Friday, July 23, 2010

Measuring Propagation Delay with a Universal Counter

A universal counter’s precision timing makes it a great low cost solution for measuring the propagation delay of high speed digital signals through cables, solder runs, or digital logic circuits. Propagation delay measurements are critical for designs that have tight timing specifications. You can measure propagation delay using a counter’s time interval measurement feature. A time interval measurement is an elapsed time measurement between a start event and a stop event. It is comparable to using a stopwatch to time a runner, where the start event is the signal telling the runner to go and the stop event is the runner crossing the finish line. For a counter making a time interval measurement, the start event is an electrical edge and the stop event is an electrical edge occurring later in time.
Why not just use a 2-channel scope that gives a nice visual picture of the edges to determine propagation delay? The reason is simple, for timing measurements a counter is much higher in accuracy and resolution. A typical scope only give you about 5 to 9 digits of useful resolution (unless you are buying a +$80k high end scope) where a counter can easily deliver 10 to 12 digits of useful resolution. For instance Agilent’s 53131A and 53132A universal counters provide single shot measurement resolution down to 750 ps and 350 ps respectively. When I say “single shot resolution” I mean a measurement that only consists of a single electrical edge event. Things get a lot better when we can measure the propagation delay of a medium, like a cable, with a continuous signal. When we do a time interval measurement with a continuous signal we can use averaging to eliminate the affects of random noise. If we do a lot of averaging (done inside the counter) we can resolve down to the noise floor, for the 53132A universal counter that is about 10 ps. Light only travels about 3.3 cm in 10 ps!

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