Microwave Tech - Analysis of High Frequency Circuit in CAD

Analysis of High Frequency Circuit in CAD

PCB manufacturers: When the operating frequency is higher (about 2GHz), the signal wavelength can gradually be compared with the device size. The impedance of the chip inductor presents an obvious distribution characteristic, that is, different impedances exist at different reference positions. Under high-frequency conditions, the circuit response of the device can vary with its size and spatial structure. Conventional impedance measurement parameters can no longer accurately reflect the response characteristics of the actual circuit. Take the RF power amplifier circuit of a certain model of mobile phone as an example. Two of the high-frequency inductors (operating frequency 1.9GHz) used for impedance matching use photolithographic film inductors. If the same specifications and accuracy are used, the Q value is significantly higher. The laminated chip inductor (measurement instrument HP-4291B) was replaced, but the result was that the transmission gain of the circuit dropped by nearly 10%. It shows that the matching state of the circuit is degraded. The low-frequency analysis method obviously cannot explain the high-frequency application problem accurately. It is not suitable to focus only on the high-frequency analysis of the chip inductors with L () and Q (), at least not enough.

Electromagnetic field theory is often used in engineering to analyze high-frequency application problems with distributed characteristics. Generally, in the measurement of chip inductors with an impedance analyzer (HP-4291B), the measurement accuracy can be increased to about 0.1nH by means of fixture compensation and instrument calibration, which is theoretically sufficient to ensure the accuracy requirements for circuit design. However, the problem that cannot be ignored is that the measurement results at this time only reflect the parameter performance between the terminal electrode interfaces of the inductance device under the matching state (the measuring fixture is designed to be accurately matched), and the internal electromagnetic distribution of the inductance device and the external electromagnetic environment requirements But it failed to reflect. The inductance of the same test parameter may have completely different electromagnetic distribution states due to different internal electrode structures. Under high frequency conditions, the actual circuit application environment of chip inductors (approximate matching, dense mounting, PCB distribution influence) and test environment are often There are differences, it is very easy to produce various complex near-field reflections and slight changes in the actual response parameters (L, Q). For the low-inductance inductance in the RF circuit, this kind of influence cannot be ignored. We call this kind of influence "distributed influence".

In the design of high-frequency circuits (including high-speed digital circuits), considerations such as circuit performance, device selection and electromagnetic compatibility are usually based on network scattering analysis (S parameters), signal integrity analysis, electromagnetic simulation analysis, circuit simulation analysis, etc. Means to comprehensively consider the performance of the actual circuit system. Aiming at the "distributed influence" problem of chip inductance devices, a feasible solution is to carry out structural electromagnetic simulation of inductance devices and accurately extract the corresponding SPICE circuit model parameters as the basis for circuit design, thereby effectively reducing the inductance The influence of errors in high-frequency design applications. The technical parameters of chip inductor products of major foreign (Japanese) component companies mostly include S-parameters, which can usually be used for accurate high-frequency application analysis.

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