Motorola GM950E Service Manual page 56

Power AmpliÞer (PA) 5-25W.
8.3 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.
9.0 Power AmpliÞer (PA) 5-25W.
The radioÕs 5-25 W PA is a four stage ampliÞer used to amplify the output from the exciter to the
radio transmit level. It consists of four stages in the line-up. The Þrst (Q5510) is a bipolar stage that is
controlled via the PA control line. It is followed by another bipolar stage (Q5520), a MOS FET stage
(Q5530) and a Þnal bipolar stage (Q5536).
Devices Q5510 and Q5520 are surface mounted. MOS FET Q5530 and bipolar Transistor Q5536
are directly attached to the heat sink.
9.1 Power Controlled Stage
The Þrst stage (Q5510) ampliÞes the RF signal from the VCO (line EXCITER PA) and controls the
output power of the PA. The output power of the transistor Q5510 is proportional to its collector
current which is adjusted by a voltage controlled current source consisting of Q5612 and Q5621.
The whole stage operates off the K9V1 source which is 9.1V in transmit mode and nearly 0V in
receive mode.
The collector current of Q5510 causes a voltage drop across the resistors R5623 and R5624.
Transistor Q5612 adjusts the voltage drop across R5621 through PA control line (PWR CNTL). The
current source Q5621 adjusts the collector current of Q5510 by modifying its base voltage until the
voltage drop across R5623 and R5624 plus VBE (0.6V) equals the voltage drop across R5621 plus
VBE (0.6V) of Q5611. If the voltage of PWR CNTL is raised, the base voltage of Q5612 will also rise
causing more current to ßow to the collector of Q5612 and a higher voltage drop across R5621. This
in turn results in more current driven into the base of Q5510 by Q5621 so that the current of Q5510
is increased. The collector current settles when the voltage over the series conÞguration of R5623
and R5624 plus VBE of Q5621 equals the voltage over R5621 plus VBE (0.6V) of Q5611.
4-18 Theory of Operation