E.F. Johnson Company 5100 Series, Руководство по обслуживанию

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CIRCUIT DESCRIPTION
5-15
5.7 800 MHz RF BOARD (VERSION A/B)
NOTE: The following describes the earlier Version A
and B RF boards described in Section 1.13.
5.7.1 FREQUENCY SYNTHESIS
The complete synthesizer subsystem consists of
the reference oscillator (U304), the voltage-controlled
oscillator (VCO U307), a buffer IC (U303), and the
synthesizer (U302).
The reference oscillator contains a temperature-
compensated 16.8 MHz crystal. This oscillator is digi-
tally tuned and contains a temperature-referenced,
five-bit, analog-to-digital (A/D) converter. The output
of the oscillator (pin 10 on U304) is applied to pin 14
(XTAL1) on U302 through capacitor C309 and
resistor 8306.
Voltage-controlled oscillator module U307 is
varactor tuned. Therefore, as the voltage being applied
to pins 1 and 7 of the VCO varies (2-11V), so does the
varactor's capacitance which changes the VCO output
frequency. The 800 MHz VCO is a dual-range oscil-
lator that covers the 806-825 MHz and the 851-870
MHz frequency bands.
The low-band VCO (777-825 MHz) provides the
first LO injection frequencies (777-797 MHz) that are
73.35 MHz below the carrier frequency. In addition, in
the transmit mode when the radio is operated through
a repeater, the low-band VCO generates the transmit
frequencies (806-825 MHz) that are 45 MHz below
the receiver frequencies. The low band VCO is
selected by pulling pin 3 high and pin 8 low on U307.
When radio-to-radio or talk-around operation is neces-
sary, the high band VCO (851-870 MHz) is selected.
This is accomplished by pulling pin 3 low and pin 8
high on U307.
The buffer IC (U303) includes a Tx, Rx, and
prescaler buffer which maintain a constant output level
and provides isolation. The Tx buffer is selected by
setting pin 7 of U303 high, and the Rx buffer is
selected by setting pin 7 of U303 low. The prescaler
buffer is always on. In order to select the proper
combination of VCO and buffer, the following condi-
tions must be true at pin 6 of U303 (or pin 38 of U302)
and pin 7 of U303 (or pin 39 of U302):
•
For first LO injection frequencies 777-797 MHz,
pins 6 and 7 must both be low.
•
For Tx repeater frequencies 806-825 MHz, pins 6
and 7 must both be high.
•
For talk-around Tx frequencies 851-870 MHz, pin 6
must be low and pin 7 must be high.
The synthesizer IC (U302) consists of a prescaler,
a programmable loop divider, a divider control logic, a
phase detector, a charge pump, an A/D converter for
low-frequency digital modulation, a balance attenuator
to balance the high-frequency analog modulation to
the low-frequency digital modulation, a 13V positive-
voltage multiplier, a serial interface for control, and
finally, a filter for the regulated 5-volt supply. This
filtered five volts is present at pin 19 of U302, pin 9 of
U307, and pins 2, 3, 4, and 15 of U303. It is also
applied directly to resistors R309, R315, and R311.
Additionally, the 13V supply generated by the positive
voltage multiplier circuitry should be present at pin 35
of U302. The serial interface (SRL) is connected to the
microprocessor via the data line (pin 2 of U302), clock
line (pin 3 of U302), and chip-enable line (pin 4 of
U302).
The complete synthesizer subsystem operates as
follows:
•
The output of the VCO, pin 4 on U307, is fed into
the RF input port (pin 9) of U303. In the Tx mode,
the RF signal is present at pin 4 of U303; in the RX
mode, the RF signal is present at pin 3 of U303.
•
The output of the prescaler buffer, pin 15 of U303,
is applied to the PREIN port (pin 21) of U302. The
prescaler in U302 is a dual modulus type with
selectable divider ratios. This divider ratio is
controlled by the loop divider, which in turn
receives its inputs from the SRL. The loop divider
adds or subtracts phase to the prescaler divider by
changing the divide ratio via the modulus control
line.
•
The output of the prescaler is then applied to the
loop divider.
•
The output of the loop divider is then applied to the
phase detector. The phase detector compares the
800 MHz RF BOARD (VERSION A/B)