E.F. Johnson Company Summit DM 975x, Руководство по обслуживанию

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CIRCUIT DESCRIPTION
6-2
August 1993
Part No. 001-9750-001
the microprocessor similar to an EEPROM. This
allows the operating program to be updated using the
standard programming setup and special software.
This eliminates the need to change a ROM or the
microprocessor to update software.
6.2 SYNTHESIZER CIRCUIT DESCRIPTION
6.2.1 INTRODUCTION
A block diagram of the synthesizer is shown in
Section 6-1. The synthesizer output signal is produced
by a VCO (voltage-controlled oscillator) located on a
separate module. The frequency of this oscillator is
controlled by a DC voltage produced by the phase
detector in synthesizer chip U801. The phase detector
senses the phase and frequency of two input signals
and if they are not the same, the VCO control voltage
increases or decreases until they are synchronized. The
VCO is then “locked” on frequency.
One input to the phase detector is the reference
frequency (fR) derived from the TCXO. The TCXO
frequency is divided by 1400 by the reference counter
in U801 to produce a reference frequency of 12.5 kHz.
This frequency is the same for all channels. In addi-
tion, the third harmonic of the TCXO frequency is
used to lock the receiver second injection oscillator on
frequency.
The other input signal to the phase detector (fV)
is derived from the VCO signal. The VCO signal is
divided by prescaler U800 and the N counter in U801
to produce this signal. The prescaler and N counter are
programmed for each channel to produce an input
frequency to the phase detector that is the same as the
fR input when the VCO is oscillating on the correct
frequency.
6.2.2 VOLTAGE-CONTROLLED OSCILLATOR
MODULE
NOTE: The VCO module is not repairable because it
is a hybrid with many of the components printed on a
ceramic substrate.
VCO (Q920), Buffer Amplifier (Q930, Q940)
Q920 is a bipolar transistor configured as a
common collector oscillator. It oscillates on the
transmit frequency in the transmit mode and 52.950
MHz below the receive frequency in the receive mode.
Therefore, its operating band is approximately 798-
869 MHz (800 MHz models) or 882-941 MHz (900
MHz models). Transistor bias is provided by R920 and
R921, and stabilization is provided by R922. L921 is
an RF choke and C926 is a bypass capacitor. C922,
C923, and R924 provide feedback to start and main-
tain oscillation and also match the oscillator to the
tank circuit.
The tank circuit consists of a section of micros-
trip; varactor diodes CR901, CR903, and CR904;
capacitors C920, C921, C901, C903, C905, C909; and
several other components. The microstrip is a short-
ened half-wave line which provides series inductance
and shunt capacitance.
The output signal on the collector of Q920 is
coupled by C932 to buffer amplifier Q930/Q940. This
is a shared-bias amplifier which provides amplifica-
tion and also isolation between the VCO and the
stages which follow. The signal is coupled from Q930
to Q940 by C935. The resistors in this circuit provide
biasing and stabilization, and C931 and C934 are
bypass capacitors. C932, C933, and C937 provide
impedance matching and DC blocking.
Frequency Control and Modulation
The VCO frequency is controlled in part by the
DC voltage applied across varactor diode CR901. As
the voltage across a reverse-biased varactor diode
increases, its capacitance decreases. The result is that
the VCO frequency increases as the control voltage
increases and vice versa. The control line is isolated
from tank circuit RF by choke L901 and decoupling
capacitor C902. The amount of frequency change
produced by CR901 is set by series capacitor C901.
The VCO frequency is modulated in a similar
manner. The transmit audio/data signal is applied
across varactor diode CR903. This varies the VCO
frequency at an audio rate. The amount of modulation
is set by C903 along with varactor diode CR904 and
capacitor C912 (see next paragraph). R901 provides a
DC ground on the anodes of CR903 and CR904, and
isolation is provided by R900, C904, and C852.