![ScopeProbe](https://upload.wikimedia.org/wikipedia/commons/6/67/ScopeProbe.JPG)
A test probe is a physical device used to connect
electronic test equipment
Electronic test equipment is used to create signals and capture responses from electronic devices under test (DUTs). In this way, the proper operation of the DUT can be proven or faults in the device can be traced. Use of electronic test equipmen ...
to a
device under test A device under test (DUT), also known as equipment under test (EUT) and unit under test (UUT), is a manufactured product undergoing testing, either at first manufacture or later during its life cycle as part of ongoing functional testing and calibra ...
(DUT). Test probes range from very simple, robust devices to complex probes that are sophisticated, expensive, and fragile. Specific types include test prods, oscilloscope probes and
current probes. A test probe is often supplied as a test lead, which includes the probe, cable and terminating connector.
Voltage
Voltage probes are used to measure voltages present on the DUT. To achieve high accuracy, the test instrument and its probe must not significantly affect the voltage being measured. This is accomplished by ensuring that the combination of instrument and probe exhibit a sufficiently high impedance that will not load the DUT. For AC measurements, the reactive component of impedance may be more important than the resistive.
Simple test leads
![Mastech test leads](https://upload.wikimedia.org/wikipedia/commons/f/f9/Mastech_test_leads.JPG)
A typical
voltmeter
A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit.
Ana ...
probe consists of a single wire ''test lead'' that has on one end a connector that fits the voltmeter and on the other end a rigid, tubular plastic section that comprises both a handle and probe body. The handle allows a person to hold and guide the probe without influencing the measurement (by becoming part of the electric circuit) or being exposed to dangerous voltages that might cause
electric shock
Electrical injury is a physiological reaction caused by electric current passing through the body. The injury depends on the density of the current, tissue resistance and duration of contact. Very small currents may be imperceptible or produce ...
. Within the probe body, the wire is connected to a rigid, pointed metal tip that contacts the DUT. Some probes allow an
alligator clip
Alligator clip
A crocodile clip or alligator clip is a plier-like spring-tensioned metal clip with elongated, serrated jaws that is used for creating a temporary electrical connection. This simple mechanical device gets its name from the rese ...
to be attached to the tip, thus enabling the probe to be attached to the DUT so that it need not be held in place.
Test leads are usually made with finely stranded wire to keep them flexible, of wire gauges sufficient to conduct a few amperes of
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
. The insulation is chosen to be both flexible and have a breakdown voltage higher than the voltmeter's maximum input voltage. The many fine strands and the thick insulation make the wire thicker than ordinary hookup wire.
Two probes are used together to measure voltage, current, and two-terminal components such as resistors and capacitors. When making DC measurements it is necessary to know which probe is positive and which is negative, so by convention the probes are colored red for positive and black for negative. Depending upon the accuracy required, they can be used with signal frequencies ranging from DC to a few
kilohertz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one h ...
.
When sensitive measurements must be made (e.g., very low voltages, or very low or very high resistances) shields, guards, and techniques such as four-terminal
Kelvin sensing
Four-terminal sensing (4T sensing), 4-wire sensing, or 4-point probes method is an electrical impedance measuring technique that uses separate pairs of electric current, current-carrying and voltage-sensing electrodes to make more accurate measur ...
(using separate leads to carry the measuring current and to sense the voltage) are used.
Tweezer probes
![Welleman-Tweezers for SMD-TLM12](https://upload.wikimedia.org/wikipedia/commons/6/6d/Welleman-Tweezers_for_SMD-TLM12.JPG)
Tweezer probes are a pair of simple probes fixed to a
tweezer
Tweezers are small hand tools used for grasping objects too small to be easily handled with the human fingers. Tweezers are thumb-driven forceps most likely derived from tongs used to grab or hold hot objects since the dawn of recorded history. ...
mechanism, operated with one hand, for measuring voltages or other electronic circuit parameters between closely spaced pins.
