gxb2_cal_clk in SDI-receiver

[Altera_Forum]

Hello 

 I use SD-SDI receiver in ArriaGX device. Global clock in project - 27MHz. rx_sd_oversample_clk_in - 67.5 MHz what calibration clock I must to connect to 'megafunction input "gxb2_cal_clk"?  The same question for HD-SDI receiver. rx_seral_refclk - 74.25 MHz  thank you

[Altera_Forum]

http://www.altera.com/literature/hb/agx/agx_52001.pdf 

 page 7:  cal_blk_clk Input Calibration clock for the transceiver termination blocks. This clock supports frequencies from 10 MHz to 125 MHz.

[Altera_Forum]

any value of frequencies from 10 to 125 MHz? 

 Are there any recomendetions, how to choose the value

[Altera_Forum]

in devices like Stratix IV GX, you need a calibration and reconfiguration clock. the overlap of the two ranges makes 50 MHz a common choice