RF technology is at the heart of our information and electronic technology. Traditionally, RF signals are transmitted and distributed electronically, via electrical cables and waveguides. Optical fiber systems have now replaced electrical systems in telecommunications. In telecommunication, RF signals are digitalized, the on/off digitally modulated optical carriers are then transmitted and distributed via optical fibers. However, RF signals often need to be transmitted, distributed and processed, directly, without going through the digital encoding process. RF photonic technology provides such an alternative. It will transmit and distribute RF signals (including microwave and millimeter wave signals) at low cost, over long distance and at low attenuation.

RF photonic links contain, typically, optical carriers modulated, in an analog manner, by RF subcarriers. After transmission and distribution, these modulated optical carriers are detected and demodulated at a receiver in order to recover the RF signals. The transmission characteristics of RF photonic links must compete directly with traditional electrical transmission and distribution systems. Therefore the performance of an RF photonic transmission or distribution system should be evaluated in terms of its efficiency, dynamic range and its signal-to-noise ratio.

RF photonic links are attractive in three types of applications. (1) In commercial communication applications, hybrid fiber coax (HFC) systems, including both the broadcast and switched networks, provide the low cost network for distribution of RF signals to and from users. RF photonic technology has already replaced cables in commercial applications such as CATV.

So far, we have made measurements without saying anything about antennas, or about how power gets from the transmitter to the receiver. In our measurements, a 50-Ω load has taken the place of the antenna. However, instead of dissipating the power as heat, an antenna radiates power as electromagnetic waves. One thing that makes antennas interesting is that they necessarily involve both the voltages and currents that we study in circuits and the electric and magnetic fields that make up radio waves. This gives antennas a special place in the history of physics. They were the crucial components that Hertz developed in the 1880s to demonstrate that Maxwell's equations for electricity and magnetism are correct. In the 1960s, a special parabolic antenna allowed Arno Penzias and Robert Wilson at Bell Telephone Laboratories to discover the cosmic background radiation. That measurement earned them a Nobel Prize, and it gave an entirely new interpretation to the history of the universe.

An antenna is characterized by its impedance and pattern, which is a plot of where the power goes for a transmitting antenna. Traditionally, antennas have been analyzed as transmitters, and most antenna engineers think entirely in terms of transmitting antennas. If we know how an antenna transmits, we can use the reciprocity theorem to figure out how the antenna works in reception. The physical description of transmission and reception are actually quite different, and the physics of receiving antennas is in many ways as interesting as for transmitting antennas.

Cables allow us to transmit electrical signals from one circuit to another. For example, we might attach coaxial cable between a function generator and an oscilloscope (Figure 4.1a) and plastic-coated twin lead between an antenna and a television (Figure 4.1b). Usually, when we analyze the circuit, we assume that the voltage at one end of the cable is the same as the voltage at the other end and that the current at the beginning is the same as the current at the end. This is appropriate if the frequency is low. However, at high frequencies the cable itself begins to have an effect. A fundamental limitation is the speed of light. If the voltage at one end of the cable changes appreciably in less time than it takes light to propagate to the other end, we should expect the voltage to be different at the two ends. Another way of saying this is that we would expect the voltages at the ends to be different when the length of the cable becomes an appreciable fraction of a wavelength.

Distributed Capacitance and Inductance

However, even when the cable is considerably shorter than a wavelength, it can have a large effect. We found in Problem 3 that a cable has capacitance. This capacitance is associated with the charges that the voltages on the line induce. We can take the capacitance into account in a circuit by adding a capacitance between the wires (Figure 4.1c).

So far the filters we have made have had only two elements: a capacitor and a resistor or inductor. We can improve the response of our filters by adding more elements. This allows us to make the pass band flatter and the roll-off steeper. Multielement filters behave somewhat like transmission lines, and we need to have the right input and output resistance to avoid problems with reflections. Analyzing these filters by hand is quite difficult, but the calculations are easy on a computer. For this we will use a computer program called Puff, which is included with this book. Instructions for running the program are given in Appendix C.

Ladder Filters

We will consider ladder networks with alternating series and shunt elements like the discrete transmission line we studied in Problem 11. If the series elements are inductors and the shunt elements are capacitors, then the circuit acts as a low-pass filter (Figure 5.1a, b). At low frequencies, the impedance of the inductors and the admittance of the capacitors are small, and the input signal passes through to the output with little loss. In contrast, at high frequencies the inductors begin to act as voltage dividers and the capacitors as current dividers. This reduces the power transmitted to the load. We can also make high-pass filters with series capacitors and shunt inductors (Figure 5.1c, d).

Many different filters have been developed, giving a wide choice of amplitude, phase, pass-band, and stop-band characteristics.

