Service manual

3.6 TP0332/2
TRANSMITTER
The transmitter block diagram is shown Figure 3.4 and the circuit diagram in Figure 6.9.
The transmitter consists of a PA module taking its input from the frequency generation block, and
feeding an antenna filter and switch. The transmit power is set by processor control of a DAC in the
baseband chip IC701. Feedback is not provided to the processor. The processor monitors the battery
voltage and the PA temperature, adjusting the transmitter output accordingly.
Frequency generator output, already modulated, is pre-amplified by TR411 and then attenuated by
R469, R472 and R473 (in the frequency generation section) before being fed to the power amplifier (signal
TX_RF), providing isolation between the VCO (TR413 and associated components) and the PA, preventing
excessive transient adjacent channel power.
The power control (IC406_B, TR101, TR102 and TR104) varies the power to ensure a smooth ramp
up and down and to set a steady state power level. The PA_ON input, when low, prevents the PA from
operating.
The antenna changeover switch routes the antenna to either the transmitter or receiver as required.
On receive, the pin diodes D100 and D101, transistors TR100 and TR103 and LED100 are off and the
antenna is connected to the receiver via the loop RX_ANT, C101, L107/L101/C103, L104, C110 and ANT.
On transmit, a DC level is applied to TR100 turning it on enabling PIN diodes D100 and D101. Control
signal PA_ON is applied to TR103 which conducts giving a DC path, the PIN diode, D100, is on and the
red LED, LED100, illuminates.
L102, L103, L104 and associated capacitors form a low pass filter.
Figure 3.4 Transmitter Block Diagram
FREQUENCY GENERATION
The frequency generation block diagram is shown in Figure 3.5 on page 3.7 and the circuit diagram
in Figure 6.10.
The two receiver local oscillator frequencies and the transmitter frequency are generated by voltage
controlled oscillators (VCOs) phase locked to a temperature compensated crystal oscillator (TCXO). The
receiver second local oscillator is shown in Figure 6.8.
The TCXO, XL400, operates at 14,4MHz and is divided down by a programmable divider (IC403)
The output frequencies are shown in Table 3.2 on page 3.7. The module has provision for electronic and
mechanical frequency adjustment. The electronic adjustment is provided via an eight-bit DAC and corrects
for frequency shifts due to ageing. Mechanical adjustment is not normally required.
The synthesizer phase detector output is applied to the VCO varicap tuning input (LOOP_FILTER)
via a passive low-pass loop filter (R428, R429, R441, C412, C415, C416 and C485).
The circuit is unusual in that the other ends of the VCO varicaps are connected to a negative voltage
(VCAP_BIAS) controlled by a digital to analogue converter. This allows a varicap tuning voltage far in
excess of the output swing of the phase comparator voltages.
Power
Antenna Switch
Amplifier
Module
Low
Generation
Frequency
From
Power
Control
Antenna
Switch
Pass
Filter
Low
Pass
Filter
Output to
Antenna
DAC
IC701
From
Output to
Receiver
Attenuator
Pre-
Amplifier
Temperature
Sensors
Output to
Controller
Low
Pass
Filter