[time-nuts] Experience with THS788 from TI?

[David]

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.