[time-nuts] quick and very dirty phase comparator

[Joseph M Gwinn]

time-nuts-bounces at febo.com wrote on 06/04/2008 08:05:16 PM:

> WB6BNQ wrote:
> > Ulrich,
> >
> > One of things I noticed when playing with that DDS design 
> tool is you can achieve
> > the same output frequency using a different clock frequency, 
> thus removing the
> > spurs.  THe trick is to look at clock frequencies that you 
> can lock to your house
> > reference for stability.  Also, it might be possible to use 
> cascading DDS chips,
> > one providing the clock for the other.
> >
> > What are the thoughts on such an approach ?
> >
> > Bill....WB6BNQ
> >
> >
> > Ulrich Bangert wrote:
> >
> > 
> Cascaded DDS systems together with PLLs have been widely investigated:
> See US patents: 4965533, 5028887, 5146186, 5216389, 5467294, 5517156, 
> 5598440. 7143125
> 
> 
> It's much better to use a DDS that employs noise shaping techniques to 
> shift the spur energy to higher offset frequencies that are easily 
> cleaned up by a PLL.

A question occurs to me.  Perhaps we can turn this upsidedown.  While the 
DDS won't necessarily generate the frequency we want without spurs, can't 
we set the DDS to generate a nearby frequency that is free or sufficiently 
free of nearby spurs?  We may not be able to get nice power-of-ten phase 
amplification ratios, but we will know the ratios we do get quite 
precisely.

Joe Gwinn


> For the particular case of an offset generator for a dual mixer system 
> it is much easier to use the JPL generation technique using cascaded LSB 

> and USB mixers even though 4 mixers are required, generating the in 
> phase and quadrature signals to drive each mixer pair is easily done by 
> the associated digital frequency divider.
> 
> Bruce
> 
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