Motorola RADIUS GM300 Service-Handbuch - Seite 27

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Theory of Operation

RF Board

Range

403-433

438-470

1 dB BW:

35 MHz

45 MHz

3 dB BW:

44 MHz

60 MHz

Loss:

4.2 dB

418 MHz

455 MHz

CENTER

Image Rej:

55 dB

At f

343 MHz

380 MHz

IMAGE

Again, image attenuation increases for frequencies

lower than f

IMAGE

A pin diode attenuator is located between the 4 pole Þl-

ter and the Þrst mixer. In the Distance mode, Q5 and Q6

are turned on, CR5 is forward-biased which bypasses

R12, and no loss is introduced. In the Local mode, Q5,

Q6 and CR5 are off, inserting 10 dB of attenuation due

to R12. Because the attenuator is located after the RF

ampliÞer, receiver sensitivity is reduced only by 5 dB,

while the overall third order input intercept is raised

by 15 dB. Thus, the Local mode signiÞcantly reduces

the susceptibility to IM-related interference.

The Þrst mixer is a passive, double-balanced type con-

sisting of T1, T2 and U1. This mixer provides all of the

necessary rejection of the half-IF spurious response,

since the improvement due to Þlter selectivity is negli-

gible at 470 MHz. Low-side injection is delivered to the

Þrst mixer from the second RX buffer in the synthesizer

circuit. The injection level is +11.6 dBm for 403-433

range, +5 dBm for the 438-470 MHz range, and

+6.5 dBm for the 465-495 and 490-520 MHz ranges.

The mixer output is connected to a diplexer network

which matches its output to the Þrst IF ampliÞer Q51 at

the IF frequency of 45.1 MHz, and terminates it in a

50 ohm resistor, R51, at all other frequencies.

Receiver Back End

Q51 ampliÞes the IF signal by approximately 17 dB.

The output of Q51 is Þltered by a four pole crystal Þlter

(comprised of two matched units Y51A and Y51B). The

3 dB bandwidth of the crystal Þlter is 14 kHz for 20/

25 kHz channel spacing models, and 8 kHz for

12.5 kHz channel spacing models. The signal is ampli-

Þed 18 dB by the second IF ampliÞer Q52, and applied

to the input of the receiver system IC U51-19 (see

Figure 2-1).

The 45.1 MHz Þrst IF signal is applied to the second

mixer section of U51. A 44.645 MHz crystal oscillator

provides the low side injection signal, which is also

applied to U51-19. The output of the second mixer is a

455 kHz second IF signal which is Þltered by ceramic

Þlter FL51, ampliÞed, Þltered by ceramic Þlter FL52,

and applied to the audio detector. As with the crystal

Þlter, the bandwidth of the ceramic Þlters are narrower

for 12.5 kHz channel spacing models than for 20/

25 kHz. The IF test point is located at the output of Þl-

ter FL52. The level of the 455 kHz signal at this test

2-2

465-495

490-520

40 MHz

60 MHz

55 MHz

3.7 dB

3.8 dB

480 MHz

510 MHz

40 dB

52 dB

54 dB

405 MHz

430 MHz

point is linearly related to the input signal level at the

antenna, allowing a convenient metering point for Þl-

ter tuning and gain measurements.

The audio detector is a peak-differential type, with the

necessary phase shift (90û at 455 KHz) provided by L-C

network L61. Recovered audio from U51-5 is routed to

the receiver audio circuitry on the logic board, and to

the squelch circuitry contained in U51. When an on-

channel signal is present, the amount of high-fre-

quency audio noise at the detector output is reduced.

This change in noise level is sensed to indicate the pres-

ence of an on-channel signal. Audio noise at U51-5 is

applied to U51-7 via R61 and C82. Extreme high-fre-

quency noise is removed by C81. The noise is ampliÞed

and appears at U51-6 where it is coupled via C80 to the

SQUELCH adjust pot R60. The output of this control is

applied via R59, C79 and C78 to pin 8 which is the

input of a limiting ampliÞer and threshold detector.

Noise levels greater than a preset threshold cause U51-

11 to go high. This is buffered by Q53 and routed to the

microcomputer circuitry on the logic board. This corre-

sponds to no channel activity. When the noise level

decreases below the threshold, due to on-channel qui-

eting, U51-11 goes low, providing a low to the logic cir-

cuitry. This implies an on-channel signal is present,

causing unmuting of the receive audio path by the

microcomputer.

Components R58, C75 and C76 determine squelch time

constants as a function of the charging currents sup-

plied by U51. These charging currents vary from weak

to strong signal conditions, providing a variable

squelch closing time-constant. For weak signals the

time constant is long to minimize ÒchatteringÓ or rapid

muting and unmuting of the audio. For strong signals,

where the carrier-absent to carrier-present conditions

are substantial, the closing time-constant is shortened

to minimize the length of the Òsquelch-tailÓ.

Frequency Synthesizer

To determine the synthesizerÕs operating frequency,

the microcomputer loads the frequency divider infor-

mation into the PLL IC U101 (see Figure 2-2). The PLL

IC contains three programmable dividers. The pro-

gram is serially loaded via the SR DATA line to U101-

10. The data is loaded one bit at a time, with each low-

to-high transition of the SR CLOCK at U101-11 latching

data from shift registers into the reference divider (R),

divide-by-N, or divide-by-A latches depending on the

control bit. A logic high of the control bit selects the ref-

erence counter latch, while a logic low selects the

divide-by-N, or divide-by-A counter latch.

After the microcomputer loads data into the PLL IC,

the SYNTH LATCH ENABLE line goes low. The syn-

thesizer is then ready to generate a receive Þrst-injec-

tion or transmit frequency.

68 No.

Name of Manual

March, 1997