Limitations of High Frequency Circuits in Electronic Circuit CAD Analysis

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Limitations of High Frequency Circuits in Electronic Circuit CAD Analysis [Copy link]

【Source: China PCB Technology Network】【Author:】【Time: 2006-1-18 9:19:05】【Click: 102】

Electronic circuit CAD simulation software PSPICE has powerful functions and has broad application prospects in electronic circuit design. However, it cannot accurately describe the complex parameters of circuits at medium and high frequencies, which brings great difficulties to the analysis of high-frequency circuits. Keywords: circuit simulation software; PSPICE; frequency circuit; natural frequency exploration of high-frequency applications of chip inductors Introduction With the rapid development of the electronic information industry, chip inductors, as new basic passive devices, have been widely used with their good performance-price ratio and convenient high-density mounting. Especially in communication terminal equipment represented by mobile phones, chip inductors have obtained typical high-frequency applications. As the operating frequency of RF circuits continues to increase, the performance characteristics of chip inductors in applications have changed significantly, and they have begun to show the working characteristics of low-end microwave frequency bands. Therefore, in order to effectively improve the electrical parameters of chip inductors and improve the performance of RF circuits, it is necessary to further analyze the different laws of their low-frequency characteristics and high-frequency characteristics. On the other hand, the constantly updated communication systems (GSM, CDMA, PCS, 3G...) have gradually made the operating frequency of chip inductors reach 2GHz or even higher. Therefore, the impedance analysis of chip inductors using traditional lumped parameter circuit theory shows increasingly obvious limitations. Exploring engineering analysis methods suitable for high-frequency conditions has also become an important topic for the research, development, production, analysis and application of chip inductors. Impedance analysis The physical meaning of inductance is to use conductive coils to store alternating magnetic field energy. In actual circuit applications, the main function of inductors is to provide the required inductive impedance to the circuit and complete the corresponding circuit functions (matching, filtering, oscillation, etc.) in cooperation with other related components. Common chip inductors include laminated chips, wound chips, photolithography films, etc., and their production processes and internal electrode structures are different. However, under medium and low frequency conditions, since the signal wavelength is much larger than the device size, the circuit response of the device is less affected by the internal electrode structure. Usually, the lumped parameter equivalent model (see Figure 1) can be used to approximate the impedance characteristics of the chip inductor. Based on this, the function formula of commonly used electrical performance parameters can be derived. Admittance function Y(j)=({1}\over{R_{O}}+{r}\over{r^{2}+ ^{2}L^{2}_{O}})+j(C_{O}-{L_{O}}\over{r^{2}+ ^{2}L^{2}_{o}}) Then impedance function Z(j)={1}\over{Y(j)}=R()+j () The impedance Z()=\sqrt{R^{2}()+ ^{2}()} ={L_{O}}\over\sqrt{({L_{O}}\over{R_{O}}+{r}\over{L_{O}})^{2}+(1-{^{2}}\over{SRF^{2}})^{2}} The inductance L()={( )}\over{ }={L_{O}(1-{ ^{2}}\over{SRF^{2}})}\over{({{ L_{O}}\over{R_{O}}+{r}\over{ L_{O}})^{2}+(1-{ ^{2}}\over{SRF^{2}})^{2}} Quality factor Q( )={ ( )}\over{R( )}={(1-{ ^{2}}\over{SRF^{2}})}\over{({ L_{O}}\over{R_{O}}+{r}\over{ L_{o}})} where SRF={1}\over{2 \sqrt{L_{O}C_{O}}} =2 FIt is not difficult to summarize from these function expressions: (1) When the operating frequency is lower than the self-resonant frequency SRF, the impedance characteristics of the chip inductor are very close to the ideal inductor and show good linear characteristics. The quality factor Q is also high, so the rated operating frequency band of the inductor is usually determined based on this; (2) When the inductance L0 is the rated value, the only way to increase the self-resonant frequency SRF is to reduce the parasitic capacitance C0; (3) In the low-frequency working area, reducing the internal electrode resistance r will effectively improve the quality factor Q value, while in the high-frequency working area, reducing electromagnetic leakage (increasing R0) will improve the Q value more significantly; (4) When the operating frequency is higher than the self-resonant frequency SRF, the chip inductor shows a capacitive impedance characteristic. In general applications, the use of an impedance analyzer to detect parameters such as Z (), L (), Q () between the end electrodes of the chip inductor can accurately reflect the response characteristics of the actual circuit at the operating frequency, and accurate circuit design and device selection can be carried out based on this. For comparison, Figure 2 lists the L (f) and Q (f) parameter curves of high-frequency inductors (SGHI1608H100N) and ferrite inductors (SGMI1608M100N) of the same specifications. It is obvious that high-frequency inductors have higher self-resonant frequencies and linear operating frequency bands, while ferrite inductors have higher Q values. High-frequency analysis When the operating frequency is high (around 2GHz), the signal wavelength gradually becomes comparable to the device size. The impedance of the chip inductor shows an obvious distribution characteristic, that is, different reference positions have different impedances. The analysis model shown in Figure 1 is no longer suitable for describing high-frequency inductor devices. Under high-frequency conditions, the circuit response of the device may change accordingly with its size and spatial structure, and the conventional impedance measurement parameters can no longer accurately reflect the response characteristics in the actual circuit. Take a