Nanjing University of Science and Technology

NI AWR Design Environment Enables NUST Students to Design Miniaturized Bandpass Filter


Company Profile

Established in October 1992, the Nanjing University of Science and Technology School (NUST) encompasses electronic engineering, optical engineering, optoelectronic technology, and detection and control engineering. NUST also offers an Electric and Electronic Teaching and Experimental Center, Institute of Wireless Communication and Sensing Network, and Jiangsu Key Laboratory of Spectral Imaging & Intelligent Sensing. With the advent of global informatization and the information age, NUST has achieved unprecedented rapid development and growth.

The Design Challenge

Bandpass filters (BPFs) are essential components used throughout many modern system-in-package (SiP) applications. They are also one of the largest components used in super heterodyne receivers. Low temperature co-fired ceramic (LTCC) technology is used to achieve miniaturization of BPFs at extremely low frequencies (less than 200 MHz) because of its vertical integration capabilities. Nonetheless, BPFs using LTCC technology for megahertz frequencies are quite typical, as the large wavelength makes size reduction a challenge. While bulk acoustic wave (BAW) filters dominate at low frequencies due to their small size, they come with higher insertion loss and group delay and simultaneously need extra capacitors and inductors for impedance matching.

The challenge is how to get BPFs to work at megahertz frequencies given that they require very large resonator capacitance and inductance. Having large capacitance and simultaneously reducing filter size is made possible at these extremely low frequencies by using a vertically-interdigital capacitor (VIC). Currently, one way of enhancing the capacitance of VICs is to increase the size or number of fingers. However, increasing the size of fingers does not help in filter size reduction and results in undesired resonances or spurious spikes, which limits the usable frequency band. 

The Solution

NUST students set out to design a miniaturized lumped-element 10-layer LTCC BPF with two transmission zeros implemented at the extremely low center-frequency of 60 MHz. 

Capacitance increment and spurious spikes suppression of the VIC were both achieved by interconnecting the open ends of interval fingers with vertical vias. The resulting BPF has, reportedly, the world’s smallest size of only 0.004 x 0.004 x 0.0004 λg.

Since size reduction was the main challenge of this work, the first step was to choose a circuit topology with a small number of elements. To start the BPF design at the center frequency of 60 MHz with a bandwidth of 15 MHz, a well-known circuit model with eight elements was selected. The capacitor C3 created a feedback path between the input and output ports for selectivity, with two transmission zeros located at 34 and 88 MHz, respectively. C1 and C2 were configured with the same value and layout for less optimization parameters. The corresponding element values were extracted using Microwave Office circuit design software optimization features.

The initial physical layout of the proposed BPF was easily set up. Fine-tuning was required in order to consider the parasitic and mutual coupling effects between elements. The entire BPF layout was finalized using AXIEM 3D planar electromagnetic (EM) simulator. 

The simulated results agreed well with the measurements.

The center frequency was 60 MHz and the bandwidth based on a return loss of 15 dB was 15 MHz. Two finite zeros were located at the prescribed locations. A higher measured passband insertion loss of 1.95 dB was noticeable, which was attributed to the greater surface roughness of top metal layer with higher resistive loss. 

Why NI AWR Design Environment

Microwave Office provides an easy to use interface and built-in process definition tools that allowed us to efficiently construct the models for vertically multilayer interdigital capacitors and multilayer spiral inductors for our LTCC design. AXIEM enabled us to easily tune, sweep, and optimize the value of the capacitors and inductors.

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