[time-nuts] Question about SA.33 Rb clock

Bob kb8tq kb8tq at n1k.org
Tue Oct 17 14:50:52 UTC 2017


> On Oct 17, 2017, at 12:37 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
> kb8tq at n1k.org said:
>> The gotcha is that the spur spec’s are not always met. As you might guess,
>> doing  testing over really wide DDS ranges is impractical. Some designs use
>> cleanup loops. The gotcha then becomes a spur (say at 0.053 Hz) that is
>> inside the cleanup loop bandwidth …. 
> How narrow can I make the loop bandwidth with a traditional analog filter 
> using reasonable parts?

Given that this is Time Nuts …. If you analog filter can be the size of a 6’ rack,
it can be pretty narrow :) The bottom line is that it also gets *very* expensive. 

>  Do people build GPSDO style digital filters?

If they are smart they do. It’s a much lower cost way to do it. Today they can 
yield results that are just as low noise as an analog approach. 

> Is there a reasonable formula for the spurs from a DDS?  That sounds like a 
> place where guaranteed-by-design would be appropriate.

You can predict some of the spurs, but not all of them. Full prediction would 
require a *lot* of data about the exact ADC you have on your chip. What gets
guaranteed is some sort of “worst case -80 dbc” sort of stuff. The gotcha is that
a spur at 80 db down is a really big deal if it’s 0.01 Hz off carrier and you are 
looking at ADEV. 

In a Telecom Rb sort of application (or in a lot of other applications) the performance
you get is “plenty good enough” to build a system that works. If that’s the case,
why spend more money? 


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