TCM4400EPET datasheet(33/64 Pages) TI

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TCM4400E

GSM/DCS BASEBAND AND VOICE A/D

AND D/A RF INTERFACE CIRCUIT

SLWS082A – JULY 1999 – REVISED MARCH 2000

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

PRINCIPLES OF OPERATION

voice codec

The voice coder/decoder (codec) circuitry processes analog audio components in the uplink path and applies

this signal to the voice signal interface for eventual baseband modulation. In the downlink path, the codec

circuitry changes voice-component data received from the voice serial interface into analog audio. The following

paragraphs describe these uplink/downlink functions in more detail.

voice uplink path

The voice uplink path includes two input stages; refer to Figure 14. The first stage is a microphone amplifier,

compatible with an electret-type microphone, that contains a FET-buffer with an open-drain output, that has a

gain of typically 27 dB, and provides an external voltage of 2 V to 2.5 V to bias the microphone. The auxiliary

audio input can be used as an alternative source for a higher level speech signal. This stage performs

single-ended to differential conversion and provides a gain of 6 dB. When auxiliary audio input is used, the

microphone input is disabled and powered down. If both microphone and auxiliary amplifiers are powered up

at the same time, only the signal of the microphone amplifier will be transmitted to the voice uplink path.

The resulting fully differential signal is fed to a programmable gain amplifier that allows adjustment of the level

of the speech signal to the dynamic range of the A/D converter, which is determined by the value of the internal

voltage reference. Programmable gain can be set from –12 dB to +12 dB in 1-dB steps. It is programmed with

bits VULPG to VULPG4 of VBCTL1 register.

Analog-to-digital conversion is made with a third-order sigma-delta modulator whose sampling rate is 1 MHz.

Output of the A/D converter is fed to a speech digital filter, which performs the decimation down to 8 KHz and

band limits the signal with both low-pass and high-pass transfer functions. The speech samples are then

transmitted to the DSP, using the voice serial interface, at a rate of 8 kHz.

Programmable functions of the voice uplink path, power-up, input selection and gain are controlled by the DSP

or the MCU using the serial interfaces. The uplink voice path can be powered down with the bit VULON of the

VBCTL1 internal register.

Bias

Generator

Microphone

Amplifier

27 dB

PGA

+ 16.6 – 7.4 dB

Auxiliary

Amplifier

6 dB

Sigma-Delta

Modulator

fs1 = 1 MHz

SINC

Filter

fs2 = 40 KHz

IIR

Bandpass

Filter

fs3 = 8 kHz

To Voice

Serial Interface

MICBIAS

MICIP

MICIN

AUXI

Side Tone

to Voice Downlink

Figure 14. Uplink Path Block Diagram

TCM4400EPET Datasheet(PDF) 33 Page - Texas Instruments

TCM4400EPET Datasheet(HTML) 33 Page - Texas Instruments

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