9

®

AFE1124

For the transmit channel, the pulse shape and the power

spectral density scale directly with the clock rate. The power

spectral density shown in Curve 1 and the pulse template

shown in Curve 2 are measured at the output of the trans-

former. The transformer and the RC circuit on the output

provide some smoothing for the output transmission. At

lower bit rates, the amount of smoothing will be less.

RXHYB AND RXLINE INPUT

ANTI-ALIASING FILTERS

An external input antialiasing filter is needed on the hybrid

and line inputs as shown in the Basic Connection Diagram

above. The –3dB frequency of the input anti-aliasing filter

for the rxLINE and rxHYB differential inputs should be

approximately 1MHz for T1 and E1 symbol rates. Suggested

values for the filter are 750

Ω for each of the two input

resistors and 100pF for the capacitor. Together the two

750

Ω resistors and the 100pF capacitor result in a 3dB

frequency of just over 1MHz. The 750

Ω input resistors will

result in minimal voltage divider loss with the input imped-

ance of the AFE1124.

The antialiasing filters will give best performance with 3dB

frequency approximately equal to the bit rate. For instance,

a 3dB frequency of 320kHz may be used for a single line bit

rate of 320k bits per second.

FIGURE 6. Basic Connection Diagram.

REF

P

V

CM

REF

N

DV

AV

DD

AFE1124

AV

DD

AV

DD

rxbaudCLK

rx48xCLK

Data Out

txbaudCLK

tx48xCLK

Data In

HDSL DSP

GNDA

GNDA

GNDA

txLINE+

txLINE–

rxHYB+

rxHYB–

rxLINE–

rxLINE+

0.01µF

–

–

+

+

Compromise

Hybrid

Input Antialias Filter

fc

≅ 2 x Symbol Rate

0.01µF

5V Analog

0.1µF

0.1µF

0.1µF

0.1µF

0.1µF

1 - 10µF

100pF

750

Ω

100pF

750

Ω

750

Ω

13

Ω

13

Ω

750

Ω

5V to 3.3V Digital

1:2 Transformer

Tip

Ring

0.1µF

0.1µF

0.1µF