In this paper, we will explain the details of high-frequency PCB design from four aspects: line width, via, cross-line crosstalk, shielding, etc., and enumerate several representative examples of high-frequency PCB board selection for reference.
High frequency board PCB design process
(1). Transmission line width
High-frequency board PCB design Transmission line width design needs to be based on impedance matching theory.
Figure 1 Impedance matching
When the input and output impedances match the transmission line impedance, the system output power is the largest (the total signal power is the smallest), and the ingress and exit reflection is the smallest.
For microwave circuits, the impedance matching design also needs to consider the operating point of the device. Signal line vias cause changes in impedance transmission characteristics, and TTL and CMOS logic signal lines have high characteristic impedance, which is not affected.
(2). Crosstalk between transmission lines
Coupling occurs when the distance between two parallel microstrip lines is small, causing crosstalk between lines and affecting the characteristic impedance of the transmission line. Special attention should be paid to 50 ohm and 75 ohm high frequency circuits, and measures should be taken in circuit design. This coupling feature is also used in actual circuit design, such as mobile phone transmit power measurement and power control. The following analysis is valid for high-frequency circuits and ECL high-speed data (clock) lines, and for small signal circuits (such as precision operational amplifier circuits).
Figure 2 Crosstalk between transmission lines
Let the coupling degree between lines be C, and the size of C is related to εr, W/d, S, and parallel line length L. The smaller the spacing S is, the stronger the coupling is; the longer L is, the stronger the coupling is. In order to increase the perceptual knowledge, for example, a 50 ohm directional coupler made using this characteristic. Such as 1.97GHz PCS frequency base station power amplifier, where d = 30 mil, εr = 3.48:
10dB directional coupler PCB size: S=5mil, l=920mil, W=53mil
20dB directional coupler PCB size: S=35mil, l=920mil, W=62mil
In order to reduce the crosstalk between signal lines, the following suggestions are given:
A. High-frequency or high-speed data parallel signal line distance S is more than double the line width.
B. Try to reduce the parallel length between signal lines.
C. High-frequency small signal, weak signal avoid strong interference sources such as power supply and logic signal line.
(3). Electromagnetic analysis of grounding vias.
No matter the IC device pin is grounded or other resistive device is grounded, the ground via is required to be as close as possible to the pin in the high-frequency circuit. The theoretical basis is that the high-frequency signal ground line is equivalent to the ground of the ideal transmission line. The standing wave state is shown in Figure 3.
Figure 3 Standing wave
Since the grounding wire is short, the grounding transmission line is equivalent to an inductive impedance (n-pH magnitude), and the grounding via is also approximately equivalent to an inductive impedance, which affects the filtering effect on the high frequency signal. This is why the ground via is as close as possible to the pin. In order to reduce the inductive load of the transmission line, the microwave circuit requires more than one via hole of the grounding pin, which is equivalent to increasing the grounding current capability in the low frequency circuit, and ensuring that each grounding point is equal to 0 level.
(4). Power supply filtering
In order to reduce the influence of signal logic on the power supply (overshoot), TTL and CMOS circuits add filter capacitors close to the power supply pins. However, it is not enough to take such measures in high-frequency and microwave circuits. The manufacturing process is taken as an example to illustrate the interference of the high frequency signal to the power supply.
Figure 4 High-frequency signals generate high-frequency interference to the power supply
Both of these high frequency signals cause high frequency interference to the power supply and affect other functional circuits. In addition to the power pin plus filter capacitor, series inductors are required to suppress high frequency interference. The selection of the series inductance is related to the operating frequency. The basis is that if the power supply pin filters high frequency interference above 1M, where C = 0.1uF, then L = 1uH inductance is selected. Be careful when adding the inductor to the open collector signal pin of the external power supply, because the inductance at this time is equivalent to a matching inductor.
Shielding measures are needed in PCB design for small and high frequency signals to reduce large signal (such as logic levels) interference or to reduce electromagnetic radiation from high frequency signals. Such as:
A. Digital, analog low-frequency (less than 30MHz) small-signal PCB design, in addition to the digital ground and analog ground segmentation, it is also necessary to lay the ground for the small signal wiring area, the ground and signal line spacing is greater than the line width.
B. In digital and analog high-frequency small-signal PCB design, it is necessary to add shielding or grounding isolation measures in the high-frequency part.
C. In high-frequency large-signal PCB design, the high-frequency part needs to be designed with independent functional modules and a shielding box to reduce the external radiation of high-frequency signals. Such as fiber 155M, 622M, 2Gb / s transceiver module.
Multi-layer PCB layout (Nokia 6110), double-sided device, mobile phone PCB design shown in Figure 5.
Figure 5 Mobile phone PCB design example
High frequency PCB Material Selection Examples
The following is an example of the high-frequency (microwave) PCB we designed and debugged to illustrate the selection of the board.
(1) Selection of 2.4 GHz spread spectrum digital microwave relay board
Its structure includes 2M digital interface, 20M spread spectrum despreading, 70M intermediate frequency modulation and demodulation board. We use FR4 sheet, four-layer PCB board, large-area paving, high-frequency analog part of the power supply is insulated from the digital part by the inductor choke.
The 2.4GHz RF transceiver adopts F4 dual-panel, and the transceiver is shielded by metal box and filtered by the power input.
(2) 1.9GHz RF transceiver
Among them, the power amplifier uses PTFE sheet, double-sided PCB board; the RF transceiver uses PTFE sheet and four-layer PCB board. All use large-area paving, functional module shield isolation measures.
(3) 140MHz IF transceiver
The top layer is made of 0.3mm S1139 sheet, which is covered with large area and separated by via holes.
(4) 70MHz IF transceiver
FR4 sheet, four-layer PCB board. Large area paving, functional module isolation strips are isolated with a series of vias.
(5) 30W power amplifier
RO4350 sheet, double-sided PCB board. Large area paving, spacing constraint is greater than or equal to 50 ohm line width, shielded with metal box, power input filter.
(6) 2000MHz microwave frequency source
It is made of 0.8mm thick S1139 sheet and double-sided PCB board.
The devices in the wireless field are widely used in the market, and the application is more complicated. Especially in the current wireless communication market, the competition is fierce, and the price and time of the products are becoming more and more competitive. Therefore, the PCB design of the electronic engineer cannot simply consider the technology. Advancement must be considered in many aspects, balancing key factors such as technological advancement, price advantage and time-to-market to improve product competitiveness.