Maui Akamai Internship Analyzing the Effects of a Class AB Power Amplifier Using Different RF Modulation Schemes Maui Akamai Internship Symposium July, 22, 2008 Kenneth Estores Trex Enterprises Mentor: Hien Do Hello everyone, my name is Kenneth Estores and I did my internship at Trex Enterprises in Kahului. My mentor was Hien Do, and my project is called analyzing the effects of a class AB power amplifier using different RF modulation schemes.

Outline Different Modern Communication Systems Impacts to Society Basic Definitions for RF Communication Systems Modulation Amplification My Project Design and Simulate Four Modulators: BPSK, QPSK, OOK, ASK-4 Advantages and Disadvantages of Operating Class AB Amplifier in Saturation Effects of Using Power Amplifier with Modulators Comparison of Amplitude Shift Keying vs. Phase Shift Keying Conclusion Future Goals Acknowledgments Here’s an outline of what I’d like to cover. Since my project deals with communication systems, I’d like to first talk about some different modern communications systems in the world today and how it impacts the society we live in. Then I’ll go over a couple of basic definitions to allow you to have a better understanding of my project and what I did. After that, I’d like to talk about my project and what I have accomplished, and finally go over my conclusion, path forward and acknowledgements.

Different Modern Communication Systems Internet Cellular phones Wi-Fi TV broadcast Radios And much more… Here’s a list of things almost all of us use everyday and probably wouldn’t be able to function without. We depend on the internet, cellular phones, wi-fi, TV broadcast, radios, and much more. Try to imagine how the world would function today if we didn’t have these communication systems, it probably wouldn’t! wikipedia.org

Basic Definitions for RF Communication Systems: Modulation and Amplification Modulation: The process in which a low frequency signal is converted into a high frequency signal in order to propagate through a medium Amplification: The process in which a low power signal is increased to a high power signal So to better understand my project, I’d like to go over a couple of definition for RF communication systems. First modulation (read the slide). Here’s a picture to illustrate the definition. We have a low frequency signal which is converted into a high frequency signal. Second amplification (read the slide). And again here’s another picture to illustrate the definition. We have a low amplitude or low power signal which is converted into a higher power signal

Four Common Digital Modulators Amplitude Shift Keying (ASK) Class On Off Keying (OOK) Uses two different waveforms with different amplitudes to transmit 0 or 1 Amplitude Shift Keying-4 (ASK-4) Uses four different waveforms with different amplitudes to transmit 00, 01, 10 or 11 Phase Shift Keying (PSK) Class Binary Phase Shift Keying (BPSK) Uses two different waveforms with different phases to transmit 0 or 1 Quadrature Phase Shift Keying (QPSK) Uses four different waveforms with different phases to transmit 00, 01, 10 or 11 I have decided to separate the four modulators I designed using visual systems simulator into two separate classes. The amplitude shift keying class modulators consist of on off keying and amplitude shift keying-4. (Read slide). The second class of modulators called phase shift keying consist of binary phase shift keying and quadrature phase shift keying. (Read slide)

Schematic of Modulators Designed Using Visual Systems Simulator BPSK Modulator OOK Modulator Here are the schematic of the four modulators I designed and simulated. (Read slide, BPSK, OOK, QPSK, ASK-4). QPSK Modulator ASK-4 Modulator

Schematic of Modulators Designed Using Visual Systems Simulator BPSK Modulator OOK Modulator An important thing to note in my design are the digital to analog converters. As you can see the BPSK and OOK modulators both have a single DAC, this allows them to convert the binary 0 or 1 into two separate waveforms. The QPSK and ASK-4 modulators instead uses two DACs. This allows them to convert the binary 00, 01, 10 or 11 into four separate waveforms. In general the advantage of using QPSK and ASK-4 over BPSK and OOK is that increases the data rate by two. However, the disadvantage is that it increases the error rate. So now that I finally built my modulators, the next step of my project was to measure the effects of using Trex Enterprises’ proprietary power amps using the four modulators. QPSK Modulator ASK-4 Modulator

