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Bull Electrical-Jun03.Qxd Constructional Project PRACTICAL RADIO CIRCUITS RAYMOND HAIGH Part 1: Introduction, Simple Receivers and a Headphone Amp. Dispelling the mysteries of radio. This new series features a variety of practical circuits for the set builder and experimenter. owards the end of the 19th century, physicist who first demonstrated the exis- (V. Poulsen), and by mechanical alterna- sending a radio signal a few hun- tence of electromagnetic waves in 1886. tors (E. Alexanderson). Semiconductors T dred yards was considered a major Before the valve era, radio frequency now play an increasing role, but valves are achievement. At the close of the 20th, man oscillations were generated by using an still used in high-power transmitters. was communicating with space probes at electrical discharge to shock-excite a tuned As their name suggests, the waves com- the outermost edge of the solar system. circuit (H. Hertz and G. Marconi), by the prise an electric and a magnetic field No other area of science and technology negative resistance of an electric arc which are aligned at right angles to one has affected the lives of people another. The electric field is more completely. And because it formed by the rapid voltage fluctu- is so commonplace and afford- ations (oscillations) in the aerial. able, it is accepted without a sec- Current fluctuations create the ond thought. The millions who magnetic field. enjoy it, use it, even those whose lives depend upon it, often have HITCHING A RIDE little more than a vague notion of Electromagnetic waves cannot, A) how it works. by themselves, convey any informa- This series of articles will view tion. They are essentially radio fre- the technology in a historical quency carriers, and arrangements perspective and try to dispel its have to be made for the audio fre- mysteries. The main purpose, quency speech and music signals to however, is to present a variety of B) hitch a ride. This is done by modu- practical circuits for set builders lating the radio frequency carrier and experimenters. And, with with the audio frequency signals. economy in mind, basic compo- If the amplitude of the carrier is nents and assemblies are repeated varied in sympathy with the signal, in different receivers. the process is known as amplitude MAKING WAVES modulation (a.m.), and typical C) waveforms are depicted in Fig.1.1. Radio uses electromagnetic Varying the carrier frequency is, of waves to transport speech, music D course, known as frequency modu- and data over vast distances at the ak lation (f.m.). speed of light. Marconi's Morse signals were The electromagnetic waves are MODULATED CARRIER RECOVERED AUDIO FREQUENCY transmitted by simply switching WAVE INPUT SIGNAL OUTPUT generated by making an electric CR the carrier on and off. It was R. A. current oscillate at frequencies Fessenden who, in 1906, used a ranging from 10kHz (ten thou- D) carbon microphone (said to be sand Hertz) to more than 100GHz RESIDUAL RADIO FREQUENCIES REMOVED water cooled) to directly modulate (one-hundred thousand million BY SHUNTING ACTION OF CAPACITOR, C the radio frequency (50kHz) out- Hertz). put of an alternator and be the first The lowest frequencies are to transmit speech and music. used for submarine communica- tions because of their ability to E) PROPAGATION penetrate water to a considerable The oscillations produced by the depth: the highest mainly for Fig.1.1. Modulation and detection: (A) Radio frequency transmitter are fed to an aerial sys- satellite communications. Most carrier wave. (B) Audio frequency signal. (C) Carrier wave tem in order to radiate the electro- radio listeners are served by the modulated by audio signal. Average value of imposed audio magnetic energy. The lower the portion of the spectrum extending signal voltage is zero. (D) Diode detector, D, working into frequency the longer the wave- from 150kHz to 110MHz. load resistor, R, rectifies the modulated carrier wave. length and the bigger the aerial. Frequency of oscillation is Reservoir capacitor, C, removes residual radio frequencies Aerial designs vary, but the measured in Hertz in honour of and enables the audio frequency output voltage to approach dipole adopted by Hertz in 1886 is Heinrich Rudolf Hertz, the its peak value. (E) Recovered audio frequency signal. still deployed at high, very high 398 Everyday Practical Electronics, June 2003 and ultra high frequencies. The elevated wire and earth arrange- ment used by Marconi in the 1890's is still used for the radiation IMPORTANT EVENTS of low and medium frequencies. 1831 Transmitter powers range from the miserly one or two watts, In published papers and a letter deposited with the Royal radiated by amateurs who specialize in low power communication, Society (opened in 1937), Michael Faraday tentatively proposes to the two-million watts output from some medium wave broad- electromagnetic wave theory. cast transmitters. 