[time-nuts] quick and very dirty phase comparator

[Bruce Griffiths]

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


Its 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.


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