GS2974B Datasheet, PDF (10/17 Pages) Gennum Corporation – HD-LINX III Adaptive Cable Equalizer

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
have a nominal voltage of 750mV_pp differential, or 375mV_pp single ended when
terminated with 50Ω as shown in Figure 4-1.
SDO
+187.5mV
V_CM = 2.925V
typical
-187.5mV
SDO
50 50
+187.5mV
V_CM = 2.925V
typical
-187.5mV
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
GS2974B GS2974B HD-LINX™ III Adaptive Cable
Equalizer
Data Sheet
45062 - 3
April 2009
10 of 17