AD9912 DDS Test

Update: it turns out the PLL filter components were not connected at all during the first tests I did 🙂 Here's a picture that compares the schematic against the actual board. When I changed the 0R resistor from position "R3" to position "R5"  the PLL started behaving much more nicely. If anyone from AD is reading - please update your documentation!

ad9912_eval_board_pll_schematic

With this change, and a 66x PLL multiplier giving a 10MHz x 66 = 660 MHz SYSCLOCK I get quite nice output:

Fout100M_Span50M Fout100M_Span400M Fout100M_Span500k

The output is set to 100 MHz and has an amplitude of ~0 dBm. There are -60 dBm spurs at +/- 50 kHz (not sure why?), -70 dBm spurs at +/- 10MHz, and a -65 dBm second harmonic at 200 MHz.

If I activate the "2x Reference" setting which detects both rising and falling edges of the input clock, and use that with a 40x PLL for a 10MHz x2 x40 = 800 MHz SYSCLOCK I still get very strong spurs at 10 MHz:

Fout100M_Span400_PLL_2x40x

I have been testing this AD9912 DDS evaluation board:
ad9912_evkit_pcb_text

So far the results are a bit strange, with output as advertized only when no external input clock is supplied. Strange.

dds_test_2013-12-30

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