Transmitter - Motorola P100 Serie Servicehandbuch

2-4
U202 will adjust the squelch circuit sensitivity depending on Noise level from recovered audio. Noise
level is amplified by internal amplifier of U201 to help U202 decide the squelch circuit sensitivity. If
the noise level is over the set threshold, the microprocessor mutes the radio.
Note Perform squelch tuning after any RX part replacement. Refer
on page 5-13.
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 (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 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
Pre Driver
Amp
Amp
Figure 2-2. Transmitter Block Diagram
Theory Of Operation: Major Assemblies
BAT+
Strip Line
Final
Amp
To Receiver
Chapter 5.5: Transmitter
Chapter 5.7: Receiver Tuning
CR401
Harmonic
Filter
Antenna
Switch
Antenna