[time-nuts] Re: Time stamping of data

[Dana Whitlow]

Thanks much for the info about time stamping- I now have a better feel.

My usual situation is that I take streaming IQ data from a spectrum
analyzer at a rate of several kSa/s, then run this data through a
process to select a desired BW and analyze the bandlimited data
for phase, amplitude, etc versus time.  The initial sampling rate is
very accurately known (locked to an Rb standard which is manually
"disciplined" to 10 MHz from a GPSDO.  And from this, the final
sample rate is derived by calculation.

I guess my question is, then, is it sufficient to label the final output
data with the accurate time of the first sample?  Or, (heaven forbid)
should I be thinking in terms of time stamping each output sample?

Dana


On Sat, Aug 24, 2024 at 8:49 AM john.haine--- via time-nuts <
time-nuts at lists.febo.com> wrote:

> Another example.  You have say a Raspberry Pi, it has some sensors on it,
> say pressure, temperature, humidity.  It is connected to the Internet and
> has an NTP client running so it "knows" the right time.  Every second (say)
> it gets a signal perhaps from a pendulum sensed by an optocoupler.  When it
> gets this it pops a record of time, temp, pressure, relhumidity, into a
> file.  The record is "time stamped".
>
> -----Original Message-----
> From: Bob Camp via time-nuts <time-nuts at lists.febo.com>
> Sent: Friday, August 23, 2024 8:14 PM
> To: Discussion of precise time and frequency measurement <
> time-nuts at lists.febo.com>
> Cc: Bob Camp <kb8tq at n1k.org>
> Subject: [time-nuts] Re: Time stamping of data
>
> Hi
>
> The quick and simple version:
>
> Fire up a clock. Off it goes. Does it do decimal seconds or something else?
> That’s up to you and is application dependent. Does it roll over at some
> point or does it count on to infinity? Again, application dependent.
>
> You likely start it with some starting value. It could be zero, it also
> could link to something like UTC. That link (or lack there of) is very
> application dependent.
>
> In comes a zero crossing.
>
> Grab the time value from the clock. (possibly to nanoseconds, picoseconds
> or some other level of resolution / accuracy),
>
> Stuff that value in a file (or maybe display it, though that’s less
> common).
>
> Step and repeat for each zero crossing.
>
> Why do this?
>
> 1) It’s a zero dead time way to look at things. A “delta time between
> crossings”
> approach might take a while to get ready for the next zero crossing.
>
> 2) It takes out potential cumulative errors (compared to delta time). If
> each delta time short by 1 ns, you are off by 10 ns after ten readings.
> Yes, that would be a really poor delta time measure.
>
> 3) You might want to know when an event happened “as received” at various
> locations. That would get you into time sync at possibly a pretty crazy
> level of accuracy.
>
> Is that everything? This is Time Nuts …. it’s never “everything” :) :).
>
> Bob
>
>
>
> > On Aug 23, 2024, at 5:39 AM, Dana Whitlow via time-nuts <
> time-nuts at lists.febo.com> wrote:
> >
> > Hello,
> >
> > I keep reading references to time stamping, but am only vaguely aware
> > of what this does and how it is done.
> >
> > Is there a sort of "primer" on the subject?
> >
> > Thanks,
> >
> > Dana Whitlow
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