Planar Electromagnetic (EM) Simulation in ADS - Chapter 4

Chapter 4

Planar Electromagnetic (EM) Simulation in ADS

PathWave Advanced Design System (ADS)

Background

Electromagnetic (EM) simulation is a key consideration for many circuits. EM simulation has always been an important part of the RF design process. However, as next-generation technologies (i.e. PCIe Gen 5) require high throughput, EM simulation is becoming more important in areas where it has traditionally been neglected. Additionally, EM and electrical simulation have typically required different design environments, which has not only increased complexity of the design process but has also introduced significant potential for error.

Keysight ADS provides two key electromagnetic simulators integrated within its environment, making it convenient for the designers to perform EM simulations on their designs. Unlike circuit simulators, EM simulators are used on layout. Additionally, ADS is capable of EM and circuit co-simulation, reducing the need to use multiple design environments.

Case Study 1: Microstrip Bandpass Filter

Step 1: Creating the Schematic Design for a BPF

Create a new workspace and select units as “mil.” Create a new schematic cell and place components for a coupled line bandpass filter topology as shown in Figures 1 and 2. This section will walk through component selection and design considerations.

Comparison of Schematic and EM Simulation Results

The last step is to compare results. In order to see both the results on the same graph, double-click the graph from the Momentum simulation (from step 3). Click on the drop-down list to locate the dataset for the schematic-based simulation. It should have the same cell name but will not contain ‘_MomUW’ (for example, ‘Lab4_Mstrip_Filter’). Plot both S(1,1) and S(2,1) in dB. Observe the response for both circuit simulation as well as Momentum simulation.

In Figure 23, the results from the momentum simulation are shown in red and blue; the results from the schematic simulation is shown in green and black. Notice that the results are generally in agreement. The bandwidth of the filters is nearly identical. There is slight difference in S(1,1). This has several possible causes, such as coupling between the microstrips and edge effects, which are be taken into consideration in the Momentum simulation but not the schematic simulation.

This chapter illustrates the flow that can be used to perform EM simulations using ADS. Towards the end of the chapter, EM/circuit co-simulation is explored.

Case Study 2: Design and Simulation of a Patch Antenna

Theory

A microstrip antenna in its simplest configuration consists of a radiating patch on one side of a dielectric substrate, which has a ground plane on the other side. The patch conductors are usually made of copper or gold and can be any shape. However, conventional shapes are normally used to simplify analysis and performance prediction. The radiating elements and feed lines are usually photo etched on the dielectric substrate. The basic configuration of a microstrip patch antenna is shown in Figure 24.

The radiating patch may be square, rectangular, circular elliptical, or any other configuration. Square, rectangular and circular shapes are the most common because of the ease of analysis and fabrication. Some of the advantages of microstrip antennas compared to conventional microwave antennas are:

• Low weight, low volume
• Low fabrication cost
• Easy mass production
• Linear and circular polarization are possible with simple feeds
• Easily integrated with MIC
• Feed lines and matching networks can be fabricated simultaneously with antenna structures

Patch antennas find various applications from military to commercial because of their many benefits. Patch arrays are extensively used in phased array radar applications and in applications requiring high directivity and narrow beam width.

Objective

Design a patch antenna at 2.4 GHz and simulate the performance using ADS.

Case Study 3: EM/Circuit Co-Simulation

What is EM/Circuit Co-Simulation?

Often, there is a requirement of having discrete components such as resistors, inductors, capacitors, transistors, etc. in the layout. However, EM solvers cannot simulate these discrete components directly. In this case, we use co-simulation, where we create a layout component and place it into the schematic, where the discrete components are connected.

Typical Process for EM/Circuit Co-Simulation

1. Connect pins in the layout where we need to make connections for discrete components.

2. Define the substrate and other EM settings (such as mesh, simulation frequency range, etc.).

3. Create an EM model and symbol for this layout component.

4. Place this layout component in the schematic and connect the required discrete components.

5. Set up the appropriate simulation in the schematic.

6. If it hasn’t been done, Momentum/FEM simulation will be performed. Otherwise, the most recent data will be used.