MTC20154

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Functional Description

The MTC20154 chip can be used on the ATU-C side (LT), and on the ATU-R (NT) side (defined by LTNT

pin). The selection consists mainly of a filter interchange between the RX and TX path. The filters (with a

programmable cutoff frequency) use automatic continuous time tuning to avoid time varying phase char-

acteristics which can be of dramatic consequence for DMT modems. It requires few external components,

uses a 3.3 V supply and is packaged in a 64 pins TQFP in order to reduce PCB area.

The Receiver (RX)

The DMT signal coming from the line to the MTC20154 is first filtered by the two following external filters:

– POTS HP filter: Attenuation of speech and POTS signalling.

– Channel filter: Attenuation of echo signal to improve RX dynamic.

The signal is amplified by a low noise gain stage (-15..+31 dB) then low-pass filtered to avoid anti-aliasing

and to ease further digital processing by removing unwanted high frequency out-of-band noise.

A 12 bits A/D converter samples the data at 8.832 MS/s, transforms the signal into a digital representation

and sends it to the DMT signal processor via the digital interface.

The Transmitter (TX/TXE)

The 12 bits data at 8.832 Ms coming from the DMT signal processor through the digital interface are trans-

formed by a D/A converter into an analog signal. This signal is then filtered to decrease DMT sidelobes

levels and meet the ANSI transmitter spectral response but also to reduce the out-of-band noise (which

can be echoed to the RX path) to an acceptable level. The pre-driver buffers the signal for the external

line driver and in case of short loops provide attenuation provision (-9..+6 dB).

The VCXO

The VCXO is divided in a XTAL driver and an auxilliary 8 bits DAC for timing recovery.

The XTAL driver is able to operate at 35.328 MHz or 17.664 MHz. An internal PLL will be used to double

the frequency in the 17.664 MHz case. It also provides an amplitude regulation mechanism to avoid tem-

perature/-supply/technology dependent frequency pulling.

The DAC which is driven by the CTRLIN pin provides a current output with 8 bits resolution and can be

used to tune the XTAL frequency with the help of external components. A time constant between DAC

input and VCXO output can be introduced (via the CTRLIN interface) and programmed with the help of an

external capacitor (on VCOCAP pin).

The Digital Interface

The digital part of the MTC20154 can be divided into two parts: The data interface converts the multiplexed

data from/to the DMT signal processor into a valid representation for the TX DAC and RX ADC. The con-

trol interface allows the board processor to configure the MTC20154 paths (RX/TX gains, filter band, ...)

or settings.

Package

The MTC20154 is housed in a 64-pin TQFP package.