Motorola GM328 Service Manual page 105

Controller Board Audio and Signalling Circuits
The ASFIC has an internal AGC that can control the gain in the mic audio path. The AGC can be
disabled / enabled by the µP. Another feature that can be enabled or disabled in the ASFIC is the
VOX. This circuit, along with the capacitor at U0221-7, provides a DC voltage that can allow the µP
to detect microphone audio. The ASFIC can also be programmed to route the microphone audio to
the speaker for public address operation.
2.2.2 PTT Sensing and TX Audio Processing
Microphone PTT coming from the controlhead is sent via SBEP bus to the microprocessor. An
external PTT can be generated by grounding pin 3 on the accessory connector if this input is
programmed for PTT by the CPS. When microphone PTT is sensed, the µP will always configure
the ASFIC CMP for the "internal" mic audio path, and external PTT will result in the external mic
audio path being selected.
Inside the ASFIC CMP, the mic audio is filtered to eliminate frequency components outside the 300-
3000Hz voice band, and pre-emphasized if pre-emphasis is enabled. 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 an
attenuator, which is tuned in the factory or the field to set the proper amount of FM deviation. The TX
audio emerges from the ASFIC CMP at U0221-40 MOD IN, at which point it is routed to the RF
section.
Dependent on the radio model, input pin 3 on the accessory connector can be programmed for
DATA PTT by the CPS. When DATA PTT is sensed, the µP will always configure the ASFIC CMP for
the flat TX audio path. Limiter and any filtering will be disabled. The signal is routed directly to the
attenuator, which sets the FM deviation according to the channel spacing of the active transmit
channel and emerges from the ASFIC CMP at U0221-40, at which point it is routed to the RF
section.
2.2.3 TX Secure Audio (optional)
The audio follows the normal transmit audio processing until it emerges from the ASFIC CMP TX
SND pin (U0221-44), which is fed to the Secure board residing at option connector J0551-33. The
Secure board contains circuitry to amplify, encrypt, and filter the audio. The encrypted signal is then
fed back from J0551-32 to the ASFIC CMP TX RTN input (U0221-36). The signal level at this pin
should be about 65mVrms. The signal is then routed through the TX path in the ASFIC CMP and
emerges at MOD IN pin 40.
2.2.4 Option Board Transmit Audio
The audio follows the normal transmit audio processing until it emerges from the ASFIC CMP TX
SND pin (U0221-44), which is fed to the option board residing at option connector J0551-33. The
option board contains circuitry to process the audio. The processed signal is then fed back from
J0551-32 to the ASFIC CMP TX RTN input (U0221-36). The signal level at this pin should be about
65mVrms. The signal is then routed through the TX path in the ASFIC CMP and emerges at MOD IN
pin 40.
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