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Build your own Super-7 AM RADIO RECEIVER by John Clarke All on a single PCB – and no SMDs! Why, in this day and age, would you want to build an AM Radio Receiver – when you can probably buy one much cheaper? Well, you’ll never learn anything by buying off the shelf . . . and you won’t have the fun of constructing something that works. Nor will you have the satisfaction of saying to your family and friends: “Look at this! I built it myself!” T he Super-7 Superhet AM Radio makes a great beginner’s project – whether you’re 8 or 88! It is nice and easy to build since all the components mount on a single PCB. They’re all standard components (no surface-mount devices to worry 46 Silicon Chip about) that are easy to get and they’re laid out in a neat manner, making assembly simple and also allowing you to see how it works. It’s powered from a 9V battery or 9V DC plugpack and it automatically switches from battery to the mains Celebrating 30 Years supply when it’s plugged in. Audio output is loud and clear from a built-in 100mm (4-inch) diameter loudspeaker but it also has a headphone jack, which automatically disconnects the speaker when in use. This set has good sensitivity and sesiliconchip.com.au It’s all built on one double-sided PCB – and while it can operate from an on-board 9V battery (making it truly portable, a 9V DC plugpack can also be used (with automatic switchover when plugged in). lectivity as well as reasonably low distortion. It fits into a custom-designed acrylic case, with a transparent back, so the components are protected but you can still see its workings. It has a large (hand-span) tuning dial showing the current frequency plus many of the available AM radio stations around Australia. Once built and aligned, you will end up with a fully functioning radio reminiscent of radio sets from the past but using modern technology. It’s called the “Super-7” partly because it is a superheterodyne but also because it uses seven silicon transistors (plus two diodes). One transistor is used for the mixer/oscillator, two for IF amplification and four for the Class-AB push-pull output stage. We’ll explain all these terms as we go. This month we will describe the Super-7 AM Radio circuit, with the assembly and alignment details to follow. If you know nothing about AM radio technology or the operation of a superheterodyne receiver, please see the accompanying panels titled “What is AM radio” and “The Superhet AM Radio Receiver” before moving on to the circuit description. Circuit description Refer to Fig.1 which shows the complete circuit of our Super-7 AM Radio. Each section of the circuit is labelled so that you can see how it relates to the block diagram in the panel on page 50 which explains how a superhet works. The circuit does not have an RF amplifier stage so the antenna signal is coupled directly to the mixer stage. The antenna coil (T1) is wound on a small ferrite rod. The high permeability of the ferrite material allows a compact antenna of this type to pick up signals that would otherwise require a fairly long standard antenna. The primary coil is tuned in a parallel resonant circuit by one section of the plastic dielectric tuning gang, VC1. Trimmer capacitor VC2 is in parallel with VC1 and is set during alignment of the AM radio so that stations appear at the correct location on the dial. A secondary coil on the ferrite rod couples the tuned signal into the base of transistor Q1, via a 22nF capacitor, and Q1 functions as a self-oscillating mixer. It oscillates at a frequency set by the parallel resonant circuitry connected to its emitter, ie, the primary of T2 plus VC3 and VC4. This oscillator is tuned by the second section of the tuning gang, VC3. Again, VC4 is a trimmer, connecting in parallel with VC3, and is set during the alignment process so that the oscillator frequency tracks the tuned frequency with the correct offset of 455kHz. The oscillator transformer, T2, has its secondary winding connected in series with the collector of Q1. This provides feedback to Q1 to sustain oscillation. The output signal of the mixer/oscillator appears at the bottom end of this secondary and is fed to the primary of transformer T3. This is adjusted (via its integral tuning slug) to be resonant at the intermediate frequency of 455kHz. Here is the “front” side of the Super-7 AM Radio Receiver – the side which normally faces you. It sports a quite large speaker (which gives it really good tone!), the volume control (the knob in the lower right), power LED and, not shown here, the tuning dial, which attaches to the shaft in the centre of the circle at right at right. Most major AM stations are shown on the dial and even some minor stations, along with frequency around the circumference. siliconchip.com.au Celebrating 30 Years November 2017  47 So it selects the intermediate (difference) frequency and filters out most of the original frequency as well as the oscillator signal and sum products. Its primary also forms the collector load for transistor Q1 and a 1.2MΩ parallel resistor sets its Q, determining its bandwidth. The output IF signal from the secondary is applied to the base of the first IF amplifier transistor, Q2. A 27kΩ resistor from the positive rail provides its base with a DC bias current. Its 1kΩ emitter resistor is bypassed with a 22nF capacitor to maximise the gain. 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, as stated above, the IF transformers filter out unwanted frequencies so that the transistors don’t waste power amplifying unwanted signals, which could potentially even cause them to saturate. 48 Silicon Chip They also improve selectivity, by limiting the bandwidth of the signal being amplified. 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. This optimises the available gain. 