If you’re looking for help designing a broadband Power Amplifier (PA), the 3D Smith Chart may be just the answer for you. 3D Smith Charts can be easily generated from Keysight’s Advanced Design System (ADS) software using Python scripts. You don’t even need to know Python. A Data Link with Python in ADS provides a simple way for you to call preprogrammed Python scripts, complete with bi-directional data transfer.
The Cylindrical 3D Smith Chart (also called the "Smith Tube") was pioneered by a team at Baylor University led by Dr. Baylis and presented in a landmark IEEE WAMICON paper in 2014 that introduced the "Smith Tube" in the literature for the very first time (see more references at the end).
Here are 5 ways a 3D Smith Chart can help you design a broadband PA:
1. It gives you a unique perspective and fast insight into resonance.
Figure 1. This LC impedance matching network topology may at first seem simple to analyze.
Figure 2. It’s not always obvious, why, for example, a particular resonant inflection occurs in the S22 response such as the one shown at 1.52 GHz.
While an LC impedance matching network for a PA design may seem simple to analyze, understanding why a resonance inflection occurs is not always easy (Figure 1 and 2). If you can get to the root of the resonance, you can exploit it to build a broadband match. A 3D Smith Chart allows you to do just that. By plotting the impedance shift of each individual matching component at each frequency, you can find the cause of a resonance and determine what adjustments are needed to mitigate its effects (Figure 3).
Figure 3. From these 3D plots, now we can see the resonance around 1.5 GHz occurs due to the impedance from C3, which is “spinning” around the impedance set by the rest of the network.
2. You can create a solid 3D surface.
Figure 4. An example EVM surface represented by a set of load-pull contours is shown. By viewing the entire surface in this format, some interesting things stand out that aren’t immediately obvious from the contours.
In PA design, load pull is typically used to sweep a transistor's load and plot contours of constant performance (e.g., output power). Load-pull contours are a flat representation of a 3D surface. Sometimes they are easy to interpret and design with, but using them to interpret the surface topology of complex structures can be challenging. Plotting the entire surface as a 3D solid structure can be very insightful in some instances, for example, finding the minimum EVM region of a PA under a modulated input signal (Figure 4).
3. You can extend contours to the third dimension.
Figure 5. A plot of load-pull contours in 3D (with the third dimension being frequency) is shown.
Suppose you’re trying to design an amplifier to deliver high output power over a broad bandwidth. Typically, this would be done by performing load-pull simulations at several frequencies and then trying to build a matching network to hit the correct loads to deliver the power required for each individual frequency. Using a 1D Smith Chart, this would be a long, difficult process, resulting in so much clutter on the plot that you would likely be unable to make sense of the results. Typically, a designer can only visualize one contour or single frequency set of contours at a time. Plotting the same contours on a 3D Smith Chart “spreads out” the contours and allows you to visualize more information at once (Figure 5).
4. You can create a surface from cross-sectional data.
Figure 6. Another way to visualize the frequency dependent power contours in Figure 5 is to create a solid surface by connecting the contours together in the Z dimension.
With a 3D Smith Chart you can create a solid “triangulated” surface by connecting the contours together in the Z-dimension. This provides you yet another way to visualize the contour data in 3 dimensions. In some cases, contour surfaces are easier to understand than repeated individual contours.
5. You can plot your 3D matching network and frequency-dependent load-pull contours on the same 3D Smith Chart.
Figure 7. The 3D matching network and the frequency-dependent load pull contours are plotted on the same 3D Smith Chart.
By plotting this data together, it’s intuitive to adjust the matching network component values so that the impedance "threads the needle" though the Pout power contour level across the frequency band. An interactive highlight marker in ADS helps you gain insight into what adjustments are needed.
These 5 applications of the 3D Smith Chart came from my friend, Matt Ozalas, RF Design Expert. Hear from him yourself in his May 4th, 2017 webcast, Four Ways to Boost Simulation Data Processing Using Python.
You might recognize him from his YouTube Series, How to Design a Power Amplifier: The Basics.
Apply today for your free 30-day full version trial of Keysight ADS.
For more information, see the following IEEE papers:
1. Joseph Barkate ; Matthew Fellows ; Jennifer Barlow ; Charles Baylis ; Robert J. Marks. "The Power Smith Tube: Joint optimization of power amplifier input power and load impedance for power-added efficiency and adjacent-channel power ratio". IEEE Wamicon, 2015. http://ieeexplore.ieee.org/document/7120398/
2. Matthew Fellows ; Matthew Flachsbart ; Jennifer Barlow ; Joseph Barkate ; Charles Baylis ; Lawrence Cohen ; Robert J. Marks. "Optimization of power-amplifier load impedance and waveform bandwidth for real-time reconfigurable radar". IEEE Transactions on Aerospace and Electronic Systems ( Volume: 51, Issue: 3, July 2015 ). http://ieeexplore.ieee.org/abstract/document/6857780
3. Matthew Fellows, Sarvin Rezayat,Jennifer Barlow, Joseph Barkate, Alexander Tsatsoulas,Charles Baylis,Lawrence Cohen. "The bias smith tube: Simultaneous optimization of bias voltage and load impedance in power amplifier design." Radio and Wireless Symposium (RWS), IEEE. 24-27 Jan. 2016. http://ieeexplore.ieee.org/document/7444408/
4. Charles Baylis; Matthew Fellows; Matthew Flachsbart; Jennifer Barlow; Joseph Barkate; Robert J. Marks. "Enabling the Internet of Things: Reconfigurable power amplifier techniques using intelligent algorithms and the smith tube". 2014 IEEE Dallas Circuits and Systems Conference (DCAS). http://ieeexplore.ieee.org/document/6965341/