On 28 Mar, 2014, at 20:53 , Bob Camp <[email protected]> wrote:
> The problem is that it can / might / could / will create a “dc bias” in the
> noise. When you filter it, you get a bump rather than zero. If your GPSDO has
> a 47 ns wide sawtooth, that could be a pretty big bump. There’s no way to
> know if the bridge is seconds, minutes, or hours wide. You can make a good
> guess that hours are a *lot* less common than seconds….
Yes, and you can get long term "dc bias" not only in the "hanging bridge"
case but, in diminishing amounts, when the sawtooth frequency passes
through 1/2 Hz, 1/3 Hz, 1/4 Hz and so on, i.e. where the sawtooth period
is an exact integer number of seconds. I think the 4th graph here
http://www.leapsecond.com/pages/m12/sawtooth.htm
shows a 1/2 Hz sawtooth with a non-zero mean error for an extended
period. I guess, viewed this way, the 5th graph would be the 1/1 Hz
sawtooth (which becomes 0 Hz when folded into the Nyquist bandwidth),
which is the worst case but not the only case.
Dennis Ferguson
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