[time-nuts] GPS orthodontics: sawteeth & hanging bridges - theeffect of t...
SAIDJACK at aol.com
SAIDJACK at aol.com
Fri Dec 22 03:40:04 EST 2006
In a message dated 12/21/2006 10:44:29 Pacific Standard Time,
tvb at leapsecond.com writes:
1 second raw samples: 10.40 ns
then removing small linear frequency offset,
1 second samples: 9.36 ns
30 second averages: 9.62 ns
300 second averages: 10.0 ns
Did I do something wrong? PHK, what do you think
about this?
/tvb
Hi Tom et. al,
Has anyone tried looking at this sawtooth/bridge noise in the frequency
domain, then using brick-wall FIR or better IIR low-pass filters below the
modulation frequency of the bridges, and sawtooth?
There are IIR low-pass filters that can be extremely efficient (>>120dB
stopband attenuation) with only a small number of coefficients. Matlab is great
to calculate these based on passband/stopband response etc. With a bit of
memory, it is would be possible to do a 512-tap IIR or FIR low-pass filter for
example with a 1/100Hz to 1/1000Hz cut-off frequency. The sampling frequency is
1Hz of course.
For fun this filter could be scaled to 96KHz sampling frequency, and would
then be equivalent to a 96Hz to 960Hz brick-wall low pass.
Essentially simple averaging is a really "poor-mans" FIR low pass filter
with all coefficients set to 1.0. We should be able to do so much better than
that with IIR filtering, especially since the frequency behavior of the
sawtooth, bridges, etc seems to be somewhat well understood and the filter can thus
be highly optimized to filter out exactly these periodic noises.
Has anyone tried something like this?
bye,
Said
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