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

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WJ-8711A DIGITAL HF RECEIVER CIRCUIT DESCRIPTION
6-1
SECTION VI
CIRCUIT DESCRIPTION
6.1
WJ-8711A DIGITAL HF RECEIVER FUNCTIONAL DESCRIPTION
The WJ-8711A Digital HF Receiver is a Digital Signal Processing (DSP) based receiver,
consisting of an analog tuner, an IF digitizer, digital signal processing and analog reconstruction circuitry, and
front panel control logic. These operating components are contained on three printed circuit assemblies,
consisting of the Type 797006 RF Assembly (A3), the Type 797214 Digital Assembly (A2), and the
Type 797015 Front Panel Assembly (A1). These three assemblies and the Type 766028 Power Supply (PS1)
comprise the complete DSP based HF Receiver, as illustrated in
Figure 6-1 . Refer to the functional block
diagram in
Figure 6-1
for the following description.
The Type 797006 RF Assembly (A3) functions as the analog tuner for the receiver. It
performs coarse signal tuning and provides two wide band IF output signals. Three conversion stages
contained in this assembly provide tuning throughout the 0 to 30.0000 MHz spectrum, with a coarse tuning
resolution of 1 kHz. The RF input from the antenna enters the assembly at the rear panel RF IN connector
(A3J1) and, after passing through two and three conversion stages, the tuned signal is translated into two IF
outputs, respectively. The first output which is after the second conversion stage is a 455 kHz IF output,
having a nominal 30 kHz bandwidth and 1-kHz resolution. It is routed directly to the rear panel Signal
Monitor Output connector (SMO, A3J2). This is a 50 ohm wide band output suitable for connection of an
external signal monitor. The second output which is after the third conversion stage is a 25 kHz IF output,
also having a nominal 30 kHz bandwidth, that is routed to the Type 7970012 Digital Assembly (A2) for
digitizing and further processing. In addition to the 25 kHz IF signal, the RF Assembly provides two clock
signals to the Digital Assembly. The 40 MHz and 430 kHz signals, derived from the receiver's local
oscillators, are used for signal processing synchronization and analog reconstruction.
Timing and synchronization of the local oscillators in the RF Assembly are maintained by a
precision 10 MHz reference oscillator contained on the assembly. In the standard receiver, the internal
reference oscillator provides a reference stability of better than 0.7 ppm, which may be optionally upgraded to
0.2 ppm with the WJ-871Y/REF option installed. The RF Assembly reference may also be locked to an
external frequency standard by connecting the external 1, 2, 5, or 10 MHz signal at the rear panel EXT REF
connector (A3J3). The connection of the external reference is automatically sensed and locks the reference
oscillator to the external source.
The Type 797214 Digital Assembly (A2) functions as the IF Digitizer, Digital Signal
Processor, and Analog Reconstruction circuit for the receiver. It also contains the microcontroller circuitry
that maintains control over all receiver operations. The Digital Assembly accepts the 25 kHz IF signal from
the RF Assembly, digitizes the signal, and using Digital Signal Processing (DSP) techniques, performs the
majority of the signal processing operations required to produce the final outputs. The DSP circuitry on this
assembly converts the digitized IF signal from a continuous time domain signal into discrete time samples that
can be stored in random access memory and processed digitally to perform a wide range of operations,
normally associated with analog circuitry, such as:
•
Receiver Fine Tuning to a 1 Hz resolution,
•
IF Bandpass Filtering,
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