Antenna Switch; Receiver - Motorola XTS 2500 Service Manual

4.1-2
The 13V, being generated by the positive voltage multiplier circuitry, should be present at pin A2 of
U202. The serial interface (SRL) is connected to the microprocessor via the data line (pin F1 of
U202), the clock line (pin F4 of U202), and the chip-enable line (pin G1 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 D10 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 D10 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 A5 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 and D270 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.

4.1.3 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 VHF.
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, bias-
ing 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.

4.1.4 Receiver

The receiver consists of the following:
• Front end
• Back end
February 14, 2012
VHF Detailed Theories of Operation: Antenna Switch
6816985H01-F