ST70134A

7/22

BLOCK DIAGRAM

Application principle is described in Figure 4.

A LP filter may be used on the TX path to reduce

DMT sidelobes and out of band noise influence on

the receiver. On the RX path, a HP filter must be

used in order to reduce the echo signal level and

to avoid saturation of the input stage of the

receiver. The POTS filter is used in both directions

to reduce crosstalk between ADSL signals and

POTS speech and signalling. Low pass POTS fil-

ter can be very simple for Lite - ADSL application

(see Figure 4).

RX Path

Speech Filter

An external bi-directional LC filter for up and

downstream POTS service splits the speech sig-

nal from the ADSL signal to the POTS circuits.

The ADSL analog front end integrated circuit does

not contain any circuitry for the POTS service but

it guarantees that bandwidth is not disturbed by

spurious signals from the ADSL-spectrum.

Channel Filters

The external analog circuits provide partial echo

cancellation by an analog filtering of the transmit

upstream signal. This is feasible because the

upstream and the downstream data are modu-

lated on separate carriers (FDM) (see Figure 4).

Signal to Noise Performance

RX- PATH SENSITIVITY AT MAXIMUM GAIN

The RX path sensitivity at the maximal RX-AGC of

the receiver is defined at -140dBm/Hz (for 100

Ω

ref) on the line. This figure corresponds to the

equivalent input noise of 31nVHz-1/2 seen on the

line.

The maximum noise density within the pass band

can exceed the average value as follows:

RX path (max AGC setting):

<100nVHz-1/2 @ 138kHz

<31nVHz-1/2 for 250kHz < f

* For ADSL over ISDN, instead of SC2, HC2 1.1MHz LP filter is programmed.

Figure 4 : Block Diagram

VCODAC

XTAL DRIVER

56

VCXOUT

60

59

35.328MHz

48

50

47

49

RXN(0:1)

RXP(0:1)

LP 1.1MHz

HC1

13 Bits A/D

Converter

7

20

12

10

9

11

LNA

PD

31

32

LP 138KHz

SC2 *

12 Bits D/A

Converter

-15.0dB

TXP

TXN

LPF

GRX

50k

Ω

R

2R

2R

R

GTX

LINE

DRIVER

1

2

TXn

HP POTSFILTER

LP POTSFILTER

POTS

1.1

Line

Zo = 100

Ω

4

Master Clock

35.328MHz

Nibbles

17.664MHz

Word

8.832/4.416MHz

RXn

CTRLIN

RESETN

50k

Ω

13

14

15