Part 1: Design and simulate modulators Part 2: Test power amplifier, operating in saturation, using the different modulators

Part 1: Design and simulate modulators Part 2: Test power amplifier, operating in saturation, using the different modulators

Characteristic Curve and Power Added Efficiency Plots of Amplifier Maximum efficiency region (~ 37dBm) Linear to non-linear region (~ 30dBm) This graph shows the characteristic plot, which is in blue, and efficiency plot, which is in pink, of the power amp. As you can see at roughly 30dBm or 1 watt, the amplifier changes from operating in the linear region to the non-linear or saturated region of the amplifier. This is due to the fact that the components in the amplifier are beginning to saturate and cannot output anymore power at a constant gain. Also the amplifier is operating at maximum efficiency at roughly 37dBm or 7 Watts, note that this is the non-linear region of the amplifier. Trex Enterprises Corporation

Advantages and Disadvantages of Operating Amplifier in Saturation Region where maximum power efficiency occurs Larger range of transmission Disadvantages Creates excess harmonic tones to modulated signal Causes amplitude distortion in signals (Read slides)

Effects of Using Power Amplifier with Amplitude Shift Keying Modulators OOK Modulator Both signals are synchronized with each other ASK-4 Modulator Amplitudes are distorted Decision circuit at receiver won’t be able to decipher transmitted data Watts Here are the plots of the pre-amplified and amplified ASK modulators signals. As you can see for the OOK modulator, both signals are synchronized with each other. They clearly indicate when either a binary 1 or 0 is transmitted. However, the ASK-4 signals clearly indicate a problem. And that is the amplitudes are distorted due to the amplifier. The problem is that the decision circuit at the receiver won’t be able to decipher transmitted data. It won’t know whether a binary 0 or 1 was transmitted. Amplitudes distorted Watts

Effects of Using Power Amplifier with Phase Shift Keying Modulators For both modulators, the pre-amplified and amplified signals are synchronized with each other Minimum effects of amplifier distortion Phase transitions are recognizable Phase transitions Watts Here are the plots of both the BPSK and QPSK modulators. (Read slide) Phase transition Watts

Spectrum Plots of Pre-amplified and Amplified Signals dBm dBm Here are the spectrum plots for all four modulators. Again the blue signals are the pre-amplified signals and the pink are the amplified signals. dBm dBm

Spectrum Plots of Pre-amplified and Amplified Signals Added harmonic tones dBm dBm As mentioned earlier, the non-linearity of the power amplifier introduces added harmonic tones to the modulated signal which creates distortion. As you can see all four amplified modulated signals have distortion. dBm dBm

Amplitude Shift Keying vs Amplitude Shift Keying vs. Phase Shift Keying Using Class AB Power Amplifier PSK can operate in saturated region of amplifier PSK can operate at maximum power efficiency level PSK modulated signals can transmit farther than ASK modulated signals PSK is less prone to amplifier distortion ASK has a better signal quality vs. PSK when operating in linear region of amplifier ASK is more robust against noisy mediums Here’s a summary of the comparison between the ASK modulators versus the PSK modulators. (Read slides). However, not shown in the previous plots but based on previous research, ASK has a better signal quality….(read slides).

Conclusion There is no “optimum” modulation scheme! Modulation schemes are decided based on specific application and needs. All modulators have positive and negative drawbacks associated with them. In the case of this amplifier, I observed that the phase shift keying modulators performed better. (read slide)

Future Goals Design and build demodulators Obtain quantitative results Bit error rate Design more “complex” modulators Quadrature Amplitude Modulators Differential Phase Shift Keying Modulators Incorporate encoders and decoders to communication system (read slide)

Acknowledgements Hien Do, Ned Davis, Daron Nishimoto Lynne Raschke, Scott Seagroves, Lisa Hunter, Lani LeBron (read slide) The Akamai Internship Program is funded by the Center for Adaptive Optics through its National Science Foundation Science and Technology Center grant (#AST-987683) and by grants to the Akamai Workforce Initiative from the National Science Foundation and Air Force Office of Scientific Research (both administered by NSF, #AST-0710699) and from the University of Hawaii.