# How often should I have to set oscilloscope probe's capacitance?

I got a new Owon SDS7102 oscilloscope and I'm using the included oscilloscope probes with it.

The manual says that the proper calibration procedure is to let the oscilloscope work at constant outside temperature for 25 minutes, do the oscilloscope's internal calibration routine and then set the probe capacitance using the included tool. The manual also says that the correct capacitance should be checked often and corrected if necessary. What the manual didn't mention is how often often is.

I expect that checking probes before each important measurement would be needed, but I'm looking for some "rule of thumb" for general use. From my current experience, several hours are enough to see small loss of sharpness of the calibration signal.

The probe capacitance adjustment is used to match the probe time constant, which consists of the probe capacitance and the probe resistance (usually 9 Megohms), to the time constant of the oscilloscope vertical amplifier input (which is usually 1 Megohm and 10 to 20 picofarads). When the time constants match, the overall attenuation of the probe/oscilloscope will be 10 to 1 over a wide frequency range. Normal practice in most labs is to adjust probes when they are first used both to ensure that they and the oscilloscope are operating OK and that nobody made any adjustments inbetween. That adjustment should hold for a day or more unless the temperature changes drastically or the probe and/or oscilloscope are defective. Defective could mean that the variable capacitor in the probe is not mechanically stable enough to hold its setting.

• Normal practice in most labs is to adjust probes when they are first used both to ensure that they and the oscilloscope are operating OK and that nobody made any adjustments in between Many years as a technician. I think I adjusted that once. – Chris K Jun 20 '12 at 23:37
• I don't think the tiny little metal adjustment is made to be fiddled with every day Your probe will probably not have a long life if you do. – jippie Jun 21 '12 at 7:02

For over 99% of the measurements I use a scope for, the probe cal doesn't matter. But every once in a while, you will need some critical high frequency measurement like rise time. That's when you need to calibrate the probe, use a direct ground connection (instead of the alligator clip ground wire) and consider any effects the scope probe loading might have on the circuit.

But worrying about all these details all the time will slow you down. Just know when it matters and when it doesn't. If you're just checking the baudrate of a serial port, you can get by without any explicit ground connection at all...

• When does it matter? – Kortuk Jun 20 '12 at 19:32
• I had to adjust the probes for making accurate RMS measurement. – Nick Alexeev Jun 21 '12 at 6:22