Motorola VRS750, Service Manual

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44
Audio Routing
Circuitry
There are four paths which are used to route audio between the mobile
and the transceiver: AUX_RX_AUD, TX_AUDIO, MOB_MIC_HI and
FIL_AUD. During the three types of communications of the VRS
(Portable-to-Base, Base-to-Portable, and In-Car-Monitor), the routing
is as follows:
Portable-to-Base
On channel RF signals are received by the transceiver through the
connector, W101. The transceiver provides demodulated audio to the
interface board at E (J30-17) and URX_SND (J30-11).
The receive audio at E is volume controlled. The transceiver
monitors the mobile’s volume setting and then updates its volume
accordingly. The audio is filtered by a 2-pole low pass filter, U107-1,
which has a corner frequency of 3.2 kHz. T1, a 600-ohm to 600-ohm
audio transformer, is used to isolate any ground noise such as
alternator whine from the receive audio before being routed to the
mobile at AUX_RX_AUD (P102-7). During portable-to-base
transmissions the mobile routes this audio to its speaker.
The receive audio at URX_SND is at a fixed level of 260 mVrms. It is
attenuated through op amp circuit U104-3 to a level of 60 mVrms
which can be adjusted via potentiometer R146. T2, a 600-ohm to
600-ohm audio transformer, is used to isolate this audio from any
ground noise such as alternator whine before being routed to the
mobile at TX_AUDIO, P102-8. During portable-to-base transmissions,
the mobile routes TX_AUDIO to its transmitter.
Base to Portable
The MIC_FIL_SEL (J30-15) and TONES-MOBILE_AUD (J30-8) control
signals are generated by the microcontroller and are used to control
audio routing circuits, U101-1 and U101-3, for “Base to portable”
activities.
FIL_AUD (P102-6)—This audio path contains receive audio from the
mobile during Base-to-portable transmissions. The receive audio
passes through op amp circuit U104-1 and can be level adjusted via
potentiometer R150. When a base-to-portable repeat occurs,
TONES_MOBILE_AUD is in a low state and MIC_FIL_SEL is in a high
state so that FIL_AUD is routed to the transmitter input (EXT_MIC,
J30-19).
In-Car-Monitor
The MIC_FIL_SEL (J30-15) and TONES-MOBILE_AUD (J30-8) control
signals are generated by the microcontroller and are used to control
audio routing circuits, U101-1 and U101-3, for “In-Car-Monitor”
activities.
MOBILE_MIC_HI (P102-12)—This audio path contains microphone
audio from the mobile during In-Car-Monitor (ICM) transmissions.
The microphone audio passes through op amp circuit U104-4 and can
be level adjusted via potentiometer R148. During an ICM
transmission, both MIC_FIL_SEL line and TONES-MOBILE_AUD line
are set to a Low state to configure both mux U101-3 and mux U101-1
to route the mobile mic audio to the transmitter input (EXT_MIC,
J30-19).