OscilloScope Tutorial, Probes

The Care and Feeding of,

Scope-Probes
r
The Heart of the Machine

-Probe Inputs, "Channels A & B"----------

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A Probing Question
Once you have the right scope, the correct probes, and some understanding of what you're doing: you still need to know how to use the probes correctly. I once knew a professor--who should have known better: but he would never use the probe's ground clip; he would leave it off the circuit he was measuring, or remove it completely from the probe. He relied, instead, on the scope's being grounded to the computer through the AC power mains. Needless to say, he got some pretty strange results. This is not what you want to do.


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Probe Assortment

Schematic of a 10X Probe

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Scope Probe Compensation

A 10X Scope Probe Consists of two voltage dividers in parallel: a Resistive (D.C.) divider and a Capacitive (A.C.) Divider. The resistive divider is fixed, and the capacitive divider is adjustable; hence when the capacitive divider = the resistive divider, the probe is said to be Compensated.


Compensation Adjustments can be either on the probe or at the connector shell.	Because every Oscilloscope's input characteristics can have a slight variation, it is a GOOD IDEA to compensate the probe when using it on a "new" scope.

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Conventional Scope Probe Use	R.F. Probe Adaptor

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Some Remedies for Ground Lead Inductance



*Quick & Dirty* IC Scope Probe Jig is Effective	Scope Probe Adaptor as Test Point

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Several "Jigs" used to overcome the ground lead problem

Surface Mount Device Probing

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Illustration of the effects of lead length Inductance
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A "Check-List" for Measurements is as Follows:

1.. Always use "10 X" probes: they load the DUT (device-under-test) ~ 10 Meg ohms @ ~ 10 pfd. A "1 X" probe offers 1 Meg ohm @ ~ 50 pfd. The designation "10 X" refers to the attenuation of the signal by the probe (not gain). In order to attain such light loading by the scope--while maintaining bandwidth--this tradeoff is required.

2.. Make sure the probes are compensated (adjust trimmer at connector housing) if attaching them to a different scope. This ensures maximum fidelity and bandwidth of the signals being eyeballed.

3.. Use the shortest ground lead or clip-lead possible: the shorter the better! Excessive ground lead length introduces unnecessary inductance and can alter the displayed signal, as well as reducing the scope's effective bandwidth (acts like a lowpass filter).

4.. When Measuring very high frequencies--especially in tight spaces--consider using a RF probe (see figure). Also, there are--so-called--FET or active probes, which are non-loading (almost) wideband probes with built-in amplifiers.

5.. When buying probes for your oscilloscope, make sure the probe is of sufficient bandwidth for your particular scope: the probe is the first-order bandwidth determinant of any scope.

Some scopes have such a wide bandwidth, that no passive probe is able to do it justice, and the only way to use the maximum bandwidth of this type of scope is to drive the scope from a 50 ohm source through a 50 ohm coax, terminated into 50 ohms at the scope's input. In fact, some high performance scopes have a 1Meg ohm/50 ohm termination switch for just such occasions.

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----Appendix-A----

Scope Probe Circuit Loading Characteristics

Scope Probe Voltage Derating

--USEFUL--LINKS:

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