First I should disclose that I work for Tektronix, and have done so for 30 years. So I have worked directly with many generations of oscilloscopes. Good oscilloscopes can make timing measurements of up to about a millisecond with timing errors of 10's of nanoseconds.
There are a very large range of oscilloscopes using different technologies over the past few decades, so there are several different answers to your questions. The range of bandwidths and new instrument prices range over a factor of roughly 1,000. Bandwidths range from about 30 MHz to 70 GHz, and prices from a few hundred dollars to a few hundred thousand dollars US. Many high-end scopes do have external clock inputs (usually 10 MHz), but this feature is not usually available on lower end or older oscilloscopes. This only affects the accuracy of longer time interval measurements (over 1 us), where the internal clock error is usually not any better than 1 ppm (and 50 ppm for low cost oscilloscopes). Internal timebase error for a 1 second measurement interval is in the several 10's of ppm for most low-end to mid-range oscilloscopes. Short-term time measurement error for high-end oscilloscopes can be under 1 ps, and in equivalent time mode is limited by trigger and aperture jitter. Lower cost oscilloscopes might have short term timing errors of several ns, limited by jitter and risetime (bandwidth). Analog oscilloscopes have analog timebases which have large errors compared to digital oscilloscopes. They have been obsolete for about 20-25 years. There are several digital oscilloscope technologies. * The easiest to understand is real time sampling, where you have a sampler running at a high sampling rate and each sample is converted in to a digital value in real time. A single trigger event results in a waveform capture, but the instrument can capture a waveform without a trigger event. The memory length can be large (1 G sample in some cases), but the sampling rate is limited by the need for the A/D to complete the conversion in real time. Using internal A/D interleaving, waveform sampling rates up to 200 GS/s are available, but the cost is very high in such cases. Lower cost recent models typically have sampling rates of 1 to 5 GS/s. * Many (but not all) real time oscilloscopes also offer random equivalent time sampling. In this case the instrument samples at a much lower sampling rate than would be required for the chosen time/division setting and waveform length. So, for example, every 10th waveform point might be filled after each trigger. In this mode, the signal has to trigger the instrument multiple times (in some cases thousands of trigger events) before all of the waveform record points are filled. The sample points are purposely randomly delayed at the start of each sub-acquisition cycle and the displayed points appropriately skewed so that all waveform samples are filled without aliasing. This gives you much better time resolution than with real time sampling, but the trigger must be extremely accurate and the signal being measured must have low jitter. Unless you want to get an eye pattern, the signal must be repetitive (same exact waveform on each trigger). A trigger is required for this mode to build up a waveform. * Sequential sampling oscilloscopes: These instruments are often referred to as "sampling scopes", although this is now a misnomer since all digital (and some analog) oscilloscopes use a sampler. This is a sequential form or equivalent time sampling, and only one sample (or less) is acquired for each trigger event. So a 1,000 point waveform requires that 1,000 triggers must be accepted, and since the trigger processing rate is usually on the order of 200 k/sec only about 1/1000 of the trigger events are actually used if the trigger signal is 200 MHz. An external separate trigger input is usually required. This instrument has the highest time resolution and short-term timing accuracy and vertical accuracy of any oscilloscope type, but must be used for stable repetitive signals. The waveform update rate of equivalent time mode (repetitive or sequential) is much slower than real time mode due to the many waveforms which must be acquired in equivalent time mode to build up a waveform record. -- Bill Byrom N5BB Tektronix RF Application Engineer ----- Original message ----- From: Hal Murray <[email protected]> To: Discussion of precise time and frequency measurement <[email protected]> Cc: [email protected] Subject: Re: [time-nuts] Using 5335 frequency counter for timing Date: Sun, 07 May 2017 20:11:58 -0700 [email protected] said: > None of them will do as well as a really fast scope. How accurate is the clock in a scope? Do the high end scopes have an external clock input? I remember playing with a scope many years ago. Trigger on a PPS from a GPS, look at the next PPS. It should be 1 second later. I think the scope I was using was off by 6 PPM. I'd expect that there would be a crossover. For short times, the scope would be better. For long times, the better crystal (or external input) in the 5335 would take over. -- These are my opinions. I hate spam. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
