[time-nuts] Short-Term Stability

Mike Feher mfeher at eozinc.com
Thu Mar 16 11:48:31 EST 2006

Poul -

Unfortunately the technology does not yet allow having a 69 tap FIR running
data at a 2 GHz clock rate. DDSs on the other hand are getting up there in
speed, with the D/A being the real limitation. For fixed frequencies of
course there is no problem as other spurious free frequency generation
methods are available. The beauty of the DDS is the frequency agility
available at sub microsecond rates for frequency hopped systems. Regards -

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Poul-Henning Kamp
Sent: Thursday, March 16, 2006 11:37 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Short-Term Stability

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:


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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