Never use alligator ground wire when measuring high frequency signals

When measuring high-frequency signals, the alligator ground wire of the probe is the source of all evil. No matter how good an instrument is, it cannot be of value. Why?


1. Measured comparison of high frequency crystal oscillator
Let's first experience the difference between the length and the short of the probe ground wire.

Take crystal oscillator signal measurement as an example, as shown in Figure 1 is the conventional alligator wire grounding measurement method. It can be seen that the signal overshoot is severely accompanied by oscillation, which is different from the imagined square wave. The short ground wire spring grounding measurement method shown in Figure 2 has a lot of correct waveforms. Obviously, the senior engineer's method is correct.


Figure 1 Conventional (crocodile line) measurement method (error)


Figure 2 Short ground (spring ground) measurement method (correct)

2. Difference: Inductance
There are various signs that the murderer is the "ground line". Where is the evidence?

Let's look at the diagram. Figure 3 shows the theoretical equivalent model of an oscilloscope using a probe for signal measurement. The probe and the oscilloscope constitute a test equipment 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 Equivalent model of theoretical measurement

Since the ground wire is a wire, it has a certain amount of distributed inductance, and the longer the wire, the greater the inductance. The conventional measurement method will introduce the distributed inductance by the alligator wire. At this time, its equivalent model is the LG inductance shown in Figure 4. This inductance will interact with the probe capacitance, at a certain frequency determined by the LG and CP values. Resonance is formed at the point, leading to the phenomenon of damped oscillation.


Fig. 4 Equivalent model of long ground wire (alligator wire) measurement

The influence of the resonance point in the frequency domain and the time domain is shown in Table 1. Among them, the short ground wire (spring ground) measurement uses the model shown in Figure 3 for analysis; the long ground wire (crocodile wire) measurement uses the model shown in Figure 4 for analysis.

It can be found that the time-domain response corresponding to the long ground wire measurement is similar to the waveform measured by our previous conventional (crocodile wire) measurement, 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 greater the inductance, which forms a resonance frequency point with the probe's capacitance, which will cause obvious overshoot and amplitude reduction oscillation on the fast edge pulse. Therefore, when measuring high-frequency signals, the shorter the probe grounding wire, the better.

to sum up
You can't just stare at the instrument when measuring high-frequency signals. You must pay attention to the ground wire. The shorter the better, you'd rather solder a short piece of wire by yourself. You should never use the alligator ground.