Motorola RADIUS GM300 Servis Kılavuzu - Sayfa 37

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Theory of Operation

RF Power Amplifiers

The third stage, Q2530, is the Class C RF Þnal power

ampliÞer. It provides 8.75 dB of gain and an output

power of up to 15 watts. Collector voltage is supplied

from UNSW B+ via a 0.2 ohm resistor, R2570. By mea-

suring the voltage drop across this resistor, the power

control circuit compensates for variations in Q2530Õs

collector current.

The antenna switch consists of two pin diodes, CR2550

and CR2551. L2553 is parallel-resonant with the off

capacitance of CR2550, increasing the diodeÕs isolation

when turned off. C2555 is a DC block. L2552 and

C2551, combined with the ÒonÓ inductance of CR2551,

form a series resonant circuit to lower the shunt imped-

ance presented by CR2551 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 L2551, C2552 and C2553

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 Q2530 is routed

through CR2550, and via the harmonic Þlter to the

antenna jack. CR2551 conducts, shunting RF power

and preventing it from reaching the receiver. L2551 is

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

low impedance of CR2551 appears as a high imped-

ance at the junction of CR2550 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 700 MHz and less than 1 dB insertion loss.

10-25 Watt UHF Power Ampliﬁer

The 10-25 watt UHF power ampliÞer is designed to

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

The Þrst stage, Q2610, operates in Class A with base

bias from 9.6T. It provides 11.8 dB of gain and an out-

put of 300 milliwatts.

The second stage, Q2620, has a nominal gain of 8.2 dB

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

stage is adjusted by the controlled B+ voltage which

supplies its collector.

The third stage, Q2630, operates in Class C with 8.1 dB

gain and a power output of up to 13 watts. Collector

voltage is directly from UNSW B+.

The fourth stage, Q2640, is the Þnal RF power ampli-

Þer, which operates Class C directly from UNSW B+. It

provides up to 30 watts output. Collector current is

monitored by the power control circuit by measuring

the voltage drop across a .05 ohm resistor, R2670, in

series with the collector DC supply lead.

The antenna switch consists of two pin diodes, CR2650

and CR2651. L2652 and C2651-2, combined with the

ÒonÓ inductance of CR2651, form a series resonant cir-

cuit to lower the shunt impedance presented by

2-12

CR2651 when it is turned on. In the receive mode, both

diodes are off. Signals applied at the antenna jack J1 are

routed, via the harmonic Þlter, through network L2651,

C2652 and C2653 to the receiver input. In the transmit

mode, 9.6T is present and both diodes are forward-

biased into conduction. The transmitter RF from Q2640

is routed through CR2650, and via the harmonic Þlter

to the antenna jack. CR2651 conducts, shunting RF

power and preventing it from reaching the receiver.

L2651 is selected to appear as a 1/4 wave at UHF, so

that the low impedance of CR2651 appears as a high

impedance at the junction of CR2650 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 500 MHz and less than 1 dB insertion loss in the

passband.

25-35/40 Watt UHF Power Ampliﬁers

There are four high-power UHF ampliÞers, which pro-

vide 25-40 watts output in the 403-433, 438-470 or 465-

490 MHz bands, or 25-35 watts output in the 490-520

MHz band. All have four stages and are similar in cir-

cuitry and layout.

The Þrst stage, Q2710, operates in Class A with operat-

ing voltage obtained from 9.6T. The second stage,

Q2720, is a gain- controlled Class C stage, the output

power of which is adjusted by varying the controlled

B+ collector DC supply. The third stage, Q2730, oper-

ates in Class C with collector voltage obtained directly

from UNSW B+. The fourth stage, Q2740, is the Þnal RF

power ampliÞer, which operates Class C directly from

UNSW B+. SpeciÞc operating parameters are:

Range

403-433

Stage 1

Gain:

11.8 dB

300 mW

OUT

Stage 2

Gain:

8.2 dB

2 W

OUT

Stage 3

Gain:

8.1 dB

13 W

OUT

Stage 4

Gain:

5.9 dB

50 W

OUT

Collector current of the Þnal stage is monitored by the

power control circuit by measuring the voltage drop

across a .01 ohm resistor, R2770, in series with the col-

lector DC supply lead.

The antenna switch consists of two pin diodes, CR2750

and CR2751. L2753, when used, is parallel-resonant

with the off capacitance of CR2750, increasing the

diodeÕs isolation when turned off. C2755 is a DC block,

present if L2753 is used. L2752 and C2750-1, combined

68 No.

Name of Manual

438-470

465-495

490-520

11.8 dB

10 dB

300 mW

200 mW

8.2 dB

9.5 dB

2 W

1.8 W

8.1 dB

8.5 dB

12.9 W

13 W

6.1 dB

6.3 dB

5.7 dB

52.8 W

55 W

48 W

March, 1997