8

®

AFE2124

BIT

DESCRIPTION

BIT STATE

OUTPUT STATE

15 (MSB)

tx Enable Signal

0

AFE Transmits a 0 Symbol

1

AFE Transmits HDSL Symbol

as defined by bits 14 and 13

14 and 13

tx Symbol

00

–3 Transmit Symbol

Definition

01

–1 Transmit Symbol

11

+1 Transmit Symbol

10

+3 Transmit Symbol

12 - 10

rx Gain Settings

000

rx gain in AFE 0dB

001

rx gain in AFE 3dB

010

rx gain in AFE 6dB

011

rx gain in AFE 9dB

100

rx gain in AFE 12dB

101

rx gain in AFE Reserved

110

rx gain in AFE Reserved

111

rx gain in AFE Reserved

9

Loopback Control

1

Loopback Mode

0

Normal Operation

8

tx Boost

0

Normal Transmit Power

1

+0.5dB Transmit Power Boost

7 - 0

SPARE

NA

TABLE I. Data In.

Rx Gain Settings—These bits set the gain of the receive

channel programmable gain amplifier.

Loopback Control—This bit controls the operation of

loopback. When enabled (logic 1), the rxLINE+ and rxLINE–

inputs are disconnected from the AFE. The rxHYB+ and

rxHYB– inputs remain connected. When disabled, the

rxLINE+ and rxLINE– inputs are connected.

txBoost—This bit controls the addition of 0.5dB additional

power to the output line driver.

rxbaudCLK: This is the receive data baud rate (symbol

clock), generated by the DSP. It is 392kHz for T1 or 584kHz

for E1. It can vary from 32kHz (64kbps) to 584kHz

(1.168Mbps).

rx48xCLK: This is the A/D converter oversampling clock,

generated by the DSP. It is 48x the receive symbol rate or

28.032MHz for 584kHz symbol rate. This clock should run

continuously.

Data Out: This is the 14-bit A/D converter output data (+2

spare bits) sent from the AFE to the DSP. The 14 bits from

the A/D Converter will be the upper bits of the 16-bit word

(bits 15-2). The spare bits (1 and 0) will be always be low.

Eight additional (interdata) bits follow, which are always

high. The data is clocked out on the falling edge of rx48xCLK.

The bandwidth of the A/D converter decimation filter is

equal to one-half of the symbol rate. The nominal output

rate of the A/D converter is one conversion per symbol

period. For more flexible post processing, there is a second

true A/D conversion available in each symbol period. In

Figure 4, the first conversion is shown as Data 1 and the

second conversion is shown as Data 1a. It is suggested that

rxbaudCLK is used with the rx48xCLK to read Data 1

while Data 1a is ignored. However, either or both outputs

may be used for more flexible post-processing.

DATA

BITS

Data 1

16

Interdata Bits

8

Data 1a

16

Interdata bits

8

Total Bits/Symbol Period

48

DATA OUT PER SYMBOL PERIOD

MSB

LSB

14

2

Reserved

A/D Converter Data

FIGURE 5. Data Out Word.

ANALOG-TO-DIGITAL CONVERTER DATA

The A/D converter data from the receive channel is coded in

Binary Two’s Complement.

ANALOG INPUT

A/D CONVERTER DATA

MSB

LSB

Positive Full Scale

01111111111111

Mid Scale

00000000000000

Negative Full Scale

10000000000000

ECHO CANCELLATION IN THE AFE

The rxHYB input is subtracted from the rxLINE input for

first order echo cancellation. For correct operation, be cer-

tain that the rxLINE input is connected to the same polarity

signal at the transformer (+ to + and – to –) while the rxHYB

input is connected to opposite polarity through the compro-

mise hybrid (– to + and + to –) as shown in Figure 6.

SCALEABLE TIMING

The AFE2124 scales operation with the clock frequency. All

internal filters and the pulse former change frequency with

the clock speed so that the unit can be used at different

frequencies just by changing the clock speed.

For the receive channel, the digital filtering of the delta

sigma converter scales directly with the clock speed. The

bandwidth of the converter’s decimation filter is always one-

half of the symbol rate. The only receive channel issue in

changing baud rate is the passive single pole anti-alias filter

(see the “rxHYB and rxLINE Input Anti-Aliasing Filters”

section). For systems implementing a broad range of speeds,

selectable cutoff frequencies for the passive anti-alias filter

should be used.