PCB Technical - Understand bandwidth design techniques in PCB design

This article relates to printed electronics and PCBs. I acquired the bandwidth design technology for PCB design, and later applied the same principles to printed electronics design. In this article, I will explain my understanding of bandwidth and how to apply it to PCBs and printed electronics.

When the signal is calculated by Fourier transform from the time domain to the frequency domain, the signal may contain several frequency components. The time domain signal is the sum of all contained frequency components, and the shape of the signal depends on the power level of each individual frequency. The digital signal contains a DC component, followed by many lower-intensity AC components, the intensity of which decreases as the frequency increases. Faster signals mean higher frequency components. Each of these AC frequencies is a very narrow frequency band, that is, a single frequency sine wave signal. Therefore, the digital signal is the sum of the DC signal plus a large number of sine wave signals. Pure AC signals can be narrow-band (such as sine waves) because they do not contain DC components.

The signal information is located somewhere in the frequency range, and all the frequency components required for this information determine the bandwidth. Frequencies outside the bandwidth are unnecessary and can be rejected, for example by filtering, because these frequencies do not carry additional information about the signal.

Bandwidth can be considered as the working area of the electrical signal, in which it does not lose information, and it is also necessary for the electrical path (ie routing) or load of the signal. Then design the electronic equipment accordingly, and in the best case, when the signal is fed into the trace, it remains unchanged. If the signal speed is higher than the bandwidth of the trace or filter, the signal will be modified, which usually means that certain frequency components will be filtered out. The tracking itself will have bandwidth limitations,

The bandwidth of the signal is determined by the signal rise time (10% to 90%), which can be expressed by the following rule of thumb:

Bandwidth = 0.35 / tr(1)

The signal frequency is not as important as the rise time requirement, just because the signal is different. Even if the signal frequency is exactly the same, the rise and fall time requirements of the digital signal (50% duty cycle) and PWM signal (10% to 90% duty cycle) are different. In the PWM signal, when the signal "on" state is shorter than the "off" state (90%) (duty cycle is 10%), this means that the rise time must be faster compared to the longer "on" state pulse Much. Of course, the signal frequency is also very important, because the higher the frequency, the faster its rise time needs to be. This bandwidth rule of thumb is my first tool for design tasks related to signal bandwidth. I learned it from a lecturer in electronic design at my university a long time ago, and I have used it many times in design since then.

If the RC filter resistance you choose is approximately at the same ohmic level as the output resistance of the signal driver, then the output resistance must also be taken into account when calculating the -3dB cutoff frequency.

The bandwidth can be considered the same as the -3dB cutoff frequency. The cut-off frequency means that the frequency at this time has been attenuated to half of its original power level. Other filters can also be used. It makes sense to minimize crosstalk through the best PCB stack design, but the filter provides us with another tool to minimize it. Filtered by RC filter. I chose a 100Ω resistor and a 100pF capacitor. In addition, we also measured the 38Ω output resistance of the signal driver and ~10pF IC load capacitance, which must be taken into account. The cutoff frequency displayed by the RC filter calculator is:

F-3dB = 1/2π(100Ω + 38Ω)*(100pF + 10pF) = 10.484MHz

According to the calculation of the bandwidth, the fastest rise time of the bandwidth is 0.35 / 10.484MHz = 33.4ns.

The signal is a digital signal. From the shape, we can see that we have not lost information after filtering. We can still reliably detect the pulse as a logic 1, and the signal will still go low fast enough before the next cycle begins. In addition, since high-frequency harmonics have been attenuated, there is much less noise. In this way, I successfully reduced the crosstalk between the digital bus trace and the sensitive sensor trace, and made the sensor work without rewiring. This is achieved by filtering only the interfering signals and not touching the analog signals at all, because the sensor bandwidth requirements are higher than the digital bus.

In printed electronics, limiting bandwidth to an appropriate level is even more important than in PCB. The main reason for limiting bandwidth in printed electronics is to reduce interference caused by crosstalk. By establishing the best stack in terms of impedance and crosstalk, printed electronics are more restricted, and I need to use filters or signals with limited slew rate. When we consider the stacking of printed electronic devices, we can see that the traces that cross each other are only separated by a locally thin printed dielectric layer. Its thickness is only tens of microns, which means that the capacitive coupling between the crossed traces is very strong. The capacitance between the traces depends on the intersection area and the thickness of the dielectric layer between them. In printed electronic products, traces are often wider than in PCBs, and the S and dielectric layers are much thinner than in PCBs, which results in greater capacitance between traces. A larger capacitance means that a lower frequency is coupled through this "capacitance". In addition, the size of the layout area may be almost the same as the product size, which means that the length of the trace is very long, thereby increasing the inductance of the trace. Like higher capacitance, higher inductance affects lower frequencies.

Because of the various materials and stacks involved, printed electronic products have brought low-frequency bandwidth challenges, but PCB manufacturers can solve these problems through known principles and methods that are widely used in PCB design. In addition, the understanding of bandwidth is very important in printed electronics design and requires careful consideration. Due to material differences, the challenges related to signal speed in printed electronics are similar to those in PCBs, but in printed electronics, we may face much fewer hallenges.