Motorola GM950 Manual - Page 10

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Theory of Operation

3.9

Liquid Crystal Display (LCD)

The LCD display H0931 is a 3 digit / 7 segment display which incorporates 11 annunciators. Data is

loaded serially into the display driver U0932 via the SPI interface. The display data of line SPI DATA

BUF is shifted serially by clock signal LCD CLCK BUF. When the last bit has been received, the LCD

display is updated.

3.10

Microphone Ampliﬁer

The microphone cartridge contains a capacitive transducer coupled to a FET ampliﬁer. The

microphone's audio signal is fed from the microphone inputs, MIC+ and MIC-, at J0903 to the

microphone ampliﬁer. The microphone ampliﬁer sets the frequency response and ampliﬁes the

audio signal to the required radio input level (about 80mV at nominal sound pressure level). The ﬁrst

stage (Q0902) connected to the microphone is an active high-pass ﬁlter which suppresses

frequency components below 300 Hz to prevent PL falsing in a receiving radio. The collector current

of Q0902, drawn from the collector of the following stage Q0901, is approximately 0.27 mA. A low-

pass ﬁlter composed of R0904 and C0907, C0908 ﬁlters out any audio frequency components riding

on this voltage. The output of the ﬁrst stage is fed via R0906 to the second stage (Q0901) which

ampliﬁes the audio signal to the radio's input level. The supply voltage of the stage is provided by the

radio's 9V3 voltage regulator via the load resistor (1kΩ) located on the radio's controller section, line

MIC and connector J0901 pin 4. Q0901 draws a collector current of 2 mA. The collector AC current

of Q0901 causes a voltage drop across the load resistor representing the audio signal from the

microphone.

3.11

Speaker

The HHCM contains a speaker for the receiver audio. The receiver audio signal from the differential

audio output of the audio ampliﬁer located on the radio's controller is fed via lines INT SPKR+, INT

SPKR-, connector J0901-12,10 and parallel resistors R0921 - R0923 to the speaker connector

J0902 pin 1 and pin 2. The speaker is connected to the connector J0902. Resistors R0921 - R0923

reduce the audio power applied to the speaker to prevent overloading.

3.12

Connections to the Radio

Interconnections between the HHCM and the host radio are via an expanding cable which is

permanently connected to the HHCM. The cable has an 18 pin Molex connector which connects to

connector P0951 in the blank head of the host radio. P0951 is mounted on an adapter PCB. After

additional ﬁltering the signals are fed to the host radio's main PCB via connector J0950, also located

on the adapter PCB, a ﬂexible ribbon cable and connector J0101. The connections between the

PCB in the HHCM and the main PCB in the radio are shown in the following table:

4C.1-4

Introduction/Theory of Operation