Serial | Murata Noise Control Class: Harmonics in Digital Signals [IV]

2-4-6. Voltage harmonics and current harmonics

(1) Comparison of voltage harmonics and current harmonics

The harmonic processing method described in the previous section is based on the assumption that the voltage waveform is a rectangular wave. It should be noted that even if the voltage wave of an actual circuit is a rectangular wave, the current waveform may be different. This means that the noise emission will show different trends depending on whether the noise is mainly derived from the voltage or the current.

Fig. 2-4-10 shows the results of calculating the waveform and spectrum using MEFSS, assuming a C-MOS digital circuit and a capacitor with a load of 5pF. The voltage waveform is close to the ideal digital pulse, and the harmonic spectrum values ​​are close to the envelope values ​​shown in Fig. 2-4-6 (the shape is slightly different due to the capacitive load, and a minimum point appears around 500MHz)

Figure 2-4-10 Difference between voltage and current

(2) The current includes more harmonic components

Unlike voltage, current only flows during the rise and fall moments (see diagram above).

As shown in the figure, the spectrum of such a waveform has a constant level in the frequency range up to several hundred MHz (depending on the rise time). Therefore, if noise emission occurs due to the current, the noise is probably caused by high frequencies. In this way, MEFSS can also calculate the spectrum of the current waveform.

In the noise measurement results shown in Fig. 2-3-14, there is almost no voltage spectrum above 500MHz in (b), while the emission noise spectrum in (c) shows strong emission. Therefore, we can see that there is a certain difference in the frequency distribution between the noise source and the emission noise, and one of the reasons is that the emission noise in this test is caused by current. (In addition to this test, there are also cases where voltage is the cause of noise emission).

(3) The current has a spike-like peak waveform

If you think that because the current waveform in Fig. 2-4-10 is in the shape of a thin spike, it is understandable why the current harmonics are not attenuated in the high frequency range.

Considering the trapezoidal wave in Fig. 2-4-6, the spike-shaped waveform, like the current waveform, can be regarded as a trapezoidal wave model when the duty ratio is very small. For the envelope of the trapezoidal wave with a small duty ratio, point A shifts to the high frequency side and maintains a constant level in a very high frequency range.

Therefore, it can be observed that the harmonics of the current waveform continue to very high frequencies without decaying.

Note that the trapezoidal wave model in Figure 2-4-6 is different from the current waveform because the spikes of the current waveform point upward and downward. Therefore, when point A is moved, the trapezoidal wave model with a smaller duty cycle has stronger harmonics. However, this trend is weaker in the current waveform .