How to reduce harmonic distortion in PCB design
Actually, the printed circuit board (PCB) is made of electrically linear materials, that is, its impedance should be constant. So, why does PCB introduce nonlinearity into the signal? The answer is that the layout of PCB is “spatially nonlinear” relative to the place where the current flows.
(The amplifier draws current from this power supply or another power supply, depending on the instantaneous polarity of the signal on the load. Current flows from the power supply, passes through the bypass capacitor and enters the load through the amplifier. Then, the current returns to the ground plane from the ground terminal of the load (or the shield of the PCB output connector), passes through the bypass capacitor, and returns to the power source that originally provided the current)
The concept that current flows through the path with minimum impedance is incorrect. The amount of current in all different impedance paths is proportional to its conductivity. In a ground plane, there are often more than one low impedance path through which a large proportion of ground current flows: one path is directly connected to the bypass capacitor; The other one excites the input resistance before reaching the bypass capacitor.
When the bypass capacitors are placed in different positions of PCB, the ground current flows to their respective bypass capacitors through different paths, which means “spatial nonlinearity”. If most 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 nonlinear way. When one polarity component is changed and the other polarity is unchanged, distortion will occur, which is manifested as the second harmonic distortion of the output signal.
Fourier transform shows that the distortion waveform is almost the second harmonic at -68dBc. When the frequency is very high, it is easy to generate this degree of coupling on PCB, which can destroy the excellent anti-distortion characteristics of amplifier without too many special nonlinear effects of PCB.
Simply put, on PCB, the ground return current flows through different bypass capacitors (used for different power supplies) and the power supply itself, and its magnitude is proportional to its conductivity. The high frequency signal current flows back to the small bypass capacitor. Low-frequency currents (such as audio signals) may mainly flow through larger bypass capacitors. Even the current with lower frequency may “disregard” the existence of all bypass capacitors and flow directly back to the power supply lead. The specific application of will determine which current path is the most critical. Fortunately, all the ground current paths can be easily protected by using a common ground point and a ground bypass capacitor on the output side.
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. Modifying this rule to improve distortion characteristics will not bring much change.
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. Distortion of signals with lower frequencies, such as audio, is much stricter. The ground current effect is smaller at low frequency, but if the distortion index is required to be improved accordingly, ground current may still be an important problem.