certain model of mobile phone RF power amplifier circuit as an example. Two high-frequency inductors (working frequency 1.9GHz) used for impedance matching are both photolithography thin film inductors. If they are replaced by multilayer chip inductors (measurement instrument HP-4291B) with the same specifications and accuracy but significantly higher Q value, the result is that the circuit transmission gain drops by nearly 10%. This indicates that the circuit matching state has declined. It is obviously impossible to accurately explain high-frequency application problems using low-frequency analysis methods. It is inappropriate or at least insufficient to only focus on L ( ) and Q ( ) for high-frequency analysis of chip inductors. Electromagnetic field theory is often used in engineering to analyze high-frequency application problems with distributed characteristics. Usually, in the measurement of chip inductors using an impedance analyzer (HP-4291B), the measurement accuracy can be increased to about 0.1nH through fixture compensation and instrument calibration, which is theoretically sufficient to ensure the accuracy required 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 end electrode interfaces of the inductor device under the matching state (the measurement fixture is designed for precise matching), but fail to reflect the internal electromagnetic distribution of the inductor device and the requirements of the external electromagnetic environment. Inductors with the same test parameters 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 effects) and the test environment are often different, which can easily produce various complex near-field reflections and cause slight changes in the actual response parameters (L, Q). For low-inductance inductors in RF circuits, this effect cannot be ignored. We call this effect "distribution effect." In the design of high-frequency circuits (including high-speed digital circuits), based on the consideration of circuit performance, device selection and electromagnetic compatibility, the working performance of the actual circuit system is usually comprehensively considered by means of network scattering analysis (S parameters), signal integrity analysis, electromagnetic simulation analysis, circuit simulation analysis, etc. In view of the "distributed influence" problem of chip inductors, a feasible solution is to perform structural electromagnetic simulation on the inductors and accurately extract the corresponding SPICE circuit model parameters as the basis for circuit design, so as to effectively reduce the error influence of inductors 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. The chip inductors commonly used in high-frequency circuits are photolithography thin film inductors, chip winding inductors and stacked chip inductors. Due to the obvious differences in the structural characteristics of the internal electrodes, even if the parameter specifications are the same, the circuit response is not exactly the same. There are certain rules and characteristics for the selection of inductor components in actual circuit applications, which can be summarized as follows: Impedance matching: Radio frequency circuits (RF) are usually composed of basic circuit units such as high-frequency amplifier (LNA), local oscillator (LO), mixer (MIX), power amplifier (PA), and filter (BPF/LPF). Between unit circuits with different characteristic impedances, high-frequency signals require low-loss coupling transmission, and impedance matching becomes indispensable. The typical solution is to use inductors and capacitors to combine into an "inverted L" or "T" type matching circuit. For the chip inductors, the matching performance depends largely on the accuracy of the inductance L, followed by the quality factor Q. When the operating frequency is high, photolithography thin film inductors are often used to ensure high-precision L. The internal electrodes are concentrated on the same level, and the magnetic field distribution is concentrated, which can ensure that the device parameters after mounting do not change much. Resonant amplification: Typical high-frequency amplifier circuits usually use a resonant circuit as the output load. For its main performance parameters such as gain and signal-to-noise ratio, the quality factor Q of the chip inductor becomes the key. The slight error of L can be compensated and corrected by a variety of circuit forms, so wound chip inductors and multilayer chip inductors are mostly used, and the Q value at the operating frequency is required to be high. However, thin film chip inductors are not suitable here in terms of price or performance. Local oscillation: The local oscillator circuit (LO) must be composed of an amplifier circuit containing an oscillation loop, usually in the form of VCO-PLL to provide an accurate reference frequency to the RF circuit, so the quality of the local oscillator signal directly affects the key performance of the circuit system. The inductor in the oscillation loop must have an extremely high Q value and stability to ensure the purity and stability of the local oscillator signal. Since quartz crystals have relatively wide impedance dynamic compensation, the L accuracy requirement for chip inductors is not the primary indicator at this time, so multilayer chip inductors and wound chip inductors are mostly used in VCO circuits. High-frequency filtering: Low-pass filtering (LPF) is commonly used in the power supply decoupling circuit of high-frequency circuits, effectively suppressing the conduction of high-order harmonics in the power supply circuit. Rated current and reliability are the primary parameters of concern; while band-pass filtering (BPF) is mostly used for coupling of high-frequency signals, or at the same time has the function of impedance matching. At this time, the insertion attenuation should be as small as possible, and L and Q are the key parameters at this time. Comprehensive comparison shows that multilayer chip inductors are most suitable for this application.