Pogo pins
Spring probes (a.k.a. "
pogo pin
A pogo pin or spring-loaded pin is a type of electrical connector mechanism that is used in many modern electronic applications and in the electronics testing industry. They are used for their improved durability over other electrical contacts, a ...
s") are spring-loaded pins used in electrical
test fixture
A test fixture is an environment used to consistently test some item, device, or piece of software. Test fixtures can be found when testing electronics, software and physical devices.
Electronics
In testing electronic equipment such as circuit ...
s to contact test points, component leads, and other conductive features of the DUT (Device Under Test). These probes are usually press-fit into probe sockets, to allow their easy replacement on
test fixture
A test fixture is an environment used to consistently test some item, device, or piece of software. Test fixtures can be found when testing electronics, software and physical devices.
Electronics
In testing electronic equipment such as circuit ...
s which may remain in service for decades, testing many thousands of DUTs in
automatic test equipment
Automatic test equipment or automated test equipment (ATE) is any apparatus that performs tests on a device, known as the device under test (DUT), equipment under test (EUT) or unit under test (UUT), using automation to quickly perform measurements ...
.
Oscilloscope probes
Oscilloscopes display the instantaneous waveform of varying electrical quantities, unlike other instruments which give numerical values of relatively stable quantities.
Scope probes fall into two main categories: passive and active.
Passive scope probes contain no active electronic parts, such as
transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
s, so they require no external power.
Because of the high frequencies often involved,
oscilloscope
An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...
s do not normally use simple wires ("flying leads") to connect to the DUT. Flying leads are likely to pick up interference, so they are not suitable for low-level signals. Furthermore, the inductance of flying leads make them unsuitable for high frequency signals. Instead, a specific ''scope probe'' is used, which uses a
coaxial cable
Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a p ...
to transmit the signal from the tip of the probe to the oscilloscope. This cable has two main benefits: it protects the signal from external electromagnetic interference, improving accuracy for low-level signals; and it has a lower inductance than flying leads, making the probe more accurate for high-frequency signals.
Although coaxial cable has lower inductance than flying leads, it has higher capacitance: a typical 50 ohm cable has about 90 pF per meter. Consequently, a one-meter high-impedance direct (1×) coaxial probe may load the circuit with a capacitance of about 110 pF and a resistance of 1 megohm.
Oscilloscope probes are characterised by their frequency limit, where the amplitude response has fallen by 3 dB, and/or by their
rise time In electronics, when describing a voltage or current step function, rise time is the time taken by a signal to change from a specified low value to a specified high value. These values may be expressed as ratiosSee for example , and . or, equivale ...
. These are related as (in round figures)
:
Thus a 50 MHz probe has a rise time of 7 ns. The response of the combination of an oscilloscope and a probe is given by
:
For example, a 50 MHz probe feeding a 50 MHz scope will give a 35 MHz system. It is therefore advantageous to use a probe with a higher frequency limit to minimize the effect on the overall system response.
Passive probes
![Test probe](https://upload.wikimedia.org/wikipedia/commons/1/11/Test_probe.jpg)
To minimize loading, attenuator probes (e.g., 10× probes) are used. A typical probe uses a 9 megohm series resistor shunted by a low-value capacitor to make an RC compensated divider with the cable capacitance and scope input. The RC time constants are adjusted to match. For example, the 9 megohm series resistor is shunted by a 12.2 pF capacitor for a time constant of 110 microseconds. The cable capacitance of 90 pF in parallel with the scope input of 20 pF (total capacitance 110 pF) and 1 megohm also gives a time constant of 110 microseconds. In practice, there will be an adjustment so the operator can precisely match the low frequency time constant (called compensating the probe). Matching the time constants makes the attenuation independent of frequency. At low frequencies (where the resistance of ''R'' is much less than the reactance of ''C''), the circuit looks like a resistive divider; at higher frequencies (resistance much greater than reactance), the circuit looks like a capacitive divider.
