Vhf Transmitter; Transmit Power Amplifier; Antenna Switch; Harmonic Filter - Motorola EP450 Service Manual

136-162 MHz VHF Theory Of Operation: VHF Transmitter
4.3

VHF Transmitter

The VHF transmitter covers the range of 136-162 MHz. Depending on model, the output power of the
transmitter is either switchable on a per-channel basis between high power (5 watts) and low power
(1 watt), or is factory preset to 2 watts. The transmitter is divided into four major blocks as shown in
Figure 4-2.
• Power Amplifier

• Harmonic Filter

• Antenna Matching Network
• Power Control
TX_ENA
PWR_SET
5T
TX_INJ
Q100
(From VCO)
4.3.1

Transmit Power Amplifier

The transmitter power amplifier has three stages of amplification. The first stage, Q100, operates in
Class AB from the 5T source. It provides 13 dB of gain and an output of 20 mW. The current drain is
typically 25mA. Components C105-C107 and L103 match the output of Q100 to the 50Ω input of the
module U110.
U110 is a two stage Silicon MOS FET power amplifier module. Drain voltage is obtained from UNSW
B+ after being routed through current-sense resistor R150 in the power control circuit. The output
power of the module is controlled by varying the DC gate bias on U110 pin 2 (VGG).
4.3.2

Antenna Switch

The antenna switch consists of two pin diodes, D120 and D121. In the receive mode, both diodes are
off. Signals applied at the antenna or at jack J140 are routed, via the harmonic filter, through network
C122-C124 and L121, to the receiver input. In the transmit mode, Q170 is on and TXB+ is present,
forward-biasing both diodes into conduction. The diode current is 50 mA, set by R120-R122. The
transmitter RF from U110 is routed through D120, and via the harmonic filter to the antenna jack.
D121 conducts, shunting RF power and preventing it from reaching the receiver. L121 is selected to
appear as a 1/4 wave at VHF, so that the low impedance of D121 appears as a high impedance at the
junction of D120 and the harmonic filter input. This provides a high series impedance and low shunt
impedance divider between the power amplifier output and receiver input.
4.3.3

Harmonic Filter

The harmonic filter consists of components C130-C136 and L130-L132. The harmonic filter is a
seven-pole elliptical low-pass configuration, optimized for low insertion loss, with a 3 dB frequency of
approximately 180 MHz and typically less than 0.8 dB insertion loss in the passband.
HKLN4216C
USWB+
Power Control
VGG VDD
Power Amplifier Module U110
RX_IN
(To Receiver)

Figure 4-2. VHF Transmitter Block Diagram

Harmonic Filter
Antenna
Switch
Antenna
Matching
Network
4-3
J140 Antenna
Jack
Antenna
June, 2005