Motorola RADIUS GM300 Service Manual - Page 36

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impedance of CR2251 appears as a high impedance at

the junction of CR2250 and the harmonic Þlter input.

The harmonic Þlter is a nine pole 0.1 dB ripple Cheby-

chev low pass Þlter with a 3 dB frequency of approxi-

mately 205 MHz and less than 1 dB insertion loss in the

passband.

10-25 Watt VHF Power Ampliﬁer

The 10-25 watt VHF power ampliÞer is designed to

cover the range of 146-174 MHz. It consists of three

stages. The Þrst stage, Q2310, operates in Class A with

base bias supplied by the 9.6T source. The collector

voltage is supplied from controlled B+. The output

level of this stage is varied by changes in the controlled

B+ voltage.

The second stage, Q2320, is operated in Class C. Collec-

tor voltage is supplied by UNSW B+. This stage deliv-

ers up to 3 watts output.

The third stage, Q2330, is the Class C RF Þnal power

ampliÞer. It can output in excess of 30 watts with

3 watts input power. Collector current is monitored by

the power control circuit by measuring the voltage

drop across a .05 ohm resistor, R2371, placed in series

with the collector DC supply lead.

The antenna switch consists of two pin diodes, CR2350

and CR2351. L2353 is parallel-resonant with the off

capacitance of CR2350, increasing its isolation when

turned off. C2354 is a DC block. L2352 and C2352, com-

bined with the ÒonÓ inductance of CR2351, form a

series resonant circuit to lower the shunt impedance

presented by CR2351 when it is turned on.

In the receive mode, both diodes are off. Signals

applied at the antenna jack J1 are routed, via the har-

monic Þlter, through network L2351, C2351 and C2353

to the receiver input. In the transmit mode, 9.6T is

present and both diodes are forward-biased into con-

duction. The transmitter RF from Q2330 is routed

through CR2350, and via the harmonic Þlter to the

antenna jack. CR2351 conducts, shunting RF power

and preventing it from reaching the receiver. L2351 is

selected to appear as a 1/4 wave at VHF, so that the low

impedance of CR2351 appears as a high impedance at

the junction of CR2350 and the harmonic Þlter input.

The harmonic Þlter is a nine pole 0.1 dB ripple Cheby-

chev low pass Þlter with a 3 dB frequency of approxi-

mately 205 MHz and less than 1 dB insertion loss in the

passband.

25-45 Watt VHF Power Ampliﬁer

The 25-45 watt VHF power ampliÞer is designed to

cover the range of 146-174 MHz. It consists of four

stages. The Þrst stage, Q2410, operates in Class A with

its operating voltage supplied by the 9.6T source. It

March, 1997

provides 11 dB of gain and has an output power of

250 mW.

The second stage, Q2320, is operated in Class C. The

collector voltage is supplied from controlled B+. The

output level of this stage is varied by changes in the

controlled B+ voltage. It has a gain of 9.1 dB and pro-

vides up to 2 watts of output power.

The third stage, Q2430, also operates in Class C. The

collector voltage is supplied directly from the UNSW

B+ source. This stage has a gain of 8.8 dB and an output

power of 15 watts. Feedback is used to control the gain

and maintain stability.

The fourth stage, Q2440, is the Class C RF Þnal power

ampliÞer. It has a gain of 6 dB and can output in excess

of 60 watts with 15 watts of input power. The power

control circuit monitors this stageÕs collector current by

measuring the voltage drop across a .01 ohm resistor,

R2471, placed in series with the collector DC supply

lead.

The antenna switch consists of two pin diodes, CR2450

and CR2451. L2453 and C2453, combined with the

internal ÒonÓ inductance of CR2451, form a series reso-

nant circuit to lower the shunt impedance presented by

CR2451 when it is conducting.

In the receive mode, both diodes are off. Signals

applied at the antenna jack J1 are routed, via the har-

monic Þlter, through network L2452, C2454 and C2455

to the receiver input. In the transmit mode, 9.6T is

present and both diodes are forward-biased into con-

duction. The transmitter RF from Q2440 is routed

through CR2450, and via the harmonic Þlter to the

antenna jack. CR2451 conducts, shunting RF power

and preventing it from reaching the receiver. L2452 is

selected to appear as a 1/4 wave at VHF, so that the low

impedance of CR2451 appears as a high impedance at

the junction of CR2450 and the harmonic Þlter input.

The harmonic Þlter is a seven pole 0.1 dB ripple Cheby-

chev low pass Þlter with a 3 dB frequency of approxi-

mately 200 MHz and a typical insertion loss of 0.8 dB

in the passband.

1-10 Watt UHF Power Ampliﬁer

The 1-10 watt UHF power ampliÞer is designed to

cover the range of 438-470 MHz and has three stages.

The Þrst stage, Q2510, operates in Class A. Its operat-

ing voltage is supplied from 9.6T. This stage provides

9 dB of gain and an output of 200 milliwatts.

The second stage, Q2520, has a nominal gain of 10 dB

and power output of up to 2 watts. The output of this

stage is adjusted by varying the controlled B+ which

supplies DC operating voltage to its collector. This

stage operates in Class C.

68 No.

Theory of Operation

RF Power Amplifiers

2-11