Motorola 6864115B62-C Detailed Service Manual page 91

Controller Board Audio and Signalling Circuits
C0254 serves as a DC blocking capacitor. Multi switch U0251 controlled by ASFIC CMP port GCB4
selects either the external microphone input signal or the voice storage playback signal for entering
the ASFIC CMP at pin 48. The audio signal at U0221-48 (TP0222) should be approximately 14mV
for 1.5kHz or 3kHz of deviation with 12.5kHz or 25kHz channel spacing.
The FLAT TX AUDIO path is used for transmitting data signals and has therefore no limiter or filters
enabled inside the ASFIC CMP. When this path is enabled via CPS and DATA PTT is asserted, any
signal on this path is directly fed to the modulator. Signals applied to this path either via accessory
connector J0501, expansion board connector J0451 or option board connector J0551 must be
filtered and set to the correct level externally or on the option board in order not to exceed the
maximum specified transmit deviation and transmitted power in the adjacent channels. The
attenuator inside the ASFIC CMP changes the FM deviation of the data signal according to the
channel spacing of the active transmit channel.
The FLAT TX AUDIO signal from accessory connector J0501-5 is fed to the ASFIC CMP (U0221)
pin42 through C0541 and line FLAT TX RTN, switch U0251 and buffer U0211-4. When the radio
switches from receive to transmit mode the µP opens switch U0251 for a short period to prevent that
any applied signal can cause a transmit frequency offset. Buffer U0211-4 sets the correct DC level
and ensures a short settle period when the radio is switched on. Inside the ASFIC CMP 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.
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.
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