Methods of Reducing Harmonic Distortion in PCB Design

Methods of Reducing Harmonic Distortion in PCB Design

In fact, printed circuit board (PCB) is composed of electrical linear materials, that is, its impedance should be constant. So why does PCB introduce nonlinearity into the signal? The answer is that the PCB layout is "spatially nonlinear" relative to where the current flows.

Whether the amplifier gets current from this power supply or another power supply depends on the instantaneous polarity of the signal on the load. The current flows out of the power supply, through the bypass capacitor, and into the load through the amplifier. Then, the current returns to the ground plane from the load ground terminal (or the shield of the PCB output connector), passes through the bypass capacitor, and returns to the power supply that originally provided the current.

The concept of the least impedance path through which current flows is incorrect. The amount of current in all different impedance paths is proportional to its conductivity. In a ground plane, there is often more than one low impedance path through which a large proportion of the current flows: one path is directly connected to the bypass capacitor; The other excites the input resistor before reaching the bypass capacitor. The earth return current is the real cause of the problem.

When the bypass capacitors are placed at different positions on the PCB, the ground current flows to the respective bypass capacitors through different paths, I.e "The meaning represented by spatial nonlinearity. If a large part of the component of a certain polarity of the ground current flows through the ground of the input circuit, then only the component voltage of this polarity of the signal will be disturbed. If the other polarity of the ground current does not disturb, then the input signal voltage will change in a nonlinear way. When one polarity component changes and the other polarity does not change, distortion will occur, which is manifested as the loss of the second harmonic of the output signal True.

When only one polarity component of the sine wave is disturbed, the generated waveform is no longer a sine wave. An ideal amplifier is simulated with a 100 Ω load, so that the load current passes through a 1 Ω resistor, and the input ground voltage is coupled only on one polarity of the signal, and the results shown in Figure 3 are obtained. Fourier transform shows that the distortion waveform is almost all the second harmonic at - 68dBc. When the frequency is very high, it is easy to generate this degree of coupling on the PCB. It can destroy the excellent anti distortion characteristics of the amplifier without resorting to too many special nonlinear effects of PCB. When the output of a single operational amplifier is distorted due to the ground current path, the ground current flow can be adjusted by rearranging the bypass circuit, and the distance from the input device can be maintained.

Multi amplifier chip

The problem with multiple amplifier chips (two, three, or four amplifiers) is more complex because it cannot keep the ground connection of the bypass capacitor away from all inputs. This is especially true for four amplifiers. Each side of the quad amplifier chip has an input, so there is no space to place bypass circuits that can mitigate the disturbance to the input channels.

Most devices are directly connected to the four amplifier pins. The ground current of one power supply can disturb the input ground voltage and ground current of another channel power supply, resulting in distortion. For example, the (Vs) bypass capacitor on channel 1 of four amplifiers can be directly placed near its input; The (Vs) bypass capacitor can be placed on the other side of the package. (Vs) ground current may disturb channel 1, while (Vs) ground current may not. PCB assembly and PCB processing manufacturers explain the methods to reduce harmonic distortion in PCB design.