Motorola GM950E, Руководство по обслуживанию

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Power AmpliÞer (PA) 5-25W
Theory of Operation 4-25
11.3 IF IC (U5201)
The Þrst IF signal from the crystal Þlters feeds the IF IC (U5201) at pin 6. Within the IF IC the
45.1MHz Þrst IF signal mixes with the second local oscillator (LO) at 44.645MHz to the second IF at
455 kHz. The second LO uses the external crystal Y5211. The second IF signal is ampliÞed and
Þltered by two external ceramic Þlters (FL5201, FL5202). Back in the IF IC the signal is demodulated
in a phase-lock detector and fed from IF IC pin 28 to the audio processing circuit AFIC U0103
located in the controller section (line DET AUDIO).
The IF IC also controls the squelch characteristics of the radio. With a few external parts (R5222,
C5229, C5230, R5223) the squelch tail, hysteresis, attack and delay were optimized for the radio. To
set the squelch threshold the signal from IF IC pin 23 (line SQ ATT IN) is attenuated by a
microprocessor controlled audio processing IC AFIC (U0103) located in the controller section. The
attenuated signal from the AFIC (line SQ ATT OUT) enters the IF IC at pin 20 and is used to create
a squelch indicator signal available at pin 15 (CSQ DET).
The microprocessor controlled ADAPT signal at pin 22 activates the fast squelch indicator signal at
IF IC pin 18 (FAST SQ). Both squelch indicator signals CSQ DET (pin 15) and FAST SQ (pin 18) are
combined, weighted by R0111 / R0112 and fed to the microprocessor U0101 pin 15 for
interpretation. From the voltage weighted by the resistors the µP determines whether CSQ DET,
FAST SQ or both are active.
At IF IC pin 11 an RSSI signal is available with a dynamic range of 70 dB. The RSSI signal is
buffered by op-amp U0106-3 and available at accessory connector J0400-15.
12.0 Power AmpliÞer (PA) 5-25W
The radioÕs 5-25 W PA is a three stage ampliÞer used to amplify the output from the exciter to the
radio transmit level. It consists of three stages in the line-up. The Þrst (Q3511) is a bipolar stage that
is controlled via the PA control line. It is followed a MOS FET stage (Q3521) and a Þnal bipolar stage
(Q3531).
Devices Q3511 and Q3521 are surface mounted. Bipolar Transistor Q3531 is directly attached to the
heat sink.
12.1 Power Controlled Stage
The Þrst stage (Q3511) ampliÞes the RF signal from the VCO (line EXCITER PA) and controls the
output power of the PA. The output power of the transistor Q3511 is proportional to its collector
current which is adjusted by a voltage controlled current source consisting of Q3641 and Q3642.
The current of the whole stage is drawn from the RX-TX Switch through coil L3652.
The collector current of Q3511 causes a voltage drop across the resistors R3645 and R3646.
Transistor Q3641 adjusts the voltage drop across R3644 through PA control line (PWR CNTL). The
current source Q3642 adjusts the collector current of Q3511 by modifying its base voltage until the
voltage drop across R3645 and R3646 plus VBE (0.6V) equals the voltage drop across R3644. If the
voltage of PWR CNTL is raised, the base voltage of Q3641 will also rise causing more current to
ßow to the collector of Q3641 and a higher voltage drop across R3644. This in turn results in more
current driven into the base of Q3511 by Q3642 so that the current of Q3511 is increased. The
collector current settles when the voltage over the series conÞguration of R3645 and R3646 plus
VBE of Q3642 equals the voltage over R3644. By controlling the output power of Q3511 and in turn
the input power of the following stages the ALC loop is able to regulate the output power of the
transmitter.
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