Puff is a circuit simulator for linear circuits. It calculates scattering parameters and makes microstrip and stripline layouts. It also makes time-domain plots. The program is named after the magic dragon in the song by the popular American singing group Peter, Paul and Mary. Puff originated as a teaching tool for Caltech's microwave circuits course. It was created as an inexpensive and simple-to-use alternative to professional software whose high costs, copy protection schemes, and training requirements create difficulties in the academic environment. Puff uses a simple interactive schematic-capture type environment. After a circuit is laid out on the screen using cursor keys, a frequency or time domain analysis is available with a few keystrokes. This process is faster than using net lists, and errors are rare since the circuit is always visible on the screen. Intended for students and researchers, public distribution of the program began in 1987. Puff use, originally limited to Caltech, UCLA, and Cornell, has since spread to many other universities and colleges. The program has also become popular with working engineers, scientists, and amateur radio operators. Over 20,000 copies of versions 1.0, 1.5, 2.0, and 2.1 have been distributed worldwide, and translations have been made to Russian, Polish, and Japanese.

When a transistor is active, the current gain β is large, in the range of 100 or more. This means that we can use the transistor as an amplifier to increase the power of a signal. The amplifier may be considered the single most important device in communications electronics, and it is key to both receivers and transmitters. Developing amplifiers has been a central focus of electrical engineering from the days of the first vacuum tubes, and it is just as important today. There are many issues to consider in designing an amplifier. In transmitters, we are very interested in efficiency. High efficiency makes it easier to dissipate the heat and allows long battery life in portable transmitters. In receivers, it is important to add as little noise as possible to the signal. In this chapter, we study linear amplifiers, where the amplitude of the output tracks the amplitude of the input. In the next chapter, we consider saturating amplifiers, where only the frequency of the output follows the input.

Common-Emitter Amplifier

The basic transistor amplifier is shown in Figure 9. 1a. It uses an npn transistor with a load resistor R at the collector. The supply voltage is written as Vcc. It is traditional to double the subscript of a supply voltage to distinguish it from an AC voltage. This circuit is called a common-emitter amplifier. You do have to be on your guard with amplifier names.

To do the exercises, you need the NorCal 40A transceiver kit, available from Wilderness Radio, tel. 650.494.3806, http://www.fix.net/jparker/wild.html, P.O. Box 734, Los Altos, CA 94023-0734. The NorCal 40A has its own home page on the World Wide Web at http://www.fix.net/∼jparker/norcal.html. The kit includes all the parts, a metal box with silk-screen lettering, and excellent instructions.

We have included a list of the equipment and parts that are used in each exercise and a vendor list. For reference, we include vendors for the components in the NorCal 40A, even though it is cheaper and more convenient to buy the kit. Vendors and parts change from time to time. There are additional considerations for a class. Extra components need to be purchased to allow for failures, and it is a good idea to talk to Bob Dyer at Wilderness Radio directly to plan. Power supplies and batteries must be short-circuit protected, both for safety and cost. For the batteries, we add fuses and switches and wrap everything in heat-shrink tubing.

On Sunday, April 14, 1912, shortly before midnight, the RMS Titanic struck an iceberg off the coast of Newfoundland. The radio operator, John Phillips, repeatedly transmitted the distress call CQD in Morse Code. He also sent the newly established signal SOS. Fifty-eight miles away, the Carpathia received the messages, and steamed toward the sinking liner. The Carpathia pulled 705 survivors out of their lifeboats. Phillips continued transmitting until power failed. He and the other passengers could have been saved if more lifeboats had been available, or if the California, which was so close that it could be seen from the deck of the Titanic, had had a radio operator on duty. However, this dramatic rescue established the power of wireless communication. Always before, ships out of sight of land and each other were cut off from the rest of the world. Now the veil was lifted. Since the Titanic disaster, wireless communications have expanded beyond the dreams of radio pioneers. Billions of people around the world receive radio and television broadcasts every day. Millions use cellular telephones and pagers and receive television programs from satellites. Thousands of ships and airplanes communicate by radio over great distances and navigate by the radio-navigation systems LORAN and GPS.

The enormous increase of wireless communications is tied to the growth of electronics in general, and computers in particular. Often people distinguish between digital and analog electronics.

This appendix gives data for most of the components that are used in the NorCal 40A. We appreciate the manufacturers giving us permission to copy data sheets. Often data sheets have a great deal of information, but it is important to make your own measurements to check them. Manufacturers vary greatly in how conservatively they rate their devices. In addition, they may be testing the devices under conditions that are different from yours. Often several manufacturers sell a device with same part number, and the performance between different manufacturers varies. These data sheets are no substitute for the data books and Web sites that give the complete list of products that a manufacturer sells. Many of these devices are part of a wide line of products that will cover a range of frequencies, functions, and power levels. For much more component information, see Data Book for Homebrewers and QRPers, by Paul Harden, published by Quicksilver Printing. The book is available from Five Watt Press, 740 Galena Street, Aurora, CO 80010-3922, email:qrpbook@aol.com.

There are two audio circuits in the NorCal 40A, the Automatic Gain Control, or AGC, and the Audio Amplifier. The AGC is an attenuator with JFETs that act as variable resistors. The Audio Amplifier is the LM386N-1, made by National Semiconductor. This integrated circuit appears in many different audio systems, and it costs about a dollar. The “–1” indicates a supply voltage range from 4 to 12 V. A “–4” version is available that allows a supply voltage up to 18 V. For the LM386N-1, the maximum output power is about one watt. In standby, it draws about 4 mA, which is a reasonable level for running off batteries.

