AM Radio Receivers. Figure 1. Authors: James Talmich Richard Velasco JC van der Schans

Transcription

1 AM Radio Receivers Figure 1. Authors: James Talmich Richard Velasco JC van der Schans March 14, 2008

2 AM Radio Receivers Authors: James Talmich, Richard Velasco, JC van der Schans The AM Radio Receiver project consisted of learning how radios work and to then design and build our own radio. Through the beginning research we learned about the four main receiver designs: crystal, regenerative, reflex, and superheterodyne. Once agreeing on which type of design would work well with what we had we were able to begin the construction process. While components are relatively cheap, there were some problems with availability and ability to buy single pieces. Building our own Printed circuit board proved to be quite the challenge, yet satisfying once completed. Once we had built our radio and placed it in an enclosure it was easy to see that our radio would not be able to compete with radios on the market today. With costs that were much larger than retail radios today our radio has no place in retail stores, but does have applications that are better suited for our process.

3 Table of Contents Illustration List P.4 Introduction P.5 AM/FM P.5 Edwin Armstrong P.6 Crystal Radio Receiver P.7 Regenerative Radio Receiver P.8 Reflex Receiver P.9 PCB Design and Build P.10 Superheterodyne Radio Receiver P.11 Hybrid Digital/Digital P.12 Testing P.12 Economic P.13 Manufacturability P.13 Applications P.13 Conclusion P. 14

4 Illustration List Figure 1. Old Time Radio Programs. Entrepreneur.com. < businessideas/514.html>. Figure 2. - Frequency Modulation. 2 March < Frequency_modulation>. Figure 3. Peterson, George. The 2005 Technology Hall of Fame. 1 September <mixonline.com/mag/audio_tecnology_hall_fame/>. Figure 4. Crystal Radio. 9 March < Crystal_radio_receiver>. Figure 5. Crystal Radio Set. 27 April < crystal-radio-set.htm>. Figure 6. Twin Triode Regenerative Radio Receiver. PV Scientific Instruments. < Figure 7. Kitchin, Charles. A Short Wave Regenerative Receiver Project. 16 December < Figure 8. - Reflex Radio Instruction Manual. Ocean Electronics. Purchased January 12 th, 2008 Figure 9 - Personal Product Photos Figure 10. Superheterodyne Receivers. Introduction to Naval Weapons Engineering Figure Excel Sheet Containing Personally Obtained Data

5 Introduction During the school year James Talmich, Richard Velasco, and JC van der Schans took up a project proposed by Professor Ender Ayanoglu for their senior design project to fulfill the requirements for EECS 189A/B. The project focused on AM radio receivers. The main portions of the project were to learn about the different AM radio receiver designs, fully understand how they work, and design and build our own radio using discrete components. During the research process they learned much about the differences between the AM and FM band, and perhaps the most influential man in the advancement of the radio, Edwin Armstrong. Edwin Armstrong is widely considered the inventor of the FM radio and also some AM radio receivers, however, during his life he was embattled in several patent wars. While there are several different AM radio receiver designs, they focused on four different ones: Crystal, Regenerative, Reflex, and Superheterodyne. Once understanding of those four designs was complete they were able to move on to the next, and perhaps most important, part of our project, the design and construction of their radios. AM/FM AM radio broadcasting makes use of amplitude modulation. During the first eighty or so years of the 20 th century AM broadcasting was the most dominant method of broadcasting [1]. Although it is still widely used today, FM broadcasting has overtaken it as the most dominant method of broadcasting. The beginnings of AM radio go back to 1906, and the demonstrations of Reginald Fessenden of the first radio broadcast. AM works by varying the strength of the signal being transmitted in relation to the information being sent [2]. It then amplifies the variations in the signal to drive a loudspeaker or earphones. Amplitude modulation is inefficient, in terms of power usage, because much of the power it uses is wasted. Most of the power it uses is in the carrier signal which contains no useful information [1]. It is also important to note that some signals act differently during the day and during the night. During the day, AM signals tend to travel by groundwave, diffracting around the curve of the earth. However, during the night AM signals tend to travel by skywave caused by changes in the ionosphere [2]. Skywaves enable the AM radio station to be heard from much farther away from the transmitter than would be possible during the day. As a result many stations are required to reduce their broadcasting power slightly during the night hours, some stations even stop broadcasting during the night. AM signals can be disrupted in large urban centers due to the abundance of large metal structures, tall buildings, sources of radio frequency interference, and countless sources of electrical noise, such as electrical motors, and lights [2]. Because of AM radio s susceptibility to both atmospheric and electrical interference, it is mostly reserved to broadcasting talk radio, such as news, sports, religious, and political stations. Music radio has now mostly shifted to FM broadcasting. FM radio was invented by Edwin Armstrong in 1933, and uses frequency modulation to provide high-fidelity sound over broadcast radio [3]. FM sends information over a carrier wave by using

