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5
CIRCUIT DESCRIPTION
A bridge consisting of diodes D6, D8, D9 and D14 ensures correct operation
regardless of the current polarity. Q17 limits the current and D7 limits the voltage
input to ISO1. Any low voltage current source capable of providing 2 mA at 4 V or
switching circuit with less than 4.8k$\Omega$ loop resistance can be used to switch
the DC loop.
The test PTT button on the front panel and the local microphone PTT button will also
key the transmitter. Both of these also mute the line audio input. The microphone
line also enables that audio input.
5.10
Microprocessor Controller
The microprocessor controller circuit uses an single-chip eight bit processor and
several support chips. The processor U13 includes non-volatile EE memory for
channel frequencies, tones, and other information. It also has an asynchronous serial
port, a synchronous serial port and an eight bit analog to digital converter.
The program is stored in U5, a CMOS EPROM. U4 is an address latch for the low
order address bits. U2 is used to read the channel select lines onto the data bus. U11
is an address decoder for U5 and U2. U3 is a supervisory chip which keeps the
processor reset unless the +5 Volt supply is within operating limits. U1 translates the
asynchronous serial port data to standard RS232 levels.
The analog to digital converter is used to measure the forward and reverse power,
tuning voltage and dc supply voltage.
The processor keys the VCO through Q10, switches the 9.2 Volt transmit line
through Q14 and Q16, and the alarm LED D1 through Q1.
5.11 Voltage Regulator
The dc input voltage is regulated down to 9.4 Vdc by a discrete regulator circuit.
The series pass transistor Q23 is driven by error amplifiers Q8 and Q18. Q9 is used
to start up the regulator and once the circuit turns on, it plays no further part in the
operation.
The +5 Volt supply for the logic circuits is provided by an integrated circuit regulator
U14 which is run from the regulated 9.4 Volt supply.
6
Field Alignment Procedure
The procedures given below may be used to align the transmitter in the field.
Normally, alignment is only required when changing operating frequencies, or after
component replacement.
RF Technology T220
Page 13
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