All about Probes

Oscilloscopes are widely used for test and repair of electronics equipment of all types. However it is necessary to have a method of connecting the input of the oscilloscope to the point on the equipment under test that needs monitoring.

Connecting a probe to a circuit can affect the operation of the circuit, and an oscilloscope can only display and measure the signal that the probe delivers to the oscilloscope input. The probe must have minimum impact on the probed circuit and it must maintain adequate signal fidelity for the desired measurements, or the result can be wrong or misleading.

Oscilloscope probes may be categorized into two main types, and they can fall into one of two main areas:

  • Passive oscilloscope probes:   This type of probe is the one that is in most widespread use. It only includes passive elements and may provide 1:1, i.e. straight through connectivity from the point under test, to the scope input. Other types may provide a defined degree of attenuation.
  • Active oscilloscope probes:   As indicated by the name, this type of scope probe has active components incorporated within the probe itself. This enables greater levels of functionality and higher levels of performance to be attained. However they are much more expensive and normally reserved for more exacting or specialist requirements.

Passive voltage probes are typically specified by bandwidth and attenuation factors such as 1X or 10X. Attenuation factor represents the ratio of input to output signal amplitude.

Scope probes are generally classified according to the level of attenuation of the signal they provide.

Types including

  • 1X (giving a 1 : 1 attenuation ratio)
  • 10X (giving a 10 : 1 attenuation ratio)

1.1X scope probes

The most basic form of oscilloscope probe, or scope probe, is what is often termed the 1X probe. It is so called because this type of scope probe does not attenuate the incoming voltage as many other probes do. It consists of a connector to interface to the oscilloscope (generally a BNC connector), and a length of coax which is connected to the probe itself. This comprises a mechanical clip arrangement so that the probe can be attached to the appropriate test point, and an earth or ground clip to be attached to the appropriate ground point on the circuit under test.

The 1X probes are suitable for many low frequency applications. They typically offer the same input impedance of the oscilloscope which is normally 1 M Ohm.

2. 10X scope probes  

To enable better accuracy to be achieved higher levels of impedance are required. To achieve this attenuators are built into the end of the probe that connects with the circuit under test. The most common type of probe with a built in attenuator gives an attenuation of ten, and it is known as a 10X oscilloscope probe. The attenuation enables the impedance presented to the circuit under test to be increased by a factor of ten, and this enables more accurate measurements to be made. In particular the level of capacitance seen by the circuit is reduced                   and this is reduces the high frequency loading of the circuit by the probe.

As the 10X probe attenuates the signal by a factor of ten, this obviously means that the signal entering the scope itself is reduced. The 10X scope probe uses a series resistor (9 M Ohms) to provide a 10 : 1 attenuation when it is used with the 1 M Ohm input impedance of the scope itself. A 1 M Ohm impedance is the standard impedance used for oscilloscope inputs and therefore this enables scope probes to be interchanged between oscilloscopes of different manufacturers. 

10X oscilloscope probes also allow some compensation for frequency variations present. A capacitor network is embodied into the probe as shown. The capacitor connected to ground can then be used to equalize the frequency performance of the probe.
Most oscilloscopes have a small square wave oscillator output. By attaching the oscilloscope probe to this a quick adjustment can be made. As the square wave requires all the harmonics to be present in the correct proportions to provide a “square” wave, the probe can be quickly adjusted accordingly. If the leading edges of square wave, when viewed on the screen has rounded corners, then the high frequency response of the probe is low and an adjustment can be made. However if the leading edges have spikes and rise too high, falling back to the required level, then the high frequency response has been enhanced and this needs to be adjusted.  Only when the square wave is truly square is the frequency response correct.

It is generally accepted that for general-purpose mid-to-low-frequency (less than around 500-MHz) measurements, high-impedance passive probes such as a 10:1 probe is the most suitable option.

3. Differential oscilloscope probes

In some instances it may be necessary to measure differential signals. Low level audio, disk drive signals and many more instances use differential signals and these need to be measured as such. One way of achieving this is to probe both lines of the differential signal using one probe each line as if there were two single ended signals, and then using the oscilloscope to add then differentially (i.e. subtract one from the other) to provide the difference

Using two scope probes in this way can give rise to a number of problems. The main one is that single ended measurements of this nature do not give the required rejection of any common mode signals (i.e. Common Mode Rejection Ratio, CMMR) and additional noise is likely to be present. There may be a different cable length on each probe that may lead to a time differences and a slight skewing between the signals.

To overcome this a differential probe may be used. This uses a differential amplifier at the probing point to provide the required differential signal that is then passed along the scope probe lead to the oscilloscope itself. This approach provides a far higher level of performance.

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. 

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 low pass filter).
  4. When Measuring very high frequencies–especially in tight spaces–consider using a RF probe. (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.
  6. 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, terminate  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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