kb8tq@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? Do people build GPSDO style digital filters? Is there a reasonable formula for the spurs from a DDS? That sounds like a place where guaranteed-by-design would be appropriate. -- These are my opinions. I hate spam.

On 10/16/17 9:37 PM, Hal Murray wrote: > > kb8tq@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? Do people build GPSDO style digital filters? Crystal filters can be pretty narrow... > > Is there a reasonable formula for the spurs from a DDS? That sounds like a > place where guaranteed-by-design would be appropriate. There's some guidelines out there for DDS spur levels but most folks just simulate it and see what the spurs are. There's a lot of variation in the details of implementation of the DDS (phase accumulator length, cosine table length, whether you're doing CORDIC or not, whether you're using phase dither or amplitude dither, whether you've got one of the clever filter schemes to move spurs around).

Hi > On Oct 17, 2017, at 12:37 AM, Hal Murray <hmurray@megapathdsl.net> wrote: > > > kb8tq@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? Bob > > > > -- > These are my opinions. I hate spam. > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there.