Designing RF-IC filters using an automated circuit and layout synthesis
RF-IC designers can automate tedious filter-design iterations by integrating design steps with an electromagnetic simulator.
By Changhua Wan and Jian-X Zheng, Zeland Software Inc -- EDN, January 5, 2006
The filters in an RF IC are important elements, affecting the IC's performance, size, and cost. Traditionally, design of RF-IC filters requires three steps: lumped-element circuit synthesis from specifications, layout synthesis of individual circuit elements, and manual assembly and adjustment of individual elements. These steps make the design time-consuming. However, with a fast and accurate EM (electromagnetic) simulator, circuit designers can automate the design of RF-IC filters by combining the three steps in a single integrated design environment.
Challenges do exist. Layout synthesis of spiral inductors and MIM (metal-insulator-metal) capacitors depends on the speed and accuracy of the EM simulator used. Automatic assembly and adjustment of spiral inductors and overlay capacitors require well-defined rules for interconnections, element separation, and input/output arrangement.
Automating circuit and layout synthesis
The first step is to electrically synthesize a lumped-element filter according to its specifications. It involves synthesizing a lowpass-filter prototype and transforming the prototype to a desired filter type if the goal is a highpass, bandpass, or bandstop response (Figure 1).
The second step is to physically synthesize inductors and capacitors. In RF ICs, a spiral is the best choice for implementing an inductor. For simplicity, this article considers only square spirals. The filter's capacitor can take two forms: a MIM capacitor or an interdigital capacitor. In capacitance value, the MIM capacitor has a larger range than the interdigital capacitor. Therefore, you use MIM capacitors to simplify the automatic synthesis. Figure 2 illustrates the translation of electrical elements to their physical layout.
You must lay out inductors in such a way that their connections with other inductors and capacitors require minimum chip area. A natural approach to inductor layout is to make input and output traces in parallel. Figure 3 shows the eight cases corresponding to spirals of two to 2.875 turns with 0.125-turn increments.
The last step is to assemble individual elements in layout format to generate a complete filter. To achieve this goal, the designer needs to specify the distance, d, between two neighboring components and the length of a feedline, l, from a component to signal or ground lines or to a junction. Figure 4a shows a lowpass LC section, and Figure 4b depicts a highpass one.
For bandpass or bandstop filters, a single element from a lowpass-filter prototype translates to a pair of L and C in series or parallel. For a series branch, figures 5a and 5b represent, respectively, a bandpass- and bandstop-LC combination. For a shunt branch, in contrast, figures 5a and 5b represent, respectively, a bandstop- and a bandpass-LC combination as opposed to the case of a series branch.
Design example
A highpass filter is hard to realize in a distributed format in RF- and microwave-frequency bands. The automatic circuit and layout synthesis for a highpass filter meeting the specifications listed in Table 1 produces the element values of the target highpass filter in lumped-element form in Figure 6.
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EM-based automatic layout synthesis of the lumped-element highpass filter finds the overlapping length of the MIM capacitor to be 165 microns and the number of turns of the spiral inductors to be 4.875 and 3.875, respectively. With the prescribed distance between adjacent elements and feedline length, the layout synthesis also automatically assembles all the elements and adds necessary extensions, connections, and pads to produce a complete filter layout (Figure 7). To suit a particular application, chip area, or both, the final layout may need some minor manual adjustment, but this modification does not alter the effectiveness of the proposed procedure. An adjustment to Figure 7 makes the ground closer to the signal conductor instead of simple extensions of traces.
IE3D, a commercial EM simulator, simulates the filter (Figure 8). The display shows both a highpass response and a significant shift of the cutoff frequency, demonstrating the validity of the design strategy and the power of the tool. The shift is most likely attributable to the interconnections, because the synthesis assumed zero-length interconnects.
