is held at logic "0".
The REF SENSE line (pin 14) provides an indication when an external reference is connected
at the rear panel EXT REF connector (A3J3). When an external reference is connected, it causes this line to
assume a logic "0" condition. The control microprocessor then tunes the reference phase-locked-loop
synthesizer to each of the allowable reference input frequencies, until the reference synthesizer locks on
external input. If no external reference is present, the REF SENSE line remains at logic "1" and the internal
reference provides the receiver time-base.
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