6 variations in its frequency, this is in contrast to amplitude modulation where the amplitude of the carrier wave is varied and the frequency in held constant as can be seen in Figure 2 [3]. Figure 2. Armstrong was driven to eliminate the biggest problem with radio at the time, static. AM radios carry sound patterns by modulating the amplitude of the carrier wave at a fixed frequency. However, this system was often disturbed by such things as electrical storms [3]. Armstrong created a wideband FM system that in tests showed could handle the worst electrical storms and would still provide the highest quality sound heard by radio at the time. Wideband FM does require a wider bandwidth than AM would at an equivalent modulating signal, but this allows the signal to more robust against any type of noise or interference. FM is also able to withstand phenomena such as signal amplitude fading, which is why today FM is more widely used than AM radio. Edwin Armstrong Figure 3. As stated above Edwin Armstrong is perhaps the most influential man in the advancement of radios. Edwin Armstrong, electrical engineer and inventor, was born on December 18, 1890 in New York City [4]. He studied at Columbia and later became a professor there. In 1912, while studying and Columbia University, he had his first big invention, the regenerative circuit. The regenerative circuit revolutionized wireless radio communication at the time [5]. Filed for a patent in 1913, and received the patent for the regenerative circuit in In 1915 Lee de Forest, another inventor, filed for a patent on the same regenerative circuit, which he sold the

7 right to AT&T [4]. With the radio business booming AT&T filed a lawsuit to overturn Armstrong s patent in favor of de Forest s. Their patent battle went through a dozen courts between the years of 1922 and 1934 [4]. In what most experts agree was a wrong decision, Armstrong, backed by Westinghouse and Radio Corporation of America (RCA), eventually lost through a judicial misunderstanding of the technical facts [4]. While serving in the military in 1917, during World War I, Armstrong invented the superheterodyne circuit and patented it in 1918 [5]. The superheterodyne circuit allowed for greater selectivity and amplification. He was called The Major, his rank in the military, by his friends. In the late 1920s Armstrong experimented with an entirely new system, where the wave frequency would be modulated, but the amplitude would be held constant. In 1933 Armstrong created FM broadcasting [5]. However, with the country in a state of depression during the 1930s the radio industry was unwilling to move toward a new FM system. Changing toward a new FM system would require basic changes to both transmitters and receivers. In 1940 Armstrong was able to get a permit for his own FM station; however he obtained it at his own expense [4]. After World War II, in the mid-1940s, FM broadcasting began to flourish, however that came to a screeching halt when the FCC, ordered FM into a new frequency, very much limiting the capabilities of FM broadcasting. At the same time he was being challenged by a multitude of corporations on the rights to his inventions. Facing another long legal battle, worn down by money problems, and frustrated with the failure of the public to recognize the importance of FM Armstrong committed suicide in January of 1954, by jumping from his 13 th floor apartment window [4]. After his death his widow, Marion MacInnes, continued the lawsuits and ultimately won 10 million in damages [4]. Armstrong would never see his dream of FM clearly being established as the superior system in the 1960s. Crystal Radio Receiver Figure 4.

