High frequency crystal oscillator measurement takes crystal oscillator signal measurement as an example

1. Comparison of high-frequency crystal oscillator measurements

Let's first feel the difference in measurement results when the probe ground wire is long or short.

Take the crystal oscillator signal measurement as an example. Figure 1 shows the conventional alligator wire grounding measurement method. It can be seen that the signal overshoots seriously accompanied by oscillation, which is different from the imagined square wave. However, the short ground wire spring grounding measurement method shown in Figure 2 has a much more correct waveform. Obviously, the method of the senior engineer is correct.

Figure 1. Conventional (alligator line) measurement method (wrong)

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

2. Core difference: inductance

All signs indicate that the murderer is the "ground wire", so where is the evidence?

Let's look at the diagram. Figure 3 shows the theoretical equivalent model of an oscilloscope using a probe to measure a signal. 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 workload.

Figure 3. Theoretical measurement equivalent model

Since the ground wire is a conductor, it has a certain amount of distributed inductance, and the longer the wire, the greater the inductance. The conventional measurement method will introduce distributed inductance through 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 and form resonance at a certain frequency point determined by the LG and CP values, resulting in the generation of the damping oscillation phenomenon.

Figure 4. Long ground lead (alligator lead) measurement equivalent model

The influence of the resonance point in the frequency domain and time domain is shown in Table 1. The short ground wire (spring ground) measurement is analyzed using the model shown in Figure 3, and the long ground wire (alligator wire) measurement is analyzed using the model shown in Figure 4.

It can be found that the time domain response corresponding to the long ground wire measurement has characteristics similar to the waveform of our previous conventional (alligator wire) measurement, which also verifies the correctness of the theoretical analysis.

The probe ground wire adds distributed inductance to the circuit; the longer the ground wire, the greater the inductance, which forms a resonant frequency with the capacitance of the probe, and will produce obvious overshoot and amplitude reduction oscillation on the fast edge pulse. Therefore, when measuring high-frequency signals, the shorter the probe ground wire, the better.

Summarize

When measuring high-frequency signals, you can't just stare at the instrument, you must pay attention to the ground wire. The shorter the better. It's better to solder a short piece of wire yourself than to use an crocodile ground.