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 transformed 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):

in,

Thus we get

Use the ABCD matrix cascade multiplication and then convert it into a Z matrix to get the Z matrix of network U

Substituting equation (6) into equation (5) we get

The two positive roots of this equation are the frequency values ​​of the two transmission zero points. It can be seen from the equation that different zero point frequencies can be obtained by changing the coupling capacitor C.

2 Circuit Design Simulation

Design filters using the insertion loss method.

If it is used to design a Chebyshev second-order bandpass filter with an in-band ripple of 0.2 dB, the values ​​of each component in the circuit can be determined according to the filter design principle. L1=L2=1.46 nH, C1=C2=0.82 pF, C3=C4=2.55 pF, M=10.02 nH, grounding capacitor C=18 pF. ADS circuit simulation software is used to simulate, and the scattering parameter S of the circuit is obtained using the software, as shown in Figure 5. From equation (7), it can be seen that the circuit has two transmission zeros. It can be seen in the figure that they are located at both ends of the passband, which plays a role in out-of-band suppression. The other curve shows that there is no grounding capacitor, and it is obvious that the out-of-band suppression effect is poor.

As shown in FIG5 , the grounding capacitor can not only introduce a transmission zero point, but also control the position of the transmission zero point.

3 Structural layout in LTCC

The LTCC layout is designed in the electromagnetic simulation software. In order to effectively utilize the coupling between inductors, the structure shown in Figure 1 is constructed. The dielectric constant of the dielectric material used is εr = 7.8, the 3rd layer is 0.08 mm, the 4th and 5th layers are 0.18 mm, the bottom layer is 0.25 mm, the 1st and 6th layers are ground planes, the 1st and 2nd layers constitute C5, the 2nd and 3rd layers constitute C3 and C4, the 4th and 3rd layers constitute C1 and C2, and the 5th layer is a spiral inductor constituting L1 and L2, which are coupled to form M. Its structure is given in Figure 6. The closer the distance between the two symmetrical inductors, the greater the mutual inductance value M. The overall size is 3 mm × 3 mm × 0.7 mm.

The simulation tools used are Ansoft HFSS and IE3D. Based on the component values ​​obtained above, the lumped component size that meets the requirements is designed in their respective environments. Figure 7 shows the results obtained by the two simulation software, which is consistent with the ADS circuit simulation results.

4 Conclusion

Combining LTCC technology, a second-order bandpass filter with a center frequency of 2.45 GHz was designed. It is not only compact in size, but also meets the modern requirements for miniaturization of wireless products. It also has two transmission zeros outside the band, which can achieve good out-of-band suppression, and can use the grounding capacitor size to control the zero position, effectively meeting the requirements of achieving better RF functions in a smaller size. Based on the research of lumped components such as inductors and capacitors, the design simulation effect was obtained by combining circuit simulation and electromagnetic simulation analysis.