8 Crystal radio receivers are probably the simplest kind of radio receiver devised for the reception of AM signals. It requires no power other than the power received from radio waves by a long wire antenna [6]. Figure 3. contains a schematic for a basic crystal radio set. Figure 5. The circuit consists of an inductor, a variable capacitor, a germanium diode (crystal), a filtering capacitor, and very high impedance headphones [5]. The inductor has taps on it to connect the antenna and one to connect the germanium diode to it. The variable capacitor is often connected across the whole of the inductor to form a tuned circuit. How the crystal radio receiver works is that the long wire antenna will pick up the thousands of radio signals from the air [5]. The setting of the variable capacitor determines the amount of capacitance and this in conjunction with the fixed inductance of the coil will make a resonant circuit. The capacitance in conjunction with the fixed inductance will resonate at a certain frequency [5]. This is how a particular frequency is chosen from among the thousands of signals collected by the antenna. Once the signal is chosen it is passed through the germanium diode, where half of the signal is used. The uf fixed capacitor then sends the signal to ground leaving a tiny audio frequency, which should be the exact same as the audio frequencies that came from the radio station [5]. This tiny audio signal drives the high impedance earphones so we are able to hear the music or words. Regenerative Radio Receiver Figure 6.

9 Edwin Armstrong invented and patented the regenerative circuit while at Columbia University. This is the invention in which he would go into the long patent war with Lee de Forest [7]. With vacuum tubes being expensive and consuming much power, the regenerative receiver was a blessing during its time. The design got the most gain out of one vacuum tube, thus filling a need of the radio community and instantly thrived [7]. The regenerative radio receiver makes the most out of few parts. Figure 6. is a schematic of one kind of regenerative radio receiver. Figure 7. As we can see in Figure 6. the receiver is separated into three parts: radio frequency (RF) stage, regenerative detector, and the audio amplifier [8]. A bipolar transistor is utilized in the RF stage, while the JFET, in the regenerative detector, and the audio amplifier use low cost integrated circuits. In the RF stage of this circuit, Q1 amplifies the signals from the antenna and prevents the radio s oscillations from causing any interference to other receivers [8]. In the regenerative detector section of the schematic JFET Q2 uses positive feedback to increase the sensitivity and selectivity of the receiver [8]. The positive feedback builds the input signal to very high levels. In the audio amplifier portion of the circuit IC1 amplifies the audio signal and is able to provide ample output to drive the small speakers or headphones [8]. Reflex Receiver

10 Reflex radios were popular many years ago, when transistors were very expensive. These radio receivers required the least amount of components when compared to other radio receivers; they can be made with one or two transistors. This in turn made them cheaper for the consumer and makes the battery last longer, which obviously attracted consumers. The way the reflex receiver saves on components Is that it makes the amplifier do twice the work, by amplifying the Radio Frequency (RF) signal and the Audio Frequency (AF) signal at the same time. This is very ingenious and because of this makes the design a bit more complicated. Fig. 8 For our project we ordered an AM reflex radio kit seen in Figure 8. The radio signal comes in through terminals 5, 6, and 7 and gets amplified by the transistor. The 800uH choke coil doesn t let any of the radio signal pass and it sends it back and it gets demodulated by the diode and capacitor. The demodulated audio signal then gets amplified by the same transistor that amplified the radio signal, and the choke coil lets the audio signal pass to the audio amplifier and then to the earphone. PCB Design and Build There are a couple of methods for etching PCB s, the main difference being the way the ink traces are put on the copper board. There is the photofabrication method, which is literally the way they develop photos. Then there is the brute method of ironing on a laser jet printing of the PCB artwork on the copper board. And of course there is always the option of sending it out to a vendor that prints circuit boards. However, before any of this is done, one has to get the PCB artwork that is going to be transferred on the copper board. This can be done by using free software available on the internet, or by purchasing a professional software like Cadence Allegro.

