GS2974B GS2974B HD-LINX™ III Adaptive Cable

Equalizer

Data Sheet

45062 - 3

April 2009

10 of 17

4.2 Cable Equalization

The input signal passes through a variable gain equalizing stage whose frequency

response closely matches the inverse of the cable loss characteristic. In addition, the

variation of the frequency response with control voltage imitates the variation of the

inverse cable loss characteristic with cable length.

The edge energy of the equalized signal is monitored by a detector circuit which

produces an error signal corresponding to the difference between the desired edge

energy and the actual edge energy. This error signal is integrated by both an internal and

an external AGC filter capacitor providing a steady control voltage for the gain stage. As

the frequency response of the gain stage is automatically varied by the application of

negative feedback, the edge energy of the equalized signal is kept at a constant level

which is representative of the original edge energy at the transmitter. The equalized

signal is also DC restored, effectively restoring the logic threshold of the equalized signal

to its correct level independent of shifts due to AC coupling. The digital output signals

terminated with 50Ω as shown in Figure 4-1.

Figure 4-1: Typical Output Voltage Levels

4.3 Programmable Maximum Cable Length Adjust (MCLADJ)

For SMPTE 259M inputs, the GS2974B incorporates a programmable Maximum Cable

Length Adjust (MCLADJ) threshold.

This feature can be useful in applications where there are multiple input channels using

the GS2974B and the maximum gain can be limited to avoid crosstalk.

The MCLADJ pin acts to change the threshold of the Carrier Detect (CD) pin. When the

input signal drops below a certain threshold, the CD pin will be driven high, indicating

that there is not a valid input signal. In order to enable automatic muting of the output

of the GS2974B, the CD pin should be connected directly to the MUTE pin. In

applications where programmable maximum cable length adjust is not required, the

MCLADJ pin may be left unconnected.

This feature has been designed for use in applications such as routers, where signal

crosstalk and circuit noise cause the Equalizer to output erroneous data when no input

signal is present. The use of a Carrier Detect function with a fixed internal reference

50

50

SDO

SDO

+187.5mV

-187.5mV

typical

+187.5mV

-187.5mV

typical