External Cable equalizer devices must be used with Reclocker feature enabled and cable drivers are also required to convert the serial signals into and out of the GTH transceivers to SDI electrical standards.
An external SDI cable equalizer must be used to convert the single-ended 75 Ω SDI signal to a 50 Ω differential signal compatible with the receiver input signal requirements of the GTH transceiver. Appropriate SDI cable equalizers are available from several manufacturers. The differential outputs of these cable equalizers usually must be AC-coupled to the GTH receiver input signals due to common mode voltage differences. An example of interfacing a typical SDI cable equalizer to a GTH receiver is shown in the following figure.
The differential inputs of the GTH RX have built-in differential termination. As described in the UltraScale Architecture GTH Transceivers User Guide (UG576), RX Termination Use Mode 3 is the recommended termination mode for the GTH RX inputs in SDI applications. The GTH internal programmable termination voltage should be set to 800 mV for SDI applications.
Similarly, the differential serial outputs of the GTH transmitter are connected to the inputs of an SDI cable driver, usually with AC coupling as shown in the following figure. The cable driver converts the differential signal from the GTH transmitter into a single-ended signal with electrical characteristics meeting the SDI standards. SDI cable drivers typically have a slew rate control input that sets the slew rate of the cable driver. The slew rate requirements for SD-SDI are significantly different than the slew rate requirements for HD-SDI, 3G-SDI, 6G-SDI, and 12G-SDI. The slew rate control input of the SDI cable driver is typically controlled by the FPGA. However, the 12G-SDI FMC expansion card used with this application note internally controls the slew rate of the cable driver. The control module supplied with this application note generates a slew rate control signal for use with the external SDI cable driver for other use cases.
