[time-nuts] sysclock source for AD9912 DDS?

[Jim Lux]

On 12/31/13 8:07 AM, Anders Wallin wrote:

>
> Could the remaining -60 dBc spurs at +/- 50 kHz be due to my 10MHz clock
> source, an Agilent 33120A?
>

Yes the spurs could be from your source. that's a function generator/ARB 
and spectral purity isn't one of the big design criteria for that kind 
of device.

The spec sheet says spurious non-harmonic signals are supposed to be 
down -65dBc + 6dB/octave above 1 MHz.

http://cp.literature.agilent.com/litweb/pdf/5968-0125EN.pdf


I don't know all the gory details of how the 33120 works inside (though 
I've got 3 or 4 of them in my lab at work), but in general, it has a DDS 
running at 40MHz that clocks samples out of a waveform table to a 12 bit 
DAC. The output from the DAC is run through some analog circuitry for 
doing AM and setting offsets and levels.

http://cp.literature.agilent.com/litweb/pdf/33120-90017.pdf

50 kHz is somewhat suspicious.. do you have any switching power supplies 
at that rate around?  It doesn't take much noise added to a sine wave 
running into a comparator to create spurs.  I've had that problem with 
leakage of a 66MHz processor clock into a 50 MHz sampling clock.