South Carolina State University
College of Industrial and Engineering Technology
University Center
Greenville, SC
A Limited Spectrum Receiver for Data Logging and Analysis of Radio Frequency Energy
Document Version 1.3
Submitted by:
Ronald Douglas Starwalt
Anderson, SC
Senior Project Proposal/ EET 459
Fall 2006
Dr. Mohammed Sarhan - Instructor/Adviser
Introduction
Wireless technology with radio frequency communication as the basis seems to be growing without bound. In the October 2, 2006 issue of Microwave Product Digest, Editor Karen Hoppe provides some amazing statistical evidence of this growth.
CTIA announced last month that data service revenues reached $6.5 billion for the first half of 2006, a 70 percent increase over the same period in 2005. …More than 12.5 billion text messages were delivered in June 2006 alone. That's a 72 percent increase over June 2005. What does this indicate? A greater need for components, subsystems and systems to continue this incredible growth.
Microwave Product Digest October 6, 2006 page 3
CTIA "The Wireless Association", is an international organization for the wireless industry. In their September 2006 Wireless Quick Facts page, CTIA provides the statistics: www.ctia.org/research_statistics/statistics/index.cfm/AID/10202.
With the continued growth of the wireless industry, the need for tools and test equipment to analyze the invisible connections made by wireless devices will not decrease. An essential tool for radio frequency analysis and troubleshooting is the spectrum analyzer. This device provides the engineer or technician with a visible representation of the radio spectrum being studied.
The basis of the proposed spectrum analyzer is a construction project originally published in QST magazine in 1998. This project will be different from the original in two aspects. The first difference is the spectrum range covered by the analyzer will be reduced. The purpose of the reduction in range is to exploit the availability of surplus digital interface components. The second difference will be a changed control and display interface. The original project required an oscilloscope to be used as a display. I propose to modify the original analyzer and adapt it to a limited spectrum of 144 to 148 MHz and provide a digital interface for the input and output connections.
Table of Contents
Problem Statement 4
List of Abbreviated Terms 5
System Description 6
Figure 1 - Simplified Block Diagram of Limited Spectrum Receiver 7
Literature Review 8
Methodology 9
Preliminary Circuit 9
Breadboard Testing 9
Circuit Board Construction Method 9
Final Assembly Method and Testing 9
Design 10
Performance Parameters and Specifications 10
Minimum Detectable Signal 10
Communication Receiver and Spectrum Analyzer Differences 10
Initial Front-end Electronics 11
Figure 2 Conversion Scheme of Hayward and White Project 12
Figure 3 Proposed Conversion Scheme of the LSR 12
Image Analysis 13
Frequency Selection with 12-bit Digital Approach 14
Table 1 Expected frequencies with 12-bit digital selection 14
Results and Error Analysis 17
Discussion 17
References 17
Appendix A Discussion of Superheterodyne Theory 18
Appendix B Industrial, Scientific, Medical (ISM) and Amateur Frequencies 19
Appendix C Kanga US version of Hayward & White SA 20
Appendix D Document Notes and Comments 24
Problem Statement
Mechanisms with circuitry that utilize radio frequency (RF) energy, present in all man-made and natural environments, may operate improperly when subjected to unwanted RF energy. This energy may, or may not, be in the same spectrum that is utilized in the operation of the product. This energy is referred to as interference and must be considered in the design of the mechanism or product. This project proposal is for a limited spectrum receiver (LSR) that can both repeatedly sweep a segment of the RF spectrum and detect the presence of RF energy above a minimum threshold or operate on a single spectra for the purpose of monitoring. For this project, the RF spectrum from 144.00 to 148.00 MHz will be studied.
Initially the project will be constructed to operate in a non-sweep mode. Frequency selection will be accomplished with 12-bit digital inputs. The output will be analog and 12-bit digital. The analog output can be applied to any meter or an external analog to digital converter (ADC) for observation and recording. The 12-bit digital output can be directly interfaced to microprocessor circuitry. The addition of a microprocessor control would allow the selection sweep function to be automated, and the results stored in a digital format.
The targeted audience for the LSR construction is an academic or serious hobbyist. All components will use through-hole circuit technology where available. Many of the components will be surplus items that are readily available. The core of this project will be a spectrum analyzer designed by Wes Hayward, W7ZOI, and Terry White, K7TAU. This project was published in QST magazine in August 1998.
