Theory Of Operation - Cobra MR HH330 FLT 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
Local oscillator PLL (Phase Lock Loop) Circuit
Memory backup
ATIS(Automatic Transmitter Identification System)
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 (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 circuit which consists of C16, L2, C62, C12, L1, C5, 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) for sampling. U2 is
the Automatic power control (APC) circuit.
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, C67 to filter out the unwanted signals, the antenna switching circuit D1,
D2 switching circuit to receive. The received RF signal is sent to a SAW filter F1 to filter out-of-band
signals. The RF amplifier Q7 amplifies signals within the range of the frequency coverage and passes
though another saw filter F2 to further filter out-of-band signals. The RF signal then is mixed with the
local oscillation frequency by the mixer Q8. A first IF (Immediate Frequency) 21.4 MHz is produced.
The IF is passed through a pair of crystal filter F3 (1/2), F4 (2/2) to further filter other unwanted
signals. The first IF then is amplified by Q2 and the IF amplifier U1 (BA4116). U1 is a integrated RF
amplifier which consists of a local oscillator, a demodulator, a second mixer, squelch control circuit,

THEORY OF OPERATION

4