[ntp:questions] How to determin hardware latency for PPS offset given simple tools.

David Taylor david-taylor at blueyonder.co.uk.invalid
Fri Jun 7 16:55:19 UTC 2013


On 07/06/2013 16:17, unruh wrote:
> On 2013-06-07, David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
>> On 06/06/2013 20:17, Doug Calvert wrote:
>>> unruh has a nice page about interrupt coalescing:
>>>
>>> http://axion.physics.ubc.ca/scatter/rt.html
>>
>> This is a quite interesting note, unfortunately rendered rather less
>> useful by the graphs, which have no axis names or units marked, and
>> which appear to have inconsistent scales (although this may simply be
>> lack of legibility).  For example:
>>
>>     http://axion.physics.ubc.ca/scatter/rtdt1.gif
>>
>> The units on the vertical axis appear to be labelled:
>>
>>     -0.0001  -5x10-6  0  5x10-6  0.0001
>
> Yes, supermongo, the plotting program, does tend to make 5 and 6 similar
> at the small sizes used in superscripts.  But as you say, it is a linear
> scale, and thus that must be a 5 rather than a 6.
>>
>> which I interpret as -100us, -5 us, 0,  +5us  +100us, but it appears to
>
> Since that is, as you point out, an interpretation that produces silly
> results, it is probably the interpretation that is at fault.
>> be a linear axis.  Is the 5x10-6 supposed to be 5x10-5?  The character
>> needs serious improvement as it looks like 5x10-6 to me.  But in any
>> case, as microseconds are referred to in the text, would it not be a lot
>> clearer if the graphs were also in microseconds, rather than some small
>> fraction of seconds?
>>
>> Similar problems in:
>>
>>     http://axion.physics.ubc.ca/scatter/gps.gif
>>
>> where the -5 and -6 suffices are very difficult to distinguish.  I do
>> hope that Bill can re-do these graphs some time as it would enhance the
>> value of this work.
>
> I am not at all sure that I can find the data anymore to redo the
> graphs.
> About all I could do is to use gimp to replace the axes labels and make
> the 5 look a bit more like 5 rather than 6. The problem is at the small
> scale of the superscripts, the little culicue up at the bottom of the 5
> in the font used becomes a single pixel closing off the the loop, making
> it look a lot like a 6. So I used gimp to open it up again, and also to
> labels on the axes.

Thanks, Bill.  Sounds like an update to "supermongo" is needed!  Can it 
label the axes?  Not critical to regenerate the graphs, but perhaps if 
you add a little to the text to clarify scales and axes it may help others.

Cheers,
David
-- 
Cheers,
David
Web: http://www.satsignal.eu



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