[time-nuts] DDS'ery narrow scoped.

Ulrich Bangert df6jb at ulrich-bangert.de
Tue Jun 21 09:03:22 UTC 2011


the information that you are concerned about close carrier spurs that will
pass through the PLL's low pass filter is not precise enough: are you
talking about a few Hz, a few ten Hz, a few 100 Hz away from the carrier or
are you going to build a device for precise timing applications where also
spurs far below (perhaps orders of magnitude below) 1 Hz may be of concern?

Best regards
Ulrich Bangert, DF6JB

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Luis Cupido
> Gesendet: Dienstag, 21. Juni 2011 02:20
> An: Discussion of precise time and frequency measurement
> Betreff: [time-nuts] DDS'ery narrow scoped.
> Folks,
> Many thanks to you all, for the info.
> This is indeed a great forum.
> My aplic. is a DDS signal that
> will serve as reference for a pll with a relatively
> narrow loop filter. As I said before.
> Most replies presume the analog world with DAC
> filters etc etc. But that I know ;-)
> I'm digging out the possibilities in the digital side
> not involving going back to analog and back to digital.
> this is how this started :-)
> Now that you all have been so kind in the great comments
> you gave, please let me just be
> very very very specific.
> Imagine an FPGA and a square wave coming out.
> Just that. Nothing more.
> (That is what I had in mind when querying about the MSB usage 
> in the first place.)
> My first approach was the ACC MSB
> (and that is working already on the bench.)
> So I'm researching a way to have that digital output cleaner (spurs) 
> without leaving the digital(FPGA) world sticking to the block 
> diagram of one FPGA one digital output. Specially worried 
> about close in spurs 
> (the far away ones won't bother me much).
> That is really scenario I'm trying to picture if there is any 
> hope to generate a cleaner digital output out of an FPGA (dds 
> with whatever 
> processing required to be done after and producing a square wave).
> Thanks for your patience.
> Luis Cupido.
> ct1dmk
> P.S. At the moment I'm testing on the bench with a real FPGA 
> cyclone III with a 48bit dds at 100MHz fclock and at circa 6 
> and 18MHz output and it 
> is not that bad. I got better than -60dBc in the desired 
> ranges. So not too unhappy for a start ;-) PLL cleans 99% of 
> it... but the close in spurs are annoying.
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