Directional Coupler U101; Antenna Switch; Harmonic Filter; Rf Detectors D101 And D102 - Motorola ASTRO XTS 5000 Detailed Service Manual

4-6

4.2.4.4 Directional Coupler U101

The directional coupler, U101, is a low-loss, dual-directional coupler with external terminating
resistors. The forward- and reverse-coupled signals are used as feedback to the transmitter
automatic level control (ALC) loop.

4.2.4.5 Antenna Switch

See Figure 9-2. Antenna Switch and Harmonic Filter Circuits on page 9-3
following discussion.
Diodes D701 and D702 form a single-pole, double-throw, positive-intrinsic-negative (PIN) diode,
transmit/receive antenna switch. When the transmitter is ON, diodes D701 and D702 are forward-
biased, and the signal goes from the transmitter to the antenna through the low resistance (about 1
ohm) of the PIN diode D701. The receiver also is shorted by the same low resistance of D702. In the
receive mode, neither diode is biased and both behave as small-value capacitors (around 1 pF).
These capacitors resonate at the signal frequency with their individual parallel inductors (L706 and
L707), which creates high blocking impedances, effectively disconnecting the transmitter circuitry
from the antenna.
The switch control circuit consists of transistors Q101, Q103, Q106, and associated resistors. The
input signal is TXINH from the VOCON board and RXH from the PCIC. Whenever TXINH is low and
RXH is high, PIN diodes are not biased and the radio is in receive mode.
When TXINH and RXH are low, the PIN diodes are ON and the radio will transmit. If TXINH goes
high, the radio is inhibited from transmitting (a secure module control feature).

4.2.4.6 Harmonic Filter

A harmonic filter is needed to suppress transmitter harmonics in order to meet the radio's
specifications for transmitter conducted and radiated emissions.
The 700/800 MHz harmonic filter uses both discrete components and transmission lines to form a
low-pass filter. The pass band is up to 870 MHz, and the stop band is above 1500 MHz. Air-core
inductors are used for their low losses and high-current handling capabilities. Shield SH700 is also
an essential part of this circuit.

4.2.4.7 RF Detectors D101 and D102

The Schottky diodes, D101 and D102, are used as forward- and reverse-power detectors. Forward-
coupled RF from the power amplifier, and reverse-coupled RF from the antenna are converted to DC
voltages FWD and RVS. Detector output is a positive DC voltage, proportional to the amplitude of
the RF signal at the input.

4.2.4.8 Summing Amplifier U106

This op-amp circuit is a non-inverting, unity gain buffer. Signals FWD and RVS are summed and sent
to the ALC input of the PCIC. When the antenna port is loaded with a low VSWR (voltage standing
wave ratio), as in normal operation, RVS is far less than FWD, the amplifier output is equal to FWD,
and the radio maintains a constant output power. Under high VSWR conditions, such as when the
antenna is damaged or held near a large metal surface, the amplitude of RVS becomes a large
fraction of FWD, the amplifier output increases proportionally, and the radio cuts back the transmitter
power by up to 50%. This reduces high battery current into a large VSWR transmitter to extend
battery life.
April 15, 2002
Detailed Theory of Operation: Transceiver Board
for schematic details of the
68P81094C31-O