[time-nuts] Short-Term Stability

Poul-Henning Kamp phk at phk.freebsd.dk
Thu Mar 16 11:36:56 EST 2006


In message <006001c6490d$abf4fdc0$0201a8c0 at n4fs>, "Mike Feher" writes:

>There are several
>mechanisms that cause the spurious, including the phase and amplitude
>truncation due to the finite amount of bits, 

A researcher at Bruel&Kjaer wrote an IEEE paper about how filtering
the rounding errors on integer FIR filters could reduce spurious
by a LOT of dB for typical filters, his canonical example is 20dB
on a 69 tap FIR:

	http://hjem.get2net.dk/jjn/quantfir.htm

The same technique applies to DDS, but is usually too expensive to
perform in real-time.  For fixed frequencies however, it is possible
to do the math beforehand, and add a "rounding" bit stream which
will be added to the accumulator along with delta-omega.

I've lost the reference to this however.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.




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