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Measurement effect of oscilloscope probe
3.1 load effect
The so-called load effect is that when the oscilloscope is connected to the circuit under test, sometimes the input resistance of the oscilloscope will affect the circuit under test, causing the signal of the circuit under test to change. If the influence of load effect is great, it is impossible to measure the waveform accurately. To reduce the load effect, it is necessary to increase the input resistance at one end of the oscilloscope. The larger the input resistance, the smaller the input capacitance and the smaller the load effect.
In the measurement of oscilloscope, another kind of load effect refers to the load effect of the probe on the circuit under test. In order to ensure the accuracy of measurement, it is necessary to reduce the load effect of the probe on the circuit under test without affecting the signal under test. Therefore, the probe with high input impedance should be selected. The input impedance of the probe can be equivalent to the parallel connection of resistance and capacitance. The load of the probe is mainly impedance at low frequency (below 1MHz) and capacitive reactance at high frequency (above 10MHz). In order to alleviate the load effect of the probe on the circuit under test, high impedance and low capacitance impedance probes should be selected, such as passive probes with a bandwidth of 100 MHz, whose input resistance is 1-10_and the input capacitance is 1-10 pF. Active probe has better load and frequency characteristics than passive probe.
3.2 impedance matching
Impedance is the ratio of voltage to current. Ideally, the normal operation of the instrument under test should not be affected, and the measured value should be the same as that without the instrument under test. When connecting instruments for measurement, the influence of impedance on measurement accuracy should be considered. In order to ensure the maximum power transmission between instruments, impedance should be matched. If the impedance is pure resistance, the input impedance should be equal to the output impedance. If the impedance contains the reactance component, the input impedance of the load should be conjugated with the output impedance of the source, so that the maximum power can be transmitted.
The impedance value of impedance matching is usually consistent with the characteristic impedance value of the transmission line used. For radio frequency systems, 50_impedance is generally used. For high impedance instruments, due to the existence of equivalent shunt capacitance, the shunt combined impedance decreases gradually with the increase of frequency, which will load the circuit under test. For example, when the input impedance of 1M reaches 100MHz, the equivalent impedance is only about 100. Therefore, high-bandwidth oscilloscopes generally use 50 input impedance, which can ensure the matching between the oscilloscope and the source. However, when using 50_input impedance, the load effect of 50_input impedance must be taken into account. At this time, it is better to use low capacitance active probe.
3.3 capacitance load
With the increase of signal frequency or conversion rate, the capacitance component of impedance becomes the main factor. As a result, capacitance load becomes the main problem, especially capacitance load will affect the rising and falling time of fast converting waveform and the high frequency component amplitude of waveform.