Service Manual

SRM9000 SERIES SERVICE MANUAL
© TMC Radio 2006 page 25 TNM-U-E-0002 Issue 1
3.2 TRANSMITTER
Refer to Figure 3-2.
3.2.1 Drivers and PA Stages
The RF output level from the VCO T/R switch D611a is typically +5dBm (UHF) and +8dBm (VHF). Tx
buffers Q606/Q612 increase this level to provide approximately 10dBm of drive power to the Tx driver
Q571. The Tx driver stage provides a maximum of 17dBm of drive to the PA module and provides part of
the power control function in conjunction with the first stage of the PA module. The buffer/driver circuits in
conjunction with their attenuator circuits also provide a high degree of VCO isolation from the Tx output.
The gains of PA driver Q571 and PA module are controlled by the power control loop to ensure that
transmitter output power remains within defined limits.
PA module U500 utilises three MOSFET stages (UHF) and two MOSFET stages (VHF) to achieve the
required maximum RF output power level of +44dBm (25 watts).
Note. Care should be taken during servicing since if for any reason the drive power is lost, while the power
control voltage is high, the current into the PA may exceed its specification. Therefore, the power supply
current should be monitored at all times and preset to as low as required. The radio has additional inbuilt
safeguards, but these should not be relied on.
Power output settings are derived from alignment data stored in flash memory during the initial factory
alignment. The DSP processes this data to optimise the power output level relative to the programmed
channel frequencies that may be changed at any time without retuning the radio.
PA current is monitored via comparator U520B, the output of which is passed via a temperature
compensation network R541 to R543 to ADC U301C. U301C samples the applied voltage after which it is
passed to the PLA and then processed by the DSP.
3.2.2 Power Control
Output power is stabilised by a power control feedback loop. A printed circuit transmission line, L580, R580,
D510 and associated components comprise the power detector. Comparator U520A and associated
components provide the power setting and control functions. Forward and reverse power is sampled by the
power detector and applied as a DC voltage to the inverting input of the comparator. The TX_PWR set
voltage is a DC voltage proportional to the programmed Tx power setting and is applied to the non-inverting
input of the comparator.
The TX_PWR voltage originates from the PLA as a PWM signal and is integrated for application to the
comparator.
PA module output level changes due to supply voltage, load or temperature variations are detected and
applied to the comparator that proportionally adjusts the PA module bias supply and the PA driver (Q571)
supply, and therefore the PA drive level. High temperature protection is provided by thermistor R552 that
progressively reduces the power level if the PA module temperature becomes excessive.
3.2.3 Antenna Changeover and Harmonic Filter
The antenna changeover circuit consisting of pin diodes D580/D540a/D541a, is switched by
Q541/Q542/Q543 and associated circuitry allowing the transmitter output to be coupled to the antenna while
providing isolation for the receiver input. With the transmitter switched on, the diodes are forward biased
allowing power to be coupled through to the antenna and isolating the receiver by grounding its input at
C588. The short circuit at the receiver input is transformed to an effective open circuit at D580 by L583,
which minimises transmitter loading. With the transmitter switched off the diodes are reverse biased
allowing the receiver input signal to reach the receiver front end with minimal loss. The harmonic rejection
low pass filter comprises L582/L584/L585 and associated capacitors.
3.2.4 Transmitter Audio Processing
The microphone audio input signal is applied to the microphone input (AUDIO_IN1) and is derived from an
external microphone unit with an applied nominal level of 40mV RMS. U801A is a control gate that switches
between AUDIO_IN1 and OPTION_AUDIO1 to provide external audio options and data input.