Classic Sampling Scope

for the lab bench or a hostile environment

Classic Sampling Scope

Postby rickj » Wed Mar 12, 2014 3:39 pm

There are lots of cheap, low-performance oscilloscopes out there, and there's no lack of expensive, high-performance ones. I'd like to try for a compromise; a cheap, high-performance oscilloscope.

A promising architecture is the Classic Sampling Scope. This gives high bandwidth and excellent overload performance. Beyond the analog input front-end, and the timebase, it needs no high-speed circuitry, so it lends itself to a high level of integration using cheap, off-the-shelf integrated circuits. Jim Williams, the analog Guru, swore by his Tektronix 661: see for instance Linear Application note 74, pp20, in which he says "Unfortunately, classical sampling oscilloscopes are no longer manufactured, so if you have one, take care of it!"

This is no longer quite true: there is now one made by Pico Technology, but they start at almost ten thousand dollars and go up from there.

If you are not sure about distinctions in 'scope architectures, there's an excellent explanation (by Jim Williams) in Linear Technology Application note 120 pp 14.

What I'd like to do is to equal or surpass the performance of my 661, in a compact, modern package using stable solid-state circuitry (yes, Fred, the 22kg 661 has vacuum tubes in it). That means two channels of 4GHz bandwidth (90ps risetime), 2mV/division sensitivity, and a timebase good to 10ps per division. Have a look at the draft specification. I'd like to do it with a three-figure price tag, which I think is possible if we can sell a thousand or two of the beasts. If we conspire to add too many features, it will be in the low four figures.

To keep down the clutter on my crowded workbench, I'd like the 'scope to be small and self-contained, rather than an add-on to a computer. It's not possible to beat the value of an LCD monitor as a display, and that will tuck away at the back of the bench, but I'd like to drive it directly from the 'scope with just a mouse to run the controls.

To do this we need a fairly powerful processor in the 'scope, but thanks to the cellphone and tablet industries these are cheap and plentiful. The processor can earn its keep with features such as FFT frequency analysis. We should make it easy to extend the software of the 'scope to new functions - that's what open-source does best.

What features and specifications would you like to see?
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electronic instrument, oscilloscope, sampling

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