Test probe

Typical passive oscilloscope probe being used for testing an integrated circuit.

A test probe (test lead, test prod, or scope probe) is a physical device used to connect electronic test equipment to the device under test (DUT). They range from very simple, rugged devices to complex probes that are sophisticated, expensive, and fragile.

Voltage probes

Voltage probes are intended to measure voltages on the DUT. Ideally, the test instrument and its probe will not affect the voltage being measured. Practically, that translates into the test instrument and its probe presenting a high impedance that will not load the DUT. In many situations, an impedance with a resistive component of a megohm is adequate. For AC measurements, the reactive component of impedance may be more important than the resistive.

Because of the high frequencies often involved, oscilloscopes do not normally use simple wires to connect to the DUT. Instead, a specific scope probe is used. Scope probes use a coaxial cable to transmit the signal from the tip of the probe to the oscilloscope, preserving those high frequencies that are so important to accurate oscilloscope operation.

Scope probes fall into two main categories: passive and active. Passive scope probes contain no active electronic parts, such as transistors, so they require no external power.

Simple test leads (flying leads)

Voltmeter probes usually consist of single wires that are equipped on one end with a connector that fits the user's voltmeter and on the other end with a rigid plastic section (the probe itself) that allows the user to safely hold the probe while being protected from the danger of electric shock. Within the plastic body of the probe, the wire is connected to a rigid, pointed metal tip that makes the actual contact with the DUT.

Typically, two probes are used: a positive and a negative. Voltmeter probes are usually colored red (for the positive probe) and black (for the negative probe). Either probe may be replaced with a wire ending in an alligator clip, allowing a connection to the DUT that does not need to be held. Some probes allow an alligator clip to be screwed onto their ends, covering the metal point.

Ordinary voltmeter probes can be used for voltages up to about 1,000 volts and currents of a few amperes. Depending upon the accuracy required, they can be used for frequencies ranging from DC to a few kilohertz.

The test leads usually consist of stranded wire to keep them flexible. In addition, the insulation is chosen to be both flexible and have a breakdown voltage consistent with the voltmeter's highest ranges (e.g., 600 volts). Consequently, the insulation will be thicker than ordinary hookup wire. Wire for this purpose is sometimes termed test prod wire.

When very small voltages are measured, shields, guards, and techniques such as Kelvin sensing are used.

Shielded leads (1X probe, direct probe)

Shield minimizes capacitive coupling from interfering sources.

Common path minizes inductive coupling from interfering sources.

Unfortunately, shielded cable carries a penalty in the form of increased capacitive loading. This penalty for a typical coaxial cable is about 30pF per foot, so 3 feet (0.91 m) of cable cable loads the DUT with another 90pF.

Attenuator probe (RC compensated probe)

Minimize capacitive loading with an attenuator. A 10X attenuator will reduce the capacitive load by a factor of about 10.

Need a balanced attenuator. DC attenuation with resistive divider. AC attenuation with capacitive divider. Effect: RC time constants match. (Wedlock & Roberge 1969, pp. 150–152)

Must match test equipment.

At higher frequencies, the capacitive loading continues to dominate.

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 5nS. Consequently, these probes will work to a few megahertz, but after that transmission line effects cause trouble.

Lossy transmission line probe (modern 10X scope probe; hi-Z probe)

Esoteric lossy transmission line. Pays attention to transmission line effects. Also analogy to low Z probe.

Tektronix high-Z probes. Bandwidth above 100MHz; some to 500MHz.

At high frequencies, the probe impedance will be low.

A passive 10x probe. Notice the switch in the probe handle that allows to choose between 1x or 10x attenuation.

