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equipment:oscilloscopeprobes [2015/08/23 15:06]
tggzzz
equipment:oscilloscopeprobes [2020/05/30 11:13]
tggzzz
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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:+===== 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. 
 + 
 +==== 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 [[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// 
 + 
 +==== 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]] 
 + 
 +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   * **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 not use on high voltages**, where high is >20V at some frequencies +    * **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 
-    * **don't lose the accessories** such as grabbers and ground springs - they are vital to the correct operation of the 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 
-  * **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+    * **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" grabbersground 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
  
-Use the right probe for your job+==== Usage ==== 
-  * **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. + 
-  * 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+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.  
 +  * 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 
 +  * 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   * be aware of how
     * the probe and scope work together to limit the observable frequency and risetimes; see table below     * the probe and scope work together to limit the observable frequency and risetimes; see table below
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     * 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.
  
-The following table shows the frequency domain and time domain performance of scope/probe combinations, with examples of HackSpace equipment.+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]]. 
 + 
 +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. 
 + 
 +Note: a scope without a working probe is as useful a bandsaw without a band: i.e. it is junk! 
 + 
 +{{ :equipment:oscilloscope_probes.jpg?nolink |}} 
 + 
 + 
 +==== Probes with Scopes ==== 
 + 
 +The following table shows the // approximate// frequency domain and time domain performance of [[oscilloscopes|scope]]/probe combinations, with examples of HackSpace equipment.
  
-| ^  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 |
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 ^ 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| ::: | ::: | ::: | ::: | ::: | ::: | ::: | ::: |
  • equipment/oscilloscopeprobes
  • Last modified: 12 months ago
  • by tggzzz