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AD9548 PCBZ Evalboard Free running frequency accuracy when never locked

Question asked by CannaGiu on Feb 4, 2015
Latest reply on Feb 4, 2015 by pkern

I'm using the AD9548 Eval board (AD9548/PCBZ) to evaluate this IC.

The on board reference is an crystal oscillator at 50MHz +- 10ppm of frequency accuracy.

I generate a system clock of 1GHz and 160MHz of free running DDS freq. The output divider of out0 is programmed to divide by 4 then the output frequency is 40MHz.

Measuring the output frequency using a counter (locked to a stable frequency reference) the frequency is 39.999850 MHz. I expect that this value should be more precise because with a stability of +- 10ppm @ 50MHz I expect to have a maximum errror of +- 400Hz @ 40MHz.

Anyone can help me to understand why the frequency error is bigger than expected?