I think that's what Jim is saying. If you try to fit to the signal using only
the zero crossing, it will be hard unless you have a lot of zero crossing,
because you will have only one point per period to fit to. If you fit 10 or 100
points per period, you improve your fitting considerably. That assumes the
signal waveform is stable of course.
Didier
Sent from my Verizon Wireless BlackBerry
-----Original Message-----
From: David McClain <d...@refined-audiometrics.com>
Sender: time-nuts-boun...@febo.com
Date: Thu, 14 Oct 2010 00:08:58
To: Discussion of precise time and frequency measurement<time-nuts@febo.com>
Reply-To: Discussion of precise time and frequency measurement
<time-nuts@febo.com>
Subject: Re: [time-nuts] Question about SoundCard stability?
> Or, now that I think about it, it's similar to what we do when
> measuring ADEV.. you can do a crude "how many zero crossings in the
> time window" or you can do a "fit a sinusoid to a series of ADC
> samples". One has an uncertainty of "one count/epoch", the other
> can be substantially better.
How could it be substantially better for the same analysis period?
Unless the frequency under test is an integral number of periods
during the analysis period, you will have a variation in the sine
fitting due to starting phase.
OTOH, as admonished in Horowitz & Hill, if the frequency to be
counted is substantially below your counter timebase, then you should
count zero crossings of the higher timebase frequency in the period
of the lower frequency under test.
Dr. David McClain
Chief Technical Officer
Refined Audiometrics Laboratory
4391 N. Camino Ferreo
Tucson, AZ 85750
email: d...@refined-audiometrics.com
phone: 1.520.390.3995
web: http://refined-audiometrics.com
On Oct 13, 2010, at 22:30, jimlux wrote:
> Jim Lux wrote:
>> That's not precisely true. You can get a frequency estimate that
>> is substantially more precise than 1/T if the snr is high.
>> Consider super-resolution in an interferometer which is
>> mathematically similar. What you give up is ambiguity. Probably
>> one of the oldest techniques is that of Prony, but there are lots
>> of others
>
> Or, now that I think about it, it's similar to what we do when
> measuring ADEV.. you can do a crude "how many zero crossings in the
> time window" or you can do a "fit a sinusoid to a series of ADC
> samples". One has an uncertainty of "one count/epoch", the other
> can be substantially better.
>
>
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