Microwave Tech - Application of High Frequency Circuit in CAD

Application of High Frequency Circuit in CAD

PCB manufacturers: The more commonly used chip inductors in high-frequency circuits include photolithographic film inductors, chip winding inductors and multilayer chip inductors. Because the structural characteristics of the internal electrodes are obviously different, even if the parameters and specifications are the same, the circuit response is different. There are certain rules and characteristics for the selection of inductance devices in actual circuit applications, which can be briefly summarized as follows:

Resonance amplifier: A typical high frequency amplifier circuit usually uses 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 chip inductors becomes the key. The influence of a little error of L can be compensated and corrected by a variety of circuit forms, so wire-wound chip inductors and laminated chip inductors are often used, which requires a higher Q value at the operating frequency. However, film chip inductors are not suitable for both price and performance.

Impedance matching: The radio frequency circuit (RF) is usually composed of basic circuit units such as high amplifier (LNA), local oscillator (LO), mixing (MIX), power amplifier (PA), and filtering (BPF/LPF). Between unit circuits with different characteristic impedances, high-frequency signals need to be coupled and transmitted with low loss, and impedance matching becomes indispensable. The typical solution is to use the combination of inductance and capacitance to form an "inverted L" or "T" type matching circuit. For the chip inductors, the matching performance largely depends on the accuracy of the inductance L, followed by the quality The level of factor Q. When the operating frequency is high, photolithographic film inductors are often used to ensure high-precision L. The internal electrodes are concentrated on the same layer, and the magnetic field distribution is concentrated, which can ensure that the device parameters after mounting do not change much.

High-frequency filtering: Low-pass filtering (LPF) is commonly used in the power supply decoupling loop of high-frequency circuits to effectively suppress the conduction of higher harmonics in the power supply loop. Rated current and reliability are the primary parameters of concern; and band-pass filtering (BPF) It is mostly used for the coupling of high-frequency signals, or at the same time has the role 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.

Local oscillation: The local oscillation circuit (LO) must be composed of an amplifier circuit with 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 oscillation signal directly affects the key performance of the circuit system . The inductance in the oscillation circuit must have a very 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 of chip inductors is not the primary indicator at this time, so laminated chip inductors and wire wound chip inductors are mostly used in VCO circuits.

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