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.

Now we begin the study of transistor circuits. Transistors have three terminals. Usually one of the terminals is the input, another is the output, and the third is a common connection that is shared between the input and the output. Transistor circuits can increase the power of a signal. For this they require an additional DC power source. Circuits that increase power are called active circuits. By comparison, a passive circuit has loss. The filters we covered in the earlier chapters are examples of passive circuits. We will study several different active circuits. An amplifier increases the power of a signal without changing the frequency. In an oscillator, an output sine wave is generated without any input signal. Transistors can also be used in passive circuits. In Problem 5, we saw that a transistor could act as a fast switch, with either a low resistance between the output terminals or a high resistance, depending on the input voltage. We will also use a transistor as a variable attenuator to control the signal level.

Manufacturers can combine many transistors on one chip of silicon. These circuits are called integrated circuits, or ICs. Many thousands of different integrated circuits are available. One common type of IC includes several amplifiers cascaded one after another, so that the output signal is much larger than the input. These circuits are called op amps, short for operational amplifiers.