1. 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.

2. 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.

3. 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

4. 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

5. 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)

6. 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

7. 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

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

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

10. 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

11. 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)

12. 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

13. 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

14. 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

15. 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

16. 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).

17. 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)

18. 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)

19. 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 ) 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 ) and from the University of Hawaii.