Liquid Crystal Display (Lcd); Microphone Amplifier; Speaker; Connections To The Radio - Motorola GM-950 Manual

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