Motorola EP350 Basic Service Manual

2-4 Theory Of Operation: Major Assemblies
2.2.2 Transmitter
The radio's TX Power Amplifier system is a three stage amplifier which is able to amplify the VCO
output up to the permitted maximum transmit power levels (UHF: 4W, VHF: 5W).
TX VCO output signal passes thru a 3 dB, pie style resistor, attenuator before going into the TX
power stage acting as isolation between the low power VCO and high power amps. The next stage
consists of a pre-driver (Q401) and a driver amplifier (Q402). The TX RF signal (UHF1: -4 dBm,
UHF2 : -4 dBm, VHF : -3 dBm) from the attenuator is amplified to +25 dBm (UHF1), +25 dBm
(UHF2) or +28 dBm (VHF) by the pre-driver and driver amp. This is followed by the final PA, an
enhancement-mode N-channel MOSFET device (Q403), which provides a 12 dB gain.
The final PA draws current directly from the DC battery supply voltage input via L413.
The PA matching network consists of C416, C417, C418, C419, C420, C422, C451 (UHF1 & UHF2)
or C417, C418, C420, C455 (VHF) and a strip line, which matches the TX Power impedance to
approximately 50 ohm. Antenna switch is shared between TX and RX circuit. In TX mode, PIN
diodes (CR401, CR301) are forward biased which enable the High Power RF signal to pass through
the antenna. In RX mode, both diodes are off. Signals applied to the antenna jack are routed, via the
Harmonics LPF in to the RX circuit. The High Power RF Signal finally passes through a TX Low Pass
Filter, a 7th order Chebyshev filter (L409, L410, L411, C426, C427, C428, C429, C445, C446).
The APC (Auto Power Control) keeps the current supplied to Final PA (Q403) constant. Resistor,
R417 is used for current sensing. The voltage difference ratio of R423 to R417 is amplified through
U401 and passed to Q404 and Q405 to produce constant power output to the antenna. Do not
exceed the maximum allowed bias voltage of the device.
Figure 2-2. Transmitter Block Diagram
Note:
Retune the TX Power if Final PA (Q403) is replaced. Refer Chapter 5.5: Transmitter
Alignment Options on page 5-3.
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

Motorola EP350 Specifications

General

Channel Capacity	16 channels
IP Rating	IP54
Drop Resistance	1.2 meters
Frequency Range	VHF: 136-174 MHz, UHF: 403-470 MHz
Battery Life	Up to 11 hours
Power Supply	7.5 V (Nominal)
Waterproof	Splashproof
Dimensions	127.6 x 62 x 42 mm (with standard battery)