Transmitter Detailed Functional Description; 15 Watt Pa - Motorola M11URD6CB1_N Service Manual

GTX LTR/Privacy Plus 800 MHz Mobile Service Manual
The IF signal routed to BBR IC is amplified, converted
to 455 kHz, filtered, limited and demodulated.
Demodulated audio comes out of BBR IC at U5201-28
and is fed to the ASFIC, which is part of the radio con-
troller section.
In addition to the audio output signal, the receiver sec-
tion provides an RSSI (Receiver Signal Strength Indica-
tor) at U5201-11. The RSSI signal is fed to the ASFIC
and accessory connector.
ADAPT signal into U5201-22 controls the BBR IC. The
normally low (0 VDC) ADAPT signal is high (5 VDC)
during change of radio channels, transition from trans-
mit to receive or at turn-on. The ADAPT signal is used
t o c o n t r o l p r e c h a r g e o f c a p a c i t o r C 5 2 3 1 a t
DEMOD_OUT. This is done in order to eliminate the
transient during frequency change or when the radio
goes from transmit to receive mode.
Transmitter Detailed Functional
Description

15 Watt PA

The 15-W PA is a three-stage radio frequency (RF)
power amplifier used to amplify the output from the
TX injection lineup to the radio transmit level. It con-
sists of driver stage Q6501 followed by a two-stage
power module U6501.
The two stages of the power module operate from the
A+ supply voltage. The module is switched on/off by
the K9.1 line through switch U6502 (which connects
the A+ line to pin 2 of U6501 when transmit is enabled).
The RF drive, which is routed into transistor Q6501, is
controlled from Q6506 via the PA control line. The ris-
ing control voltage on the PA control line causes a ris-
ing collector voltage on Q6501. This causes more
power out of the stage. Conversely, a decreasing con-
trol line voltage decreases the power delivered into the
next stage. By controlling the drive power to U6501
and the following stages in the power amplifier lineup,
ALC loop is able to regulate the output power of the
transmitter.
The antenna switch is switched synchronously with
the keyed 9.1 voltage. In the transmit mode, this 9.1
voltage is high—turning on diodes CR6502, CR6503
and CR6504. When CR6502 is turned on, it forms a low
impedance to the RF transmit path and allows the sig-
nal to pass through. Diodes CR6503 and CR6504 short
the ends of quarter wavelength lines to ground, pre-
senting a high impedance towards the receiver. In this
way no power is delivered into the receiver, the trans-
mit path remains undisturbed, and the receiver is pro-
tected during transmission.
In the receive mode, all these diodes are off. The off
capacitance of CR6502 is tuned by L6508 to form a high
June, 2000
impedance looking into the transmitter. Therefore,
energy coming in the receive mode is channeled to the
RX port.
Harmonics of the transmitter are attenuated by the har-
monic filter. The harmonic filter is formed by inductors
L6513 and L6518 as well as capacitors C6540-C6542,
C6551-C6553. This network forms a low-pass filter to
attenuate harmonic energy of the transmitter to speci-
fications level.
A forward-power detector follows the harmonic filter.
This forward-power detector is a microstrip printed
circuit, which couples a small amount of the forward
energy and to diode CR6506 where it is rectified. This
rectified signal (VFORWARD) forms the forward volt-
age which the power control circuit holds constant.
Holding this voltage constant (which is proportional to
the RF rectified energy appearing across the diode)
ensures the forward-power out of the radio is held to a
constant value.
Thermistor R6519 senses temperature in the area of the
power module. This signal is fed back into the power
control circuit to protect the power amplifier against
over-temperature conditions.
Resistor R6520, in series with the A+ line supply, feeds
voltage to the power module. The voltage across R6520
is monitored and the difference voltage is channeled to
the power control circuit. The power control circuit
monitors the voltage drop across the resistor which is
determined by the magnitude of the drain current in
U6501. It uses this as a limiting mechanism whereby
the power control circuit limits the magnitude of cur-
rent that can be drawn by U6501. This protects the
device from over dissipation.
Bias voltage reverse polarity protection for the transmit-
ter is provided by diode CR6508. Under reverse polarity
conditions to the radio, this diode conducts and protects
the radio from damage. This diode also provides tran-
sient over-voltage protection by breaking down when
the supply voltage to the radio exceeds 24 V.
35 Watt PA
The 35-Watt PA is a four stage radio frequency (RF)
power amplifier used to amplify the output from the TX
injection lineup to the radio transmit level. It consists of
driver stage Q6501, two-stage power module U6501 and
RF final device transistor Q6505.
Q6505 and U6501 operate from the A+ supply voltage.
U6501 is switched on/off by the K9.1 line through
switch U6502 (which connects the A+ line to pin 2 of
U6501 when transmit is enabled). The RF drive, which is
routed into transistor Q6501, is controlled from Q6506
via the PA control line. The rising control voltage on the
PA control line causes a rising collector voltage on
Q6501. This causes more power out of the stage. Con-
versely, a decreasing control line voltage decreases the
6880905Z99-O
Theory of Operation

Transmitter Detailed Functional Description

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