Motorola RADIUS GM300 서비스 매뉴얼 - 페이지 34

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Name of Manual

Data Path

Receiver audio is also used for decoding of signaling

tones or codes. There are two general classiÞcations of

the types of data involved. High-speed data refers to

data which occupies the voice range of frequencies

(between 300 and 3000 Hz, although it can also occupy

the entire range from less than 1 Hz to 3000 Hz). This

data is not transmitted simultaneously with voice. The

other type is low-speed data, which is contained to fre-

quencies below 300 Hz, generally at reduced deviation

levels, and may be transmitted simultaneously with

voice. MDC1200 and Select 5 are examples of high-

speed data; PL and DPL are examples of low- speed

data.

Low speed data is obtained from U551B-7, which con-

tains frequencies between 0.6 Hz and 3000 Hz. It is

buffered by U602A and Þltered by U603A and U603B

in masked logic boards, or U602B and U603A in

expanded logic boards. These form a 5-pole biqua-

dratic low pass Þlter, attenuating all signals above

250 Hz. This Þlter prevents voice from interfering with

low-speed data detection. This is fed to a center-slicer

(U601B in masked, U603B in expanded) which con-

verts the tone or data into a symmetrical square wave

which can be decoded by the microcomputer. The cen-

ter slicer tracks out near-DC offsets to maintain wave-

form symmetry. In expanded boards, U603B is an

opamp rather than a comparator, and transistor Q601

is used to provide level shifting to a 0-to-5V logic level.

High speed data can be obtained from either U551B-7,

the same point as for low-speed data, or from U552B-7,

which only contains frequencies between 300 and 3000

Hz. If the type of data does not contain information

below 300 Hz, the use of additional 300 Hz high-pass

Þltering improves detector sensitivity. No additional

Þltering is required, only that the signal is squared up

by center-slicer U601A before being applied to the

microcomputer for decoding.

Transmitter Audio

Voice Path

Microphone audio is routed from the front panel mic

jack via J8- 14, or from accessory connector J3-2, to the

mic mute gate Q651. DC bias is applied to operate the

transistor ampliÞer within the micro-phone. Q651 is

controlled by Q652 and mutes the microphone audio

path during receive mode or during transmission of

high- speed signaling or data. When low mic gain (80

mV rms for 60% deviation) is selected by JU651, the

signal is pre-emphasized by C652 and R655; for high-

gain (40 mV rms), by C667 and R647. Supply rail limit-

ing by U651A prevents over-deviation.

The signal is then routed to the RF board reference

oscillator modulation input via J6-13, and, via buffer

U652A, to voltage controlled attenuator U652. Devia-

March, 1997

tion is adjusted by programming the DC output volt-

age of D/A converter U801-6. This allows electronic

adjustment of deviation, compensation for sensitivity

variations across the operating frequency band, and

choice of full system voice deviation on non-PL or DPL

channels, or reduced voice deviation on channels with

PL or DPL. The output of attenuator U653 is routed

through U652B, a 3-pole biquadratic 3000 Hz low-pass

ÒsplatterÓ Þlter, before being applied to the RF board as

VCO modulation via J6-10.

Radios used in 12.5 kHz channel spacing systems

require full deviation of 2.5 kHz, instead of 5 kHz for

25/30 kHz systems. This reduction is accomplished by

resistor values on the RF board surrounding the VCO

MOD and REF MOD adjust posts.

Data Path

High speed data is generated by microcomputer out-

puts U802-30 and 31 which are summed by opamp

U602B (masked) or U701A (expanded). The ratio of

R701 and R702, and the relative timing of the transi-

tions of the two microcomputer outputs, allow low-

distortion simulated sinewaves to be generated. The

opamp output is routed to the input of limiter U651A,

where it is summed with pre-emphasized mic audio.

The audio path from U651A to the limiter is not pre-

emphasized. The remainder of the audio path to the

modulator is the same as for the voice signal path.

In masked logic boards, audio applied to accessory

connector J3-5 is applied, without pre-emphasis, to the

input of the limiter U651A. In expanded logic boards,

jumper JU701 allows either this path to be chosen

(position ÒAÓ), or allows J3-5 audio to be routed via

low-speed data Þlter U701B (JU701 in position ÒBÓ)

and introduced into the transmitter audio path after

the limiter. In this conÞguration, externally-generated

PL may be applied to the transmitter without being

ÒcapturedÓ (attenuated or removed entirely) when lim-

iting due to voice peaks occurs.

Low speed data is generated by microcomputer out-

puts U802-27 and 28 which are summed by opamp

U651B (masked) or U701B (expanded). The ratio of

R706 and R707 allow low-distortion simulated sine-

waves to be generated. The opamp is conÞgured as a 3-

pole biquadratic low pass Þlter to remove any distor-

tion components of the low speed data signal above

300 Hz which would fall within the voice range. This

audio is resistively summed with the output of the lim-

iter U651A. In this manner, limiting on voice peaks will

not affect the continuous low speed data transmission.

The summed audio is then routed to the REF MOD

input of the RF board via J6-13, and to the voltage con-

trolled attenuator, splatter Þlter, and VCO MOD input

of the RF board (J6-10), following the same path as

voice signals.

68 No.

Theory of Operation

Logic Board

2-9