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 circuitous 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 (T3) and then via the secondary of oscillator transformer T2), before arriving at the collector of Q1. Now turn your attention to the second IF transformer, T4. Its primary is the collector load for Q2 while the output from its secondary is fed to the base of Q3. Besides a few details of the biasing Celebrating 30 Years of Q3, this amplification stage is essentially identical to Q2 and it provides more gain for the signal before it’s fed to the detector. Detector diode This role is performed by diode D1 but while the detector looks simple, there is more to it than first appears. The detector diode is driven by the secondary winding of the third IF transformer, T5. This diode performs two tasks. Firstly, it detects or demodulates the amplitude modulated IF signal to produce an audio signal and secondly, it produces the AGC voltage which is used to control the gain of the 1st IF amplifier, Q2. D1 is a Schottky diode, selected for its low forward voltage drop of about 0.3V. Germanium diodes, with a 0.2V forward voltage drop, have traditionally been used as detectors but they are starting to be hard to find. D1 rectifies the negative-going portion of the IF signal, resulting in a negative output voltage. Its anode is connected to a 22nF capacitor and provides the first stage of RF filtering, siliconchip.com.au Fig.1 : the Super-7 receiver uses seven commonly available transistors. The incoming RF signal is picked up by the ferrite rod antenna and fed to Q1 which functions as a self-oscillating mixer. The “difference” signal (between the oscillator and tuned input signal) is then coupled via T2 to the two IF amplifier stages and onto detector diode D1, to recover the audio signal and generate AGC, which is fed back to Q2. The audio signal is then fed, via volume control VR1, to the amplifier stage comprising Q4-Q7. and then via a 2.2kΩ resistor to a second 22nF capacitor for more filtering of the final audio signal before being applied to the 10kΩ volume control potentiometer, VR1. The demodulated signal is also coupled via a 3.3kΩ resistor to a 10µF filter capacitor, which forms a low-pass filter with a -3dB point of 5Hz. Thus, the audio portion of the signal is eliminated before being fed back to the base of Q2 via T3’s secondary. 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 base bias voltage of Q2. So Q2 will 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 secsiliconchip.com.au ondary of the 1st IF transformer (T3). Another thing to consider is that the current flowing through the 27kΩ and 3.3kΩ resistors will tend to forwardbias D1 slightly, offsetting its forward voltage and thus slightly increasing its sensitivity and reducing audio distortion. Having the bias current flow through the volume control pot is not ideal because pots with DC flowing through them will cause a little noise during rotation. Potentiometers become even noisier if DC current flows via the wiper but this does not happen in this circuit since we use a 10µF coupling capacitor. Audio amplifier The audio signal from the volume control is fed to a 4-transistor amplifier consisting of Q4, Q5, Q6 & Q7. This amplifier is directly 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. Celebrating 30 Years Q5 also forms a common-emitter stage and provides most of the voltage gain of the audio amplifier. Its collector current flows partly into the bases of the push-pull output transistors, Q6 and Q7, while the rest goes through the 1kΩ resistor and loudspeaker to ground. Output transistors Q6 and Q7 are connected as complementary emitter followers in class-AB mode. To explain class-AB, this is a variant of class-B operation. In class B, Q6 conducts for one half of the signal waveform, then turns off, and Q7 takes over for the second half of the signal waveform. This switching process inevitably causes crossover distortion which can make the sound quality quite unpleasant. Class-AB fixes this by making sure the transistors never fully turn off. So 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 (but not completely eliminate) crossover distortion. November 2017  49 The Superhet AM Radio Receiver The basic operation of a superheterodyne AM radio receiver (usually abbreviated to “superhet”) is shown in the block diagram below. There are many variations on this theme but all rely on the principle of heterodyning, or mixing, different frequencies. Heterodyning is applied in order to provide high gain, without instablility. The antenna is tuned by a variable capacitor in a parallel resonant circuit.This variable capacitor is one section of a “ganged” capacitor (ie, two sections on the one shaft or control). The other section of the ganged 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 selected while signals at other frequencies are rejected. The signal from the antenna is then fed to the mixer and this is where the “superheterodyne” process takes place, The word “heterodyne” refers to the “beating” effect generated by mixing two signals of different frequencies. “Hetero” is derived from the Greek word for “other” while “dyne” is derived from the French word for power. “Super” here refers to the fact that the second frequency is higher than the frequency of interest. In the Mixer stage, the Local Oscillator signal is mixed with that from the antenna. The result is a signal with components at four different frequencies: the two original frequencies (ie, the carrier and local oscilla- tor), plus the sum and difference frequencies. Assuming the carrier and local oscillator frequencies are close together, the sum will be at around twice the tuned frequency while the difference will be at a much lower frequency. This resulting signal is passed to an amplifier stage or stages tuned to the difference frequency, which results in the rejection of signals at the three other frequencies. The difference frequency is referred to as the Intermediate Frequency or IF. In most radios of this type, the Intermediate Frequency is 455kHz or 450kHz. The first superhets had an intermediate frequency of 50kHz which gave very sharp selectivity but poor audio response, because of the necessarily low bandwidth of the IF filters. Later, the standard IF was 175kHz and later still this was standardised at 455kHz. 