Motorola EP350

435 – 480 MHz UHF2 Theory Of Operation: UHF2 Transmitter 5-3
5.3 UHF2 Transmitter
The UHF2 transmitter covers the range of 435 – 480 MHz. Depending on model, the output power of
the transmitter is switchable on a per-channel basis between high power (4 Watts) and low
power (1 Watt). The transmitter is divided into four major blocks as shown in Figure 5-2.
• Power Amplifier
• Harmonic Filter
• Antenna Matching Network
• Power Control.
Figure 5-2. UHF2 Transmitter Block Diagram
5.3.1 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 -4dBm TX RF signal from attenuator is then amplified by pre driver and driver amp
to around +25dBm 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 C416 to C420 and a
strip line, transforms the impedance to approximately 50 Ohm.
5.3.2 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 network L409 to L411, to the receiver input.
5.3.3 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.
Attenuator
Circuit
Pre Driver
Amp
From VCO
Driver
Amp
Final
Amp
Power Control
Strip Line
Current Detect
SWB+
BAT+
Antenna
Switch
To Receiver
CR401
Harmonic
Filter
Antenna