[time-nuts] sysclock source for AD9912 DDS?

Jim Lux jimlux at earthlink.net
Mon Dec 30 11:36:49 EST 2013


On 12/30/13 7:56 AM, Anders Wallin wrote:
> I've tested the AD9912 evaluation board:
> http://www.anderswallin.net/wp-content/uploads/2013/12/dds_test_2013-12-30.png
>
> I want to use it with a 10MHz external input clock, but it looks like the
> on-board PLL that generates a 1200MHz sample clock from my input isn't that
> great, since I get strong side-bands on the output that are only 18-20 dB
> down from the fundamental.
>
> So it looks like I need to supply a clean 800-1000MHz clock to the DDS to
> get a clean output. Any ideas/suggestions for generating this from a 10 MHz
> sine?
> Driving the DDS system clock from an expensive RF generator (e.g. HP 8648A)
> would be possible but I'd prefer a PLL from 10MHz if it's doable
> simply/cheaply.
>
Most of the time, they're expecting you to filter the output of the DDS 
to remove the spurs.

Probably not a PLL, at least not one of the generic PLL chips, because 
you'll get spurs and sidebands from the comparison frequency in the 
loop, and they'll be fairly close in.

You have two basic alternatives.. a PLL using a divider from your 1 GHz 
and using the 10 MHz as the comparison frequency, and then good 
filtering to get rid of the 10 MHz spurs. A bit challenging since that's 
a <1% bandwidth filter. (maybe you could do that with the onchip 
reference generator, and feed it back in)

Or, You want a straight out multiplier chain with appropriate filtering 
in between stages.  Maybe a x7, followed by 70MHz BPF (which should be 
readily available), then another x7 to 490 MHz, then a x2 or x3.  Or end 
with a x5 (to 350 MHz) and a x3 to 1050.

A sort of hybrid approach might also work.. take your 10 and make 
100MHz(x5 x2) with it, then feed that into your DDS. That will put the 
spurs 100 MHz away, which is a lot easier to filter than spurs that are 
10 MHz away.

I'm pretty sure Wenzel has some application notes on this.

When you say "clean", how clean do you need it?  What sort of tuning 
range are you going to run the DDS over? (maybe some of the spurs will 
wind up in places you don't care about and can filter out)





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