Influence of oscilloscope probe on signal source impedance

According to the relative value of impedance , Adding probes and oscilloscopes to the test points leads to various load effects .

The value of signal source impedance may significantly affect the net effect of probe load . for example , When the impedance of the signal source is low , It is difficult to notice the typical high impedance 10X Load effect of probe . This is because the high impedance added in parallel with the low impedance will not significantly change the total impedance

4.1. At the test point (TP) The signal measured on the can be expressed by the signal source and the related load impedance (a). Probe and oscilloscope impedance are added to the signal source load at the detection test point , Cause the measuring system to absorb part of the current (b)

4.2. The higher the impedance of the signal source , The greater the load caused by detection . under these circumstances , All impedances are equal , The detection results in a decrease in the signal amplitude at the test point 30% above

however , At higher signal source impedance , The situation has changed significantly . For equal Z value , When the probe and oscilloscope are not connected to the test point , The load of signal source is 2Z( See chart 4.2a). This results in 0.5ES Signal amplitude . however , When connecting the probe and oscilloscope ( chart 4.2b), The total load on the signal source becomes 1.5Z, The signal amplitude at the test point is reduced to the undetected value 2/3.

In the latter case , There are two ways , Reduce the impact of detection on impedance load . One way is to use a probe with higher impedance . Another method is to detect signals elsewhere in the circuit at the test point with low impedance . for example , The cathode 、 The impedance of transmitter and signal source is usually lower than that of metal disk 、 Collector or drain wire .

The above is the influence of oscilloscope probe on source impedance , If you have other problems in use , Welcome to Xi'an puke Electronic Technology