Probes must be used with oscilloscopes, so it is necessary to understand and apply the information on the oscilloscopes page. If in doubt (particularly about safety), please ask before damaging equipment or people.

Safety

Do not use any of HackSpace's scope probes for measuring mains equipment and voltages, and that includes inside SMPSs.

Carefully read and understand the safety references in the “Praxis” section of Scope Probe Reference Material and Tektronix' ABCs of Probes Primer pp55-56 - they might save someone's life

Types of Probe

There are many types of scope probe, including *1, *10 “high” impedance, *10 “low” impedance Z0 resistive divider, current, EHV, envelope, active, differential, isolated. Each has advantages and disadvantages, and some are essential in some situations. See Tektronix' Probes and Accessories, Your Guide to Selecting the Right Probe

Hackspace only has the common-or-garden *1 and *10 “high” impedance probes, but it may be possible to borrow others from members.

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

  • 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. The latest Tektronix probe tips cost £250/5, and each one can be used 5 times; yes, each time they touch your circuit costs £10.
  • 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. Definitely do not connect to mains voltages; that requires special (very expensive) types of probe
    • do read the probe's manual, especially to check the max-voltage-vs-frequency graph and also the compensation range; the former often catches out those experienced with probing digital logic voltages
    • shield/ground lead is connected to mains earth, so only attach the shield/ground to circuits that can tolerate it. Get that wrong and your circuit might blow up or large currents could damage the probe, scope and circuit
    • don't lose the accessories such as “witches' hat” grabbers, ground springs, insulating tips - they are vital to the correct and safe operation of the probe. Place accessories in the marked bag underneath the scope probe rack
  • vital: your measurements will only be as good as your scope probes and the accessories

Usage

Basic hints and tips:

  • 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.
  • read Tektronix' ABCs of Probes Primer to understand the different types of probes, when they should and should not be used (don't bother to fill in the marketing information :) ) Safety: read pages 55 and 56
  • 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 which really is 500ohms and therefore higher impedance than a “10Mohm” 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. Consider whether that will damage anything.
  • 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.

For more pointers about safety, choosing and using probes, plus when and how to make your own probes, see Scope Probe Reference Material.

Probes with Scopes

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

Scope Bandwidth Example Probe
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
Example Scope 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
  • Last modified: 4 weeks ago
  • by tggzzz