Audio Amplifier

Figure 13.1 shows the schematic for the LM386N-1. It is more complicated than the previous circuits that we have looked at, and because it is an integrated circuit, most points are not accessible for measurements. There are three stages of amplification. The input is a differential amplifier, but it is made with pnp transistors rather than npn transistors. This turns things upside down. Each input has a pair of stacked pnp transistors. The stack gives the effect of a single transistor with a current gain of β2. The differential amplifier is followed by a common-emitter stage. The output is a Class-B emitter follower that is similar to the one we discussed in Section 10.6.

Fundamentally, a receiver is limited in sensitivity by noise that competes with the signal we want. A receiver is also limited in handling strong signals by its nonlinearities, which produce intermodulation products that block reception. Noise is a random voltage or current that is present whether a signal is there or not. We distinguish noise from interference, which is an unwanted signal coupled into the circuit, and from fading, which is a variation in the signal level, caused by interference between radio waves arriving by different paths. There are many different sources of noise. Several forms are caused by bias currents. In diodes, the random arrival times of electrons cause shot noise. Another current noise is 1/f noise, where power varies inversely with the frequency. This 1/f noise is found in contacts, and it is associated with energy states at interfaces called traps. It can often be reduced by improving the fabrication process. However, even in the absence of bias currents there is noise associated with resistors. It is called Johnson noise after John Johnson at the Bell Telephone Laboratories, who first measured it.

Noise

On an oscilloscope, noise makes a trace appear as a band that evokes the feeling of grass. We can write the noise as a function of time V(t), but we would not be able to predict its value at a future time.

It is common in public-address systems to hear a loud tone when someone moves the microphone too close to a speaker. People call this feedback, and it is caused by sound from the speaker getting back into the microphone. When this happens, the sound is amplified again and again until the amplifier overloads. It is perhaps surprising that we hear a single tone rather than a broad range of frequencies, and it suggests that we can use feedback to make a sine-wave oscillator. We can distinguish between two kinds of feedback. The public-address oscillations are an example of positive feedback, where the output adds to the signal. In negative feedback, the output cancels part of the input, reducing the gain. We have had two examples of negative feedback so far, in the emitter resistor of the Driver Amplifier and in the source resistor of the Buffer Amplifier. In both cases, the output generates a voltage across a resistor that cancels part of the input. We will see two more examples of negative feedback when we consider the Automatic Gain Control and the Audio Amplifier. Positive feedback increases the gain of an amplifier. This was used in early radios in a circuit called a regenerative receiver, where positive feedback brought the receiver to the brink of oscillation. This gave a large gain and allowed a receiver to operate with only a single stage of amplification.

A modern electrical-engineering textbook is formidable. One thousand pages of matrices and theorems and problems sap enthusiasm from the hardiest students. Even after wading through this massive amount of material, students may be no closer to designing or building electronic circuits. A delightful contrast to these books is Paul Nahin's The Science of Radio. Nahin, who is also a historian of great skill, approaches the mathematics of communications engineering in top-down fashion, by telling a history of early radio and introducing the mathematics only when (“just in time”) he needs it for his story. However, in one sense, Professor Nahin only tells half the story, and we would like to tell the rest of it. The mathematics of communications, although beautiful, is limited – engineering products must be built. Today's electrical-engineering students have usually not built stereos or tinkered with cars, and this means that they do not know the smoke and smell of construction or the excitement of electronic circuits coming to life. Many universities encourage this trend, with exercises where students switch components in and out of a circuit, never even heating up the soldering iron.

This is an introduction to electronics based on the progressive construction of a radio transceiver, the NorCal 40A, through thirty-nine exercises. At Caltech, beginning electrical-engineering students complete one problem as homework for each lecture. These exercises may also be useful for students in radio engineering classes.

Class-A amplifiers produce outputs with little distortion because the transistors are biased and driven so that they are always active. However, when a transistor is active, the voltage and current are large at the same time, so that the dissipated power is substantial and the efficiency is poor, in the range of 35%. In addition, the amplifier dissipates power even when there is no output. These are severe limitations for even modest output power levels; consequently, few power amplifiers run Class A. To eliminate the power drain when there is no signal, we can leave the transistor unbiased, so that it does not dissipate power when it is off. In addition, if we drive the transistor clear to saturation, using the transistor as a switch, the dissipated power can be greatly reduced because the saturation voltage is low. This is Class-C amplification, which achieves excellent efficiencies, in the range of 75%. We will also see variations of Class C, the Class D, E, and F amplifiers, that achieve even higher efficiencies. The disadvantage of operating Class C is that the output amplitude no longer follows the input level. There is significant distortion at both low and high levels. We say the amplifier is nonlinear, and this presents challenges in amplifying signals that vary in frequency and amplitude at the same time, such as music in stereo amplifiers. However, Class C is quite suitable for signals that simply turn on and off, such as Morse Code in the NorCal 40A, or signals that only vary in frequency, such as FM transmissions.