GS2984 Datasheet, PDF (12/20 Pages) Gennum Corporation – GS2984 Adaptive Cable Equalizer

4.4 Programmable Squelch Adjust (SQ_ADJ)
The GS2984 incorporates a programmable Squelch Adjust (SQ_ADJ) threshold.
This feature can be useful in applications where there are multiple input channels using
the GS2984 and the maximum gain can be limited to avoid crosstalk.
The SQ_ADJ pin acts to change the threshold of the Carrier Detect (CD) pin, through
voltage level variances. 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 GS2984, the CD pin should be connected directly
to the MUTE pin. In applications where programmable squelch adjust is not required, the
SQ_ADJ pin may be left unconnected. Figure 4-2 shows the relationship between the
SQ_ADJ voltage and cable length at which CD will assert or deassert.
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
does not solve this problem, since the signal to noise ratio on the circuit board could be
significantly less than the default signal detection level set by the on chip reference.
3.2
3.0
2.8
2.6
2.4
2.2
2.0
0
50 100 150 200 250 300 350 400 450
Cable Length (m)
Figure 4-2: SQ_ADJ vs. Cable Length (VCC=3.3V, room temperature, 800mV
launch swing)
4.5 Mute and Carrier Detect
The GS2984 includes a MUTE input pin that allows the application interface to mute the
Serial Digital Output at any time. Set the MUTE pin HIGH to mute SDO and SDO. In this
case, the outputs will mute regardless of the setting of the BYPASS pin.
A Carrier Detect output pin (CD) indicates the presence of a valid signal at the input of
the GS2984. When CD is LOW, the device has detected a valid input on SDI and SDI.
When CD is HIGH, the device has not detected a valid input.
NOTE 1: CD will only detect loss of carrier for data rates greater than 19Mb/s. The CD
output pin may be connected directly to the MUTE input pin to enable automatic muting
of the GS2984 when no valid input signal has been detected.
NOTE 2: If the maximum cable length is exceeded (set by the SQ_ADJ pin) and the
device is not in Bypass mode, the CD pin will not be driven low, even if a carrier is
present.
GS2984 Adaptive Cable Equalizer
Data Sheet
50985 - 3
March 2010
12 of 20