9

®

AFE1105

DISCUSSION OF

SPECIFICATIONS

UNCANCELLED ECHO

The key measure of transceiver performance is uncancelled

echo. This measurement is made as shown in the diagram of

Figure 4. The AFE is connected to an output circuit includ-

ing a typical 1:2 line transformer. The line is simulated by a

135

Ω resistor. Symbol sequences are generated by the tester

and applied both to the AFE and to the input of an adaptive

filter. The output of the adaptive filter is subtracted from the

AFE output to form the uncancelled echo signal. Once the

filter taps have converged, the RMS value of the uncancelled

echo is calculated. Since there is no far-end signal source or

additive line noise, the uncancelled echo contains only noise

and linearity errors generated in the transmitter and receiver.

The data sheet value for uncancelled echo is the ratio of the

RMS uncancelled echo (referred to the receiver input through

the receiver gain) to the nominal transmitted signal (13.5dBm

into 135

Ω, or 1.74Vrms). This echo value is measured under

a variety of conditions: with loopback enabled (line input

disconnected); with loopback disabled under all receiver

LAYOUT

The analog front end of an HDSL system has a number of

conflicting requirements. It must accept and deliver digital

outputs at fairly high rates of speed, phase-lock to a high-

speed digital clock, and convert the line input to a high-

precision (14-bit) digital output. Thus, there are really three

sections of the AFE1105: the digital section, the phase-

locked loop, and the analog section.

The power supply for the digital section of the AFE1105 can

range from 3.3V to 5V. This supply should be decoupled to

digital ground with a ceramic 0.1

µF capacitor placed as

Ideally, both a digital power supply plane and a digital

ground plane should run up to and underneath the digital

may be supplied by a wide printed circuit board (PCB) trace.

A digital ground plane underneath all digital pins is strongly

recommended.

be in the 4.75V to 5.25V range. This portion of the AFE1105

should be decoupled with both a 10

µF Tantalum capacitor

FIGURE 4. Uncancelled Echo Test Diagram.

rxHYB

N

AFE1105

rxLINE

P

rxLINE

N

REF

N

rxD13 - rxD0

rxHYB

P

txLINE

N

txLINE

P

txDAT

P

100pF

0.01µF

576

Ω

1.54k

Ω

13

Ω

13

Ω

5.6

Ω

1:2

5.6

Ω

0.047µF

150

Ω

576

Ω

0.047µF

2k

Ω

2k

Ω

100pF

750

Ω

Transmit

Data

Uncancelled

Echo

0.1µF

750

Ω

0.1µF

2k

Ω

2k

Ω

135

Ω

S

1