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equipment:oscilloscopeprobes [2016/05/02 08:04]
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equipment:oscilloscopeprobes [2020/05/30 11:13] (current)
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| ==== Oscilloscope Probes ==== | ===== Oscilloscope Probes ===== |
| | Probes must be used with oscilloscopes, so it is necessary to understand //and apply// the information on the [[equipment:oscilloscopes|oscilloscopes]] page. If in doubt (particularly about safety), please ask before damaging equipment or people. |
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| //Safety: do not use any scope probe for measuring mains equipment and voltages. Carefully read and understand the safety references in the "Praxis" section of [[https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/|Scope Probe Reference Material]] and [[http://www.tek.com/document/primer/abcs-probes|Tektronix' ABCs of Probes Primer]] pp55-56 - they might save someone's life// | ==== Safety ==== |
| | **Do not use any of HackSpace's scope probes for measuring mains equipment and voltages**, and that includes inside SMPSs. |
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| Scope probes are much //much// more than "bits of wire". There are many different types for different purposes. They are surprisingly sophisticated pieces of technology containing very thin wires and precision components: | Carefully read and understand the safety references in the "Praxis" section of [[https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/|Scope Probe Reference Material]] and [[http://www.tek.com/document/primer/abcs-probes|Tektronix' ABCs of Probes Primer]] pp55-56 - //they might save someone's life// |
| * **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 £5. | |
| | ==== 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 [[https://download.tek.com/document/60W_14232_9_MR_Letter.pdf|Tektronix' Probes and Accessories, Your Guide to Selecting the Right Probe]] |
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| | Hackspace only has the common-or-garden *1 and *10 "high" impedance probes, but it may be possible to borrow others from members. |
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| | 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 | * **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 kink, twist, stretch, or coil the leads**, or, as you often see in EBay listings, put a rubber band around them! |
| * **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 | * **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 | * **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 grabbers and ground springs - they are vital to the correct operation of the probe | * **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 | * **vital**: your measurements will only be as good as your scope probes //and// the accessories |
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| Use the right probe for your job: | ==== Usage ==== |
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| | 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 [[https://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect|Dunning-Kruger effect]] is unforgiving, and nobody wants the extra paperwork. | * **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 [[https://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect|Dunning-Kruger effect]] is unforgiving, and nobody wants the extra paperwork. |
| * read [[http://www.tek.com/document/primer/abcs-probes|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 | * read [[http://www.tek.com/document/primer/abcs-probes|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 |
| * 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 | * 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 | * 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 | * 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//. | * 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. | * 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 [[https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/|Scope Probe Reference Material]]. | For more pointers about safety, choosing and using probes, plus when and how to make your own probes, see [[https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/|Scope Probe Reference Material]]. |
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| The following table shows the frequency domain and time domain performance of [[oscilloscopes|scope]]/probe combinations, with examples of HackSpace equipment. | There are some unused probes in plastic bags on or near the scopes. //Those should **not be used** routinely.// Why? See the warnings below about probes' fragility and ease of damage - and how expensive they are to replace. Instead **use the probes hanging on the equipment rack**. If it is necessary to use one of the probes in the plastic bags, please //put a message on the mailing list// noting why it was necessary. That will enable people to ensure we have the right equipment when members need it. |
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| | Note: a scope without a working probe is as useful a bandsaw without a band: i.e. it is junk! |
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| | {{ :equipment:oscilloscope_probes.jpg?nolink |}} |
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| | ==== Probes with Scopes ==== |
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| | The following table shows the // approximate// frequency domain and time domain performance of [[oscilloscopes|scope]]/probe combinations, with examples of HackSpace equipment. |
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| | ^ Scope Bandwidth ||||||| ^ Examples ^ | | ^ Scope Bandwidth ||||||| ^ Example Probe ^ |
| ^ Probe Bandwidth ^ 20MHz ^ 40MHz ^ 60MHz ^ 100MHz ^ 250MHz ^ 500MHz ^ 1000MHz | ::: | ::: ^ | ^ 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 | | ^ 20MHz | 14MHz 24.7ns | 18MHz 19.6ns | 19MHz 18.4ns | 20MHz 17.8ns | 20MHz 17.6ns | 20MHz 17.5ns | 20MHz 17.5ns ^ | T5020 | |
| ^ 1500MHz | 20MHz 17.5ns | 40MHz 8.8ns | 60MHz 5.8ns | 100MHz 3.5ns | 247MHz 1.4ns | 474MHz 0.7ns | 832MHz 0.4ns ^ | HP10020A | | ^ 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 | | | | | | | ^ Example Scope | Kenwood cs4125 | Topward 7042 | Agilent 54621 | Schlumberger 5602 | | | | | | |
| ^ ::: | Hameg hm203 | ::: | Tek 2213 | Tek tds340 | ::: | ::: | ::: | ::: | ::: | | ^ ::: | Hameg hm203 | ::: | Tek 2213 | Tek tds340 | ::: | ::: | ::: | ::: | ::: | |
| ^ ::: | Metrotest ox2030| ::: | ::: | ::: | ::: | ::: | ::: | ::: | ::: | | ^ ::: | Metrotest ox2030| ::: | ::: | ::: | ::: | ::: | ::: | ::: | ::: | |