Build Your Own Transistor Radios

McGraw-Hill Companies, Inc.

Contents

Chapter One

This book will be a journey for both the hobbyist and the engineer on how radios are designed. The book starts off with simple designs such as an offshoot of crystal radios, tuned radio-frequency radios, to more complicated designs leading up to superheterodyne tuners and radios. Each chapter presents not only the circuits but also how each circuit was designed considering the tradeoffs in terms of performance, power consumption, availability of parts, and the number of parts.

In the engineering field, often there is no one best design to solve a problem. In some chapters, therefore, alternate designs will be presented.

Chapters 4 through 12 will walk the hobbyist through various radio projects. For those with an engineering background by practice and/or by academia, Chapters 13 through 23 will provide insights into the theory of the various circuits used in the projects, such as filter circuits, amplifiers, oscillators, and mixers.

For now, an overview of the various radios is given below.

Tuned Radio-Frequency (TRF) Radios

The simplest radio is the tuned radio-frequency radio, better known as the TRF radio. It consists mainly of a tunable filter, an amplifier, and a detector.

A tunable filter just means that the frequency of the filter can be varied. Very much like a violin string can be tuned to a specific frequency by varying the length of the string by using one's finger, a tunable filter can be varied by changing the values of the filter components.

Generally, a tuned filter consists of two components, a capacitor and an inductor. In a violin, the longer the string, the lower is the frequency that results. Similarly, in a tuned filter, the longer the wire used for making the inductor, the lower is the tuned frequency with the capacitor.

In TRF radios, there are usually two ways to vary the frequency of the tuned filter. One is to vary the capacitance by using a variable capacitor. This way is the most common method. Virtually all consumer amplitude-modulation (AM) radios use a variable capacitor, which may be a mechanical type such as air- or poly-insulated variable-capacitor type or an electronic variable capacitor. In the mechanical type of variable capacitor, turning a shaft varies the capacitance. In an electronic variable capacitor, known as a varactor diode, varying a voltage across the varactor diode varies its capacitance. This book will deal with the mechanical types of variable capacitors.

The second way to vary the frequency of a tunable filter is to vary the inductance of an inductor or coil via a tuning slug. This method is not used often in consumer radios because of cost. However, for very high-performance radios, variable inductors are used for tuning across the radio band. In this book, tunable or variable inductors will be used, but they will be adjusted once for calibration of the radio, and the main tuning will be done via a variable capacitor. Figure 1-1 shows a block diagram of a TRF radio.

Block Diagram of a TRF Radio

A TRF radio has a radio-frequency (RF) filter that is usually tunable, an RF amplifier for amplifying signals from radio stations, and a detector (see Figure 1-1). The detector converts the RF signal into an audio signal.

Circuit Description of a TRF Radio

For the AM radio band, the RF filter is tuned or adjusted to receive a particular radio station. Generally, an antenna is connected to the RF filter. But more commonly, a coil or an inductor serves as the "gatherer" of radio signals. The coil (L) may be a loop antenna (see Figure 3-1). A variable capacitor (VC) is used to tune from one station to another.

The output of the filter will provide RF signals on the order of about 100 microvolts to tens of millivolts depending on how strong a station is tuned to. Typically, the amplifier should have a minimum gain of 100. In this example, although the amplifier usually consists of a transistor, a dual op amp circuit (e.g., LME49720) is shown for simplicity. Each amplifier stage has gain of about 21, which yields a total gain of about 400 in terms of amplifying the RF signal.

The output of the amplifier is connected to a detector, usually a diode or a transistor, to convert the AM RF signal into an audio signal. A diode CR1 is used for recovering audio information from an AM signal. This type of diode circuit is commonly called an envelope detector.

Alternatively, a transistor amplifier (Q1, R1B, R2B, and C2B) also can be used for converting an AM signal into an audio signal by way of power detection. Using a transistor power detector is a way of demodulating or detecting an AM signal by the inherent distortion (nonlinear) characteristic of a transistor. Power detection is not quite the same as envelope detection, but it has the advantage of converting the AM signal to an audio signal and amplifying the audio signal as well.

Power-detection circuits are commonly used in regenerative radios and sometimes in superheterodyne radios.

It should be noted that in more complex TRF radios, multiple tuned filter circuits are used to provide better selectivity, or the ability to reduce interference from adjacent channels, and multiple amplifiers are used to increase sensitivity.

Regenerative Radio

This is probably the most efficient type of radio circuit ever invented. The principle behind such a radio is to recirculate or feed back some of the signal from the amplifier back to the RF filter section. This recirculation solves two problems in terms of providing better selectivity and higher gain. But there was another problem. Too much recirculation or regeneration caused the radio to oscillate, which caused a squealing effect on top of the program material (e.g., music or voice) (Figure 1-2).

