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

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6-1
August 1993
Part No. 001-9750-001
CIRCUIT DESCRIPTION
6.1 GENERAL TRANSCEIVER DESCRIPTION
6.1.1 INTRODUCTION
The Summit 97xx transceiver contains the
following PC board assemblies: RF, PA (power ampli-
fier), audio/logic, and display. The RF board contains
the synthesizer and receiver sections, and the PA
board contains the transmitter section. The audio/logic
board contains the digital control and audio and data
processing circuitry. The display board is used in
front-mount models and the remote control unit of
remote-mount models. General descriptions follow
and more detailed descriptions are located in Sections
6.2-6.7. A transceiver block diagram is shown in
Figure 6-1.
Circuit protection is provided by a 15-ampere
fuse in the power cable and a 4-ampere fuse on the PA
board and also by regulators which automatically limit
current. The fuse on the PA board protects low-power
circuits such as the audio power amplifier and the 5-,
8-, and 9-volt regulators. The power cable fuse
protects the high-power circuits in the transmitter such
as the driver and final.
6.1.2 SYNTHESIZER
The synthesizer output signal is the transmit
frequency in the transmit mode and the receive first
injection frequency in the receive mode. This signal is
routed by a switching circuit to either first mixer Q206
on the RF board or power amplifier module U500 on
the PA board.
Channels are selected by programming the
counters in synthesizer chip U801 to divide by a
certain number. This data comes from microprocessor
U401 located on the audio/logic board. The frequency
stability of the synthesizer is determined by the
stability of TCXO (temperature compensated crystal
oscillator) A800. The stability of A800 is ±1.5 PPM
for both 800 and 900 MHz from –22° to +140° F
(–30° to +60° C).
6.1.3 RECEIVER
The receiver is a double-conversion type with
intermediate frequencies of 52.950 MHz and 450 kHz.
Receiver selectivity is enhanced by two 52.950 MHz
crystal filters and a 450 kHz ceramic filter. Two band-
pass filters in the front end attenuate the image, half
IF, injection, and other frequencies which could
degrade receiver performance.
6.1.4 TRANSMITTER
The transmitter amplifies the +10 dBm synthe-
sizer signal to produce a power output of 15 or 30/35
watts, depending on model. All data and audio modu-
lation of the transmit signal occurs in the synthesizer.
The power control circuit allows the microprocessor
to directly control transmitter power output. This
permits each selectable system to be programmed with
a different setting if desired. The ambient temperature
of 30/35-watt models is also monitored by the micro-
processor so that power can be cut back if temperature
becomes excessive.
6.1.5 AUDIO/LOGIC BOARD
Microprocessor-based logic on the audio/logic
board controls such functions as synthesizer program-
ming, scan, data encoding and decoding, and the
display. In addition to the digital control circuitry, the
audio/logic board contains analog circuits which
provide amplification, filtering, and other processing
of the audio, data, and Call Guard signals.
Information which changes from transceiver to
transceiver such as channels to be selected and various
operating parameters is programmed into the trans-
ceiver by a personal computer connected to the front-
panel microphone jack. This information is stored in
EEPROM U406. Since this type of memory device is
nonvolatile and reprogrammable, battery backup or
extra PROMs are not required.
The operating program is stored in flash EPROM
U404. This type of memory can be reprogrammed by
SECTION 6 CIRCUIT DESCRIPTION