Hi Adrian,
I know. I avoided discussing that detail in my initial postings.
For the purpose, the 500 ps resolution of the HP53131A is sufficient, or
else I would have used another counter for the purpose.
I can shift the phase of the DUT intentionally, but if so I want to be
able to compensate that shift in the software. Now, such a shift should
be kept separate from the calibration factors which fills a different
purpose.
Cheers,
Magnus
On 10/09/2016 06:34 PM, Adrian wrote:
Hi Magnus,
unfortunately, you can't measure 0 delay between two signals with a counter.
With a 53131A, there is 500ps of LSB jitter and jitter from the measured
signal as well as from the reference signal.
When both signals are exactly in phase, the counter will randomly jump
between 0 and 1 second (assuming you are measuring 1pps signals).
In order to avoid this dead zone, you must add a sufficiently large
phase offset between both signals.
And, keep the acquisition time small enough to avoid phase wraps due to
drift between both sources.
The dead zone random jumps can not be unwrapped by any software.
Cheers,
Adrian
Magnus Danielson schrieb:
Hi,
Well, yes. You can do some fancy stuff with additional hardware, but I
think with already a handful of relatively simple software fixes and
some basic setup conditions, a sufficiently robust method emerges.
I could not sign-swap the measurements in TimeLab when I tried.
I don't seem to be able to force the unwrapped phase to be +/- half
cycle.
I don't seem to be able to offset my readings. I have two sources of
offset, one is the additional delay of cables, and the other is the
offset due to wrong cycle (I hope this one can be baked into
alternative phase-unwrapping mode). I would prefer if I could hit
calibration to establish the zero-level. Typically I use a BNC barrel
and well, it does add smoe more delay
What I propose should be doable with a simple counter like 5335A,
53131/2A or similar. If you have a locked say 100 Hz or 1 kHz signal
(TADD-2 can be useful if the GPSDO does not output proper signal), you
can do the picket fence and resolve things, it is just that there is a
few things to aid in the post-processing to make values useful.
I further hint about a few things which makes easier to analyze is the
improved support for zooming.
Oh, I do care about phase variations and absolute phase measures. I do
such measures a lot. ADEV and TDEV is not all the things I measure,
especially when considering systematic effects.
Cheers,
Magnus
On 10/09/2016 03:42 PM, Bob Camp wrote:
Hi
Given that *some* of us have more than errr … one counter :)
There are several setups that involve two or three counters to
resolve some of these issues. Having
multiple serial ports or multiple devices on a GPIB isn’t that big a
problem. Addressing multiple devices
(setting up the addresses in TimeLab) is an added step. Coming up
with standard setups would be the
first step. Getting them documented to the degree that they could be
run without a lot of hassle would be
the next step.
Another fairly simple addition (rather than a full blown counter)
would be some sort of MCU to time tag
the input(s). It’s a function that is well within the capabilities of
a multitude of cheap demo cards. Rather than
defining a specific card, it is probably better to just define a
standard message (115200 K baud, 8N1, starts
with “$timenuts$,1,”, next is the channel number, after that the (32
bit?) seconds count.The final data field is
a time in nanoseconds within the second, *two byte check sum is last,
cr/lf). If there is a next generation version that is
incompatible, the 1 after timeouts changes to a 2.) Yes, even 10
seconds after typing that definition I can see
a few problems with it. Any structural similarity to NMEA is purely
intentional. That’s why it needs a bit of
thought and work before you standardize on it. It still would be a
cheap solution and maybe easier to integrate
into the software than multiple counters. You do indeed have all the
same setup and documentation issues.
In any of the above cases, the only intent of the added hardware is
to get a number that is good to 10’s of ns.
Anything past that is great. Once you know where all the edges really
are, sorting out the phase data becomes
much easier.
Bob
On Oct 9, 2016, at 7:32 AM, Magnus Danielson
<mag...@rubidium.dyndns.org> 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.
Cheers,
Magnus
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