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GS7025_09

PRO-LINX™ Serial Digital Receiver

Gennum -> Semtech 

3. Detailed Description

The GS7025 Serial Digital Receiver is a bipolar integrated circuit, containing a built-in

cable equalizer and reclocker.

Serial digital signals are applied to either the analog SDI/SDI or digital DDI/DDI inputs.

Signals applied to the SDI/SDI inputs are equalized and then passed to a multiplexer.

Signals applied to the DDI/DDI inputs bypass the equalizer, and go directly to the

multiplexer. The analog/digital select pin (A/D) determines which signal is then passed

to the reclocker.

Packaged in a 44 pin MQFP, the receiver operates from a single 5V supply at a data rate

of 270Mb/s. Typical power consumption is 600mW.

3.1 Cable Equalizer

The automatic cable equalizer is designed to equalize a serial digital data rate of

270Mb/s.

The serial data signal is connected to the input pins (SDI/SDI) either differentially or

single-endedly. The input signal passes through a variable gain equalizing stage, whose

frequency response closely matches the inverse 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 gain stage provides up to 35dB of

gain at 135MHz which typically results in equalization of greater than 350m of Belden

8281 cable at 270Mb/s.

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 an external

differential AGC filter capacitor (AGC+/AGC-) 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 DC-restored, effectively restoring the logic threshold of the

equalized signal to its corrective level irrespective of shifts due to AC-coupling.

3.1.1 Signal Strength Indication/Carrier Detect

The GS7025 incorporates an analog Signal Strength Indicator/Carrier Detect (SSI/CD)

output indicating both the presence of a carrier and the amount of equalization applied

to the signal. The voltage output of this pin versus cable length (signal strength) is shown

in Figure 2-2 and Figure 3-1. With 0m of cable (800mV input signal levels), the SSI/CD

output voltage is approximately 4.5V. As the cable length increases, the SSI/CD voltage

decreases linearly providing accurate correlation between the SSI/CD voltage and cable

length.

GS7025 PRO-LINX™ Serial Digital Receiver

Data Sheet

13813 - 7

October 2009

11 of 23

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