The result is a frequency compensated probe for modest frequencies that presents a load of about 10 megohms shunted by 12 pF. Although such a probe is an improvement, it does not work when the time scale shrinks to several cable transit times (transit time is typically 5 ns). In that time frame, the cable looks like its characteristic impedance, and there will be reflections from the transmission line mismatch at the scope input and the probe that causes ringing. The modern scope probe uses lossy low capacitance transmission lines and sophisticated frequency shaping networks to make the 10× probe perform well at several hundred megahertz. Consequently, there are other adjustments for completing the compensation.
A directly connected test probe (so called 1× probe) puts the unwanted lead capacitance across the circuit under test. For a typical
coaxial cable
Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a p ...
, loading is of the order of 100pF per meter (the length of a typical test lead).
Attenuator probes minimize capacitive loading with an attenuator, but reduce the magnitude of the signal delivered to the instrument. A 10× attenuator will reduce the capacitive load by a factor of about 10. The attenuator must have an accurate ratio over the whole range of frequencies of interest; the input impedance of the instrument becomes part of the attenuator. A DC attenuator with resistive divider is supplemented with capacitors, so that the frequency response is predictable over the range of interest.
The RC time constant matching method works as long as the transit time of the shielded cable is much less than the time scale of interest. That means that the shielded cable can be viewed as a lumped capacitor rather than an inductor. Transit time on a 1-meter cable is about 5 ns. Consequently, these probes will work to a few megahertz, but after that transmission line effects cause trouble.
At high frequencies, the probe impedance will be low.
The most common design inserts a 9 megohm resistor in series with the probe tip. The signal is then transmitted from the probe head to the oscilloscope over a special lossy coaxial cable that is designed to minimize
capacitance
Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...
and
ringing
Ringing may mean:
Vibrations
* Ringing (signal), unwanted oscillation of a signal, leading to ringing artifacts
* Vibration of a harmonic oscillator
** Bell ringing
* Ringing (telephony), the sound of a telephone bell
* Ringing (medicine), a ri ...
. The invention of this cable has been traced to John Kobbe, an engineer working for
Tektronix
Tektronix, Inc., historically widely known as Tek, is an American company best known for manufacturing test and measurement devices such as oscilloscopes, logic analyzers, and video and mobile test protocol equipment.
Originally an independent ...
. The resistor serves to minimize the loading that the cable capacitance would impose on the DUT. In series with the normal 1 megohm input impedance of the oscilloscope, the 9 megohm resistor creates a 10× voltage divider so such probes are normally known as either ''low cap(acitance)'' probes or ''10×'' probes, often printed with the letter ''X'' or ''x'' instead of the multiplication sign, and usually spoken of as "a times-ten probe".
Because the oscilloscope input has some parasitic capacitance in parallel with the 1 megohm resistance, the 9 megohm resistor must also be bypassed by a capacitor to prevent it from forming a severe RC
low-pass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter des ...
with the 'scope's parasitic capacitance. The amount of bypass capacitance must be carefully matched with the input capacitance of the oscilloscope so that the capacitors also form a 10× voltage divider. In this way, the probe provides a uniform 10× attenuation from DC (with the attenuation provided by the resistors) to very high AC frequencies (with the attenuation provided by the capacitors).
In the past, the bypass capacitor in the probe head was adjustable (to achieve this 10× attenuation). More modern probe designs use a
laser-trimmed thick-film Thick-film technology is used to produce electronic devices/modules such as surface mount devices modules, hybrid integrated circuits, heating elements, integrated passive devices and sensors. Main manufacturing technique is screen printing ( stenc ...
electronic circuit in the head that combines the 9 megohm resistor with a fixed-value bypass capacitor; they then place a small adjustable capacitor in parallel with the oscilloscope's input capacitance. Either way, the probe must be adjusted so that it provides uniform attenuation at all frequencies. This is referred to as ''compensating the probe''. Compensation is usually accomplished by probing a 1 kHz
square wave
A square wave is a non-sinusoidal periodic waveform in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum. In an ideal square wave, the transitions b ...
and adjusting the compensating capacitor until the oscilloscope displays the most square waveshape. Most oscilloscopes have a 1 kHz calibration source on their front panels since probe compensation must be done every time a 10:1 probe is attached to an oscilloscope input. Newer, faster probes have more complex compensation arrangements and may occasionally require further adjustments.
