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BUILD THIS AM RADIO TRAINER Ever wanted to build a radio but haven’t seen a suitable circuit with easy to get parts? Well, now is the time to give it a try with this demonstration AM Radio Trainer project. It is intended for beginners, schools & TAFE students & will give you an understanding of how an AM radio works. By MARQUE CROZMAN & LEO SIMPSON When radio stations first began broadcasting in Australia and other countries, they all used the amplitude modulation (AM) system. In this system, the radio frequency carrier signal is modulated in proportion to the amplitude of the audio signal. 12  Silicon Chip The AM radio signal is radiated from the broadcast trans­mitter antenna and picked up by the radio. It demodulates the signal – the reverse of the amplitude modulation process –and the recovered audio signal is then amplified and fed to the radio’s loudspeaker so that you can listen to it. All of today’s AM radios are designed along the superheter­ odyne principle, which was invented by Edwin Armstrong in 1918. The first AM superhet radios were put on the market by Radio Corporation of America (RCA) in 1924. Later, RCA licensed other manufacturers so that the design was used world-wide. Prior to the superheterodyne, all radios were either crys­tal sets or used the tuned radio frequency (TRF) principle of which there are a number of variations. Essentially though, the TRF can be thought of as a crystal set with gain. In a TRF re­ceiver, all amplification up to the detector takes place at the frequency of the incoming signal. Left: all the parts for the AM Radio Trainer are mounted on a single large PC board. The circuit diagram is screened onto the component side, to show you where to mount the parts. The superheterodyne radio brought with it two major advan­ tages over previous circuits. The first was greatly increased gain. This was a big boost compared with TRF tuners which are strictly limited as far as maximum gain is concerned; any attempt to increase the gain over this limit and the circuit goes into oscillation – a loud squeal is the result. Second, the selectivity of the superheterodyne was a big improvement over previous circuits and this meant that weak stations could be separated out from strong stations which would otherwise tend to blanket half the dial. Finally, the superheterodyne re­ ceiver brought with it the possibili­ty of automatic volume control (AVC), although this did not become a feature until around 1930. AVC did away with the need for manual gain controls and meant that all stations came in with roughly the same loudness, as they do today, in spite of the fact that some stations may be very strong and some very weak. Since the advent of the superheterodyne receiver, or “superhet” for short, there have been relatively few changes to the basic circuit configuration although the components used have changed radically. Originally, valves (or vacuum tubes) were used and now transistors are used or a single integrated circuit with just a few external components may suffice. So if you decide to build this AM superhet receiver, you will be building a circuit configuration which has been around for over 70 years but one which is still just as relevant today. Let’s have a look at the operating principles of the super­het which are set out in block diagram form in Fig.1. Block diagram Fig.1 shows the general configuration of a superhet receiv­ er. The antenna at left feeds into an RF amplifier which has a parallel resonant circuit which is tuned by a variable capacitor. This is one section of a tuning gang capacitor. The other section of the gang capacitor varies the local oscillator which we’ll come to in a moment. The parallel resonant circuit is “tuned” by the variable capacitor so that the wanted signal is amplified and other sign­als are rejected. The signal from the RF Amplifier is then fed to the Mixer and this is where the “superheterodyne” process takes place. The word “heterodyne” refers to a difference in frequency or beat. “Hetero” is derived from the Greek word for “other” while “dyne” is derived from the French word for power. In the Mixer stage, the Local Oscillator signal is mixed with that from the RF Amplifier. The result is four signals: the original two signals plus the sum and difference frequencies. These are passed to an amplifier stage or stages which are tuned to the difference frequency which is now known as the Intermediate Frequency or IF (pronounced “Eye-Eff”). The IF stage amplifies only the difference frequency and rejects all the others. In most radios of this type, the Intermediate Frequency is 455kHz or 450kHz. The output of the IF stage is then applied to the detector which in transistor radios is usually a germanium diode, selected because of its small forward voltage drop. This rectifies the IF signal which is then filtered to remove RF components, leaving the original audio signal which modulated the transmitter. This audio signal is fed to the Audio Amplifier and this