Theory Of Operation - Cobra MR HH325VP Service Manual

This PLL - controlled VHF marine transceiver provides an accurate and stable multi-channel operation.
The transceiver consists of 6 main sections:
Transmitter
Receiver
Low Voltage Detection
Vibration Alert
PLL (Phase Lock Loop) Circuit
Memory Backup
Transmitter
The audio is picked up from the internal MIC, the audio signal is then amplified by Audio Amplifier, U4
LM324 (2/4), U4 (3/4) and filtered by a low pass filter U4 LM324 (4/4). The audio is adjusted with VR2 to
obtain a suitable audio frequency response, and then modulated with the carrier by VCO, through Varicap
(D11).
The modulated signal output from the VCO is pre-amplified by Q5, Q4 and Q3. Then it is amplified by Q12,
Q6 .The amplified signal then passes through a low-pass filter network which consists of C72, L8, C66,
C15, L6, C4 filters out spurious emission, and the antenna switching circuit, D1, D2. The signal is filtered
by another low-pass filter network which consists of C16, L2, C62, C12, L1, C5, and C1. These low pass
filters are necessary to suppress the second and third harmonics. The signal is then fed into the antenna
input and radiated out. TX current is converted into a voltage signal, that signal fed into U2 LM2904 (2/2)
the Automatic power control (APC) circuit for sampling.
When the unit is transmitting, the audio signal is added to the TX VCO Varicap D11. The capacitance of
D11 is varied following the audio signal and when mixed with the carrier to form the modulated signal.
Receiver
The receiver uses a double frequency super-heterodyne circuit. The first Immediate Frequency (IF) is 21.4
MHz and the second is 450 kHz.
The RF signal is received by the antenna, and passes through a low-pass filter network C1, L1, C5, C12,
L2, C62, C16, C20, L7, and C67 to filter out the unwanted signal, the antenna switching circuit D1, D2

THEORY OF OPERATION

4