100× passive probes are also available, as are some designs specialized for use at very high voltages (up to 25 kV).
Passive probes usually connect to the oscilloscope using a
BNC connector
The BNC connector (initialism of "Bayonet Neill–Concelman") is a miniature quick connect/disconnect radio frequency connector used for coaxial cable. It is designed to maintain the same characteristic impedance of the cable, with 50 ohm and ...
. Most 10× probes are equivalent to a load of about 10-15 pF and 10 megohms on the DUT, while 100× probes typically present a 100 megohm load and a smaller capacitance, and therefore load the circuit less.
Lo Z probes
Z
0 probes are a specialized type of low-capacitance passive probe used in
low-impedance, very-high-frequency circuits. They are similar in design to 10× passive probes but at much lower impedance levels. The probe cables usually have a characteristic impedance of 50 ohms and connect to oscilloscopes with a matched 50 ohm (rather than a 1 megohm) input impedance). High-impedance scope probes are designed for the conventional 1 megohm oscilloscope, but the 1 megohm input impedance is only at low frequency; the input impedance is not a constant 1 megohm across the probe's bandwidth but rather decreases with frequency. For example, a Tektronix P6139A input impedance starts falling above 10 kHz and is about 100 ohms at 100 MHz. A different probe technique is needed for high frequency signals.
A high frequency oscilloscope presents a matched load (usually 50 ohms) at its input, which minimizes reflections at the scope. Probing with a matching 50-ohm transmission line would offer high frequency performance, but it would unduly load most circuits. An attenuator (resistive divider) can be used to minimize loading. At the tip, these probes use a 450 ohm (for 10× attenuation) or 950 ohm (for 20× attenuation) series resistor. Tektronix sells a 10× divider probe with a 9 GHz bandwidth with a 450 ohm series resistor. These probes are also called resistive divider probes, since a 50 ohm transmission line presents a purely resistive load.
The ''Z
0'' name refers to the
characteristic impedance
The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in ...
of the oscilloscope and cable. The matched impedances provide better high-frequency performance than an unmatched passive probe can achieve, but at the expense of the low 500-ohm load offered by the probe tip to the DUT. Parasitic capacitance at the probe tip is very low so, for very high-frequency signals, the Z
0 probe can offer lower loading than any hi-Z probe and even many active probes.
In principle this type of probe can be used at any frequency, but at DC and lower frequencies circuits often have high impedances that would be unacceptably loaded by the probe's low 500 or 1000 ohm probe impedance. Parasitic impedances limit very-high-frequency circuits to operating at low impedance, so the probe impedance is less of a problem.
Active scope probes
Active scope probes use a high-impedance high-frequency
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
mounted in the probe head, and a screened lead. The purpose of the amplifier is not gain, but isolation (buffering) between the circuit under test and the oscilloscope and cable, loading the circuit with only a low capacitance and high DC resistance, and matching the oscilloscope input. Active probes are commonly seen by the circuit under test as a capacitance of 1 picofarad or less in parallel with 1 megohm resistance. Probes are connected to the oscilloscope with a cable matching the characteristic impedance of the oscilloscope input. Tube based active probes were used before the advent of high-frequency
solid-state electronics
Solid-state electronics means semiconductor electronics: electronic equipment using semiconductor devices such as transistors, diodes and integrated circuits (ICs). The term is also used as an adjective for devices in which semiconductor electr ...
, using a small
vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied.
The type kn ...
as
cathode follower
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the ...
amplifier.
Active probes have several disadvantages which have kept them from replacing passive probes for all applications:
* They are several times more expensive than passive probes.
* They require power (but this is usually supplied by the oscilloscope).
* Their dynamic range is limited, sometimes as low as 3 to 5 volts, and they can be damaged by overvoltage, either from the signal or
electrostatic discharge
Electrostatic discharge (ESD) is a sudden and momentary flow of electric current between two electrically charged objects caused by contact, an short circuit, electrical short or dielectric breakdown. A buildup of static electricity can be caused ...