11 (Left) PCB Artwork on PCBArtist (Middle) Etching Copper (Right) After copper was etched Figure 9. For our project we used a free software called PCB Artist. It is not difficult to get the artwork done, one simply draws the schematic and the program will automatically route all the components together with traces. We decided to use the brute force method of ironing on the artwork on the copper board, which proved to be very difficult. Once that is done you peel the paper off and hope the ink is stays on the copper. If there places where there is no ink, you can fill it in with a sharpie. When it s ready you can drop it in a plastic container with enough Ferric Chloride to submerge the copper board. The acid will start etching away the copper that is not covered by the ink. This works faster if the acid is warm so it s a good idea to keep a lamp on it. Superheterodyne Receiver Heterodyning is the mixing of two frequencies. Superheterodyning is taking the difference between an incoming signal and a generated signal resulting in a signal with a lower beat frequency than the original incoming signal. This is not for the purpose encoding information for transmission. What superheterodyning does is to purposely mix in another frequency in the receiver, so as to reduce the signal frequency prior to processing. [9] For AM reception, the receiver is tuned to the carrier frequency between 570 and 1700 khz. The signal is amplified and then mixed with a signal generated by the receiver. The generated signal is an intermediate frequency (IF), and is always the same, regardless of the frequency of the incoming carrier signal. This is done by a variable oscillator that is tied to the tuner. The IF, typically 450 khz, can then be amplified, demodulated, passed through an audio amplifier, and then the speaker.

12 Figure[10]: Stages of superheterodyne receiver. There are three main advantages to use of a superheterodyne, the latter two pertaining to the AM receiver. First, it reduces the signal from very high signal sources. As noted, this is not an issue for the am receiver, but for frequencies that are high enough to cause damage to components, this is a way to get around that problem. Second, receivers can be built much more inexpensively. As most of the components need only operate at a fixed frequency, it is much easier to optimize and build them. Third, superheterodyning can improve signal isolation by arithmetic isolation. [9] Simply stated, if there is a 2% spread in the signal. That 2% will be a smaller value at the IF than the incoming signal. These qualities of the superheterodyne make it a cheaper and more robust receiver. Hybrid Digital/Digital Hybrid digital or HD Radio, is the trademake for in-band on channel (IBOC) selected for terrestrial audio broadcasting. [10] This technology was developed by ibiquity Digital Corporation. The application is more prevalent with FM stations, as AM stations have been slow to adopt the technology. Hybrid digital works, as the name implies, in both analog and digital modes. First the receiver locks onto an analog signal. and then tries to find a digital signal. The AM signal is usually transmitted at 40 kbits/s., while the FM signal is transmitted at 100 to 150 kbits/s. Some of the main problems with hybrid technology are the expense and the lack of coverage. Receivers for the car are around $100, while tuners for the home cost in the range of $75. Coverage is a problem because up to now, the HD signal was 1% of the power of most

13 stations analog signal. Because of the weak signal, the receiver often reverts back to analog mode. An example of digital available on the market is satellite radio. There are two providers of satellite radio. They are XM Radio and Sirius Radio. Each system has three satellites with backup ground transmitters and transmits at 2.3 GHz. Testing To conduct some testing on which radio design worked better and to what extent it worked better we obtained some radios from Professor Ender Ayanoglu and did some testing on what signals and the clarity of the different radios. There were four radios used in the tests: reflex, regenerative, superheterodyne (with IF), and superheterodyne (without IF). The tests were conducted at two different times of the day, one at night and one during the day. We also conducted the tests in two different counties, Orange County and San Diego County. The results of our tests can be seen in Figure 11. attached to the end of the paper. Based on the results, the all radios seemed to work better during the day. This can be attributed to there being more signals being transmitted during the day because some stations stop broadcasting during the night. We can also see that there are more weak signals during the day due to more interference caused by other signals during the day. Though there are less signals during the night the signals are clearer because signals are stronger during the night travel on skywaves, as stated above, which allow for signals to travel further. Both San Diego County and Orange County produced nearly identical results in terms of number of station being received, though they were not necessarily picking up the same stations in both counties. The reflex and regenerative radio performed similarly in our testing, both picking up somewhere in the range of 5-11 signals. They were both always within two, in number of stations, of each other. It could be argued that reflex did did slightly better, but not by much. The big winner in the testing was the superheterodyne receivers which performed above and beyond the capabilities of the reflex and regenerative receivers. The superheterodyne receivers were picking up anywhere from 2X to 4X the amount of the reflex and regenerative receivers. There were two superheterodynes being tested. One with an IF and one without. The superheterodyne receiver with the IF performed the best picking up a low of 27 signals and maxing out at 37 signals received. The superheterodyne without IF did not perform as well, yet was much better than the reflex and regenerative. The superheterodyne picked up a low of 16 signals and peaking at 22 signals. Based on this testing conducted it is very easily seen that the superheterodyne with IF was the best type of receiver. Economic We built two radios, a reflex and a superheterodyne. We spent in total about $100 to the reflex radio. While it was relatively cheap to find components for this radio, the bulk of the money spent on this radio went towards material to be used to make the printed circuit board. The ferric chloride and the copper board used in the printed circuit board process cost roughly $50. The enclosure cost about $10. When we take into account the costs to build this radio, this radio was very expensive to build. The superheterodyne radio built cost a little over $100. Most of the cost was caused by difficulty to find components and the necessity of having to buy components