then drives a loudspeaker. Automatic gain control Apart from demodulating the IF signal, the detector is also used to produce the AGC voltage. AGC stands for “automatic gain control” which was previously referred to as AVC or “automatic volume control”. AGC was regarded as a wonderful innovation when it was introduced, as it eliminated the need for manual gain controls. These were needed to stop the IF stages from overload­ing on strong signals and to increase the gain for very weak signals. To derive the AGC voltage, the raw DC output from the detector is heavily filtered to remove all audio components, to produce a DC voltage which is proportional to the strength of the IF signal. This is then used to control the gain of the IF stages and June 1993  13 RF CARRIER DETECTED AUDIO COMPONENT APLIFIED IF CARRIER IF CARRIER AMPLIFIED AUDIO ANTENNA RF AMPLIFIER MIXER IF AMPLIFIER AUDIO AMPLIFIER DETECTOR SPEAKER OSCILLATOR WAVE LOCAL OSCILLATOR AGC TO OTHER STAGES POWER SUPPLY GANGED TUNING Fig.1: the general configuration for a superheterodyne radio receiver. The incoming RF signal is first mixed with the output from a local oscillator to produce an intermediate frequency (IF) signal & this is then fed to a detector stage to recover the original audio signal. perhaps also the RF stage, so that the signal is held to a more or less constant level. So why is this type of radio circuit referred to as a “superheterodyne”? Why couldn’t it just have been called a plain old heterodyne radio? It is not because the circuit has a “super you-beaut” performance, although it was a big step forward com­ pared to the TRF. The reason is that the intermediate frequency produced by the superhet was “supersonic” as opposed to circuits such as the beat frequency oscillator (BFO) which produced audi­ble heterodynes or beats. Hence, superhet is a contraction of “supersonic heterodyne”. The first superhets had an intermediate frequency of 50kHz which gave very sharp selectivity but poor audio response. Later, the standard IF was 175kHz and later still this was standardised at 455kHz. Interestingly, some references give the definition of superhet as referring to the fact that the Local Oscillator signal is above the incoming RF signal from the antenna – hence super, meaning “above”. Local oscillator Note that the Local Oscillator frequency always “tracks” the tuned frequency of the RF Amplifier. So if the radio is tuned to 1370kHz, the local oscillator will be set to 1370 + 455 = 1825kHz. Similarly, if the radio is tuned to 702kHz, the local oscillator will be at 702 + 455 = 1157kHz. All this happens automatically by virtue of the 14  Silicon Chip 2-section tuning gang – one section for the RF amplifier and the other for the local oscillator. Variations on a theme While we have just described the broad concept of the superhet, there are many variations on this theme. For example, many superhet circuits leave out the RF Amplifier stage and some do not have a local oscillator. Instead, the local oscillator is combined with the mixer stage in what is known as a “self oscil­lating mixer”. Others may have two or three IF stages and still others may have a separate detector to produce the AGC voltage. Another important variant is the double conversion superhetero­ dyne configuration which is used in some high performance communications re­ceiv­ers. The circuit to be described is a “single conversion” superhet, meaning that it performs just one conver­sion from the incoming RF frequency to the intermediate frequen­cy. In communications receivers which tune the higher frequency bands, double conversion may be used. The first local oscillator and mixer will produce an intermediate frequency of, usually, 10.7MHz. This will be passed through one or more IF stages before being mixed with a second (fixed) local oscillator to produce a second intermediate frequency of 455kHz. Other variations which are common include “permeability tuned” superhets and today’s frequency synthesised receivers which have digital readouts and microprocessor control. Perme­ability tuning was common in car radios and moved the slugs in inductors in tuned circuits rather than using tuning gangs which were more susceptible to vibration. Regardless of all the variations, you will find that all superhets have the same operating mode and same circuit functions as described by the block diagram of Fig.1. By the way, the Edwin Armstrong who produced the AM super­het receiver was the same brilliant inventor who later developed the principles of FM transmission and reception. One further note before we leave the origins of the super­het: apparently, radio (wireless?) circuits working along the same principle were used in British submarines during the First World War. 7-transistor circuit Now refer to Fig.2 which shows the complete circuit of our AM Radio Trainer. Each section of the circuit is labelled so that you can see how it relates to Fig.1. The circuit does not have an RF amplifier stage so the antenna signal is coupled directly into the mixer