TMC Radio SRP 9100, Руководство по обслуживанию

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Page 4.3
3 ~ TECHNICAL DESCRIPTION
4.2 T
RANSMITTER
Refer to Figure 4-2.
4.2.1
Drivers and PA Stages
The RF output level from the VCO T/R switch D611a is typ8dBm. Tx buffers Q550/Q560 increase this
level by approximately 6dB (136-530MHz), 9dB (66-88MHz) and also provide a high degree of VCO isolation
from the Tx output.
The PA module U500 requires a drive power of approximately 17dBm (335-520MHz), 13dBm (136~245MHz)
and 15dBm (66-88MHz). The module contains power control circuitry and MOSFET stages to provide a
maximum output power of +37dBm (5 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 R534 to R537, and analogue gate U800B to ADC U301C. U301C samples the applied voltage after
which it is passed to the PLA and then processed by the DSP.
4.2.2
Power Control
Output power is stabilised by a power control feedback loop. A printed circuit transmission line, L590, R580,
D510 and associated components comprise the power detector. Comparator U520A and associated
components provide the power setting and control functions. Forward 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 pre-driver supply, and therefore the PA drive level. High
temperature protection is provided by thermistor R532 that progressively reduces the power level if the PA
module temperature becomes excessive.
4.2.3
Antenna Changeover and Harmonic Filter
The antenna changeover circuit consisting of pin diodes D580/D540a/D541a, is switched by Q541/Q542 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.
4.2.4
Transmitter Audio Processing
The internal microphone unit comprising an Electret microphone provides 10mV RMS (nominal) at the
microphone input (INT_MIC) to provide approximately 60% of maximum system deviation. U800C is a control
gate to switch between the microphone audio signal and EXT_MOD to provide external audio options and data
input.
U800A provides CODEC input switching which selects either the receiver I signal or transmitter audio/data
signals depending on the Tx/Rx mode. All pre-emphasis, filtering, compression and limiting processes for
narrow and wideband operation are carried out in the DSP after A-D conversion by the CODEC (U820). The
processed transmitter audio/data from the CODEC output at VOUTL is applied to the VCO as a modulation
signal with a level of approximately 200mV P/P.