14 in bulk. The components included: resistors, capacitors, transistors, and inductors. Components alone cost about $80. There was also the need for somewhere to place the components. This is where the cost of $25 for a breadboard was. This was the extent to which that radio was taken, and would have obviously cost much more if we were to change the circuit into a printed circuit board and enclose it in some kind of surrounding. Manufacturability Under no circumstance should the radios we built be sold in retail stores. AM radios today are priced anywhere from $ $ These radios are of much higher quality than our radios. Their signal is much clearer and stronger. They are also placed in a nicer protective case. They also have other functions that our radios are not capable of, such as: FM radio, alarm, and clock. We were able to build two radios, with some degree of success. The reflex radio was by far the best radio that we built. However, for the amount we spent it does not come close to competing in the market of multi-function radios. There would have to be major changes to our design and the costs if there were ever going to be any possibility of being manufactured. Applications As was shown above, there is little to no possibility of the radios we constructed to be sold in retail stores. However, there are a few possibilities that our radios can be used for. Applications include educational use, and use by hobbyists. There is a market for people who wish to learn more about how a radio works and how to construct their own printed circuit board. As can easily be seen on the internet there are numerous websites with step-by-step processes on how to construct a radio with detailed descriptions on how these radios work. These types of websites are very educational because they allow users the opportunity to learn by the hands-on-approach. Users of the website will need to gather their own components, place, and solder those components onto some sort of board. From there they will move on to creating a printed circuit board, where as shown above has many steps of its own. This first-hand experience will give the user in depth knowledge on steps to be taken in building a working radio. If we were looking to make some money from our design we would look to sell a kit to build our radio to hobbyists. There are dozens if not hundreds of websites and listings online that sell kits with step-by-step instructions on how to build their radios. There are a number of people out there who take great joy in building their own radio and listening to the radio they built. Conclusion Despite the low cost of simple radios in stores, it was quite expensive in our process to build our radios. The construction of a single radio, that is not as efficient as those in stores, was well over five times the cost of radios sold in retail stores. Optimization of a design is very difficult and tedious work, requiring countless hours of testing and correction. It is safe to say that of the four radio receiver designs we studied: crystal, regenerative, reflex, and superheterodyne, that superheterodyne was by far the beset design. However, it is also a much harder design to

15 implement. As we look at radios today it is easy to see that we are in a new era of HD and digital radio that will very well push the radios of yesterday further back into the past.

