TMC Radio SRP 9100, Руководство по обслуживанию, Страница: 41 / 66

Page 4.5
3 ~ TECHNICAL DESCRIPTION
4.3 F
REQUENCY
S
YNTHESISER
4.3.1
General
Refer to Figure 4-3.
The frequency synthesiser consists of one transmitter and two receiver voltage controlled oscillators, loop
filters, varactor negative bias generator, reference oscillator and an integrated dual phase locked loop device
U701.
4.3.2
PLL
The PLL device contains two prescalers, programmable dividers and phase comparators to provide a main and
auxiliary PLL. The main PLL of U701 controls the frequency of the Tx/Rx VCO’s via Control Voltage outputs at
pins 2 and 3 and VCO feedback to pin 6. The auxiliary PLL is used to control the receiver 90MHz second local
oscillator via the Control Voltage output at pin 17 and VCO feedback to pin 15. The PLL operation involves the
division of the 14.4MHz reference oscillator frequency to 225kHz (VHF) or 450kHz (UHF) by divider. It is then
divided by the internal divider of U701 down to a low frequency that corresponds to a sub-multiple of the radio
channel spacing, ie. 6.25kHz for 12.5/25kHz channel spacing, 5kHz for 10/20kHz channel spacing or 7.5kHz if
required. The VCO frequency is sampled and divided down to the same frequency after which it is phase
compared to the reference. Any error produces an offset to the Control Voltage output that is used to correct
the VCO frequency. A valid lock detect output is derived from pin 20 and is sampled by the PLA. During
transmit, if an unlocked signal is detected the radio will switch back to receive mode. An unlocked signal in
receive mode will cause the radio to beep.
4.3.3
VCO’s
The transmitter and receiver VCO’s use low noise JFET transistors Q600 (Rx), Q602 (Tx) and associated parts
to generate the signals for the required band coverage. Electronic tuning is provided by varactor diodes D600
to D608 with their control voltages derived from the Loop Filter, PLL and Negative Bias Generator. VCO
selection and timing is controlled by the DSP/PLA via the Rx and Tx power supplies and applied through
switches Q601 (Rx) and Q603 (Tx). VCO buffer Q604/Q605 isolates the VCO from load variations in following
circuits and active power supply filter Q615 minimises supply related noise. A PLL feedback signal is sampled
from the VCO buffer output via buffer Q607.
The 90MHz receiver VCO comprises Q402 and associated parts. Automatic tuning is achieved by applying a
Control Voltage to D408/D409 via Loop Filter R433, R445, R464, C467, C493 and C702.
4.3.4
Negative Bias Generator and Loop Filter
A positive and negative varactor bias supply similar to the front-end varactor arrangement has been used to
achieve the required broadband tuning range of the VCO’s. PLL device U701 is programmed to deliver a
n2.5V output from phase detector/charge pump CPPF or CPP regardless of the channel frequency
selected. This voltage is filtered to remove synthesiser noise and reference products by the Loop Filter,
comprising C671 to C686 and R683/R684/R685. The resulting low noise voltage is applied to the cathode side
of the VCO varactor tuning diodes as a positive bias voltage. The negative bias supply originates as a positive
DC voltage (0.1V to 3.0V) at the DAC output of U701 (DOUT) with a level relative to the programmed state of
the radio (eg. channel frequency and Tx/Rx state). The voltage is translated to a negative voltage between 0V
and –16V by the circuit comprising Q700 to Q703. The -16V rail of this supply is generated by U904E/F with
D903 to D906 providing the voltage multiplying effect needed to achieve -16V. The output of the negative
supply is applied directly to the VCO varactor anodes as the negative tuning voltage VCAP BIAS.
4.3.5
Phase Modulator
The modulation path for audio, data and higher frequency CTCSS signals is via varactor D609 and its
associated components in the Tx VCO. The reference input to the PLL (REF) provides the low frequency
modulation path via the Phase Modulator.
The phase modulator comprises the following sections:
- Integrator U760B is a low pass filter providing 6dB per octave attenuation to frequencies above
approximately 10Hz.
-
Divider U710 divides the 14.4MHz reference frequency down to 225kHz (VHF) or 450kHz (UHF).
- Ramp generator Q711/Q712 provides a saw tooth output, the slope of which is adjustable via
the MOD_BAL line. This adjustment is set via a DAC output controlled from the Alignment