Antenna Switch - Motorola ASTRO XTS 2500 Service Manual

4.4-2
• A charge pump
• An A/D converter for low-frequency digital modulation
• A balance attenuator to balance the high-frequency analog modulation to the low-frequency
digital modulation
• A 13V positive voltage multiplier
• A serial interface for control
• A filter for the regulated 5 and 3 volts
The 13V, being generated by the positive voltage multiplier circuitry, should be present at pin A2 of
U201. The serial interface (SRL) is connected to the microprocessor via the data line (pin F1 of
U201), the clock line (pin F4 of U201), and the chip-enable line (pin G1of U201).
The complete synthesizer subsystem works as follows:
1. The buffered output of the VCO will be present at pin 8 of U250 in RX mode and pin 10 during the
TX mode.
2. The output of the prescaler buffer will be present at pin 12 of U250 and applied to pin 32 of U201.
The prescaler in U201 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 U201. 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 U201. The loop filter consisting
of C229, C228, C221, C295, C291, C292, C298, R210, R212, and R207, will convert this current into
a voltage that will be applied to varactors D210, D211, and D212 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 the (1) A/D converter as well as to the (2) 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.

4.4.3 Antenna Switch

An electronic PIN diode switch steers RF between the receiver and transmitter. The common node of
the switch is at capacitor C120.
1. In transmit mode:
a. RF is routed to the anode of diode CR101.
b. Bias current, sourced from U102, pin 32 is routed through PIN diodes CR101 and CR102,
biasing them to a low-impedance state.
2. In receive mode:
a. RF is routed through pin 1 of FL1.
b. PIN diodes CR101 and CR102 are unbiased and thus at high impedance.
August 16, 2007
700/800 MHz Detailed Theories of Operation: Antenna Switch
6816985H01-B