Probe head

The probe also attenuator probe (English: probe = Probe) is a measuring means for the electrical measurement technology , mainly used for measurements using the oscilloscope . With the multitude of measurement tasks, the oscilloscope cannot always be connected directly to the measurement object . Depending on the requirements , the interconnection of a probe can help, which forwards the measuring signal via a coaxial cable to the indicating measuring device. There are also probes for measurements with the logic analyzer .

There are a variety of probe types . A probe head preferably detects electrical voltage , depending on the design over a very large frequency range from constant to gigahertz. But the electrical current can also be displayed with a current probe with the oscilloscope.

Probe head in preferred design;
the ground connection cable below left

General

Usually a probe is operated on an oscilloscope. This usually has an input resistance of 1 MΩ and an input capacitance of 20 to 50 pF. The peak value of the input voltage at the probe is limited to a few hundred volts, depending on the specialization, significantly less. Oscilloscopes with higher bandwidths usually have 50 Ω inputs or can connect a corresponding termination. A first signal treatment required in the probe head can be carried out with passive components or require an active circuit. In order to adapt the oscilloscope to very different measuring tasks, many probes with different properties are available. Most probes have to be adapted to the respective oscilloscope used in such a way that the transmission behavior becomes frequency-independent; for this purpose they can be adjusted using a test signal.

Probe head with test probe in use

Simple testing and contacting aids are the test tip (needle) and the spring clip (clip), which has a hook under spring tension and can encompass an electrical line . The clamp test probe is a longer version for hard-to-reach places . Of course, a connection between a reference potential of the test object to be examined and the ground of the oscilloscope must be established in advance for voltage measurements .

conditions

The probe head should influence the circuit to be examined as little as possible and transmit the applied signal without distortion. This results in the following requirements:

• The input impedance should be as high as possible.

The ohmic input resistance should thus be as large as possible.

At the same time, the input capacity should be as low as possible.

• Reflections on the measuring line should not occur if possible.

These requirements cannot be combined: Either the input impedance is low and corresponds to the characteristic impedance of the cable (coaxial cable, e.g. 50 Ω). Or the input impedance is high, in which case the voltage source is little loaded, but compromises must be made with regard to the upper limit frequency and pulse fidelity.

Since the cable length determines the signal propagation time , identical probes or at least the same cable lengths must be used for measurements in the nanosecond range when using multiple channels of the oscilloscope. The cable length also limits the bandwidth of the probe. To prevent overshooting , the ground line should always be as short as possible.

Probe types

Probe heads differ in their input behavior, their demands on the user and the acquisition costs, so that often not only technical considerations but also economic considerations play a role in the selection of probes. Probe heads with high bandwidths or active components are usually mechanically and electrically much more sensitive than simple probe heads for lower frequencies.

Standard probes

The most common design is a passive probe head with a voltage division that makes the voltage on the oscilloscope 1:10 lower than on the probe tip. The input resistance is 10: 1 greater, i.e. 10 MΩ. It is created by 9 MΩ in the probe tip in addition to 1 MΩ in the oscilloscope.

Schematic circuit diagram of a conventional passive probe head including the input stage of the oscilloscope

Adaptation circuit of a passive probe head close to the connector, labeled with its characteristics

Advantages:

• Cheap price
• No active components
• No power supply required
• Mechanically and electrically robust
• The probe and oscilloscope can usually be from different manufacturers
• Input capacitance about the divider ratio smaller

Disadvantage:

• Smallest measuring range larger by the division ratio
• Input capacitance (typ. 10 to 20 pF) still too high for some measurements, therefore unsuitable for connection to high-impedance circuit nodes with steep signal edges
• Due to the input capacity, a relatively low bandwidth that can be used in practice

Example: If the input impedance is 10 MΩ parallel to 16 pF, the resistance is 10 MΩ for DC voltage, but only 10 kΩ for sinusoidal voltage with 1 MHz. As with any voltage measurement , on the other hand, the source resistance must be significantly smaller in order to avoid feedback deviation .