List of Abbreviated Terms
AC - Alternating Current
ADC - Analog to Digital Converter
ARRL - American Radio Relay League
BPF - Band Pass Filter
CR - Communications Receiver
dB - Decibel
dBm - Decibel relative to a Milliwatt
DAC - Digital to Analog Converter
DC - Direct Current
FCC - Federal Communications Commission
IF - Intermediate Frequency
ISM - Industrial, Scientific, Medical
LPF - Low Pass Filter
LSR - Limited Spectrum Receiver
LO - Local Oscillator
PCB - Printed Circuit Board
RF - Radio Frequency
SA - Spectrum Analyzer
mV - Millivolt - 1 x 10-3 Volt
mW - Milliwatt - 1 x 10-3 Watt
MHz - Megahertz - 1 x 106 Hertz
uW - Microwatt - 1 x 10-6 Watt
UHF - Ultra High Frequency
URL - Uniform Resource Locator
VCO - Voltage Controlled Oscillator
VHF - Very High Frequency
System Description
· Heterodyne circuitry is employed in the design
· A voltage-controlled oscillator (VCO) is used for mixer input
· The VCO is controlled by a digital-to-analog converter (DAC)
· The analog output is scaled for a panel meter or an analog-to-digital converter (ADC)
The limited spectrum receiver (LSR) has a superheterodyne receiver as the base component for receiving the desired input spectra. Superheterodyne receiver theory has been documented well in published literature and will be discussed only briefly in the appendix of this paper. The VCO used in the LSR has a wide range of adjustment and is spectrally pure for our purpose. A DAC will supply the DC voltage for the VCO. This approach will allow either discrete digital input selection or a microprocessor selection of the desired voltage. The output of the LSR mixer, after filtration, will be amplified and scaled for measurement by an external digital panel meter, or the same previously mentioned microprocessor application for the DAC could be used with an ADC. This approach provides the opportunity for future software development.
A secondary goal for the LSR project, with necessary modification, could be possible application for solar radio astronomy. Radio spectra of interest to radio astronomy is protected by international agreements. This spectrum is usually different from communications and the Industrial-Scientific-Medicine (ISM) allocation, but with modifications the project could be used for such purposes.
Simplified Block Diagram of Proposed Limited Spectrum Receiver
Figure 1
Literature Review
ARRL Publications (American Radio Relay League)
The Handbook for Radio Amateurs 2001, 78th Edition, The American Radio Relay League, Inc., 2000
Chapter 26 of the 2001 Handbook for Radio Amateur (2001 Handbook) is titled "Test Procedures and Projects." A section of this chapter is dedicated to spectrum analyzers. It explains the theory and application of the device.
QST Official Journal of ARRL - published monthly
QEX A Forum for Communication Experimenters -published bimonthly
The ARRL is devoted to the promotion, education, and development of radio and electronics engineering. It is the amateur radio operator's lobby for Congress and a voice for his interests. Their publications were used extensively for research on this project.
Internet sources
A few web sites exist (as of this writing) showing spectrum analyzer projects and support.
Yahoo Builders Group
http://groups.yahoo.com/group/spectrumanalyzer/
Source for the QST Aug/Sep 1998 Analyzer parts
http://www.bright.net/~kanga/kanga/w7zoi/products/spectrum_analyzer.htm
Manufacturer sources
Mini-Circuits http://www.minicircuits.com
Burr-Brown (now purchased by Texas Instruments)
Analog Devices http://www.analog.com
Methodology
1. Preliminary Circuit - Research of current methods and simulation.
Literature and internet sources will be used to determine a practical method of pursuing the project. After sufficient research, an attempt will be made to use PSpice or other software to simulate the LSR circuitry in both the stand-alone analog and active digital modes. A complete suite of development tools, those that allow schematic capture and artwork development at the same time, are not at my disposal. Several software packages will be employed in this project.
2. Breadboard testing - Monitor & Adjust
After simulation has proven original portions of the LSR circuitry, a breadboard construction will be attempted. RF circuitry is particularly sensitive to this type of construction and often does not work. At the very least, the digital control sections can be tested for correct functionality. The RF portions may require direct construction to allow testing. In an effort to help ensure the success of the project, I will purchase previously proven sections. All spectrum analyzers have common components. The focus of this project is a limited analyzer adapted for my target spectrum.
