Specifications

23
4. THEORY OF OPERATION
4.1. INTRODUCTION
The VHF and UHF radios comprise of two PCBs (RF and digital PCB). These boards are
connected by an 18 pin female and male connector. The digital board which controls the radio
and data receiving and sending is interfaced with external data equipment through the 15 pin d-
sub female (DB-15) connector.
4.2. DIGITAL CIRCUITS
The Digital circuit contains the CPU, the channel select switch, and associated digital circuits.
TX-SIGNAL CIRCUIT
There are two signal paths in the Tx-signal circuit. One is FSK data signal path and the other is
the audio signal path. The FSK data signal from Pin 1 of DB-15 connector (CON401) goes
through IC406-C. The signal is amplified by IC404-C and then its amplitude is limited by IC404-
D. After that, this signal is filtered by an 8’th order low pass filter (IC405) in order to reduce the
required transmission bandwidth. The first two stages of the 8’th order LPF consist of a
Gaussian filter for the improvement of ramp function response and the last two stages use a
Butterworth filter for attenuation. The output of the LPF is then fed to the RF board for TX
modulation.
Audio signal, from Pin 7 of DB-15 connector is fed into the 300Hz High pass filter(IC408)
through the IC406-B and IC409. The HPF removes sub-audible voice products for application of
Sub-audio(Tone) squelch system (CTCSS, DCS) and then the output from IC408 is fed into
IC404-A&B with associated parts to form a mic amplifier and pre-emphasis circuit. After that, the
pre-emphasized Tx-audio signal is inputted to the RF board for Tx modulation through the FSK
data signal path.
RX-SIGNAL CIRCUIT
The Rx-signal circuit also has two signal paths. One is the data signal path and the other is the
audio signal path. The Rx signal comes from the RF board, which is connected with pin 10 of
CON405. Data signals are switched in IC406-D by a Busy signal which is activated when the
radio receives a valid RF signal, and is filtered by IC416-A and then its amplitude level is