16 Works Cited [1] AM Broadcasting. 13 March < [2] Amplitude Modulation. 13 March < Amplitude_modulation>. [3] Frequency Modulation. 2 March < Frequency_modulation>. [4] Lessing, Lawrence. Edwin H. Armstrong. Dictionary of American Biography. Supplement Five p < [5] Halper, Donna. Major Edwin Howard Armstrong. Old Radio.com < [6] Crystal Radio. 9 March < [7] Crystal Radio Set. 27 April < crystal-radio-set.htm>. [7] Regenerative Circuit. 12 February < Regenerative_circuit>. [8] Kitchin, Charles. A Short Wave Regenerative Receiver Project. 16 December < [9] Superheterodyne Receivers. Introduction to Naval Weapons Engineering [10] <

17 Addendum Figure 11. Midnight in San Diego ReflexTotal=6 Regenerativ e Total=8 Superheterodyne (with IF) Superheterodyne (without Total=29 IF) Total=16 600clear 600clear 540clear 600clear 760clear 690weak 600weak 620clear 900weak but clear 750clear 620clear 660weak 1090weak 1090clear 640clear 690clear 1340weak 1130weak 660weak 760clear 1360clear 1230clear 690clear 900clear 1360clear 710weak 950clear 1470clear 760clear 1090clear 800clear 1130clear 810weak 1170weak 860clear 1210weak 900clear 1220clear 930weak 1360clear 950clear 1420clear 1040weak 1470clear 1070clear 1700clear 1090clear 1130clear 1170clear 1230clear 1270clear 1310weak 1360clear 1420weak 1470clear 1530clear 1550weak 1580weak 1630weak Midday in San Diego ReflexTotal=9 Regenerativ e Superheterodyne (with Total=11 IF) Superheterodyne (without Total=33 IF) Total=19 570weak 600clear 540clear 600clear 600clear 640clear 570weak 620clear 640clear 690weak 600clear 640clear

18 760clear 750clear 620clear 690clear 800weak 790weak 640clear 760clear 900clear 1090clear 660weak 790clear 1090clear 1130clear 690clear 800clear 1340weak 1170w eak 710weak 900clear 1360clear 1230clear 750weak 950clear 1360clear 760clear 1090clear 1470weak 790clear 1130clear 800clear 1070clear 810weak 1210weak 860clear 1220clear 900clear 1310weak 930weak 1360clear 950clear 1420clear 1040clear 1470clear 1070clear 1700clear 1090clear 1130clear 1170clear 1210clear 1230clear 1270clear 1310weak 1360clear 1420weak 1470clear 1530clear 1550weak 1580weak 1630weak Midnight in Irvine ReflexTotal=7 Regenerativ e Total=5 Superheterodyne (with IF) Total=27 Superheterodyne (without IF) Total=16 570clear 830clear 570clear 550weak 640clear 1090clear 640clear 570clear 690clear 1190clear 690weak 590clear 740clear 1480clear 710weak 620clear 830clear 1530clear 720clear 640weak 1090clear 740clear 660clear 1190clear 760clear 740weak 790clear 770weak 810weak 830clear

19 830clear 950weak 840weak 1020clear 930weak 1090clear 950weak 1190weak 960weak 1370weak 980clear 1480clear 1020weak 1580weak 1030weak 1070clear 1090clear 1190weak 1370clear 1480clear 1510weak 1530clear 1540weak 1580weak 1600clear Midday in Irvine ReflexTotal=10 Regenerativ e Total=11 Superheterodyne (with IF) Total=37 Superheterodyne (without IF) Total=22 570clear 570weak 540clear 570clear 640clear 640clear 570clear 600clear 690clear 690weak 600weak 640clear 740clear 740clear 640clear 690clear 790clear 790weak 690clear 710clear 830clear 830clear 710clear 740clear 1070clear 1070clear 740clear 790clear 1090clear 1090clear 790clear 830clear 1110clear 1110weak 830clear 870weak 1190clear 1190clear 860clear 930weak 1480clear 870clear 950weak 930weak 980clear 950weak 1020weak 980clear 1070clear 1020clear 1090clear 1070clear 1110weak 1090clear 1190clear 1110clear 1390weak 1150clear 1480clear 1190clear 1580weak 1240weak 1600clear 1280clear 1650weak

20 1300clear 1350weak 1370weak 1390weak 1410weak 1430clear 1440weak 1480clear 1510clear 1540weak 1550weak 1580weak 1600clear 1640weak 1650weak