Watkins-Johnson Company WJ-8718-19/FE Instruction Manual page 133

CIRCUIT DESCRIPTION WJ-8718-19/FE HF RECEIVER
o 1st LO Prescaler
o 1st LO Main Programmable Divider '
o 1st LO Terminal Count Detector
o 1st LO Phase Detector
o 3rd LO Synthesizer
o Time Base
3.4.18.2.1 1st LO Prescaler
A two-modulus prescaler, U1/U2A, is used at the input to the divide-by-N
counter to divide down the frequency from the VCO so that it can be handled by
conventional low-power Schottky counters.
The prescaler input frequency ranges from 171.64 MHz to 291.60 MHz. The prescaler
divides this by 50 or 51, depending on the states of the E inputs of Ul. Ul is a
divide-by-10/11 counter and U2A is a divide-by-5 counter. The prescaler divides by 51
when E4 is low and when E5 pulses low once for every five pulses from U2A. E5 is
low for only one count out of five so the complete count cycle of Ul and U2A takes
51 counts (4 x 10 + 1 x 11). Control of the prescaler via E4 comes from the swallow
^portion of the programmable divider (see paragraph 3.4.18.2.2). The prescaler output
Inves the clock input to the programmable divider and the terminal count detector.
3.4.18.2.2 1st LO Programmable Divider
The programmable divider consists of swallow counter U8/U9 and main
counter U9, UIO, Ull and U12A. The counters within the programmable divider have a
divide range from 4291 to 7290. The inputs of the counters are always preset from the
. BCD equivalents of the four most significant digits of the tuned frequency. This range
is from 0000 to 2999. J K
The 1st LO swallow counter is formed by decade counter U8 and the
divide-by-5 part of bi-quinary counter U9. Cascaded, they form a divide-by-50 counter
which controls the divide mode of prescaler Ul. The programmable counter is formed
by Ull, U10, and part of U9. Ull and U10 count down and U9 counts up U10 is a
divide-by-10 counter (BCD). Ull is a divide-by-10 counter (BCD). With the Dinput
of Ull tied high (to Vcc 3), the counter is always preset with at least 8 (1000) The
divide-by-2 counter within bi-quinary counter U9 is part of the programmable counter
using preset input A and output QA. '
Since Ull is wired to automatically add 8 to its preset, the programmable
divider has a preset input range of 8000 (0000 + 8000) to 10999 (2999 + 8000) The
terminal count detector (see paragraph 3.4.18.2.3) stops the programmable divider when
a terminal count of 3709 is reached. Since the counters are wired to count down the
?onTra innnV1o.e. ?nge needed from the counters is obtained; the divide range is 'from
"291 (8000-3709) to 7290 (10999-3709).
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