Motorola SSETM 5000 - Manual (Page 35)

6881094C12-A November 11, 2004
Theory of Operation: Transceiver Board 2-13
2.4.3.4 Directional Coupler
A directional coupler senses the transmitter forward power as a control signal in the transmitter’s
automatic level control (ALC) loop. Isolated ports are terminated with external resistors.
The directional coupler is U101, a low-loss, bidirectional coupler.
2.4.3.5 Antenna Switch
The SSE 5000 has two antenna switches: one standard antenna switch and a remote antenna
switch.
The standard antenna switch is a quarterwave switch that determines whether the radio is in receive
or transmit mode. The standard antenna switch consists of part numbers D701, D702, C706, C704,
C701, C707, L702, L703, and R701. When the standard antenna switch is in receive mode, the
diodes D701 and D702 are unbiased and radio signals are able to travel to the receive front-end.
When the standard antenna switch is in transmit mode, radio signals travel from the transmitter to
the selected port and radio signals from the transmitter to the receive front-end are redirected by the
large impedance presented by L702 and C704. The receive front-end is also protected from the
transmitter because of the combined effect of the radio wave redirection and the short produced by
C703 when the standard antenna switch is enabled.
The circuitry that enables the standard antenna switch consists of part numbers L703, C707, and
U104. When the radio is in transmit mode, pin 32 (also known as ANO) on U104 provides
approximately 6.7 volts to diode D702. This voltage is dropped approximately 1.4 volts, or two diode
drops, and applied to R701. R701 sets the current through the antenna switch (approximately 14
mA). L703 and C707 are used as a DC bias network designed to only transmit DC signals.
The remote antenna switch is also a quarterwave switch, but this switch determines which antenna
the radio uses for transmit or receive. The remote antenna switch consists of part numbers D602,
D601, C724, C725, C726, C727, C728, L701, L704, L711, R703, R704, R705, Q702, and Q703.
When the radio is receiving from the standard antenna, no diodes are forward biased. In order to
activate the remote antenna port, the VOCON must supply 2.9 volts (+/– 3%) to pin 19 of the 26-pin
connector. Pin 19 is also known as TX_INH. TX_INH going high causes the voltage on the collector
of Q703 to become the voltage on the emitter. The voltage on the emitter for this circuit is zero volts.
The collector is connected to the gate of Q702, which in turn forces the voltage on the drain of Q702
to become the voltage on the source of Q702. The voltage on the source of Q702 is switched B+, or
battery voltage. Switched B+ is dropped approximately 1.4 volts, or two diode drops, and applied to
R705. R705 sets the current through the remote port switch (approximately 12 mA).
L701 and C724 are used as a DC bias network designed to only transmit dc signals. When the radio
is receiving or transmitting through the remote port, radio signals travel through diode D602. The
signals are directed away from the standard antenna by the large impedance presented by L704 and
C727. Accidental radiation through the standard antenna is prevented, because of the combined
effect of the radio wave redirection and the short produced by C725 when the remote port is
enabled.
NOTE: Part numbers C728 and L711 are used to resonate the parasitic capacitance created by diode
D602. The parasitic capacitance was creating a degradation in the transmit response through
the standard antenna port. Essentially, C728 is a DC block to prevent reverse biasing D602,
and L711 resonates with the parasitic capacitance of D602 to create a large impedance.
2.4.3.6 Harmonic Filter
RF from the power amplifier is routed through the coupler (U101), passed through the antenna
switch, passed through the remote port switch, and applied to a harmonic filtering network. The SSE
5000 harmonic filters are five-pole elliptical low-pass filters.