Test probe

The probe, also using a probe, is a measuring device used in electronics, mainly for measurements with the oscilloscope. The probe is connected to the device under test and supplies the measuring signal to the actual measuring instrument. There are probes for measuring various physical quantities such as Voltage or current. A probe contains electrical components to compensate for measurement errors.

Simple test and Kontaktierhilfe without electrical components is the probe ( needle with banana plug socket ) and test clips, the mechanically actuated hook or claw has to include components. They are used primarily for voltage measurements and Logic Analyzers.

The "longer " design for hard to reach areas is the test probe.

  • Passed 4.1 division factor
  • 4.2 Energy supply active probes

General

Usually, a probe is operated on an oscilloscope. An oscilloscope has an input resistance of 1 M and an input capacitance of 20 pF in general. Some oscilloscopes may be set to an impedance of 50 Ω. The input voltage is often limited to about 300 volts. These input parameters are not suitable for measuring in all situations. To adjust the measurement input of the oscilloscope to the measurement probes of different properties are used.

Requirements

The probe is not to change the signal to be measured, therefore, various requirements are placed:

  • The input impedance should be as high as possible.
  • Therefore, its input capacitance should be particularly low.
  • The input resistance should be as large as possible.
  • Reflections on the measuring line as possible should not occur.

These demands can not be combined with each other: either the input impedance is low and corresponds to the impedance of the cable ( coaxial cable, 50 Ω ) or it will compromise on the upper cutoff frequency or pulse fidelity tolerated.

Since the cable length determines the signal propagation time must be used when using multiple channels of the oscilloscope probes identical or at least same cable lengths for measurements in the nanosecond range. In addition, the cable length limits the bandwidth of the probe.

To prevent Ringing the ground wire should always be as short as possible.

Probe types

There are a number of different probe types. Depending on the application of the appropriate probe must be selected for each measurement. The different probes are also very different in terms of the cost. Quality probes are electrically and mechanically much more sensitive in the rule.

Passive Probes

The most common design is the passive probe with a 10:1 split. The input resistance is 10 M.

Advantages:

  • Cheaper price.
  • No active components.
  • No power supply required.
  • Mechanically and electrically robust.
  • Probe and oscilloscope can be mostly (see Additional Functions) by different manufacturers.
  • Input capacitance is smaller by about a dividing factor.

Cons:

  • Relatively high input capacitance
  • Problems when measuring small voltages

Passive probes consist essentially of a metal tip, which is connected with a compensated voltage divider on the supply line to the instrument.

For this purpose, located on the front in the probe, a resistor and a capacitor connected in parallel, their relationship to that of the input impedance of the oscilloscope same needs. To this end in the connector of the probe, an adjustable second capacitor is housed, with its adjustment, the probe can be adapted to the input capacitance of the oscilloscope. To load the signal source as little as possible, that is to have a very low input capacitance, such probes usually have a voltage division ratio of 10:1.

To compare the probes give Oscilloscopes typically a square wave signal from, by which one can compare the probe so that even such is shown. This Tastkopfbauform is in most measurement situation a useful result achieved.

But as soon as the signals are faster, this Tastkopfbauform distorts the signal.

Example calculation:

A capacitance of 2 pF, provides a 100 MHz signal has a reactance of approximately 800 Ω dar. the reactance of the probe is thus about 10,000 times smaller in a non-reactive input impedance of 10 M. The circuit is charged over 10,000 times at 100 MHz. The signal is distorted.

Transmission-Line Probes

Also, the transmission -line probe is a passive probe. With a 10:1 dividing the input resistance is only 500 Ω.

Advantages:

  • Very low input capacitance in the range of 0.2 pF to 0.5 pF.
  • Especially for the measurement of high frequency signals.
  • Lower error of fast signals.
  • The probe is over a larger area a constant load dar.

Cons:

  • Low input resistance (500 Ω to 1000 Ω ).
  • Can only be used for small signal levels.
  • Strong load the signal source.
  • The oscilloscope needs to handle high power at the measuring input.

In a Transmision Line Probe adjustment is made between probe cable and oscilloscope. That the oscilloscope operates with an impedance of 50 Ω, and the feed line used has a characteristic impedance of 50 Ω also. Between tip and lead a resistance of 450 Ω (with 10:1 - division ) to about 950 Ω (with 20:1 - division ) is switched.

Is accepted by the input capacitance of 0.5 pF (a high value ), the capacitance and the ohmic resistance is equal only at a frequency of about 600 MHz. The usable bandwidth is significantly higher than the "normal " passive probe. High-quality probes can thus be used up to the GHz range.

Active Probes

In active probes the signal is amplified already in the probe. The probe needs power.

