bob at evoria.net
Sun Oct 9 12:21:43 EDT 2016
The problem with two counters is that they will never read exactly the same. What would be better is if the TICs were able to steer the first incoming signal to start and the next to stop, and then apply a sign based on where the first pulse came from. Of course, then you have the problem of deciding whether you started the counter at the right point in time, or merely have things backward as a result of being too crafty with your hardware.
GFS GPSDO list:
From: Alexander Pummer <alexpcs at ieee.org>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Sunday, October 9, 2016 11:01 AM
Subject: Re: [time-nuts] TimeLab
I am a totally unerducated time nut better, to say; not time nut, just
an old RF ingenieur, and so I have trouble to understand how could a
counter stop to count before it started to count. I case you would have
a circuit, which would tell you which pulse came at first and start the
counter regardless of which of the two pulse came first and the same way
stop the counter regardless pulse came last, you could count out the
time difference with the interval counter independently from the
sequence the pulses,
Or you could use two counters and reverse the inputs at the second
counter, thus one counter would show the positive error and the other
the negative error.
On 10/9/2016 4:32 AM, Magnus Danielson wrote:
> Fellow time-nuts,
> I don't know if it is me who is lazy to not figure TimeLab out better
> or if it is room for improvements. I was considering writing this
> directly to John, but I gather that it might be of general concern for
> many, so I thought it be a good topic for the list.
> In one setup I have, I need to measure the offset of the PPS as I
> upset the system under test. The counter I'm using is a HP53131A, and
> I use the time-interval measure. I have a reference GPS (several
> actually) which can output PPS, 10 MHz, IRIG-B004 etc. In itself
> nothing strange.
> In the ideal world of things, I would hook the DUT PPS to the Start
> (Ch1) and the reference PPS to the Stop (Ch2) channels. This would
> give me the propper Time Error (DUT - Ref) so a positive number tells
> me the DUT is ahead of the reference and a negative number tells me
> that the DUT is behind the reference.
> Now, as I do that, depending on their relative timing I might skip
> samples, since the counter expects trigger conditions. While TimeLab
> can correct for the period offset, it can't reproduce missed samples.
> I always get suspicious when the time in the program and the time in
> real world does not match up.
> I could intentionally shift the PPS output of my DUT to any suitable
> number, which would be one way to solve this, if I would tell TimeLab
> to withdraw say 100 ms. I might want to do that easily afterhand
> rather than in the setup window.
> To overcome this, I use the IRIG-B004 output, which is a 100 Hz signal
> with a stable rising edge aligned to the PPS to within about 2 ns.
> Good enough for my purpose. However, for the trigger to only produce
> meaningful results, I will need to swap inputs, so that the PPS from
> DUT is on Start/Ch1 and the IRIG-B is on Stop/Ch2. This way I get my
> triggers right. However, my readings have opposite sign. I might have
> forgotten about the way to correct for it.
> However, TimeLab seems unable to unwrap the phase properly, so if I
> have the condition where I would get a negative value of say -100 ns
> then the counter will measure 9,999,900 ns, so I have to force a
> positive value as I start the measurement and then have it trace into
> the negative. I would very much like to see that TimeLab would
> phase-unwrap into +/- period/2 from first sample. That would be much
> more useful.
> I would also like to have the ability to set an offset from which the
> current zoom window use as 0, really a form variant of the 0-base but
> letting me either set the value or it be the first value of the zoom.
> I have use for both of these. I often find myself fighting the offset
> issues. In a similar fashion, I have been unable to change the
> vertical zoom, if I don't care about clipping the signal then it
> forces me to zoom in further than I like to. The autoscale fights me
> many times in a fashion I don't like.
> OK, so there is a brain-dump of the last couple of weeks on and off
> measurement experiences. While a few things might be fixed in the
> usage, I wonder if there is not room for improvements in the tool. I
> thought it better to describe what I do and why, so that the context
> is given.
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