PCB News - The golden rule of reducing harmonic distortion in high frequency PCB design

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

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

The concept of current flowing through the path of least impedance is incorrect. The amount of current in all the 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 ground current flows: one path is directly connected to the bypass capacitor; the other is to stimulate the input resistance before reaching the bypass capacitor. Figure 1 illustrates these two paths. The ground return current is the real cause of the problem.

When the bypass capacitors are placed in different positions on the PCB board, the ground current flows to the respective bypass capacitors through different paths, which is the meaning of "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, only the component voltage of this polarity of the signal will be disturbed. If the other polarity of the ground current is not disturbed, the input signal voltage changes in a non-linear manner. When one polarity component is changed and the other polarity is not changed, distortion will occur, and it will appear as the second harmonic distortion of the output signal. Figure 2 shows this distortion effect in an exaggerated form.

When only one polarity component of the sine wave is disturbed, the resulting waveform is no longer a sine wave. A 100 Ω load is used to simulate an ideal amplifier, and the load current is passed through a 1 Ω resistor, and the input ground voltage is coupled to only one polarity of the signal, and the result shown in Figure 3 is obtained. Fourier transform shows that the distorted waveform is almost all the second harmonic at -68dBc. When the frequency is 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 non-linear effects of the 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 loop and keeping the distance from the input device, as shown in Figure 4.

Multi-amplifier chip
The problem of multi-amplifier chips (two, three or four amplifiers) is more complicated because it cannot keep the ground connections of the bypass capacitors far away from all inputs. This is especially true for quad amplifiers. Each side of the four-amplifier chip has an input terminal, so there is no space for a bypass circuit that can reduce the disturbance to the input channel.

Figure 5 shows a simple method of the four-amplifier layout. 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 the other channel power supply, causing distortion. For example, the (+Vs) bypass capacitor on channel 1 of the quad amplifier can be placed directly near its input; and the (-Vs) bypass capacitor can be placed on the other side of the package. (+Vs) ground current can disturb channel 1, while (-Vs) ground current may not.

To avoid this problem, let the ground current disturb the input, but let the PCB current flow in a spatially linear manner. In order to achieve this, you can use the following method to layout bypass capacitors on the PCB: make the (+Vs) and (–Vs) ground currents flow through the same path. If the disturbance of the positive/negative current to the input signal is equal, there will be no distortion. Therefore, the two bypass capacitors are arranged next to each other so that they share a ground point. Because the two polar components of the ground current come from the same point (output connector shield or load ground) and both return to the same point (the common ground connection of the bypass capacitor), both positive and negative currents flow through the same path . If the input resistance of a channel is disturbed by the (+Vs) current, the (–Vs) current has the same effect on it. Because no matter what the polarity is, the disturbances are the same, so there will be no distortion, but small changes in the channel gain will occur, as shown in Figure 6.

To verify the above inference, two different PCB layouts are used: a simple layout (Figure 5) and a low-distortion layout (Figure 6). The distortion produced by Fairchild's FHP3450 quad operational amplifier is shown in Table 1. The typical bandwidth of FHP3450 is 210MHz, the slope is 1100V/us, the input bias current is 100nA, and the operating current of each channel is 3.6mA. It can be seen from Table 1 that the more severe the distortion of the channel, the better the improvement effect, so that the 4 channels are nearly equal in performance.

Without an ideal quad amplifier on the PCB, it would be difficult to measure the effects of a single amplifier channel. Obviously, a given amplifier channel not only disturbs its own input, but also the inputs of other channels. The ground current flows through all the different channel inputs and produces different effects, but they are all affected by each output. This effect is measurable.
Table 2 shows the harmonics measured on the other undriven channels when only one channel is driven. The undriven channel shows a small signal (crosstalk) at the fundamental frequency, but without any significant fundamental signal, it also produces distortion directly introduced by the ground current. The low-distortion layout in Figure 6 shows that the second harmonic and total harmonic distortion (THD) characteristics are greatly improved because the ground current effect is almost eliminated.

Summary of this article
Simply put, on the PCB, the ground return current flows through different bypass capacitors (for different power supplies) and the power supply itself, and its size is proportional to its conductivity. The high frequency signal current flows back to the small bypass capacitor. Low-frequency currents (such as audio signal currents) may mainly flow through larger bypass capacitors. Even lower frequency currents may "ignore" the existence of all bypass capacitors and directly flow back to the power leads. The specific application will determine which current path is the most critical. Fortunately, by using a common ground point and a ground bypass capacitor on the output side, all ground current paths can be easily protected.
The golden rule of high-frequency PCB layout is to place the high-frequency bypass capacitor as close as possible to the power supply pin of the package. However, comparing Figures 5 and 6, it can be seen that modifying this rule to improve the distortion characteristics will not bring much change. The improvement in distortion characteristics comes at the cost of adding about 0.15 inches of high-frequency bypass capacitor traces, but this has little effect on the AC response performance of the FHP3450. PCB layout is very important to give full play to the performance of a high-quality amplifier, and the issues discussed here are by no means limited to high-frequency amplifiers. Lower frequency signals like audio have much stricter requirements for distortion. The ground current effect is smaller at low frequencies, but if the required distortion index is required to be improved accordingly, the ground current may still be an important issue.

The above is an introduction to the golden rule of reducing harmonic distortion in high-frequency PCB design. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.