STLC60134 datasheet(2/22 Pages) STMICROELECTRONICS

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the ATU-C/ATU-R receivers and transmitters. Ex-

ternal low noise driver and input stage used with

STLC60134S guarantee low noise performances.

The STLC60134S chip can be used at ATU-C

and ATU-R ends (behaviour set by LTNT pin).

The selection consists mainly of a filter inter-

change between the RX and TX path. The filters

(with a programmable cutoff frequency) use auto-

matic Continuous Time Tuning to avoid time vary-

ing phase characteristic which can be of dramatic

consequence for DMT modem. It requires few ex-

ternal components, uses a 3.3V supply (a sepa-

rate 3.0V supply of the digital part is possible)

and is packaged in a 64-pin TQFP in order to re-

duce PCB area.

The Receiver (RX) part

The DMT signal coming from the line to the

STLC60134S 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

An analog multiplexer allows the selection be-

tween two input ports which can be used to select

an attenuated (0, 10dB for ex.) version of the sig-

nal in case of short loop or large echo. The sig-

nal is amplified by a low noise gain stage (0-

31dB) then low-pass filtered to avoid anti-aliasing

and to ease further digital processing by remov-

ing unwanted high frequency out-of-band noise.

A 12-bit A/D converter samples the data at

8.832MS/s (or 4.416MS/s in alternative mode),

transforms the signal into a digital representation

and sends it to the DMT signal processor via the

digital interface.

The Transmitter (TX) part

The

12-bit

data

words

8.832MS/s

(or

4.416MS/s) coming from the DMT signal proces-

sor through the digital interface are transformed

by D/A converter into a analog signal.

This signal is then filtered to decrease DMT side-

lobes level and meet the ANSI transmitter spec-

tral 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

loop provide attenuation (-15...0dB).

The VCXO part

The VCXO is divided in a XTAL driver and a aux-

iliary 8 bits DAC for timing recovery.

The XTAL driver is able to operate at 35.328MHz

and provides an amplitude regulation mechanism

to avoid temperature / supply / technology de-

pendent frequency pulling.

The DAC which is driven by the CTRLIN pin pro-

vides a current output with 8-bit resolution and

can be used to tune the XTAL frequency with the

help of external components. A time constant be-

tween DAC input and VCXO output can be intro-

duced (via the CTLIN interface) and programmed

with the help of an external capacitor (on VCOC

pin).

See chapter ’VCXO’ for the external circuit re-

lated to the VCXO.

The Digital Interface part

The digital part of the STLC60134S can be di-

vided in 3 sections:

The data interface converts the multiplexed

data from/to the DMT signal processor into

valid representation for the TX DAC and RX

ADC. It performs also the error correction

mechanism needed at the (redundant) ADC

output.

The control interface allows the board proces-

sor

to configure the STLC60134S paths

(RX/TX gains, filter band, ...) or settings (OSR,

vcodac enable, digital / analog loopback,...).

The test interface to enable digital (Full Scan,

nandtree, loop backs, functional,...) or analog

(TIN, TOUT assignation) tests to be per-

formed.

DMT Signal

A DMT signal is basically the sum of N inde-

pendently QAM modulated signals, each carried

over a distinct carrier. The frequency separation

of each carrier is 4.3125kHz with a total number

of 256 carriers (ANSI). For N large, the signal can

be modelled by a gaussian process with a certain

amplitude probability density function. Since the

maximum amplitude is expected to arise very

rarely, we decide to clip the signal and to trade-

off the resulting SNR loss against AD/DA dy-

namic. A clipping factor (Vpeak/Vrms = ”crest fac-

tor”) of 5 will be used resulting in a maximum

SNR of 75dB.

ADSL DMT signals are nominally sent at -40dBm/Hz

±3dB (-3.65dBm/carrier) with a maximal power of

100mW for down link transmitter and 15.7mW for

uplink transmitter.

DMT symbols are transmitted without ’window-

ing’ causing sin (x)/x like sidelobe s. For spectral

response shaping, the 1st sidelobe level is as-

sumed to be 13dB under the carrier level with

an atten uation of -20dB/d ec.

The minimum SNR + D neede d for DMT carrier

demodulation is about (3

⋅ N + 20) dB with a

minimum of 38dB were N is the constellation size

of a carrier (in bits).

STLC60134S

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