The most common design inserts a 9 megaohm resistor in series with the probe tip. The signal is then transmitted from the probe head to the oscilloscope over a special coaxial cable that is designed to minimize capacitance and ringing. 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 10x voltage divider so such probes are normally known as either low cap(acitance) probes or 10X probes. The X or x is a multiplication symbol, not a letter ex, and so is usually spoken as "times": as in, for example, "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 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 10x voltage divider. In this way, the probe provides a uniform 10x 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 10x attenuation). More modern probe designs use a laser-trimmed thick-film 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 square wave and adjusting the compensating capacitor until the oscilloscope displays the most accurate waveshape. Newer, faster probes have more complex compensation arrangements and may occasionally require further adjustments.

100x 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. Most 10x probes impose a loading of about 10-15 pF and 10 megohms on the DUT, with 100x probes imposing a lighter load.

Lo Z probes

Z₀ probes are a specialized type of low-capacitance passive probe used in low-impedance, very-high-frequency circuits. Very similar in design to ordinary passive probes, they are designed to connect to oscilloscopes with 50 ohm (rather than 1 megohm) input impedance. Because of this, these probes use a 450 ohm (for 10x attenuation) or 950 ohm (for 20X attenuation) series resistor and 50 ohm coaxial cable (rather than the 9 megohm resistor and specialized coaxial cable of the 10x probe). The high impedance scope probes are designed for the conventional 1 megohm oscilloscope. At high frequencies, the mismatch between the scope input impedance and the cable impedance isn't worth keeping. Instead, a high frequency oscilloscope will present a matched load (usually 50 ohms) at its input. That minimizes reflections at the scope.

The disadvantage is that 50 ohms will usually present a heavy DC load to the circuit under test. Once again, an attenuator can be used to minimize that loading. These probes are also called resistive divider probes, since a 50 ohm transmission line presents a purely resistive load. A 21X divider probe consists of a 1000 ohm series resistor and a short 50 ohm transmission line. Tektronix sells a 10x divider probe with 9GHz bandwidth with a 450 ohm series resistor.

The Z₀ name refers to the fact that the coaxial cable matches the characteristic impedance of the oscilloscope. This provides far better high-frequency performance than any ordinary passive probe can achieve, but at the expense of the 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 Z₀ probe can actually offer lower loading than any hi-Z probe and even many active probes.

Active scope probes

Active scope probes use a small, usually FET-based amplifier mounted directly within the probe head. By doing this, they are able to obtain exceptionally low parasitic capacitance and high DC resistance (It is common to see capacitance of 1 picofarad or less with 1 megohm resistance). They are connected to the oscilloscope in the same fashion as passive Z₀ probes (using 50 ohm coaxial cable terminated at the oscilloscope's input).

Active probes do have several disadvantages, however. These have kept them from entirely replacing passive probes:

• They are several times more expensive than passive probes
• They require power (but this is usually supplied by the oscilloscope)
• They have a limited dynamic range, often as low as 3 to 5 volts.
• They can be damaged by overvoltage or, sometimes, even ESD.

To overcome their often-limited dynamic range, many active probes allow the user to introduce an offset voltage. 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 a large amount of insulation to ensure operator safety against electric shock. This allows the probe heads on active probes to be extremely small, making them very convenient for use with modern high-density electronic packaging. Because of their size and excellent electrical characteristics, they are strongly preferred for troubleshooting digital electronics.

Before the advent of high-performance solid-state electronics a very few active probes were built using vacuum tubes as the amplifiers.

A modern 50GS/s digital scope requires that the user solder a preamp to the DUT to get 20GHz performance.

Differential probes

Differential probes are a specialized variation on the other probe families optimized for acquiring differential signals. To maximize the common-mode rejection ratio (CMRR), differential probes must provide two signal paths that are as nearly-identical as possible, matching in overall attenuation, frequency response, and time delay.

