Antenna Switch - Motorola ASTRO Digital Saber Service Manual

Antenna Switch

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6 and 7 must both be high. For talkaround TX frequencies 851-
870MHz, pin 6 must be low while pin 7 must be high.
The synthesizer IC (U303) consists of a prescaler, a programmable loop
divider, a divider control logic, a phase detector, 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, and finally a filter for the regulated five
volts. This filtered five volts is present at pin 19 of U302, pin 9 of U307,
and pins 2, 3, 4, and 15 of U303. It is also applied directly to resistors
R309, R315, and R311. Additionally, the 13V, being generated by the
positive voltage multiplier circuitry, should be present at pin 35 of
U302. The serial interface (SRL) is connected to the microprocessor via
the data line (pin 2 of U302), clock line (pin 3 of U302), and chip
enable line (pin 4 of U302).
The complete synthesizer subsystem works as follows. The output of
the VCO, pin 4 on U307, is fed into the RF input port (pin 9) of U303.
In the TX mode, the RF signal will be present at pin 4 of U303. On the
other hand, in the RX mode, the RF signal will be present at pin 3 of
U303. The output of the prescaler buffer, pin 15 on U303, is applied to
the PREIN port (pin 21) of U302. The prescaler in U302 is a dual-
modulus type with selectable divider ratios. This divider ratio is
controlled by the loop divider, which in turn receives its inputs via the
SRL. The loop divider adds or subtracts phase to the prescaler divider
by changing the divide ratio via the modulus control line. The output
of the prescaler is then applied to the loop divider. The output of the
loop divider is then applied to the phase detector. The phase detector
will then compare the loop divider's output signal with the signal from
U304 (that is divided down after it is applied to pin 14 of U302). The
result of the signal comparison is a pulsed dc signal which is applied
to the charge pump. The charge pump outputs a current that will be
present at pin 32 of U302. The loop filter (which consists of capacitors
C322, C317, C318, C329, C324, and C315, and resistors R307, R305,
and R314) will transform this current into a voltage that will be
applied to pins 1 and 7 of U307, and alter the VCO's output frequency.
In order to modulate the PLL, the two-port modulation method is
utilized. The analog modulating signal is applied to the A/D converter
as well as the balance attenuator, via U302 pin 5. The A/D converter
converts the low-frequency analog modulating signal into a digital
code that is applied to the loop divider, thereby causing the carrier to
deviate. The balance attenuator is used to adjust the VCO's deviation
sensitivity to high-frequency modulating signals.
Switching between the standard and external antenna ports is
accomplished with the external mechanical switch which is actuated
by a plunger located on the accessory connector.
An electronic PIN diode switch steers RF between the receiver and
transmitter. The common node of the switch is at capacitor C101. In
the transmit mode, RF is routed to the anode of diode CR104. In
receive mode, RF is routed to pin 1 of U201. In transmit, bias current
sourced from U504 pin 21, is routed through PIN diodes CR104 and