stage. The antenna coil is wound on a small ferrite rod and the primary coil is tuned in a parallel resonant circuit by one section of the tuning gang, VC1. VC2, also in the circuit, is a trimmer which is set during the alignment process. A secondary coil on the ferrite rod couples the tuned signal into the base circuit of transistor Q1 which functions as a self-oscillating mixer or mixer/oscillator. It oscillates at a frequency set by the parallel resonant circuit connected to its emitter. The oscillator is tuned by the second section of the tuning gang, VC3. VC4 is a trimmer which is set during the align­ment process. The oscillator coil (L2) has its secondary winding connected in series with the collector of Q1. The IF components of the collector current drawn by Q1 pass through the primary winding of the 1st IF transformer, T1. The secondary of this transformer couples the IF signal to the base of Q2, the 1st IF amplifier stage. The collector current of Q2 passes through the primary of IF transformer T2 and its secondary couples the signal to base of Q3, the second IF amplifier stage. It is virtually identical to the 1st IF stage and drives the third IF transform­er, T3. Transformer coupling These transformer coupled stages may seem odd to readers who are used to seeing circuits in which transistor stages are directly coupled; ie, without capacitors or transformers. There are several reasons for using transformers. The first is that each IF transformer is designed to resonate with the capacitor connected in parallel with its primary winding. During the alignment process, each IF transformer is tuned to 455kHz by adjusting its iron dust core (the threaded “slug”). By this means, the IF stages become very efficient amplifiers over a narrow bandwidth centred on 455kHz, while frequencies outside the wanted band are strongly rejected. Second, the IF transformers provide the right degree of impedance matching between the relatively high impedance of the collector circuits of the transistors and the relatively low impedance base circuit of the following transistor. Note that in each case, the collector current of the transistor passes through only a portion of the transformer primary and this is part of the intended matching process. Note also the tortuous path followed by the DC collector current for the mixer transistor Q1. The current passes through part of the primary of the 1st IF transformer (T1) and then via the secondary of oscillator coil L2 (which is also a transformer), before arriving at the collector of Q1. Detector diode We now come to a part of the circuit which looks to be quite simple but which has more going on than meets the eye: the detector diode (D1). This is driven by the secondary winding of the third and last IF transformer. The detector diode performs two tasks: (1) it detects or demodulates the amplitude modulated IF signal to produce an audio signal; and (2) it produces the AGC voltage which is used to control the gain of the 1st IF amplifi­er, Q2. D1 is an OA91 germanium diode, selected for its low forward voltage drop of about 0.2V. Note that the diode appears to be connected the opposite way around to what you might expect. The anode of the diode is connected to a .022µF capacitor which provides the first stage of RF filtering, and then via a 2.2kΩ resistor to a second 0.022µF capacitor which provides more filtering of the final audio signal which appears across the 10kΩ volume control potentiometer. The reason that the diode is connected back to front is so that it can develop a negative DC voltage as it rectifies the IF signal. This negative voltage is coupled via a 3.3kΩ resistor to a 10µF filter capacitor and thus becomes part of the bias voltage for the base of the 1st IF amplifier stage, Q2. The AGC works as follows: if a large signal is being picked up, diode D1 will produce a larger than normal negative DC vol­tage and this will tend to throttle back the bias voltage of Fig.2 (right): the circuit employs seven transistors in a fairly conventional arrangement. The incoming RF signal is picked up by a ferrite rod antenna & fed via the tuner stage to Q1 which functions as a self-oscillating mixer stage. The resulting signal is then coupled via T1 to the 1st & 2nd IF amplifier stages & detected by diode D1 to recover the audio signal. This then drives audio amplifier stage Q4-Q7 via volume control VR1. June 1993  15 Q2. Q2 will therefore conduct less current and its gain will conse­quently be reduced. The stronger the signal, the greater the gain reduction and hence the chance of signal overload is greatly reduced. Note the rather complicated bias network for the base of Q2. Current passes first via the 27kΩ resistor, the 3.3kΩ and 2.2kΩ resistors associated with diode D1, and then via the 10kΩ volume control pot VR1. The base current flows from the junction of the 27kΩ and 3.3kΩ resistors via the secondary of the 1st IF transformer (T1). Having the bias current flow through the volume control pot is not PARTS LIST 1 PC board, code 06106931, 