Motorola ASTRO MT 1500, Руководство по обслуживанию, Страница: 104 / 817

August 16, 2007 6816985H01-B
4.3-2 UHF2 Detailed Theories of Operation: Antenna Switch
The 13V, being generated by the positive voltage multiplier circuitry, should be present at pin 47 of
U202. The serial interface (SRL) is connected to the microprocessor via the data line (pin 7 of U202),
the clock line (pin 8 of U202), and the chip-enable line (pin 9 of U202).
The complete synthesizer subsystem works as follows:
1. The buffered output of the VCO is present at R294 in receive mode and R275 during transmit
mode.
2. The output of the prescaler filter is present at C293 and applied to pin 32 of U202. The
prescaler in U202 is a dual-modulus type with selectable divider ratios. This divider ratio is
controlled by the loop divider, which in turn receives its input from the SRL. The loop divider
adds or subtracts phase to the prescaler divider by changing the divider ratio via the modulus
control line.
3. The output of the prescaler is then applied to the loop divider.
4. The output of the loop divider then feeds the phase detector. The phase detector will compare
the loop divider's output signal form the VCOs. The VCO signal is divided down after it is
applied to pin 32 of U202. The result of the comparison is a pulsed DC signal which is applied
to the charge pump.
5. The charge pump outputs a current that will be present at pin 43 of U202. The loop filter
consisting of R202, C202, R203, C203, R201, C201, C236, C237, C238, C239 and C240 will
convert this current into a voltage that will be applied to varactors D251, D252, D253, D254,
D270, D271, D272, and D273 to alter the output frequency of the VCO.
In order to modulate the PLL, the following two-port modulation method is utilized: The analog
modulating signal is applied to (1) the A/D converter as well as to (2) the balance attenuator:
1. The A/D converter converts the low-frequency analog modulating signal into a digital code.
When the digital code is applied to the loop divider, it causes the carrier to deviate.
2. The balance attenuator is used to adjust the VCO’s deviation sensitivity to high-frequency
modulating signals.
4.3.3 Antenna Switch
An electronic PIN diode switch steers RF between the receiver and transmitter. The common node of
the switch is at capacitors C171 and C180 for UHF Range 2.
1. In transmit mode:
a. RF is routed to the anode of diode D170.
b. Bias current, sourced from RAW_B+ is routed through PIN diodes D170 and D171,
biasing them to a low-impedance state.
2. In receive mode:
a. RF is routed through C179 to pre-selector filter.
b. PIN diodes D170 and D171 are unbiased and thus at high impedance.