Those are some very good suggestions, Robert. Just a few clarifications: On Mon, Nov 3, 2008 at 12:40 PM, Robert Young wrote: > Some important points to remember about a scope (even the cheapo Vellman stuff): > 1) If using a x10 or x100 probe (x10 is much more common) learn to compensate the probe so that you get the best impedance match and good looking edges. I can't think of any x1 probes I've ever seen that had compensation that worked for a damn if at all. They just have too much capacitance. There are times when 50 Ohm or 500 Ohm direct connect cables and probes are nice and other times when you have to build your own too. As far as I know, compensation wouldn't mean anything for a 1X probe. What compensation is doing is matching the capacitance across the attenuation resistor (which doesn't exist in a 1X probe) to the load capacitance (cable plus scope). This makes the attenuation flat across frequency. > > 2) If you could have just one probe on your bench, use an x10 probe to minimize the loading of your signal. You can get active probes that have even less capacitive loading and all kinds of special probing jigs and connectors but a nice x10 gets you a long, long way. My personal preference is for fixed x10 probes, not the switchable kind. And many Tek and HP scopes have a little ring and pin setup that can automatically tell the oscilloscope what kind of probe is attached. A very nice feature to look for. > This only works if you have a probe which is equipped to report its type to the scope. > 3) Don't use the 6" ground clip for anything except quick probing. It is useless for looking at fast edges. Do this instead: > http://www.pericom.com/pdf/applications/AN021.pdf There are other descriptions of the technique out there but the picture in this app note is pretty clear. Its all about the ground loop... This advice about the clip is absolutely right, BUT, the term "ground loop" refers to something completely different. "Ground loop" means treating a ground wire or trace as having no voltage drop across it when in fact it does because it is carrying significant current or has a high impedance. In this case, what is happening is that the clip acts like an inductor or an antenna, causing ringing or bandwidth reduction or pickup of stray signals. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist