TCM4400E Datasheet, PDF (33/64 Pages) Texas Instruments – GSM/DCS BASEBAND AND VOICE A/D AND D/A RF INTERFACE CIRCUIT

TCM4400E
GSM/DCS BASEBAND AND VOICE A/D
AND D/A RF INTERFACE CIRCUIT
SLWS082A – JULY 1999 – REVISED MARCH 2000
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.
MICBIAS
MICIP
MICIN
AUXI
Bias
Generator
Microphone
Amplifier
27 dB
Auxiliary
Amplifier
6 dB
Side Tone
to Voice Downlink
PGA
+ 16.6 – 7.4 dB
Sigma-Delta
Modulator
fs1 = 1 MHz
SINC
Filter
fs2 = 40 KHz
IIR
Bandpass
Filter
fs3 = 8 kHz
To Voice
Serial Interface
Figure 14. Uplink Path Block Diagram
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