.
Many active probes allow the user to introduce an offset voltage to allow measurement of voltages with excessive DC level. The total dynamic range is still limited, but the user may be able to adjust its centerpoint so that voltages in the range of, for example, zero to five volts may be measured rather than -2.5 to +2.5.
Because of their inherent low voltage rating, there is little need to provide high-voltage insulation for operator safety. This allows the heads of active probes to be extremely small, making them very convenient for use with modern high-density electronic circuits.
Passive probes and a modest active probe design is discussed in an application note by Williams.
The Tektronix P6201 is an early DC to 900 MHz active FET probe.
At extreme high frequencies a modern digital scope requires that the user solder a preamp to the DUT to get 50GS/s, 20 GHz performance.
[Tektronix design insight seminar, October 27, 2009. Tektronix P75TRLST solder tip probe for the MSO70000. In addition, the scope compensates for the inevitable loss and group delay in the cable.]
Differential probes
Differential probes are optimized for acquiring
differential signals. To maximize the
common-mode rejection ratio
In electronics, the common mode rejection ratio (CMRR) of a differential amplifier (or other device) is a metric used to quantify the ability of the device to reject common-mode signals, i.e. those that appear simultaneously and in-phase on both i ...
(CMRR), differential probes must provide two signal paths that are as nearly identical as possible, matched in overall attenuation, frequency response, and time delay.
In the past, this was done by designing passive probes with two signal paths, requiring a
differential amplifier
A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. It is an analog circuit with two inputs V_\text^- and V_\text^+ and one outpu ...
stage at or near the oscilloscope. (A very few early probes fitted the differential amplifier into a rather-bulky probe head using vacuum tubes.) With advances in solid-state electronics, it has become practical to put the differential amplifier directly within the probe head, greatly easing the requirements on the rest of the signal path (since it now becomes single-ended rather than differential and the need to match parameters on the signal path is removed). A modern differential probe usually has two metal extensions which can be adjusted by the operator to simultaneously touch the appropriate two points on the DUT. Very high CMRRs are thereby made possible.
Additional probe features
All scope probes contain some facility for grounding (earthing) the probe to the circuit's reference voltage. This is usually accomplished by connecting a very short pigtail wire from the probe head to ground. Inductance in the ground wire can lead to distortion in the observed signal, so this wire is kept as short as possible. Some probes use a small ground foot instead of any wire, allowing the ground link to be as short as 10 mm.
Most probes allow a variety of "tips" to be installed. A pointed tip is the most common, but a seizer probe or "test hook" with a hooked tip that can secure to the test point, is also commonly used. Tips that have a small plastic insulating foot with indentations into it can make it easier to probe very-fine-pitch
integrated circuits
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
; the indentations mate with the pitch of the IC leads, stabilizing the probe against the shaking of the user's hand and thereby help to maintain contact on the desired pin. Various styles of feet accommodate various pitches of the IC leads. Different types of tips can also be used for probes for other instruments.
Some probes contain a push button. Pressing the button will either disconnect the signal (and send a ground signal to the 'scope) or cause the 'scope to identify the trace in some other way. This feature is very useful when simultaneously using more than one probe as it lets the user correlate probes and traces on the 'scope screen.
Some probe designs have additional pins surrounding the BNC or use a more complex connector than a BNC. These extra connections allow the probe to inform the oscilloscope of its attenuation factor (10×, 100×, other). The oscilloscope can then adjust its user displays to automatically take into account the attenuation and other factors caused by the probe. These extra pins can also be used to supply power to active probes.
Some ×10 probes have a "×1/×10" switch. The "×1" position bypasses the attenuator and compensating network, and can be used when working with very small signals that would be below the scope's sensitivity limit if attenuated by ×10.
