Introduction

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.

Gain-Controlled IF Amplifiers

Normally, it is desirab:le to сопtгоl the gаiп of опе ог тоге IF amplifiers to equalize the loudness when tuned to а weak ог strong radio-frequency (RF) signal. In АМ receiveгs, the gain control is "automatic" via the detectoг stage. Often, in соmmuпiсаtiопs radios the IF gаiп сап Ье сопtгоllеd manually ог automatically via а detectoг stage. Usually, the gain control should Ье set at the very first IF stage. In this way, апу succeeding IF stage сап avoid distortion of the IF signal that would result in distortion in the detected signal. Foг example, if а receiver has two IF amplifiers, if the second IF amplifier is adjusted for collector curгent as а way to control its gаiп, then а strongly received signal will cause the first IF stage to pгovide large signals into the second IF amplifier. However, the distortion of the IF amplifieris Ibased оп signal voltage into the input and not оп collectoг cuгrent. Theгefore, а high-Ievel signal into the second IF stage сап cause oveгload. Normally, gain сопtгоl оп just the fiгst IF amplifier should Ье adequate to equalize the lоudпеss. The gain contгolis just а function of Vbias connected to the base via а windilng of the IF transformer ог via capacitive coupling. Therefore, the gain of each соттоп emitteг ог cascode IF stage is

аз [m(t)][cos(2pfIF)t][1 + m(t)]2 e/2] = аз cos(2nfIF)t[m(t) + 2[m(t)]2 + [m(t)]з]е/2] represents ап amplitude modulated signal at fгequency but contains sесопd-апd third-order distortions оп the modulating signal т( t). Thus third-oгder distortion in ап 1F amplifieг гesults in both second- and thiгd-oгdeг distortion оп the modulating signal, which гesults in а distoгted demodulated audio signa:l. Thus typically а single-ended ог differential-pair transistoг amplifier in the 1F section will cause second- and th!ird-order distortion at the output of the detector stage. 1t should Ье noted that intuitively it makes sense as to why third order distortion in the 1F stage causes а distorted demodulated signal. Ап АМ signal has а caгrier, upper-sideband, and lower-sideband signals that аге close together in frequencies . 1f two signals of close frequencies, F1 and F2, аге connected to amplifier with third-order distortion, th!e resulting third-order intermodulation distortion products will have fгequencies of (2F1 - F2) and (2F2 - Fl) that fall within the 1F bandwidth (band -pass), which will cause distortion iп the demodulated signal. However, ап amplifier with second-order distortion will have intermodulation products whose fгequencies аге (F1 - F2) апd (Fl 1 F2) that fall outside the IF Ьапdwidth (Ьапd раss), and thus will not cause distогtiоп iп the demodulated signal. Recall that second-order distoгtion in ап IF amplifier does поt add апу distortion at all to the modulating gпаl, so when the IF signalis demodulated, the гecovered audio signal is free of distoгtion. Therefore, sq'uare-Iaw devices such as pentodes and FEТs provide "distoгtion-free" demodulated signals. Question: 15 it possibIe to make а single-ended amplifier free of third-order distortion? The answer is yes.

Practical Considerations for Using Amplifiers

In Chapter 18 concerning automatic volume control (AVC) amplifiers, it was shown that for а common-emitter amplifier, а minimum base voltage is required for turning оп the transistor for amplification. In ор amps, опе must consider the minimum, voltage needed to bias оп the transistors for amplification. This section will cover biasing conditions, output swing, and frequency response of ор amps. Figure 19-16 shows basically two types of input stages. Опе important consideгation (ог using ор amps isdetermining the voltage range at the input terminals that allows ргорег biasing within th!e ор атр. This voltage range is named the соmmоп-mоdе iпрut vo/tage, which is based оп а given supply voltage. For now, assume that the -V supply voltage is grounded. In Figuгe 19-16, the NPN diffeгential amplifieг input stage consisting of Q1 and Q2 has ап emiitter current souгce QЗ. This type of input stage сап Ье found in the NE5534 ор атр. Generally, for each tгa nsistor allow О. 7volt minimum foг VBE base-to-emitteг tuгn-on voltage, and allow the collector-to-base voltageto Ье generally О volt ог higher. Therefore when the negative power supply volt is gгounded such that -V = о volt, the minimum input voltage is at least ог equal to 0.7 volt (гот the VBE base-to-emitter turn-on voltage of f Q1 ог Q2 plus another 0.7 volt fгom the collector QЗ. So the minimum voltage at the input terminal is at least 1.4 volts. However, to Ье оп the safe side, the minimuminput voltage should Ье about 2 volts, which is about 1 VBE drop extra to guarantee operation over low tem peratu ге.

Aurther

Ronald Quan

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