[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?

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?

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