Interchangeability
Because of their standardized design, passive probes (including Z
0 probes) from any manufacturer can usually be used with any oscilloscope (although specialized features such as the automatic readout adjustment may not work). Passive probes with voltage dividers may not be compatible with a particular scope. The compensation adjustment capacitor only allows for compensation over a small range of oscilloscope input capacitance values. The probe compensation range must be compatible with the oscilloscope input capacitance.
On the other hand, active probes are almost always vendor-specific due to their power requirements, offset voltage controls, etc. Probe manufacturers sometimes offer external amplifiers or plug-in AC power adapters that allow their probes to be used with any oscilloscope.
High-voltage probes
![HighVoltageDividerProbe](https://upload.wikimedia.org/wikipedia/commons/3/3b/HighVoltageDividerProbe.jpg)
A high voltage probe allows an ordinary voltmeter to measure voltages that would otherwise be too high to measure or even destructive. It does this by reducing the input voltage to a safe, measurable level with a precision
voltage divider
In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (''V''out) that is a fraction of its input voltage (''V''in). Voltage division is the result of distributing the in ...
circuit within the probe body.
Probes intended for up to 100 kV typically employ a
resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
voltage divider, with an input resistance of hundreds or thousands of
megohm
Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm.
Ohm or OHM may also refer to:
People
* Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm''
* Germán Ohm (born 1936), Mexican boxer
* Jörg Ohm (b ...
s to minimize circuit loading. High linearity and accuracy is achieved by using resistors with extremely low voltage coefficients, in matched sets that maintain a consistent, precise divider ratio across the probe's operating temperature. Voltmeters have input resistance that effectively alters the probe's divider ratio, and
parasitic capacitance
Parasitic capacitance is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other. When two electrical conductors at different voltages a ...
that combines with the probe's resistance to form an
RC circuit
A resistor–capacitor circuit (RC circuit), or RC filter or RC network, is an electric circuit composed of resistors and capacitors. It may be driven by a voltage or current source and these will produce different responses. A first order RC c ...
; these can easily reduce DC and AC accuracy, respectively, if left uncompensated. To mitigate these effects, voltage divider probes usually include additional components that improve frequency response and allow them to be calibrated for different meter loads.
Even higher voltages can be measured with capacitor divider probes, though the larger physical size and other mechanical features (e.g.,
corona ring
A corona ring, more correctly referred to as an anti-corona ring, is a toroid of conductive material, usually metal, which is attached to a terminal or other irregular hardware piece of high voltage equipment. The purpose of the corona ring is t ...
s) of these devices often preclude their use as handheld probes.
Current probes
A
current probe generates a voltage proportional to a current in the circuit being measured; as the proportionality constant is known, instruments that respond to voltage can be calibrated to indicate current. Current probes can be used both by measuring instruments and oscilloscopes.
Sampling resistor
The classic current probe is a low valued resistor (a "sampling resistor" or "current shunt") inserted in the current's path. The current is determined by measuring the voltage drop across the resistor and using
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equat ...
. The sampling resistance needs to be small enough not to affect circuit operation significantly, but large enough to provide a good reading. The method is valid for both AC and DC measurements. A disadvantage of this method is the need to break the circuit to introduce the shunt. Another problem is measuring the voltage across the shunt when common-mode voltages are present; a differential voltage measurement is needed.
Alternating current probes
![Stromwandler Zeichnung](https://upload.wikimedia.org/wikipedia/commons/3/3b/Stromwandler_Zeichnung.svg)
Alternating currents are relatively easy to measure as transformers can be used.
A
current transformer
A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). It produces a current in its secondary which is proportional to the current in its primary.
Current transformers, along with volt ...
is commonly used to measure alternating currents. The current to be measured is forced through the primary winding (often a single turn) and the current through the secondary winding is found by measuring the voltage across a current-sense resistor (or "burden resistor"). The secondary winding has a burden resistor to set the current scale. The properties of a transformer offer many advantages. The current transformer rejects common mode voltages, so an accurate single-ended voltage measurement can be made on a grounded secondary. The effective series resistance
of the primary winding is set by the burden resistor on the secondary winding
and the transformer turns ratio
, where:
.
