[time-nuts] A counter for phase measures

Magnus Danielson magnus at rubidium.dyndns.org
Sat Nov 3 15:28:42 UTC 2012


On 11/03/2012 03:10 PM, Volker Esper wrote:
>
> Thank you for the interesting information. Now, the time has come to
> look for an adequate counter - anyone who has experience with the HP
> 53132A and the SR620? If they both where at - say 1000 USD - what would
> you prefer for the job of phase measurement? I've read about that
> massive single shot capability of the SR, but - as being a newbie - is
> there anything I overlook at this moment?

For short time-scales, single shot resolution and trigger jitter 
dominates your measurement floor.

Single-shot resolution is the time resolution by which you make a single 
measurement.

Trigger jitter is the noise at the trigger point. it's a combination of 
thermal noise and the slew-rate at the trigger points. It is often that 
trigger jitter is dominated by slew-rate, but there is also internal 
sources of trigger jitter. The slope dependent trigger jitter follows 
the formula:

t_jitter = v_noise / s_slew

t_jitter is the trigger jitter (s)
v_noise is the noise power (V)
s_slew is the slew rate (V/s)

When the time-span of a measurement is long, long-term stability comes 
in as well as systematic drifts. Also, systematic noise such as hum also 
becomes important.

To see how much you depend on slew-rate limitation, you can reduce the 
amplitude, and as this reduces the slew-rate you can separate the 
slew-rate dependent jitter from the intrinsic jitter of the input. It 
also helps you to identify if you need to work on the slew-rate limit 
rather than anything else.

So, it may not be the single-shot resolution which limits you, but a 
combination of things.

I would recommend you to pick up a SR620. It has 4 ps single shot 
resolution and about 25 ps jitter (but you can get less). That is 
significantly better than the 53152A provides.

SR620 manual (one of many links):
http://ilrs.gsfc.nasa.gov/docs/timing/sr620_manual.pdf

Cheers,
Magnus



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