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Scope probes are much much more than “bits of wire”. They are surprisingly sophisticated pieces of technology containing very thin wires and precision components:

  • fragile and easily damaged: a damaged probe is as useful as a chocolate teapot - and can easily waste a day of your (remaining) life
    • do not kink, twist, stretch, or coil the leads, or, as you often see in EBay listings, put a rubber band around them!
    • do not use on high voltages, where high is >20V at some frequencies
    • don't lose the accessories such as grabbers and ground springs - they are vital to the correct operation of the probe
  • expensive: when buying a second-hand scope the rule-of-thumb is to spend more on the probes than on the scope. The Agilent probes cost £130+VAT each, and they are relatively low-cost for a decent probe. Many probes cost >£700, Farnell sells a probe costing £5,500, and some probes cost much more than that
  • vital: your measurements will only be as good as your scope probes and the accessories

Use the right probe for your job:

  • safety don't use any of the HackSpace probes for measuring mains equipment unless you really understand what you are doing; get it wrong and you will kill the scope, the probe, and yourself. The Dunning-Kruger effect is unforgiving, and nobody wants the extra paperwork.
  • 10Mohm *10 “high impedance” probes don't exist, despite what you might have been misled into believing! Typically they are 1000ohms at 10MHz, and only 100ohms at 100MHz. When measuring 100MHz and above, use a 500ohm *10 “low impedance” Z0 probe
  • be aware of how
    • the probe and scope work together to limit the observable frequency and risetimes; see table below
    • the probe's capacitance can affect the circuit you are testing, particularly with *1 probes
    • the effect of inductance in wires can resonate with capacitances and cause circuits to become unstable. E.g. a 6“ ground lead has 150nH of inductance
  • use a *1 probe for low-voltages, and a *10 probe for high frequencies
  • if using a *1/*10 probe, you will eventually find you have it on the wrong setting
  • when using a *10 probe, make sure it is compensated for the scope you are using; if not the waveforms will be distorted. Some scope/probe combinations cannot be properly compensated.
  • digital logic is surprisingly fast, with a correspondingly high bandwidth (N.B. the clock or bit rate is irrelevant, only the rise/falltime matters). 1980s Schottky TTL (which had a fearsome reputation back then!) has a 14ns risetime. Modern jellybean LVC components have a 0.6ns risetime. Some FPGAs are even faster, as is some ECL. If someone can let me use a RPi, then I might be able to verify rumours that the GPIO risetime is ~2ns.

The following table shows the frequency domain and time domain performance of scope/probe combinations, with examples of HackSpace equipment.

Scope Bandwidth Examples
Probe Bandwidth 20MHz 40MHz 60MHz 100MHz 250MHz 500MHz 1000MHz
20MHz 14MHz 24.7ns 18MHz 19.6ns 19MHz 18.4ns 20MHz 17.8ns 20MHz 17.6ns 20MHz 17.5ns 20MHz 17.5ns T5020
40MHz 18MHz 19.6ns 28MHz 12.4ns 33MHz 10.5ns 37MHz 9.4ns 39MHz 8.9ns 40MHz 8.8ns 40MHz 8.8ns
60MHz 19MHz 18.4ns 33MHz 10.5ns 42MHz 8.2ns 51MHz 6.8ns 58MHz 6.0ns 60MHz 5.9ns 60MHz 5.4ns Picotest, ?Kenwood PC-54?
100MHz 20MHz 17.8ns 37MHz 9.4ns 51MHz 6.8ns 71MHz 4.9ns 93MHz 3.8ns 98MHz 3.6ns 100MHz 3.5ns
150MHz 20MHz 17.7ns 39MHz 9.1ns 56MHz 6.3ns 83MHz 4.2ns 129MHz 2.7ns 144MHz 2.4ns 148MHz 2.4ns Agilent 10074C
250MHz 20MHz 17.6ns 39MHz 8.9ns 58MHz 6.0ns 93MHz 3.8ns 177MHz 2.0ns 224MHz 1.6ns 243MHz 1.4ns M12
500MHz 20MHz 17.5ns 40MHz 8.8ns 60MHz 5.9ns 98MHz 3.6ns 224MHz 1.6ns 354MHz 1.0ns 447MHz 0.8ns
1500MHz 20MHz 17.5ns 40MHz 8.8ns 60MHz 5.8ns 100MHz 3.5ns 247MHz 1.4ns 474MHz 0.7ns 832MHz 0.4ns HP10020A
Examples Kenwood cs4125 Topward 7042 Agilent 54621 Schlumberger 5602
Hameg hm203 Tek 2213 Tek tds340
Metrotest ox2030

FFI, have a look at the references in Scope Probe Reference Material

  • equipment/oscilloscopeprobes.1440338794
  • Last modified: 6 years ago
  • by tggzzz