275 x 90mm 1 50mm 8Ω loudspeaker 1 455kHz IF transformer/ oscillator kit (DSE R-5040) 1 60-160pF tuning gang capacitor (DSE R-2970) 1 ferrite rod with coil (DSE R-5100) 1 3.5mm socket 1 SPST toggle switch 1 9V battery holder 1 9V battery 1 10kΩ log. pot (VR1) 1 200Ω trimpot (VR2) Semiconductors 4 BC547 NPN transistors (Q1,Q2,Q3,Q4) 2 BC327 PNP transistors (Q5,Q7) 1 BC337 NPN transistor (Q6) 1 OA91 germanium diode (D1) 1 1N4148 signal diode (D2) Capacitors 1 470µF 16VW electrolytic 1 100µF 16VW electrolytic 5 10µF 16VW electrolytic 5 .022µF monolithic or ceramic 1 .01µF monolithic or ceramic 1 .0047µF monolithic or ceramic Resistors (0.25W, 1%) 1 1.2MΩ 1 10kΩ 1 1MΩ 1 4.7kΩ 1 820kΩ 2 3.3kΩ 1 56kΩ 1 2.2kΩ 1 47kΩ 2 1kΩ 1 39kΩ 1 470Ω 1 27kΩ 2 100Ω 1 12kΩ 16  Silicon Chip good engineering practice because pots with DC flowing through them generally become noisy after awhile. Potentiometers become even noisier if current is drawn off via the wiper but that does not happen in this circuit. Having DC flow though the volume pot is common in cheap transistor radios, hence we repeat the practice here. The signal from the volume control is fed to a 4-transistor amplifier consisting of Q4, Q5, Q6 & Q7. This amplifier is direct coupled throughout apart from the output capacitor which we’ll come to in a moment. Q4 is connected as a common emitter stage with all its collector current becoming the base current of the following PNP transistor, Q5. This is also a common emitter stage and provides most of the voltage gain of the amplifier. Its collector current flows partly into the bases of the output transistors, Q6 and Q7, while the rest goes through the 1kΩ resistor and 8Ω loudspeaker to ground. Output transistors Q6 and Q7 are connected as complementary emitter followers in class-AB mode. The two output transistors are slightly biased into forward conduction by the voltage devel­oped across diode D2 and trimpot VR2. VR2 provides quiescent current adjustment to minimise cross­ over distortion. Negative feedback from the output of the amplifier is pro­ vided by the 4.7kΩ resistor to the emitter of Q4. The AC voltage gain of the amplifier is set to about 47 by the 100Ω resistor from the emitter of Q4, while the series 10µF capacitor sets the bass roll-off of the amplifier. By now, you’ve probably realised that this is “minimum component count” radio, very similar in circuitry to most portable AM radios. Another place where components have been minimised is in the output stage where the 1kΩ resistor is connected to 0V via the speaker. The same DC bias conditions could have been obtained in the output stage by simply connecting the 1kΩ resistor direct­ly to the 0V line but there is good reason for doing it the way we have. Bootstrapping By connecting the 1kΩ resistor via the speaker we take advantage of the fact that the output stage transistors are emitter followers. In this mode, these transistors have a voltage gain just slightly less than one. This means that the AC signal voltage at the emitters of Q6 and Q7 (and hence across the speak­er) is only slightly less than the signal voltage at the bases of these two transistors. Because of this, the AC voltage applied across the 1kΩ resistor is very small and so little AC current flows. Hence, transistor Q5 “sees” a much higher collector load than the nomi­nal 1kΩ connected. This means it is able to provide more drive to the output stage and higher overall voltage gain. This technique is known as “boot­ strapping” and is commonly used in audio amplifiers. However, while this is an effective method which improves the overall performance, it does have one drawback. If the loudspeaker or headphone is not in circuit, no current can flow through the 1kΩ resistor. If this happens, the output stage is not biased on and the whole amplifi­er “latches up” and draws no current at all. This may not seem important because the speaker will nor­ mally always be connected. But if you try connecting a ceramic earpiece to the earphone socket, no current will flow through it and the amplifier won’t work. So don’t be trapped! One other little circuit trick needs to be noted before we finish this article and this involves the 470µF capacitor just after on/off switch S1. This relatively large capacitor may seem unnecessary since the circuit is intended to be powered from a 9V battery but it does have a distinct benefit. As the battery ages, its internal impedance rises. This means that it is less able to deliver the relatively high current pulses demanded by the amplifier and the result is more distor­ tion from the amplifier; ie, poor sound. By placing the 470µF capacitor across the 9V supply, we effectively reduce the AC impedance of the battery and thus enable it to deliver those higher current pulses. The result is better sound quality. Whew! Well, that’s it for this month. Next month we will show you how to assemble this AM Radio Trainer and give the alignment procedure. You will build an alignment oscillator to do this, so no special equipment will SC be required.