For high-frequency signal measurement, the probe's crocodile grounding wire is the source of all evils. No matter how good the instrument can't play its value, why?
Let's first take a look at how the probe ground length is different from the short one.
Taking the crystal signal measurement as an example, as shown in Figure 1, the conventional crocodile line grounding measurement method can be seen that the signal overshoot is severely accompanied by oscillation, which is different from the square wave in the imagination. The short-ground spring grounding measurement method shown in Figure 2 has a lot of waveforms. Obviously, the method of senior engineers is correct.
Figure 1 General (crocodile line) measurement method (error)
Figure 2 Short (spring ground) measurement method (correct)
2, the core difference: inductanceWhere are the signs that the murderer is the “ground�
And look at the diagram, as shown in Figure 3 is the theoretical equivalent of the oscilloscope using the probe for signal measurement. The probe and the oscilloscope form a test device with a certain input resistance and input capacitance; the measured signal is equivalent to a source with a certain internal resistance and working load.
Figure 3 Theoretical measurement equivalent model
Since the ground wire is a wire, it has a certain amount of distributed inductance, and the longer the wire, the larger the inductance. The conventional measurement method will introduce a distributed sense from the crocodile line. At this time, its equivalent model is shown in Figure 4 as the LG inductor. This inductance will interact with the probe capacitance, and the frequency is determined by the LG and CP values. Resonance is formed at the point, which causes the phenomenon of ringing oscillation.
Figure 4 Long ground line (crocodile line) measurement equivalent model
The effects of the resonance point in the frequency domain and the time domain are as shown in Table 1. The short ground (spring ground) measurement is analyzed by the model shown in Fig. 3; the long ground line (crocodile line) measurement is analyzed by the model shown in Fig. 4.
It can be found that the long-line measurement corresponds to the time domain response, and its characteristics are similar to those measured by our conventional (crocodile line), which also verifies the correctness of the theoretical analysis.
The probe ground wire adds distributed inductance in the circuit; the longer the ground wire, the larger the inductance, and the resonant frequency of the probe, which will produce significant overshoot and amplitude oscillation on the fast edge pulse. Therefore, when measuring high frequency signals, the probe ground wire should be as short as possible.
to sum upWhen measuring high-frequency signals, you should not only stare at the instrument. Be sure to pay attention to the ground wire. The shorter the better, you should weld a short wire yourself, and you should never use a crocodile.
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