SMX920 Base Station & Repeater
Technical Manual
RELM/
BK RADIO
16
and VF processing. The Received Signal Strength Indication (RSSI) is buffered and
connected to the Micro Controller.
RF from the VCO at a nominal level of +3 dBm is applied to the fractional-N
synthesiser’s main divider input. This signal is compared with the reference oscillator
frequency and the correction voltage from the synthesiser’s charge pump output is
filtered then amplified by the non inverting low noise op amp. This correction voltage
is fed back to the VCO to maintain loop lock as well as being fed to the Micro
Controller. A lock detect signal is also fed to the Micro Controller. The op amp uses a
15 volt power supply from the DC to DC converter so as to provide a wide tuning
range voltage to the frequency control varicaps located in the VCO section. Frequency
programming data for the exciter is sent to the synthesiser chip from the Micro
Controller via a serial data line under the control of the Clock and Strobe lines.
The local oscillator signal to the mixer is controlled by the Micro Controller through
an enable signal. This signal switches the supply current to the local oscillator
amplifier and is used to enable or disable the receiver.
5.2.1 General Radio Management
In addition to analogue signal processing circuitry the Micro Controller section
accommodates an M30620FCAFP microprocessor IC2, a 16 kbyte EEPROM IC1, as
well as I/O latches and other miscellaneous circuitry. The Micro Controller is
responsible for ensuring that the radio acts as programmed by the user. It stores the
user-entered parameters for each channel in EEPROM. This information includes RX
and TX RF frequencies; RX and TX CTCSS frequencies as well as RF output power
and operating mode. An 8-channel analogue to digital converter allows the
microprocessor to read 8 analogue values internal to the radio, which in conjunction
with the digital inputs to the microprocessor allow the operating status of the radio to
be monitored and controlled. Virtually all logic control is done through the
microprocessor.
The following analogue items are read:
Port
Parameter
ADC0 DC
operating
voltage
ADC1
PA reflected power
ADC2
PA forward power
ADC3 PA
temperature
ADC4
Received Signal Strength Indication (RSSI)
ADC5
RX VCO tuning Voltage
ADC6
TX VCO tuning Voltage
ADC7 PA
drive
level
Table 5-1 Microprocessor Port Parameters
The Micro Controller sends programming data to the synthesiser ICs in the Receiver
and Exciter sections each time the channel is changed as well as on PTT. This
information is communicated to the Receiver and Exciter sections by way of bussed
data and clock lines and an individual module strobe. A lock detect signal from each
section is read by the Micro Controller.
In addition to the synthesiser programming bus, an I
2
C bus goes to the main EEPROM
and PA module EEPROMs.
