AD9956 CML Driver


I have some questions connected to the PLL and the CML on the AD9956 evaluation board (including a VCO at  ~2450MHz). I am using the eval board as a fractional divider loop as stated in the data sheet, i.e. the VCO output is fed into the DDS/DAC and compared to a reference signal (not the on board crystal) connected to the PLLREF input (25MHz). The R-divider output is fed into the CML driver to generate a clock signal.

1. For reference, the PLL LOCK/SYNC pin should indicate whether the PLL is locked to the ref. signal (locked at logic 1). However, I observe the pin to be high, in case the loop is locked but also if the charge pump is at the edge of its range (0V or 3.3V) and the loop cannot work anymore. What exactly does the IC test to decide if the PLL is working or not?

2. The CML driver is equiped with two outputs. Using this driver, I want to feed the REF_IN of an AD9959. This input is single ended, so I am not quite sure how to connect these boards properly, I expect at least some modifications, because the CML output is supposed to be terminated with 100Ohms for proper PECL mode. Any hints here are welcome!

3. Eventually, I need the AD9956 to have a clock signal output at 500MHz. Therefore, the VCO need to be changed (one at 500MHz seems to be reasonable). Can you give any hints, how to design the loop filter?

Thank you,


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