1864 Radiation from the transmitter reaches the receiver by either the James Clerk Maxwell's mathematical analysis of Faraday's work ground wave (line of sight or diffraction around the earth's curva- published in his paper: A Dynamical Theory of the Electro-mag- ture), or the sky wave (reflected between the ionosphere and the netic Field. surface of the earth. Propagation path is frequency dependant: by 1888 ground wave up to 500kHz, then gradually shifting to sky wave Heinrich Rudolf Hertz uses a crude spark transmitter and until, above 30MHz, the waves are no longer reflected back by the receiver to demonstrate the existence of electromagnetic waves. ionosphere and escape out into space. 1889 Solar radiation has a profound effect on the charged particles which Sir Oliver Lodge lectures on the need to tune the transmitter to make up the ionosphere, and propagation conditions vary between night receiver, a condition he called “syntony”. and day, seasonally, and according to the eleven-year sunspot cycle. 1893 Sir Oliver Lodge uses an invention of Edouard Branley's as a RECEPTION sensitive detector of electromagnetic waves. (The coherer). Reception involves three essential functions: picking up the 1901 energy radiated by transmitters, selecting one station from all the Guglielmo Marconi transmits radio signals across the Atlantic. rest, and extracting the modulation from the carrier wave in order 1904 to make the transmitted speech or music audible to the listener. Sir John Ambrose Fleming patents the diode valve. 1906 Signal Pick Up Dr Reginald Fessenden modulates a carrier wave and broad- Receiving aerials respond to either the electric or the magnetic field casts speech and music. Dr Lee de Forest makes a patent appli- radiated by the transmitter. Sets that use telescopic rod or wire aerials cation for his triode valve, the first electronic amplifying device. pick up the electric field. Receivers that have loop aerials, i.e., a coil 1913 wound on a frame or a ferrite rod, respond to the magnetic field. Major Edwin Howard Armstrong invents the regenerative receiver. Portable receivers usually incorporate both: long and medium 1918 waves (150kHz to 1·6MHz) are covered by a loop with a ferrite Armstrong invents the superheterodyne receiver. rod core, and the v.h.f. f.m. band (88MHz to 108MHz) and short- 1921 wave bands by a telescopic aerial. Armstrong invents the super regenerative receiver and W. G. Cady uses quartz crystals to stabilize oscillators. Station Selection 1933 In order to select one station from the thousands that are spread Armstrong demonstrates his system of frequency modulated across the radio frequency spectrum, the receiver has to be tuned to radio transmission. the carrier frequency of the transmitter. 1947 Sir Oliver Lodge was stressing the importance of tuning, a con- John Bardeen, Walter Brattain and Dr William Shockley develop dition he called “syntony”, as early as 1889, and he patented his the transistor at the Bell Telephone Laboratories. system in 1897. This is one of the most fundamental patents in radio, and his method is still universally adopted. Lodge's invention exploits the way an inductor (coil) and capac- Sharpness of tuning depends mainly on resistive and other loss- itor combination resonate at a particular frequency. If the capacitor es in the inductor or coil: the lower these losses the sharper the is connected in parallel with the inductor (see Fig.1.2a) the circuit tuning. Resistive and dielectric losses in the capacitor also affect presents a high impedance at its resonant frequency and a lower performance, but, with modern tuning components, these are usu- impedance at all others. ally so small they can be ignored. Connecting the capacitor in series with the inductor (Fig.1.2b) The ability of the coil to resonate sharply, i.e., be more selective, is results in a low impedance at resonance and a higher impedance at known as its “Q” factor: the sharper the resonance the higher the Q. other frequencies. If the inductor or the capacitor (usually the Resonant tuned circuits magnify signal voltages, a phenomenon capacitor) is made variable, it is possible to tune the circuit across that is crucial to radio reception. If a signal of the same frequency a range of frequencies. as the resonant frequency of the tuned circuit is applied to the coil/capacitor combination, its voltage will be increased in pro- portion to the Q of the coil. With a Q of 100, a 1mV signal will be magnified to 100mV or 0·1V. We will be returning to this later. PARALLEL TUNED CIRCUIT Demodulation Presents a high impedance With amplitude modulated signals (a.m.) the process of recov- at its resonant frequency and a lower impedance at other ering the modulation is essentially one of rectification. In Fig.1.1d, frequencies. diode, D, rectifies the incoming radio frequency carrier wave and A) capacitor, C, shunts residual radio frequencies to ground (earth) leaving only the audio frequency modulation.
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