Test probe

. A test probe is a physical device used to connect electronic test equipment to a device under test (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.

.]] A test probe is a physical device used to connect electronic test equipment to a device under test (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. ====Tweezer probes==== Tweezer probes are a pair of simple probes fixed to a tweezer mechanism, operated with one hand, for measuring voltages or other electronic circuit parameters between closely spaced pins. ===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 transistors, so they require no external power. Because of the high frequencies often involved, oscilloscopes 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 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 t_r . These are related as (in round figures) f_{3 dB} = 0.35/t_r 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 t_r (combined) = \sqrt{t_r(probe)^2 + t_r(scope)^2} 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==== 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, 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 and ringing. The invention of this cable has been traced to John Kobbe, an engineer working for Tektronix. 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 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 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 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. 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. A current transformer 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 R_s of the primary winding is set by the burden resistor on the secondary winding R and the transformer turns ratio N , where: R_s = R / N^2 . 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. 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. ==Demodulator probes== To measure or display the modulating waveform of a modulated high-frequency signal—for example, an amplitude-modulated radio signal—a probe fitted with a simple diode demodulator 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

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