PCB Tech - High frequency wiring process and PCB board material selection

1. Transmission line width

High frequency board PCB design transmission line width design needs to be based on impedance matching theory.

When the input, output impedance and transmission line impedance match, the system output power is the largest (the total signal power is the smallest), and the input and output reflections are the smallest. For microwave circuits, impedance matching design also needs to consider the operating point of the device. Signal line vias will cause changes in impedance transmission characteristics. TTL and CMOS logic signal lines have high characteristic impedance, and this effect is ignored. However, this effect needs to be considered in low-impedance, high-frequency circuits such as 50 ohms, and it is generally required that the signal line has no vias.

2. Crosstalk between transmission lines

When the distance between two parallel microstrip lines is very small, coupling occurs, causing crosstalk between the 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 has reference value for micro-signal circuits (such as precision operational amplifier circuits).

Suppose the degree of coupling between lines is C, and the size of C is related to εr, W/d, S, and the length of parallel lines L. The smaller the distance S, the stronger the coupling; the longer the L, the stronger the coupling. In order to increase perceptual knowledge, for example: use this characteristic to make a 50 ohm directional coupler. Such as 1.97GHz PCS frequency end 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. The distance S between high-frequency or high-speed data parallel signal lines is more than twice the line width.

B. Minimize the parallel length between signal lines.

C. Avoid strong interference sources such as power supply and logic signal lines for high-frequency small signals and weak signals.

3. Electromagnetic analysis of ground vias

Regardless of whether the IC device pin is grounded or other resistive components are grounded, the grounding via is required to be as close to the pin as possible in the high-frequency circuit. The standing wave state is shown in Figure 3.

Since the ground wire is very short, the ground transmission line is equivalent to an inductive impedance (n-pH order), and the ground via is also approximately equivalent to an inductive impedance, which affects the filtering effect of high-frequency signals. This is why the ground vias are as close as possible to the pins. In order to reduce the inductive load of the transmission line, the microwave circuit requires more than one via hole on the ground pin, which is equivalent to increasing the ground plane current capability in the low-frequency circuit to ensure that each ground point is equal to 0 level.

4. Power filter

In order to reduce the influence of signal logic on the power supply (overshoot) in TTL and CMOS circuits, filter capacitors are added near the power supply pins. However, it is not enough to just take such measures in high-frequency and microwave circuits. The following takes the manufacturing process as an example to illustrate the interference of high-frequency signals on the power supply.

The high-frequency signals of these two methods both produce high-frequency interference to the power supply and affect other functional circuits. In addition to the power supply pin and the filter capacitor, a series inductor is also needed 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, select L=1uH inductance. Please be cautious when adding inductance to the open-collector signal pin of the external power supply, because the inductance at this time is equivalent to an inductance for matching.

5. Shield

In the PCB design of small signals and high-frequency signals, shielding measures should be taken to reduce the interference of large signals (such as logic levels) or reduce the electromagnetic radiation of high-frequency signals. like:

A. In digital and analog low-frequency (less than 30MHz) small-signal PCB design, in addition to dividing the digital ground and analog ground, it is also necessary to lay the ground in the small-signal wiring area, and the distance between the ground and the signal line is greater than the line width.

B. In the design of digital and analog high-frequency small-signal PCB, it is also necessary to add a shielding cover or pave the ground via isolation measures in the high-frequency part.

C. In the high-frequency large-signal PCB design, the high-frequency part needs to be designed with an independent functional module and a shielding box is added to reduce the external radiation of the high-frequency signal. Such as optical fiber 155M, 622M, 2Gb/s transceiver module.

Multi-layer PCB layout (Nokia 6110), double-sided placement device, mobile phone PCB design as shown in Figure 5.

Examples of high-profile PCB board selection

Below we take the high frequency (microwave) PCB we designed and debugged as an example to illustrate the selection of plates.

(1) Selection of 2.4GHz spread spectrum digital microwave relay board

Its structure includes 2M digital interface, 20M spread spectrum despreading, 70M intermediate frequency modulation and demodulationboard. We use FR4 board, four-layer PCB board, large area floor, high-frequency analog part of the power supply is isolated from the digital part by inductance choke.

The 2.4GHz radio frequency transceiver adopts F4 double panel, the transceiver is shielded by a metal box, and the power input is filtered.

(2) 1.9GHz radio frequency transceiver

Among them, the power amplifier adopts PTFE sheet and double-sided PCB board; the radio frequency transceiver adopts PTFE sheet and four-layer PCB board. All adopt large-area paving and isolation measures of functional module shielding cover.

(3) 140MHz intermediate frequency transceiver

The top layer is made of 0.3mm S1139 board, which is spread over a large area and is isolated by via holes.

(4) 70MHz intermediate frequency transceiver

Using FR4 board, four-layer PCB board. Pave the ground in a large area, and the functional module isolation belt is isolated by a series of via holes.

(5) 30W power amplifier

Use RO4350 board, double-sided PCB board. Pave the ground in a large area, with a spacing constraint greater than or equal to 50 ohm line width, shielded by a metal box, and filtered at the input end of the power supply.

(6) 2000MHz microwave frequency source

Using 0.8mm thick S1139 sheet, double-sided PCB board.