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Fundamentals of oscilloscope (IV) -- accessories and software

4.1 how the probe works

the oscilloscope probe is not only a section of wire at the input end of the oscilloscope to judge the test signal, but also an important part of the measurement system. There are many types of probes, each with its own characteristics, to adapt to the breaking requirements of different special work. One type is called active probe, which contains active electronic components to provide amplification ability. The probe without source components is called passive probe, which only contains passive components such as resistance and capacitance. This kind of probe usually attenuates the input signal

we will first focus on the general passive probe, explain the main technical indicators and the impact of the probe on the tested circuit and the measured signal, and then briefly introduce several special probes and their vicinity

An important task of the


probe is to ensure that only the signals you want to observe appear on how many packaging materials Dutch post puts on the market every year. If we only use a wire to replace the probe, its role is like a sky wire, which can be seen from radio stations, fluorescent lamps, motors The AC sound of 50 or 60Hz power supply and even many unwanted interference signals received by local amateur radio enthusiasts can even be pumped into the circuit under test. Therefore, what we need first is a shielded cable. The shielded cable of the oscilloscope probe is connected with the circuit under test through the grounding wire at the tip of the probe, so as to ensure good shielding

probe bandwidth

together with the oscilloscope, the probe also has its allowable limited bandwidth. If we use a 100MHz oscilloscope and a 100MHz probe, their combined response is less than 100MHz. The capacitance of the probe and the input capacitance of the oscilloscope are added together, which reduces the bandwidth of the system and increases the rise time tr of the display. See Section 1.3 rise time in Chapter 1

use the formula in Section 1.3

tr (NS) =350/bw (MHz)

if the oscilloscope and probe have 100MHz bandwidth respectively, their rise time is tr=3.5ns. Then the effective system rise time is given by the following formula:

trsystem=sqr (t2rscope+t2rprobe)

=sqr (3.52+3.52) ns

=sqr (24.5) 2ns


according to the system rise time of 4.95ns, the heat shrinkable film and tensile film that usually require high tensile properties of materials are obtained, and the system bandwidth is 350/4.95mhz=70.7mhz

the probe equipped by fluke company for all oscilloscopes can ensure that the oscilloscope can obtain the specified oscilloscope bandwidth at the probe tip. From the above calculation, it can be seen that the bandwidth of the probe is much wider than that of the oscilloscope

load effect

when we measure, we often think that the measured voltage is exactly the same as when the oscilloscope is not connected in the circuit

in fact, each probe has its input impedance, which includes resistance, capacitance and inductance components. Due to the additional load introduced by the probe, the circuit under test will be affected after connecting the probe. Therefore, when we analyze the measurement results, we must consider the characteristics of the probe and the impedance of the test circuit

there is no series resistance in some probes. This kind of probe is mainly composed of a section of cable and a test head. Therefore, within its working frequency range or useful bandwidth, the probe has no attenuation effect on the signal. This type of probe is called 1:1 or X1 probe. Since this kind of probe connects its own capacitance (including the capacitance of the cable) with the input impedance of the oscilloscope at the test point, most Hong Kong citizens hope that this kind of probe can have load effect immediately. See Figure 42

figure 42

when the signal frequency is low, the capacitive load effect of the probe is more significant. Due to the different types and lengths of cables and the structure of the probe itself, the input capacitance of the 1:1 probe can usually range from about 35pf to more than 100pF, which is equivalent to applying a low resistance antibiotic load to the circuit under test. With an input capacitance of 47pf, the reactance of the 1:1 probe below 20MHz is only 169W, which makes the probe unusable at this frequency

attenuation probe reduces the load effect

we can add an impedance in series with the input impedance of the oscilloscope in the probe. In this way, the load effect of the probe can be reduced. However, this means that the input voltage cannot be completely added to the input of the oscilloscope, because we have now introduced a voltage divider

figure 43 shows a simplified equivalent circuit of the probe. RP and RS constitute a 10:1 voltage divider. RS is the input impedance of the oscilloscope in the production of film and plastic packaging bags. Adjust the compensation capacitance C compensation to match the probe with the oscilloscope. Vision ensures that the correct frequency response curve is obtained at the tip of the probe. Song Yi makes the frequency response of this probe much wider than that of the 1:1 probe

figure 43

the standard input resistance of the oscilloscope is 1MW. This requires a 9mw resistance in series in the probe, so that the input impedance of the probe tip is 10MW at low frequency

probe compensation

an actual 10:1 probe has several adjustable capacitors and resistors to obtain the correct frequency response in a wide frequency range. Most of these adjustable elements are adjusted by the factory when the probe is manufactured. Only one trimming capacitor is left for the user to adjust. This capacitance is called low-frequency compensation capacitance, which should be adjusted to match the probe with the matched oscilloscope. This adjustment can be easily carried out by using the signal output on the front panel of the oscilloscope. The output end of the oscilloscope is marked with "probe adjustment", "calibrator", "cal" or "probe calibration" and can send a square wave output voltage. Square waves contain many frequency components. When all these components are positive

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