The probe resistance forms a voltage divider with the input resistance of the oscilloscope, its input capacitance and (depending on the internal structure) the parallel cable capacitance . In order for the division to be frequency-independent, there must also be a capacitor connected in parallel to the resistor in the probe . For the independence of the divider ratio of the frequency of the product of resistance and capacitance must in the two impedances of the voltage divider equal be . While the ohmic resistance in the probe head is adjusted at the factory, the capacitance of the probe head must be adjustable on a case-by-case basis. The setting option is located in the tip or close to the connector, depending on the structure. ${\displaystyle R_{\mathrm {t} }}$ ${\displaystyle R_{\mathrm {e} }}$ ${\displaystyle C_{\mathrm {e} }}$ ${\displaystyle C_{\mathrm {k} }}$ ${\displaystyle C_{\mathrm {t} }}$ ${\displaystyle R_{\mathrm {t} }\;C_{\mathrm {t} }=R_{\mathrm {e} }\;(C_{\mathrm {e} }+C_{\mathrm {k} })}$

To adjust the probe head, oscilloscopes usually output a square wave signal, with the help of which the probe head can be adjusted so that the steep signal edges are displayed steeply and without overshoot. With this probe head design, a useful result can be achieved in frequently encountered measuring situations.

Transmission line probes

The transmission line probe is also a passive probe. With a division of 10: 1 the input resistance is only 500 Ω, the capacitive component of the input impedance is significantly lower than in the high-resistance version.

Schematic circuit of a probe according to the transmission line principle

Advantages:

• Lower input capacitance (typ. 0.2 to 0.5 pF)
• Especially for measuring high-frequency signals
• A constant load over a wide frequency range

Disadvantage:

• Low input resistance (500 Ω)
• Can only be used for low signal levels
• Strong DC load on the signal source
• High quality 50 Ω termination on the oscilloscope required

In the case of a transmission line probe, matching is established between the probe line and the oscilloscope. I.e. the oscilloscope works with an impedance of 50 Ω, and the lead used has a characteristic impedance of 50 Ω. A resistance of 450 Ω (with a division 10: 1) or 950 Ω (with a division 20: 1) is connected between the measuring tip and the supply line.

Example: If the input capacitance is assumed to be 0.5 pF (a high value), the capacitive resistance and the ohmic resistance are only equal at a frequency of over 600 MHz. The usable bandwidth is therefore much higher than with the "normal" passive probe head.

Active probes

With active probes, the signal is already amplified in the probe. The probe needs a supply of auxiliary power .

Basic circuit diagram of an active probe head with differential amplifier , one input of which is connected to reference potential

Advantages:

• High input resistance
• Low input capacitance
• High upper limit frequency / bandwidth

Disadvantage:

• Energy supply necessary
• High acquisition costs
• Mechanically and electrically much more sensitive than passive probes
• Active probes often only suitable for the oscilloscopes from the same manufacturer
• Signal amplitude limited by the amplifier

Active probes are used when fast signals with a small voltage swing are to be measured. The use requires in-depth knowledge of the device technology in order not to destroy the probe head and to obtain a meaningful measurement result. In most cases, active probes are powered by the oscilloscope; however, there are also solutions that use an external power supply unit.

Differential probes

The probes described so far always measure against ground (housing, protective contact). Usually this is also the reference potential in the circuit . If a signal is to be measured whose reference potential is not 0 V (e.g. a symmetrical signal ), there are different approaches:

Block diagram of a differential probe head

• The oscilloscope has no ground reference (connection via isolating transformer). This carries the risk of electric shock or destruction of the oscilloscope and is not permitted
• The measurement object is operated potential-free . For lower frequencies and low source impedances, this is a useful alternative to a differential probe, otherwise useless
• Measurement with two channels, the difference of which is displayed by the oscilloscope
• Use of a differential probe

There are several disadvantages to measuring with two channels of the oscilloscope:

• For a differential signal, two channels with exactly the same adjustment are required
• Fast signals are not represented with sufficient accuracy
• With small signal differences and a large common mode signal, the measurement can be very erroneous

The differential probe is therefore available for fast symmetrical signals. This is one of the special active probes that usually has three connections: Ground, A, B.

