Vhf Transmitter; Power Amplifier; Antenna Switch; Harmonic Filter - Motorola EP350 Service Manual

136 – 174 MHz VHF Theory Of Operation: VHF Transmitter
8.3

VHF Transmitter

The VHF transmitter covers the range of 136 – 174 MHz. Depending on model, the output power of
the transmitter is switchable on a per-channel basis between high power (5 Watts) and low
power (1 Watt). The transmitter is divided into four major blocks as shown in Figure 8-2.

• Power Amplifier

• Harmonic Filter

• Power Control
Attenuator
From VCO
Circuit
8.3.1 Power Amplifier
The transmitter power amplifier has three stages of amplification – Pre Driver (Q401), Driver Amp
(Q402) and Final Amp (Q403). Signal from TX VCO is applied to the pre driver via an attenuator
circuit. The attenuator is pie style resistor attenuator, and is used as isolation between VCO and the
power amps. The -3dBm TX RF signal from attenuator is then amplified by pre driver and driver amp
to around +28dBm and is applied to the final amp. The final amp (Q403) is an enhancement-mode
N-channel MOSFET device providing a gain of 12dB. The device drain current is drawn directly from
the DC battery supply voltage input via L413. A matching network consisting of C417, C418, C419,
C420, C455 and a strip line, transforms the impedance to approximately 50 Ohm.
8.3.2

Antenna Switch

An antenna switch works mainly as a switching device between transmit and receive paths. In
transmit mode (PTT), Q407 is turned on and both PIN diodes (CR401, CR301) are forward biased
into conduction. This enables the RF signal to pass to the harmonic filter and then to the antenna. In
the receiver mode, both diodes are off. Signals applied to the antenna jack are routed via the LPF
(harmonic filter), through a TX Low Pass Filter (L409, L410, L411, C426, C427, C428, C429, C430,
C445 and C446), to the receiver input.
8.3.3 Harmonic Filter
The harmonic filter consists of components L409, L410, L411, C426, C427, C428, C429, C430, C445
and C446. The harmonic filter is a seven-pole Chebychev filter.
SWB+
Power Control
Current Detect
Driver
Pre Driver
Amp
Amp

Figure 8-2. VHF Transmitter Block Diagram

BAT+
Strip Line
Final
Amp
To Receiver
CR401
Harmonic
Filter
Antenna
Switch
8-3
Antenna