The core of some current transformers is split and hinged; it is opened and clipped around the wire to be sensed, then closed, making it unnecessary to free one end of the conductor and thread it through the core.
Another clip-on design is the
Rogowski coil
A Rogowski coil, named after Walter Rogowski, is an electrical device for measuring alternating current (AC) or high-speed current pulses. It sometimes consists of a helical coil of wire with the lead from one end returning through the centre o ...
. It is a magnetically balanced coil that measures current by electronically evaluating the line integral around a current.
High-frequency, small-signal, passive current probes typically have a frequency range of several kilohertz to over 100 MHz. The Tektronix P6022 has a range from 935 Hz to 200 MHz.
Direct-current probes
Transformers cannot be used to probe
direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...
s (DC).
Some DC probe designs use the nonlinear properties of a magnetic material to measure DC.
Other current probes use
Hall effect
The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor that is transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current. It was disco ...
sensors to measure the
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
around a wire produced by an
electric current
An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
through the wire without the need to interrupt the circuit to fit the probe. They are available for both voltmeters and oscilloscopes. Most current probes are self-contained, drawing power from a battery or the instrument, but a few require the use of an external amplifier unit. (See also:
Clamp meter
In electrical and electronic engineering, a current clamp, also known as current probe, is an electrical device with jaws which open to allow clamping around an electrical conductor. This allows measurement of the current in a conductor without t ...
)
Hybrid AC/DC current probes
More advanced current probes combine a Hall effect sensor with a current transformer. The Hall effect sensor measures the DC and low frequency components of the signal and the current transformer measures the high frequency components. These signals are combined in the amplifier circuit to yield a wide band signal extending from DC to over 50 MHz. The Tektronix A6302 current probe and AM503 amplifier combination is an example of such a system.
Near-field probes
Near-field probes allow the measurement of an
electromagnetic field
An electromagnetic field (also EM field or EMF) is a classical (i.e. non-quantum) field produced by (stationary or moving) electric charges. It is the field described by classical electrodynamics (a classical field theory) and is the classical c ...
. They are commonly used to measure electrical
noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...
and other undesirable
electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...
from the DUT, although they can also be used to spy on the workings of the DUT without introducing much
loading into the circuitry.
They are commonly connected to
spectrum analyzer
A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that most co ...
s.
Temperature probes
![ThermocoupleProbe](https://upload.wikimedia.org/wikipedia/commons/2/2c/ThermocoupleProbe.jpg)
Temperature probes are used to make contact measurements of surface temperatures. They employ a temperature sensor such as a
thermistor
A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''.
Thermistors are divided based on their conduction ...
,
thermocouple
A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the ...
, or
RTD, to produce a voltage that varies with temperature. In the case of thermistor and RTD probes, the sensor must be electrically stimulated to produce a voltage, whereas thermocouple probes do not require stimulation because a thermocouple will independently produce an output voltage.
Voltmeters can sometimes be used to measure temperature probes, but this task is usually delegated to specialized instruments that will stimulate the probe's sensor (if necessary), measure the probe's output voltage, and convert the voltage to temperature units.
Demodulator probes
To measure or display the modulating waveform of a
modulated
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
high-frequency signal—for example, an
amplitude-modulated radio signal—a probe fitted with a simple
diode
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.
A diode ...
demodulator
Demodulation is extracting the original information-bearing signal from a carrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from the modulated ...
can be used. The probe will output the modulating waveform without the high-frequency
carrier.
See also
*
Langmuir probe, used to measure electric potential and electron temperature and density of a plasma
Logic probes
A logic probe is used for observing
digital signal
A digital signal is a signal that represents data as a sequence of discrete values; at any given time it can only take on, at most, one of a finite number of values. This contrasts with an analog signal, which represents continuous values; at ...
s.
References
*
*
*
External links
ABCs of Probes Primer , TektronixOscilloscope Probes and Accessories(PDF, 6.69 MB), from Keysight
{{Authority control
Measuring instruments
Electronic test equipment
it:Oscilloscopio#Sonde di misura