Oscilloscope question

Just a quick comment on this one-- You want an scope with several times as much "bandwidth" as the frequency of the fastest digital signal you will examine-- 4X minimum. The analog frequency content of a digital signal is much faster than its clock rate. Without a substantial margin, the signal will be badly distorted. Analog scopes are cheap and widely available, but they have limited value for troubleshooting digital electronics. They can only show you repetitive signals, or the envelope of a randomly changing signal. They can't show you something that happens only once, or very rarely, but that's the nature of most digital problems. Expensive analog "storage" scopes can hold a signal on the screen, but only if you can figure out how to capture it in the first place. You might as well spend the money on a digital scope. If you're getting a digital scope, you need a "sampling rate" that is also much faster than the frequency of your fastest signal. The Nyquist theory says you need 2X the sampling rate to reconstruct a signal, but that theory applies ONLY to reconstructing a known waveform given a large number of samples. Again, in practice, you want a sampling rate at least 4X your digital clock rate, and preferably more like 10X. This ratio determines how accurately you can measure the position of your signal edges. You will find old digital scopes that quote two sampling rates. The higher figure will be a "repetitive sampling rate" specification that applies ONLY to reconstructing periodic signals such as radio carrier waves or computer clock signals. For non-periodic sources such as logic signals, only the lower "raw" or "real-time" or "one-shot" or "instantaneous" sampling rate matters. Today, even for under $1,000, you can get a digital scope with a true sampling rate much faster than you'll probably ever need. These are usually priced according to analog bandwidth. For troubleshooting old computers, even 100MHz is probably more than sufficient. A good probe reduces the load on the signal being observed, as David Holland said. An o-scope input has a 50-ohm impedance; that kind of load will stop many circuits from working. So-called "active" probes present an almost undetectable load, but you will rarely need anything that sophisticated. If you're going to use a scope for ANYTHING, get this book: Troubleshooting Analog Circuits, Bob Pease, ISBN 0750694998 http://www.amazon.com/exec/obidos/tg/detail/-/0750694998/ Or buy an autographed copy from the author by emailing rap at galaxy.nsc.com. Though written about analog circuits, all of these electrical principles and troubleshooting techniques apply to digital circuits. Let's face it, the circuits themselves are analog anyway. . png