Advantages:

  • Low input capacitance.
  • High cut-off frequency / bandwidth.

Cons:

  • Power supply is required.
  • High cost
  • Mechanically and electrically much more sensitive than passive probes.
  • Active probes are often proprietary, which means that they only mate with oscilloscopes manufacturers.
  • By the amplifier, the maximum amplitude signal is limited.

Active probes to be used when high-speed signals with low voltage swing to be measured. The use involves in-depth knowledge of metrology in order not to destroy the probe and to get a meaningful measurement result. Active probes are supplied in most cases by the oscilloscope with energy, but it also exist solutions that use an external power supply.

Differential probes

The probes described so far always measure to ground or earth. Usually the ground is connected to 0 V in the circuit. If you want to measure signals whose reference potential is not 0 V (eg a balanced signal ), there are various approaches:

  • The oscilloscope has no ground reference ( isolation transformer ). This is extremely rare. The ground terminal of the probe at a potential equal mass results may to the destruction of the oscilloscope.
  • Measuring with two channels. The oscilloscope is the difference.
  • Using a differential probe.

Measuring with two channels of the oscilloscope has several disadvantages:

  • Are required for a differential signal of two channels.
  • Fast signals are represented insufficiently accurate.
  • With small differential signals and a large DC offset, the measurement is very inaccurate.

For quick balanced signals there is therefore the differential probe. A differential probe is one of the special active probes. The differential probe usually has three connections: Ground, A, B.

Ground must be connected to the ground of the circuit. This mass point defines the working range of the probe. The other two terminals must be connected to the differential pair.

Advantages:

  • It is only needed an oscilloscope channel.
  • High impedance.
  • Low input capacitance.
  • High common mode rejection.

Cons:

  • Price.
  • Restricted workspace.

Differential probes are becoming increasingly important, as many robust systems and especially new bus systems with symmetric transmission of high speed work. Representatives slower transfer rate are professional audio systems, CAN and RS485. Symmetrical high-speed transmission is eg for USB, PCI Express, and various graphic interfaces ( LVDS) used.

Some active differential probes can switch between the operating states of channel A, channel B, channel AB (differential component), 0.5 * (A B ) ( common-mode component ) are switched.

Current probes

The current probe is also one of the active probes. It is used to measure DC and AC currents. The function is based on the transformer effect, the Hall effect. The circuit in which the current to be measured flows, is performed by a toroidal core made ​​of ferrite. The measured signal is fed to the Auskoppelspule after treatment to the oscilloscope. Because it is not possible in practice often the current path unravel folding systems are used. The conductor is, for example, inserted into a U-shaped ferrite member, through the then an I-shaped ferrite member is inserted.

The systems usually require an adjustment before each measurement ( modified air gap when opening, etc.). The processing of the measured value needed due to nonlinearities, a slightly more complex evaluation. Due to the operating principle of the bandwidth is usually limited depending on the probe to a few hundred kHz.

High Voltage Probes

To measure higher voltages than 500 V, there are also compensated probes with divider ratios of 100:1 and 1000:1.

Hochfrequenztastköpfe

For measuring the level of very high frequencies, there are high-frequency probes. They have behind the tip, a diode and provide the rectified value of the high frequency voltage.

Additional functions

Passed dividing factor

The pictured Tektronix probe is passive, but it is by contact pins in a position to inform the oscilloscope that a 10:1 division is connected. The oscilloscope has the BNC input a metal ring, which is affected by the contact pin. As a result, the vertical scale is set correctly. This feature is usually only available if the probe and meter from the same manufacturer and both devices support this function. For high quality probes adapters are available, eg To connect a Tektronix probe to an Agilent oscilloscope.

Energy supply active probes

If the manufacturer of the meter and probe are identical, the energy supply of active probes on additional contacts can be made. Active probes can be powered via an external power source just as well. Many current probes work with external power supply.

Accessories

In order to facilitate the daily work, we have developed a range of accessories for probe heads over time. These include:

  • Earth clip
  • Tastkopfablage
  • Flexible adapter
  • Solder tails
  • Contact sprung hook
  • Color rings
  • Insulation cap for the probe
  • Tastspitzenkappe

In particular, the earth clip is in circuit with a voltage reference different from that of the measuring device, it is necessary to obtain useful signals. However, it is important to note that at different potentials transient currents occur which can destroy fuses or circuits. For leaded integrated circuits the contact hooks, which can be easily mounted to the connections is recommended.

Swell

  • Tektronix: ABCs of Probes Primer
Electrical impedance Transmission delay Electrical reactance RS-485 Low Voltage Differential Signaling
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