[time-nuts] quick and very dirty phase comparator
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Tue Jun 10 01:12:35 UTC 2008
Joseph M Gwinn wrote:
> 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
>
>
Joe
Thats essentially what JPL do with their latest DMTD system which uses a
commercial synthesizer as an offset generator.
They use a 100MHz mixer input frequency and use a 123 Hz beat frequency.
Bruce
Bruce
More information about the Time-nuts_lists.febo.com
mailing list