Ground must be connected to the ground of the circuit. This ground point defines the working area of ​​the probe. The other two connections are to be connected to the line pair on which the difference is to be measured.

Advantages:

• Only one oscilloscope channel is required
• High impedance
• Low input capacitance
• High common mode rejection

Disadvantage:

• High price
• Limited work area

Differential probes are becoming increasingly important as many robust systems and especially new bus systems work with symmetrical transmission at high speeds. Representatives of slow transmission rates are professional audio systems, CAN and RS485 . Symmetrical high speed transmission is e.g. B. used with USB, PCI Express and various graphic interfaces ( LVDS ).

Some active differential probes can be switched between the operating states channel A, channel B, channel AB (difference formation), 0.5 · (A + B) (common mode component).

Current probes

A current probe works like a clip-on ammeter or with a Rogowski coil . He can be passive and active; it is used to measure direct and alternating currents. DC probes are always active. Pure AC probes are mostly passive. The function is based on the transformer effect or the Hall effect .

The systems require u. U. an adjustment before each measurement (changed air gap when opening, etc.). The processing of the measured value requires a somewhat more complex evaluation circuit due to non-linearities. Due to the operating principle, the bandwidth is usually limited to a few hundred kHz, depending on the probe.

High voltage probes

For measuring voltages higher than about 500 V there are probes with divider ratios up to 1000: 1.

Demodulator probes

There are high-frequency probes for measuring the level of very high frequencies . They have a diode behind the tip and supply the rectified value of the high-frequency voltage - an envelope demodulator , hence the name.

Cable divider

A probe head with a test probe or spring clip does not offer a secure and permanent hold. A cable divider can then be used that contains a BNC connector instead of a tip or clamp. It behaves electrically like a standard probe head. For example, it is suitable for connection to a vibrating machine part if it has a permanently installed connection socket.

Additional functions

Transfer ratio

In addition to the connector, some probes have a contact pin which, through its position, informs the oscilloscope of the division ratio. The voltage measuring range is then converted. This function is usually only available if the probe and oscilloscope come from the same manufacturer and both devices support the function.

Power supply for active probes

If the manufacturer of the measuring device and probe are identical, the power supply of active probes can be provided via additional contacts. Active probes can just as easily be supplied by an external energy source. Many current probes use an external supply.

equipment

Accessories for probes

In order to make daily work easier, a number of accessories for probes have been developed over time. Which includes:

• Earth clamp
• Probe holder
• flexible adapter
• Soldering lugs
• spring-loaded contact hook
• Colored rings
• Isolation cap for the probe
• Probe tip cap

In particular, the earth clamp is necessary for circuits whose voltage reference differs from that of the measuring device in order to receive meaningful signals. It should be noted, however, that equalizing currents occur at different potentials, which can destroy fuses or protective circuits. For wired integrated circuits, we recommend the contact hook, which can be easily fitted to the connections.

Web links

Commons : Oscilloscope probes  - collection of images, videos and audio files

• Oscilloscope: adjust probe head (online simulator in browser)

Individual evidence

1. ^ Hans-Rolf Tränkler: Pocket book of measurement technology. Oldenbourg, 1990, p. 112
2. ↑ Thomas Mühl: Introduction to electrical measurement technology: Fundamentals, measurement methods, applications, Springer-Vieweg, 4th edition 2014, pp. 208 ff
3. ↑ Wilfried Plaßmann, Detlef Schulz (Hrsg.): Handbook of electrical engineering: Basics and applications for electrical engineers. Springer-Vieweg, 6th edition 2013, p. 730