WJ-8611 DIGITAL VHF/UHF RECEIVER
5-10 CIRCUIT
DESCRIPTIONS
changing the tuning voltage that is applied to the VCO’s tuned circuit. The
tuning voltage present at TP35 is typically b7 and +15 V for a
normally operating 100 MHz synthesizer. Capacitor C67 adjusts the VCO
for its proper operating characteristics. It is adjusted for the proper tuning
voltage level at TP35.
The 3rd LO synthesizer is a fixed frequency synthesizer. It produces a
fixed 21.65 MHz output for the third conversion stage. The circuit consists
of a 21.65 MHz VCO, a feedback path and a phase-locked-loop (PLL)
control circuit that contains the phase detector and frequency dividers.
When the receiver is first powered on, the Digital Control PC Assembly
(A2) sends the 3rd LO serial tuning data (DATA, CLK, and STB3) to the
PLL control circuits and the synthesizer remains at the fixed frequency.
The 10 MHz reference is used as the reference for the synthesizer’s phase
detector. It is also applied to a frequency doubler to produce a 20 MHz
signal that is applied to a mixer in the synthesizer’s feedback path. A
sample of the VCO’s 21.65 MHz output is fed back to the control circuit
through the mixer. The mixer provides the difference frequency of 1.65
MHz to the control circuitry. Under normal operating conditions, the
phase detector provides a tuning voltage of approxi6 V to the
VCO for frequency control. Capacitor C204, in the tuned circuit is
adjusted for the +6 V tuning voltage at TP16.
The operational status of the 3rd synthesizer is available to the Digital
Control PC Assembly (A2) at the 3RD LOCK line (A2P3, pin 32). A logic
“1” on this line indicates normal operation. If the synthesizer should
unlock, the 3rd LO LOCK line is set to logic “0”. Additionally, a 3rd LO
lock indicator provides a visual indication the synthesizer’s status. If the
synthesizer should unlock, indicators DS4 and DS5 will illuminate.
The 2nd LO synthesizer is a three-loop design, two of which are controlled
by two phase-locked-loop synthesizers. It produces an output frequency
from 1327.750 MHz to 1330.249 MHz, in 1 kHz increments. Control over
the 2nd LO output frequency is performed by the operation of the 2nd LO
Step Loop and the 2nd LO Resolution Loop synthesizers. Their
frequencies are set by serial data from the Digital Control PC Assembly
(A2). These synthesizers, in turn, set the tuning voltage of the 2nd LO
output VCO. The frequency of the 2nd LO VCO output is determined by
the following formula:
FREQ
LO2
= FREQ
LO1
- 21.400 MHz - RF
Where:
FREQ
LO2
is the 2nd LO Output Frequency, in MHz
FREQ
LO1
is the 1st LO frequency, in MHz
RF is the Tuned Frequency, in MHz
Courtesy of http://BlackRadios.terryo.org
Summary of Contents for WJ-8611
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