Soft Limit; Inpsense Programmable Gain Stage; Transmit Bandpass Filter; Tx Hf Attenuator - Ericsson EF738 Service Manual

Technical Description

Soft Limit

A 0 to -30 dB attenuator follows the audio input block where this soft limit is ena-
bled through software and is used to limit clipping before the compressor. The soft
limit is controlled by two threshold comparators. When the signal exceeds the
threshold, a clock will step a tap on a resistor ladder to increase the attenuation.
When the level drops below the threshold, a slower clock steps the tap back up the
resistor ladder to reduce the attenuation.

INPSENSE Programmable Gain Stage

The soft limit output is AC coupled into the INPSENSE programmable stage and
this stage is used to adjust the audio level into the compressor. The gain is pro-
grammed through software in a range from +2 to +14.4 dB in steps of 0.8 dB where
the nominal setting is +8 dB.
This stage has calibration settings for three modes of operation:
• external audio
• internal audio
• DTMF tones
The three calibration values are stored in the EEPROM.

Transmit Bandpass Filter

The next block in the transmit audio chain is a switched capacitor bandpass filter
which is designed to allow frequencies from 300 to 3000 Hz to pass through.

TX HF Attenuator

Following the bandpass filter is a 0 to -49 dB attenuator for full duplex handsfree
operation, which is programmable in steps of 7 dB. When the external audio signal
on the system connector is low, the software implements a handsfree algorithm that
attenuates the unused audio path to prevent feedback from external speakers to
microphone.

Compressor

The audio is AC coupled from the transmit handsfree attenuator block into a 2:1
switch capacitor compressor. This stage is enabled during the conversation modes
of the phone and can be bypassed for testing.
Test mode 29 has two options for control of the compressor.
An integrator using an amplifier in the ASIC is configured to provide feedback from
the output to the input of the compressor.

Pre-emphasis

A 3 dB/Octave high pass filter follows the compressor. This filter is used to equalize
the audio spectrum by attenuating the low frequency components of the signal.
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