There are many types of scope probe, each with their own advantages and disadvantages. You need to be able to choose the right type of probe for the job in hand, and use it correctly. If you don’t, at best you will waste time chasing after strange unexpected effects. At worst you will damage the circuit, oscilloscope, or yourself.
Fortunately there is a wealth of information available; these links are a starting point.
- basic introduction: Analog Devices’ High-Speed Time-Domain Measurements—Practical Tips for Improvement is a useful first introduction containing basic theory and practice.
- effective use: Tektronix’ ABCs of Probes Primer is a good all-round discussion of how to choose a probe and use it effectively and safely. Some of the interesting information, including measurements, is scattered through the document. Tektronix XYZs of Oscilloscopes Primer is the equivalent for scopes.
- fundamental theory: Tektronix’ Oscilloscope Probe Circuits is a very detailed description of the theory and practice of probes and their construction. It is from 1969, but the physics hasn’t changed – even if it has been forgotten or is no longer taught!
- high speed digital: Howard Johnson’s Probing High Speed Digital Designs is widely cited introduction to the merits of FET probes and low impedance Z0 probes. Everything else on his site is worth reading and understanding. Recommended.
- practical effects: Jim William’s justly famous AN-47 High Speed Amplifier Techniques shows how to correctly and incorrectly use probes, a good balance between theory and practice, and many photographs and many measurements. Highly recommended.
Homebrew / DIY
- simulation has limitations, but it enables you get a feel for the relative magnitudes of various effects:
- Doug Ford’s The Secret World of Scope Probes is an informal simulation-based description of the high impedance 10:1 probes’ performance.
- simulating 10:1 high impedance probes requires Spice models of lossy transmission lines. There’s a library of model parameters, but note that some Spice engines require the conductance, g, to be zero. That’s probably not a significant limitation – unless you are interested in modelling vintage POTS cables that have been in the ground too long and the waxed paper insulation has become saturated with water.
- low impedance Z0 probes:
- Douglas C Smith constructs a simple DC to 1GHz probe, but doesn’t present measurements. He also creates and measures a novel Balanced Passive Probe, which might be beneficial when probing signals that aren’t referenced to ground, e.g. ECL
- Janne Ahonen constructed a quick and dirty low impedance Z0 probe, Wideband 1:21 1k DIY Oscilloscope Probe and measured its performance
- Paolo Renato constructs and gives Praise for the Lo-Z Probe, with one measurement
- active probes:
- Daniel Kramnik constructed an active probe, 1GHz Active Differential Probe. The hoped-for 1GHz ended up as a 300MHz, thus illustrating how performance requires a deep understanding and attention to detail.
- David Jewsbury constructed another active probe Poor Man’s 1GHz Active Probe, and claims a 1.5GHz bandwidth but doesn’t present measurements
- standard 10:1 high impedance probes:
- these workhorse probes can introduce aberrations above ~80MHz, which are often visible as ringing in medium speed digital signals. A 3D printed accessory can be retrofitted to any standard 10:1 high-impedance probe to significantly reduce the signal integrity distortion
- alternative ways of connecting a probe are to solder a testpoint into the circuit, either a commercial product or more likely homebrew wire coils or paper clips. This is justifiably a favoured technique when breadboarding analogue circuits, but is more problematic with densely populated digital circuits
- a succinct introduction to the key characteristics of high impedance and active probes is “Are you measuring your circuit or your scope probe?” by Frost, Whiteman, Tsai. It also contains two diagrams showing a mechanical technique for reducing tip capacitance
Praxis & Safety
- practical safety: loud noises and foul smells can be the consequence of inappropriately connecting a scope probe’s ground lead. If you have the time, EEVblog #279 – How NOT To Blow Up Your Oscilloscope! is a good lecture, presented in “his own inimitable style”.
- high voltages: loud noises followed by deathly quiet can be the result of measuring high voltages incautiously. All too frequently floating or isolating the scope is a suggested “solution”, but that is almost always dangerous. Fortunately Tektronix’ Floating Oscilloscope Measurements, Fundamentals of Floating Measurements and Isolated Input Oscilloscopes and Floating Oscilloscope Measurements … And Operator Protection indicate how to probe mains voltages safely.
- “floating” the scope: to emphasise the previous bullet point, you should never disconnect the scope’s protective mains earth. Too often you find posts to the effect that “I floated a scope and lived to tell the tale (so it can’t be that dangerous)”. Here’s an anecdote about someone that took precautions and still killed themself.
- cheap probes: examples of the problems with very cheap probes are illustrated in a blog article POS ebay ‘scope probes
- scope+probe system bandwidth: a table denoting the system bandwidth and risetime of various common scopes and probes, plus simple hints and tips
- triggering: without a solid trigger it can be impossible to adequately capture a signal. Often triggering is more important than scope bandwidth or the sampling rate, but too often it is neglected in scope design, scope reviews and scope recommendations. Troubleshooting problems on different customers’ sites is a good way to develop informal practical guidelines about what to look for and avoid in signals, connections, scopes and scope reviews