[time-nuts] Experience with THS788 from TI?
David
davidwhess at gmail.com
Thu Mar 22 15:20:28 UTC 2012
On Wed, 21 Mar 2012 18:52:33 -0400 (EDT), SAIDJACK at aol.com wrote:
>Alternatively, if you want to design your own, you could use a
>time-expander. It only requires a small micro with counter/timer, and a little bit of
>external circuitry for charging/discharging a precision cap. You charge
>fast gated by the signal to measure, then you discharge slowly (expanded time)
>and measure the amount of charge deposited on the cap. The Linear Appnote
>I mentioned earlier already has most of the capacitor charge pump circuitry
>in it that you would need for this.
>
>The idea is to design the cap discharge and charge cycles at different
>time scales, say 1000x to 1, so that the capture time get's expanded out to
>intervals that the micro can measure. If the micro has say 60MHz counter
>resolution (16.66ns) then a 1000x to 1 expansion would allow a 0.016ns (16ps)
>resolution. Using say 2000x expansion and a 100MHz counter in the micro
>would get you to 5ps resolution.
The Tektronix 2230 and 2232 oscilloscopes I mentioned earlier work
this way. The 2230 service manual gives a very detailed description
of the circuit. The 2230 uses the 50ns sampling clock as a
calibration source and divides it into 200 counts for 250ps of
resolution which is all that was needed given the oscilloscope
bandwidth and record length.
The Tektronix 7T11/7T11A TTH (Time to Height converter) is
considerably more complex but has a resolution in the ps range and
high throughput (50Ksamples/sec) as well. It uses an FET input
capacitance feedback integrator (Miller Integrator) with an FET reset
that includes gate charge compensation. At the fastest sweep speed,
the output changes by 10ps/mv. Part of the complexity comes from
being designed to measure negative time as well if the trigger and
strobe are reversed.
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