3. Circuit Board Construction method - Through Hole
Primary in the LSR is the ability to use through-hole technology. Although this technology has been virtually nearly abandoned by the electronics industry at the time of this proposal (Fall 2006), it is the only viable alternative for student or hobby based construction. The components were chosen with the student or hobbyist in mind.
The parts will be purchased new as needed or recycled from circuit boards. For this project, I plan to construct a circuit board(s), where needed, using either PCB-mill technology or photo-etch technology. Either method will require the same work in schematic capture. The PCB-mill method provides a quick and chemical-less solution. Although the investment in money and time is considerably higher for the mill, the results are more predictable.
4. Final Assembly and testing
After all subsections have been tested, the final assembly of the LSR will be performed. An analysis of operation will be performed to compare with the predicted operation.
Hopefully the LSR will be in a finalized package for display and consideration.
Design
In August 1998 QST, published a project article, by Wes Hayward and Terry White, describing a spectrum analyzer for radio amateurs. This unit was intended to operate with an oscilloscope and to provide coverage of 50kHz to 70MHz with the ability to analyze into the UHF region if modified. A readily available kit of parts and circuit boards is available as of this writing. Like most analyzers, this article described a system that sweeps over the desired energy band. It had no digital control or any interface for a computer.
Performance Parameters and Specifications
Although the LSR is a digitally controlled receiver in the literal sense, it is more of a spectrum analyzer with limited capabilities. The capabilities of any device are defined by its design parameters and confirmed by testing. Spectrum analyzers and communication receivers have common design parameters. Some of these parameters follow below.
Minimum Detectable Signal
According to the August 1998 QST article, the analyzer was "…designed for a basic reference level of -30 dBm…." Hayward and White state that this is "…a common value for commercial analyzers." The unit dBm is used extensively in radio engineering. It is defined as the power level in dB with respect to 1 mW. This is because the power level in most radio receiver front-end circuitry is very small compared to the usual unit of electrical power, the Watt. A very handy online calculator was found at the following URL: http://www.temcom.com/pages/dBCalc_en.html This online tool converts dBm to milliwatts and voltage values. Using this tool, the basic reference level for the Hayward and White unit is 7.0711 mV or 1 uW.
Communication Receiver and Spectrum Analyzer Differences
The function of a communications receiver should be well known to the reader, but the differences between a communications receiver (CR) and a spectrum analyzer (SA) may not be well known. The CR is made to detect intelligence encoded in a radio signal and provide an audio output for the listener. Information or entertainment is the primary purpose of the device. A SA is a measurement instrument designed to display radio energy in a frequency domain along a horizontal axis and the power level of the radio energy along a vertical axis. Although the two devices may employ superheterodyne conversion, they use the results of the conversion for different purposes.
Initial Front-end Electronics
The 2001 Handbook addresses the superheterodyne conversion differences between the communication receiver and the spectrum analyzer. From page 26.46:
Most spectrum analyzers use an up-converting technique so that a fixed tuned input filter can remove the image. Only the first local oscillator need be tuned to tune the receiver. In the up-conversion design, a wide-band input is converted to an IF higher than the highest input frequency. As with most up-converting communication receivers, it is not easy to achieve the desired ultimate selectivity at the first IF, because of the high frequency. For this reason, multiple conversions are used to generate an IF low enough so that the desired selectivity is practical.
The ARRL Handbook for Radio Amateurs 2001 page 26.46
The LSR will use the front-end electronics of the Hayward and White SA, but will be modified for the range of my interest. Some sections will have to be retuned to the band I intend to analyze. Appropriate filters will also have to be constructed to employ the new conversion scheme.
Figure 2 - Conversion scheme of the Hayward and White project
Figure 3 - Proposed conversion scheme of the LSR
The output of the 2nd mixer section of the Hayward and White SA is 10 MHz. The LSR will use the same frequency. This will allow the back end of the LSR to use the same modules as the Hayward and White SA. Another deviation from their project will be the removal of the drive circuitry for an external oscilloscope. I hope to make allowances for the external drive to be reintroduced after this project. This feature would make the unit very versatile in that a visual method of analysis can be complemented with a digital control.