Block Diagram of a Regenerative Radio

The regenerative radio in Figure 1-2 consists of a tunable filter that is connected to an RF amplifier. The RF amplifier serves two functions. First, it amplifies the signal from the tunable filter and sends back or recirculates a portion of that amplified RF signal to the tunable-filter section. This recirculation of the RF signal causes a positive-feedback effect that allows the gain of the amplifier to increase to larger than the original gain. For example, if the gain of the amplifier is 20, the recirculation technique will allow the amplifier to have a much higher gain, such as 100 or 1,000, until the amplifier oscillates. The second function of the amplifier is to provide power detection of the RF signal, which means that the amplifier also acts as an audio amplifier.

Circuit Description of a Regenerative Radio

In Figure 1-2, the tunable RF filter is formed by variable capacitor VC and antenna coil L1. Antenna coil L1 also has an extra winding, so this is more of an antenna coil-transformer.

Also, because transistors have a finite load resistance versus the "infinite" input resistance of a vacuum tube or field-effect transistor, the base of the transistor is connected to a tap of antenna coil L to provide more efficient impedance matching.

Note

In a parallel-capacitor-coil resonant circuit (aka parallel-capacitance-inductance circuit) for a tunable RF filter, the quality factor, Q indicates that the higher the selectivity, the better is the separation of radio stations. A low Q in an antenna coil and variable-capacitor resonant circuit will allow unwanted adjacent stations to bleed into the tuned station. But a higher Q allows the RF signal of the desired tuned radio station to pass while attenuating RF signals from other stations. The Q in a parallel tank circuit is affected by the input resistance of the amplifier to which it is connected. The higher the input resistance, the higher the Q is maintained. So an amplifier with an input resistance on the order of at least 100 k (e.g., typically 500 k or more) allows for a high Q to be maintained. If an amplifier has a moderate input resistance (e.g., in the few thousands of ohms), tapping the coil with a stepped-down turns ratio allows the Q to be maintained, but at a tradeoff of lower signal output. For example, if an antenna coil has a 12:1 step-down ratio or 12:1 tap, the signal output will be 1/12 in strength, but when connected to an amplifier of 3 k[ohm] of input resistance, the effective resistance across the whole coil and variable capacitor is 12 × 12 × 3 k, or 432 k, which maintains a high Q.

Transistor Q1 serves a dual purpose as the RF amplifier and detector. The (collector) output signal of Q1 is connected to an audio transformer T1 that extracts audio signals from detector Q1, but Q1's collector also has amplified RF signals, which are fed back to coil L1 via the extra winding. By varying resistor R1, the gain of the Q1 amplifier is varied, and thus the amount of positive feedback is varied. The user tunes to a station and adjusts R1 to just below the verge of oscillation. Too much positive feedback causes the squealing effect. But when adjusted properly, the circuit provides very high gain and increased selectivity.

Reflex Radio

In a reflex radio, which also uses a recirculation technique, an amplifying circuit is used for purposes: (1) to amplify detected or demodulated RF signals and (2) to amplify RF signals as well. The demodulated RF signal, which is now an audio signal, is sent back to the amplifier to amplify audio signals along with the RF signal. So although reflex radios have similar characteristics as regenerative radios, they are not the same. Reflex radios do not recirculate RF signals back to the amplifier. And unlike regenerative radios, reflex radios do not have a regeneration control to increase the gain of the amplifier. A reflex radio would be essentially the same as a TRF radio but with use of a recirculation technique to amplify audio signals. Thus, in terms of sensitivity and selectivity, a reflex radio has the same performance as a TRF radio (Figure 1-3).

Block Diagram of a Reflex Radio

In Figure 1-3, the reflex radio consists of a tunable filter, an amplifier, and a detector. In essence, this reflex radio has the same components as the TRF radio in Figure 1-1. The difference, however, is that the output of the detector circuit (e.g., an envelope detector or diode), a low-level audio signal, is fed back and combined with the RF signal from the RF filter section. The audio output, which typically in other radios is taken from the output of the detector, is taken from the output of the radio-frequency/audio-frequency (RF/AF) amplifier instead.

Circuit Description of a Reflex Radio

The RF filter section is formed by variable capacitor VC and coil/inductor L1, which also has a (stepped-down) secondary winding connected to the base of transistor Q1. Note that the base of Q1 is an input for amplifier Q1. RF signals are amplified via Q1, and the RF signals are detected or demodulated by coupling through an RF transformer T2 to diode CR1 for envelope detection. At resistor R2 is a low-level audio signal that is connected to the input of Q1 via AF coupling capacitor C1 and the secondary winding of L1. RF coupling capacitor C2 is small in capacitance to direct RF signals to the emitter of transistor Q1 without attenuating the low-level audio signal. Audio transformer T1 is connected to the output of the amplifier at the collector of Q1. T1 thus extracts amplified audio signal for Q1.