In the past, this was done by designing passive probes with two signal paths, leading to a differential amplifier stage at or near the oscilloscope. (A very few probes fitted the differential amplifier into a rather-bulky probe head using vacuum tubes.) With advances in solid-state electronics, it has become completely 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 (return) voltage. This is usually accomplished by connecting a very short pigtail wire from the probe head to ground. Because inductance in the ground wire can lead to distortion in the observed signal, this wire should always be as short as possible. Some probes allow the use of 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 small, pointed tip is the most common, but "hook tips" that hold onto the test point (also known as "witches hats") are also very commonly used. Specialized tips that have a small plastic insulating foot with indentations into it can make it easier to probe very-fine-pitch integrated circuits; 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.

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 (10x, 100x, 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 X10 probes have a "X1/X10" switch. The "X1" 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 X10.

Interchangeability

Because of their standardized design, passive probes (including Z₀ 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

By inserting a large resistor in series with the probe and providing good electrical insulation, it is possible to create a probe that allows an ordinary voltmeter to measure very high voltages (up to about 50 kV). The value of the resistor must be chosen to form an appropriate voltage divider with the input resistance of the voltmeter. Because of the very high value of the resistor needed (several megohms), high voltage probes are mainly used for measuring DC and low frequency AC; the RC circuit that is formed with the parasitic capacitance of the voltmeter input will attenuate higher frequencies.

Current probes

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. (Wedlock & Roberge 1969, p. 152.) The sampling resistance needs to be small enough that it won't affect the measurement but large enough to provide a good reading. The method is good for both AC and DC measurements. The primary 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.

AC current probes

A current transformer is commonly used to measure AC currents. The current to be measured is forced through the primary winding (often a single turn) and the voltage across the burdened secondary winding is measured to determine the current. 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; the voltage measurement can be made on a grounded secondary. The effective series resistance $R_{s}$ of the current transformer is set by the burden resistor $R$ and the transformer turns ratio $N$: $R_{s}=R/N^{2}$.

Some current transformers are split in half so they can be clipped around the wire to be sensed. That avoids the problem of breaking a connection to insert the current probe.

Another interesting clip on design is the Rogowski coil. It is a magnetically balanced coil that measures current by electronically evaluating the line integral around a current flow.

High frequency, small signal, passive current probes are also available. They typically have a frequency range of several kilohertz to over 100 MHz. The Tektronix P6022 has a range from 935 Hz to 200 MHz. (Tektronix 1983, p. 435)

DC current probes

The conventional current transformer does not measure DC currents.

There are some DC current probe designs that use the nonlinear properties of a magnetic material to measure DC currents.

Some current probes use Hall effect sensors to measure the magnetic field around a wire produced by an electric current flowing through the wire without any need to cut or interrupt the wire. 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)

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. (Wedlock & Roberge 1969, p. 154) The Tektronix A6302 current probe and AM503 amplifier combination is an example of such a system. (Tektronix 1983, p. 375) (Tektronix 1999, p. 571)

Near-field probes

Near-field probes allow the measurement of an electromagnetic field. They are commonly used to measure electrical noise and other undesirable electromagnetic radiation 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 analyzers.

Temperature probes

A Vernier temperature probe

Voltmeters commonly allow the connection of a temperature probe, allowing them to make contact measurements of surface temperatures. The probe usually consists of a thermistor with characteristics that are specific to the voltmeter being used. Thermocouples can also be used.

Pogo pins

Spring probes (a.k.a. "pogo pins") are spring-loaded pins used in electrical test fixtures 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 fixtures which may remain in service for decades, testing many thousands of DUTs in automatic test equipment.

References

• Johnson, Howard W.; Graham, Martin (1993), High Speed Digital Design: a Handbook of Black Magic, Prentice-Hall, ISBN 978-0133957242

• Tektronix (1983), Tek Products, Tektronix

• Tektronix (1998), Measurement Products Catalog 1998/1999, Tektronix

• Wedlock, Bruce D.; Roberge, James K. (1969), Electronic Components and Measurements, Prentice-Hall, ISBN 0-13-250464-2

See also

• Logic probe — a test probe specially designed for TTL and CMOS signals
• Oscilloscope