User's Manual

The HT750 microcontroller can be reset by an external device by generating an active
high reset pulse at pin P102-19, RADIO_RESET. This will cause Q109 to drive J30-5
low.
Audio Routing Circuitry:
There are four paths which are used to route audio between the mobile and the HT750
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 HT750 through the connector, W101. The
HT750 provides demodulated audio to the interface board at EXT_SPKR+ (J30-17) and
URX_SND (J30-11).
The receive audio at EXT_SPKR+ is volume controlled by the HT750. The
HT750 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
3.2kHz. 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 260mVrms. It is attenuated
through op amp circuit U104-3 to a level of 60mVrms 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 HT750’s 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 HT750’s 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 HT750’s microcontroller and are used to control audio routing circuits,
U101-1 and U101-3, for “In-Car-Monitor” activities.