Earpiece Driver; Afms; Transmit Audio Circuit Description; Atms - Ericsson EF738 Service Manual

Technical Description

Earpiece Driver

The earpiece driver is composed of a differential pair, a non-inverting amplifier and
an inverting amplifier.
The drivers can be software configured in four modes:
• single-ended
• differential
• external
• muted
A resistor is added in series with the earpiece as a pad to limit the maximum acous-
tic level to 103 dBspl. The gains in the ASIC are set to force the signal into clipping
at maximum deviation and maximum volume and the excess amplitude is divided
down between the resistor and the earpiece.

AFMS

The inverting earpiece amp is used to generate the AFMS signal. The signal is AC
coupled to reduce popping when connecting a portable handsfree unit.

Transmit Audio Circuit Description

ATMS

The ATMS signal is AC coupled to the transmit audio path via a capacitor and is
then reduced by 2 dB and biased to a voltage. A switch to this voltage is also con-
nected to the output of the ATMS port to provide a bias for the portable handsfree
microphone. This switch is controlled by the microprocessor, but can be controlled
directly by the PORTHF port of the system connector.

Microphone Input

The ASIC has a switchable +22/+32 dB gain stage for amplifying low level micro-
phone signals. The ASIC also provides a low noise bias voltage for microphone
biasing.
ANTON is designed to provide gain to the portable handsfree microphone by the
use of software controlled switches which are located before and after the Mic gain
stage, to allow routing of the signal from the ATMS port through the Mic amplifier.
The switches can be configured to route the mic audio through the mic amp, to route
ATMS through the mic amp or to bypass ATMS around the mic amp. In all three
cases, the output of this block is routed to the soft limiter.
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