Research on double zero bandpass filter based on LTCC technology

With the rapid development of radio frequency wireless products, the requirements for miniaturization, integrated modularization, and high frequency of microwave filters are becoming higher and higher. The market demand for small-volume, high-performance, and low-cost microwave filters is increasing. The design and implementation of such microwave filters has become one of the key issues in modern microwave technology. Its main design concept is to transform the two-dimensional circuit layout into a three-dimensional circuit layout, thereby achieving the purpose of reducing the volume. Due to the advantages of high integration density, high performance, high reliability, and embedded passive components, low-temperature co-fired ceramic (LTCC) technology has become the mainstream of multi-layer passive devices and circuit design, which has greatly promoted the miniaturization of microwave passive devices. The multi-microwave passive filter based on LTCC studied and designed in this paper strives to achieve structural miniaturization and superior performance.

1 Design principle of filter with transmission zero

The transmission zero theory refers to the filter transmission function equal to zero, that is, at this frequency point, energy cannot pass through the network, thus playing a complete isolation role. Usually, the transmission function of a bandpass filter tends to zero at an infinite frequency point, which is called an infinite transmission zero point, but because it is infinitely far away, it has no practical significance. In the actual design of the bandpass filter, in order to achieve greater suppression outside the passband, it is necessary to introduce zero points at some specific frequencies, which are usually referred to as finite zero points.

There are many methods to introduce zero points in LTCC. Since LTCC often adopts a multi-layer structure, the devices are arranged closely and the electromagnetic coupling between them will be very large, which usually deteriorates the circuit characteristics. In this paper, the coupling between spiral inductors is used to improve the circuit characteristics. The filter structure is shown in Figure 1. In order to match the impedance of the external circuit, capacitors C1 and C2 are introduced, and C3 and L1 as well as C4 and L2 each form a resonant circuit. Among them, the cross-coupling coefficient of L1 and L2 is M, and C5 is a grounding capacitor. The structure can be regarded as two parts. The upper part is a typical second-order bandpass filter, as shown in Figure 2. The lower part is a ground coupling capacitor, as shown in Figure 3. The bandpass structure produces the required passband characteristics, and the transmission zero points are located at the DC point and the infinite frequency. The introduced ground coupling capacitor can obtain the required two transmission zero points, and has little effect on the passband characteristics of the bandpass filter connected in series with it.

Using the microwave network analysis method, the two-port network can be viewed as a series connection of the two networks in Figure 2 and Figure 3, and the Z matrix of the entire network is equal to the sum of the Z matrices of the upper and lower networks.

The transmission coefficient S21 of the network can be converted from the network's Z matrix

Where Z0 is the characteristic impedance of the port, which is 50 Ω. Let S21 = 0, and we can get from equation (2):