Software tool comes to the aid of Chinese university

The designers developed a broadband interconnection using double bond wires with square shaped defected ground structure under open stubs, which enabled the designers to increase the characteristic impedance of the micro-strip line without narrowing its width.

“Using Microwave Office and AXIEM to simulate and optimise this novel broadband interconnection structure gave us the insight we needed to overcome the challenges and develop a successful design,” said Dr Zhou, Professor at NUPT.

Bond wires are used extensively in monolithic microwave integrated circuits (MMICs) and multi-chip modules (MCMs) for signal transmission in substrates with different heights. Substrate 2, with a height of H2, is stacked on substrate 1 with a height of H1. Bond wire interconnections are used to interconnect substrate 1 and substrate 2.

Substrate-to-substrate interconnection is important for several reasons. In order to achieve a higher isolation among the transmitter, receiver and local oscillation (LO) parts of a transceiver, each part must be designed on a separate circuit substrate to avoid signal leakage through the shared substrate, especially the high power LO leakage to other parts. Additionally, it is not possible in every case to place all components on one circuit substrate so they must be placed on different substrates for a complex circuit.

Conventionally, substrate-to-substrate interconnection is established using bond wires. However, a bond wire adds a low-pass element to the microwave system and limits its operating bandwidth. Approaches using multiple bond wires or compensated micro-strip stubs can broaden the bandwidth of such interconnections, but none of them exceed 20GHz frequencies. It is more important that compensated open stubs are used to broaden the bandwidth of bond wire interconnections, but in most cases the required compensated open stubs must be narrower than 0.1mm, which is impossible to fabricate based on current printed circuit board (PCB) substrate or low temperature co-fired ceramic (LTCC) fabrication processes.

To solve these challenges, engineers at NUPT wanted to design a novel broadband substrate-to-substrate interconnection structure.

The designers developed a broadband interconnection using double bond wires with square shaped defected ground structure (DGS) under open stubs. The square shaped DGS etched under compensated micro-strip open stubs not only expanded its operating bandwidth, but also increased the characteristic impedance of the micro-strip line without narrowing its width, which overcame the PCB fabrication limitation of narrow stubs.

The advantage of using this structure was that it enabled the designers to increase the characteristic impedance of the micro-strip line without narrowing its width. A 250ohm characteristic impedance was easily achieved using a 0.6mm micro-strip line with the DGS patterned in the ground plane. The interconnection also exhibited low-pass characteristics. The measured results agreed well with the simulations.

Electromagnetic simulation was accomplished using NI AWR Design Environment, specifically Microwave Office circuit design software and AXIEM 3D planar EM simulator, which is a full wave electromagnetic simulation software based on the method of moments (MoM). The optimised dimension parameters were - a = 3mm, b = 3.6mm c = 0.6mm, g = 0.5mm and w = 1.2mm. Equivalent circuit elements were C1 = C2 = 0.15pf and L = 0.26nH.

Note that the bandwidth of the novel interconnection is 38GHz, whereas the bandwidth of conventional interconnection is only 2.837GHz. So the structure provides more than 1,200% bandwidth increment compared to a conventional one.

Because NUPT is a member of the NI AWR University Program, students and professors are able to leverage the full NI AWR Design Environment suite of tools. The designers commented that Microwave Office provides an easy to use interface and AXIEM enabled them to easily design and optimise this novel structure.