Superheterodyne Radio

The superheterodyne radio overcomes shortfalls of the TRF, regenerative, and reflex radios in terms of sensitivity and selectivity. For example, the TRF and reflex radios generally have poor to fair selectivity and sensitivity. The regenerative radio can have high selectivity and sensitivity but requires the user to carefully tune each station and adjust the regeneration control so as to avoid oscillation or squealing.

A well-designed superheterodyne radio will provide very high sensitivity and selectivity without going into oscillation. However, this type of radio design requires quite a few extra components. These extra components are a multiple-section variable capacitor, a local oscillator, a mixer, and an intermediate-frequency (IF) filter/amplifier. In many designs, the local oscillator and mixer can be combined to form a converter circuit. Selectivity is defined mostly in the intermediate frequency filter (e.g., a 455-kHz IF) circuit. And it should be noted that an RF mixer usually denotes a circuit or system that translates or maps the frequency of an incoming RF signal to a new frequency. The mixer uses a local oscillator and the incoming RF signal to provide generally a difference frequency signal. Thus, for example, an incoming RF signal of 1,000 kHz is connected to an input of a mixer or converter circuit, and if the local oscillator is at 1,455 kHz, one of the output signals from the mixer will be 1,455 kHz minus 1,000 kHz, which equals 455 kHz.

One of the main characteristics of a superheterodyne radio is that it has a local oscillator that tracks the tuning for the incoming RF signal. So the tunable RF filter and the oscillator are tied in some relationship. Usually, this relationship ensures that no matter which station is tuned to in the oscillator, it changes accordingly such that the difference between the oscillator frequency and the tuned RF signal frequency is constant.

Thus, if the RF signal to be tuned is 540 kHz, the local oscillator is at 995 kHz, the RF signal to be tuned is at 1,600 kHz, and the local oscillator is at 2,055 kHz. In both cases, the difference between the oscillator frequency and the tuned RF frequency is 455 kHz.

Although the superheterodyne circuit is probably the most complicated system compared with other radios, it is the standard bearer of radios. Every television tuner, stereo receiver, or cell phone uses some kind of superheterodyne radio system, that is, a system that at least contains a local oscillator, a mixer, and an IF filter/amplifier.

Block Diagram of a Superheterodyne Radio

One of the main characteristics of a superheterodyne radio is that it has a local oscillator that tracks the tuning for the incoming RF signal (Figure 1-4).

The tunable RF filter is connected to an input of the converter oscillator circuit. The converter oscillator circuit provides an oscillation frequency that is always 455 kHz above the tuned RF frequency. Because the converter output has signals that are the sum and difference frequencies of the oscillator and the incoming tuned RF signal, it is the difference frequency (e.g., 455 kHz) that is passed through the IF filter and amplifier stage. So the output of the IF amplifier stage has an AM waveform whose carrier frequency has been shifted to 455 kHz.

To convert the 455-kHz AM waveform to an audio signal, the output of the IF amplifier/filter is connected to a detector such as a diode or transistor for demodulation.

Circuit Description of a Superheterodyne Radio

The tunable RF filter is provided by variable capacitor VC_RF and a 240-µH antenna coil (L_Primary) with a secondary winding (L_Secondary). The converter oscillator circuit includes transistor Q1, which is set up as an amplifier such that positive feedback for deliberate oscillation is determined by the inductance of Osc Transf 1 and variable capacitor VC OSC. In superheterodyne circuits both the VC RF and VC OSC variable capacitors share a common shaft to allow for tracking. At the base of Q1 there is the tuned RF signal, and at the emitter of Q1 there is the oscillator signal via a tapped winding from oscillator coil Osc Transf 1. The combination of the two signals at the base and emitter of Q1 results in a mixing action, and at the collector of Q1 is a signal whose frequency is the sum and difference of the tuned RF frequency and the oscillator frequency.

A first IF transformer (T1 IF) passes only the signal with a difference frequency, which is 455 kHz in this example. The secondary winding of T1 IF is connected to Q2's input (base) for further amplification of the IF signal. The output of Q2 is connected to a second IF transformer, T2 IF. The secondary winding of T2 IF is connected to the input (base) of the second-stage IF amplifier, Q3. It should be noted that in most higher-sensitivity superheterodyne radios, a second stage of amplification for the IF signal is desired. The output of Q3 is connected to a third IF transformer, T3 IF, whose output has sufficient amplitude for detector D2 to convert the AM 455-kHz signal into an audio signal.

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Excerpted from Build Your Own Transistor Radiosby Ronald Quan Copyright © 2013 by The McGraw-Hill Companies, Inc.. Excerpted by permission of McGraw-Hill Companies, Inc.. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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