Hi > On Oct 17, 2017, at 12:37 AM, Hal Murray <[email protected]> wrote: > > > [email protected] 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? Bob > > > > -- > These are my opinions. I hate spam. > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
