[time-nuts] Some results of PRS10 and Trimble Resolution
cfmd at bredband.net
Wed Jun 28 12:45:40 EDT 2006
From: "Stephan Sandenbergh" <stephan at rrsg.ee.uct.ac.za>
Subject: Re: [time-nuts] Some results of PRS10 and Trimble Resolution
Date: Wed, 28 Jun 2006 18:22:58 +0200
Message-ID: <005f01c69acf$1e766060$401c9e89 at Stephan>
> So, the only option is to go for a higher clock speed. What are your
> experiences with the resulting EMI? Is it a problem? How do you combat it?
You interpolate using analogue interpolators. Take a look into the HP5335A
counter for instance. It happilly acheives 1 ns single-shot resolution but it
counts at a mear 10 MHz clock. The analog interpolators of the HP5335A acts
like pulse-stretchers and will make the pulse 200 times longer. It will see a
pulse which is 100-200 ns long, but will convert it into a pulse which is
20-40 us long, which turns into 200 to 400 counts long in its 10 MHz clock.
These interpolator counters is actually in normal TTL in the HP5335A, and is
actually only 8-bit, so they will wrap-around, but the software will de-wrap it
since there is no ambiguity in reality. The cost for the interpolators
themselfs is not very high, not compared to all the fancy stuff around them
anyway. It will cost some additional logic for error-pulse generation and also
for the additional counters (you usually have one start and one stop counter).
Another cost is the longer conversiontime. However, if you work on PPS clocks,
then a conversion-time of 40 us is very quick anyway and not much of a problem.
If you scale this design a little, getting 200 ps or 100 ps should be reachable
without too much of an effort. Infact, I suspect that this is exactly what HP
did in the Z3801. Their measurement FPGA was really not up to any magic in
Soo, if you want to improve your measurement resolution only, it is simple.
In a very similar fashion, you can actually do the revese, in order to
compensate for the time error of a signal in your clock. However, thus designs
you can buy canned if you don't want to DIY. A DAC working as a programable
current source (preferably through a current mirror) and a cap with a buffer
and comparator aught to do it.
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