Tagad9912

AD9912 DDS frequency resolution?

See also discussion on the AD forum: https://ez.analog.com/dds/f/q-a/107510/ad9912-ftw-lsb-always-zero

We've tried to measure the frequency resolution of the AD9912 DDS, which when used with a 1 GHz SYSCLK should be 1 GHz / 2**48 = 3.55 uHz.

We tried an ARTIQ Urukul-board and an AD dev-board and got the following results:

In both cases the output frequency corresponds to an even frequency tuning word (FTW) although we step the frequency by one LSB. In other words the LSB appears to be zero in all cases, even when we write an odd FTW with '1' as the LSB. Instead of the expected 3.55 uHz frequency resolution we see double-sized steps of 7.1 uHz.

The Urukul measurement was done with a Microsemi 3120A phase-meter and the dev-board was measured using a PICDIV 1PPS-divider followed by a Keysight 53230A time interval counter. The even FTW frequencies agree with the predicted frequency to much better than 0.1 uHz.

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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