Motorola GM950, Manual

Page: 35 / 92

Transmit Audio Circuits
Theory of Operation 5C.3-11
4.2 External Mic Path
The external microphone signal enters the radio on accessory connector J0400 pin 2 and is routed
to the ASFIC (U0201-A7) through resistor R0414, capacitor C0413 and line EXT MIC, with DC bias
provided by R0415 / R0416. R0415 and C0417 provide a 1k
Ω
AC path to ground that sets the input
impedance for the microphone and determines the gain based on the emitter resistor in the
microphone’s amplifier circuit. Filter capacitor C0414 provides low-pass filtering to eliminate
frequency components above 3 kHz, and C0413 serves as a DC blocking capacitor.
4.3 PTT Sensing and TX Audio Processing
Mic PTT coming from the Control Head via connector J101-4 is sensed by the
µ
P U0101 pin 22. An
external PTT can be generated by grounding pin 3 on the accessory connector. When microphone
PTT is sensed, the
µ
P will always configure the ASFIC for the ”internal” mic audio path, and external
PTT will result in the external mic audio path being selected for models with separated MIC and EXT
MIC signals.
Inside the ASFIC, the MIC audio is filtered to eliminate frequency components outside the 300-
3000Hz voice band, pre-emphasized if pre-emphasis is enabled. The capacitor between ASFIC pre-
emphasis out U0201-C8 and ASFIC limiter in U0201-E8 AC couples the signal between ASFIC
blocks and prevents the DC bias at the ASFIC output U0201-H8 from shifting when the ASFIC
transmit circuits are powered up. The signal is then limited to prevent the transmitter from over
deviating. The limited MIC audio is then routed through a summer which, is used to add in signalling
data, and then to a splatter filter to eliminate high frequency spectral components that could be
generated by the limiter. The audio is then routed to two attenuators, which are tuned in the factory
or the field to set the proper amount of FM deviation. The TX audio emerges from the ASFIC at
U0201-H8 MOD IN, at which point it is routed to the RF section.
4.4 TX Secure Audio (optional)
The audio follows the normal transmit audio processing until it emerges from the ASFIC MIC AMP
OUT pin (U0201-A6), which is fed to the Secure board residing at option connector J0103-3. The
Secure board contains circuitry to amplify, encrypt, and filter the audio. The encrypted signal is then
fed back from J0103-1 to the ASFIC TX IN input (U0201-C7). The signal level at this pin should be
about 80mVrms. The signal is then routed through the TX path in the ASFIC and emerges at VCO
ATN pin H8.