[time-nuts] Discussion of precise time and frequency measurement
albertson.chris at gmail.com
Thu Jun 30 00:15:27 UTC 2011
OK, now I understand why you wanted a sound card with a wel regulated
clock. Yes they make those. The common cheap cards use a square can
oscillator good to about 100PPM. Better cards can use a better
and way more expensive clock source. I think knowing the sample rate
is accurate is good enough. Then you simly feed the 60Hz signal to
the sound card and record it. Latter plot a spectra or whatever. I
thought of that too but then rememebered that the audio card's best
sample rate is only about 96K samples per second so the time
resolution is only on order it 1/48,000 second.
Then I though I could square the 60Hz wave and feed it to pin one on a
serial port the same way you conect the PPS signal from a GPS. I
could build a device to sense the 60Hz power by aiming a photo
transistor at the florescent lights on the ceiling. I'm only half
joking. I think a neon pilot lamp and a photo cell both inside a
light proof metal box might work.
The device driver that reads the PPS from a GPS will not care if the
signal is comming from the power line it will be hapy to time tage
every cycle with a nano second counter value.
I think your plot is simply the delta between cycles. Or the period
of the 60Hz input. If you plot the period it might go up or down over
On Wed, Jun 29, 2011 at 4:30 PM, Will Matney <xformer at citynet.net> wrote:
> What I would think that would be needed, software wise, is an app that's
> similar to the long term plots used in calibration. I would say that you
> know the ones I'm speaking of, as it is a graph that stretches over a month
> (or longer), and lets say the horizontal center line would represent 60 Hz,
> and above or below it would be how much it's off in which ever measuremet
> you want, either in frequency, or in PPM. Long term time is on the
> horizontal axis, and frequency+/- is the vertical. That was my thinking the
> other day, so it would log any shifts over a long timespan, really similar
> to the old paper plots off a frequency standard. Now whether you could use
> this with a sound card, I don't know, as one would need a frequency
> standard, along with the line frequency, as inputs so they could be
> *********** REPLY SEPARATOR ***********
> On 6/29/2011 at 4:11 PM Chris Albertson wrote:
>>On Wed, Jun 29, 2011 at 3:00 PM, Bill Dailey <docdailey at gmail.com> wrote:
>>> The FireWire thing makes sense but is there some way to make the
> computer itself a slave to an external source also?
>>Why? The computer has many clocks in it and they are not all derived
>>all from one master. So to answer we'd need to know which clock you
>>care about. Obviously the clock to keep the current time of day can
>>be sync'd from s master source, like GPS or an Internet NTP server.
>>Maybe it's best you describe the big picture. What are you trying to do?
>>But if you mean the audio interface. Then all you really need is a
>>stable clock on that. There is an "elastic" buffer between the audio
>>interface and the software so the computer can go off and do
>>"whatever" for 1/10th of a second and the interface wil buffer the
>>data, none gets lost as long as the buffer does not overflow.
>>Modern CPUs do not run at a nice fixed rate. they are
>>non-deterministic and might execute many instructions at once or even
>>out of order. The newer Intel chips adjust their internal clock
>>rates to control power use and core temperature. If you need to
>>sync to the external world then software techniques are used
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