Service Manual

SRM9000 SERIES SERVICE MANUAL
© TMC Radio 2006 page 28 TNM-U-E-0002 Issue 1
The divided reference signal is differentiated and discharges C744 via Q711 after which Q711 is turned
off allowing C744 to recharge via constant current source Q712/Q713.
- Schmitt Trigger comprising Q714 to Q716 converts the modulation combined with the saw tooth ramp
to a square wave output, the duty cycle of which is controlled by the ramp slope and modulation.
Modulation balance adjustment is carried out using a CODEC generated 100Hz square wave applied to
the TX_MOD input and set to give an optimum demodulated square wave output.
3.3.6 Reference Oscillator
TCXO U700 determines the overall frequency stability and frequency setting of the radio. The frequency
setting is achieved by adjusting its ADJ voltage with the Alignment Tool. In addition, the ADJ input is used in a
frequency control loop with the receiver I and Q signals to provide receiver AFC. U700 operates at 14.4MHz
and is specified at ±2.0ppm frequency stability over the temperature range -30° to +75°C.
90MHz
VCO Q402
Synthesiser
U701
RX/TX VCO
Q600/ Q602
TX/RX
Switch
D601/
D611
VCO
Buffer
Q604/
Q605
Data From
DSP
Phase
Modulator
Q714-
Q716
Ramp
Generator
Q711-
Q713
Integrator
U711A
TX_MOD
MOD_BAL
Prog.
Divider
U710
Buffer
Q710
14.4MHz
Reference
Oscillator
U700
To TX Buffer
REF
PLL Loop
Filter
VCO Neg
Bias
Supply
Q700-
Q703
CPP
CPPF
DOUT
VCAP_BIAS
AUX_LO2
AUX_CP
Divide 32 (UHF)
Divide 64 (VHF)
Synth
Buffer
Q607
SYNTH
LO1_RX
TX_MOD
To IQ
Demodulator
U400
LO2
AFC
from
PLA
To RX Mixer
Buffer
U711B
RX_PSU
RX_PSU TX_PSU
Figure 3-3 VHF/UHF Synthesiser, Block Diagram
3.4 CONTROL
Refer to Figure 3-4.
3.4.1 DSP and PLA
The SRM9000 transceiver operates under the control of a DSP (U201) and PLA (U300) combination that
together with a number of other dedicated devices perform all the operational and processing functions
required by the radio. The PLA is configured by the DSP under software control to provide the following
functions:
• Channel set-up of all operating frequencies
• Modulation processing and filtering
• De-modulation processing and filtering
• Tx power output reference