Challenges of Electronic Circuit CAD Simulation Software in High-Frequency Circuit Analysis

introduction

随着电子信息产业的迅猛发展,片式电感作为新型基础无源器件,以其良好的性能价格比和便于高密度贴装等显著优点,迅速得到了广泛应用,尤其在以移动手机为代表的通信终端设备中,片式电感获得了典型的高频应用。由于RF电路的工作频率不断提升,片式电感在应用方面的性能特点发生了明显变化,已经开始显现出低端微波频段的工作特性。因此，为有效提升片式电感的电性参数，改善RF电路性能，必须进一步分析其低频特性与高频特性的不同规律。

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 inductor devices based on the traditional lumped parameter circuit theory has shown increasingly obvious limitations. Exploring engineering analysis methods suitable for high-frequency conditions has also become an important topic in the research, development, production, analysis and application of chip inductors.

Impedance analysis

The physical meaning of inductance is to use a conductive coil to store energy in an alternating magnetic field. In actual circuit applications, the main function of the inductor is to provide the required inductive impedance to the circuit and to complete the corresponding circuit functions (matching, filtering, oscillation, etc.) in cooperation with other related components. Common chip inductor devices include stacked 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, a concentrated 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}})

The impedance function

Z(j )={1}\over{Y(j )}=R( )+j ( )

Impedance can be derived approximately

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}}

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 Factors

Q( )={ ( )}\over{R( )}={(1-{ ^{2}}\over{SRF^{2}})}\over{({ L_{O}}\over{R_{O}}+{r}\over{ L_{o}})}

in

SRF={1}\over{2 \sqrt{L_{O}C_{O}}}

=2 F

It is not difficult to summarize 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)在低频工作区,降低内电极电阻r将有效提升品质因素Q值，而在高频工作区，减小电磁漏损(增大R0)对Q值的提高则更为显著;

(4) When the operating frequency is higher than the self-resonant frequency SRF, the chip inductor exhibits capacitive impedance characteristics.

In common applications, the use of an impedance analyzer to detect parameters such as Z( ), L( ), and Q( ) between the end electrodes of a chip inductor can accurately reflect the response characteristics of the actual circuit at the operating frequency, and thus accurate circuit design and device selection can be performed. For comparison, Figure 2 lists the L(f) and Q(f) parameter curves of a high-frequency inductor (SGHI1608H100N) and a ferrite inductor (SGMI1608M100N) of the same specification. It is clear that the high-frequency inductor has a higher self-resonant frequency and linear operating frequency band, while the ferrite inductor has a higher Q value.

High frequency analysis

When the operating frequency is high (around 2GHz), the signal wavelength can gradually be compared with the device size. The impedance of the chip inductor shows a clear 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 can 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. Taking a certain model of mobile phone RF power amplifier circuit as an example, two high-frequency inductors (operating frequency 1.9GHz) used for impedance matching are both photolithography thin film inductors. If they are replaced with multilayer chip inductors with the same specifications and accuracy but significantly higher Q values ​​(measurement instrument HP-4291B), the result is that the circuit transmission gain decreases by nearly 10%. This shows that the circuit matching state has decreased. It is obviously impossible to accurately explain the high-frequency application problem using low-frequency analysis methods. It is not appropriate to only focus on L () and Q () for high-frequency analysis of chip inductors, or at least it is not enough.

Electromagnetic field theory is often used in engineering to analyze high-frequency application problems with distributed characteristics. Usually, when using an impedance analyzer (HP-4291B) to measure chip inductors, the measurement accuracy can be improved 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 to be accurately matched), 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 (approximate matching, dense mounting, PCB distribution effects) of chip inductors is often different from the test environment, 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 considerations such as circuit performance, device selection, and electromagnetic compatibility, network scattering analysis (S parameters), signal integrity analysis, electromagnetic simulation analysis, and circuit simulation analysis are usually used to comprehensively consider the working performance of the actual circuit system. In response to the "distributed impact" problem of chip inductors, a feasible solution is to perform structural electromagnetic simulation of the inductor and accurately extract the corresponding SPICE circuit model parameters as the basis for circuit design, thereby effectively reducing the error impact of the inductor 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.

Circuit Application

The three types of 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 parameters and specifications are the same, the circuit responses are not exactly the same. There are certain rules and characteristics for the selection of inductor components in actual circuit applications, which can be briefly summarized as follows:

Impedance matching: Radio frequency circuits (RF) are usually composed of basic circuit units such as 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 essential. 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 do not change much after mounting.

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 effect of L can be compensated and corrected by a variety of circuit forms, so wound chip inductors and stacked chip inductors are mostly used, and the Q value at the operating frequency is required to be high. Thin-film chip inductors are not suitable for this purpose 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 a VCO-PLL to provide an accurate reference frequency to the RF circuit. Therefore, 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 stacked 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, which effectively suppresses 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, multilayer chip inductors are most suitable for this application.