Transmitter; Figure 2-2. Transmitter Block Diagram - Motorola CP185 Basic Service Manual

2-4
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).
Attenuator
From VCO
Circuit
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 (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 and C451 (UHF1 and
UHF2) or C417, C418, C420 and 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 elliptic filter (C426, C427, C428, C429, C445, C446, L409, L410
and L411).
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.
Note: Retune the TX Power if Final PA (Q403) is replaced. Refer
Alignment Options on page 5-4.
SWB+
Power Control
Current Detect
Driver
Final
Pre Driver
Amp
Amp Amp

Figure 2-2. Transmitter Block Diagram

Theory Of Operation: Major Assemblies
BAT+
CR401
Strip Line
Antenna
Switch
To Receiver
Chapter 5.5: Transmitter
Antenna
Harmonic
Filter