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 low forward voltage drop. (We’ve used a “schottky” diode in the Super-7 circuit for the same reason – ie, low voltage drop). The diode rectifies the IF signal which is then filtered to remove RF carrier, leaving the audio signal. This is then fed to the audio amplifier, which drives a loudspeaker. Automatic gain control Apart from demodulating the IF signal, the detector is also used to produce the AGC voltage. AGC was regarded as a wonderful innovation when it was introduced as it eliminated the need to adjust the set’s gain each time you tuned into a new station. Gain adjustment is necessary to stop the IF stages from overloading on strong signals while still providing sufficient gain for very weak signals (eg, from distant or low-powered stations). To derive the AGC voltage, the raw DC output from the detector is heavily filtered to remove the audio signal, producing a DC voltage that is proportional to the amplitude of the IF signal. This is then used to control the gain of the IF stages and sometimes also the RF stage, so that the signal is held to a more or less constant level, ie, using negative feedback. So why “superheterodyne”, rather than “subheterodyne”, ie, with the local oscillator below the station frequency? After all, this would produce the same difference frequency. This was tried but it results in a lower sum frequency component which can be within the broadcast band, resulting in “ghost stations” (or “image frequencies”) on the dial, at higher frequencies than the actual station. This is pretty much totally eliminated in a superhet. Local oscillator The local oscillator frequency always tracks the tuned frequency of the RF amplifier. So for an IF of 455kHz, if the radio is tuned to 1370kHz, the local oscillator will be set to 1825kHz (1370 + 455). Similarly, if the radio is tuned to 702kHz, the local oscillator will be at 1157kHz (702 + 455). All this happens automatically by virtue of The general configuration for a superheterodyne radio receiver. The incoming RF signal is mixed with a local oscillator signal to produce an intermediate frequency (IF) signal, which is then fed to a detector stage to recover the original audio signal. 50 Silicon Chip Celebrating 30 Years siliconchip.com.au the 2-section capacitor tuning gang – one section is for tuning the antenna and the other for the local oscillator. These variable capacitors track each other over the adjustment range. Various tricks are used to create the necessary frequency offset while maintaining good tracking. 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 have a tuned RF Amplifier stage and some do not have a separate local oscillator. Instead, the local oscillator is combined with the mixer stage in what is known as a self-oscillating mixer or mixer/oscillator (as in the Super-7 circuit). Others may have two or three IF stages and some may have a separate detector to produce the AGC voltage. Another important variant is the double conversion configuration used in some high-performance communications receivers. This combines two superhet stages to shift the signal frequency in two “steps” and is usually used for receiving shortwave signals, as these are at much higher frequencies (up to 30MHz) than broadcast AM stations. The Super-7 circuit is a “single conversion” superhet, meaning that it performs just one conversion from the incoming RF frequency to the intermediate frequency. Other variations which are common include “permeability tuned” superhets and today’s frequency synthesised receivers with digital readouts and microprocessor control. Permeability tuning was common in car radios, where tuning was done by varying inductance rather than capacitance. One advantage of permeability tuning, especially useful in cars, is reduced susceptibility 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 above. By the way, Edwin Armstrong, who invented the AM superhet receiver was the same person who later developed the principles of FM transmission and reception. One further note before we leave the origins of the superhet: apparently, radio (or “wireless”) circuits working along the same principle were used in British submarines during the First World War. siliconchip.com.au Negative feedback from the output of the amplifier is provided 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 47µF capacitor sets the bass roll off of the amplifier. Amplifier output with no signal sits at about half supply, ie, around 4.5V. This DC offset is removed by using a 470µF coupling capacitor between the amplifier output and the loudspeaker. The capacitor allows the AC signal to pass to the loudspeaker but blocks the DC voltage. The DC needs to be blocked to prevent the loudspeaker cone being forced away from its normal resting position and increasing distortion. By now, you’ve probably realised that this design aims to achieve good performance without using too many components, similar in concept to a portable AM radio. For example, the output stage component count has been minimised by connecting the 1kΩ resistor to 0V via the speaker coil. The same DC bias conditions could have been obtained in the output stage by simply connecting the 1kΩresistor directly to the 0V line but there is a 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. This means it is able to provide more drive to the output stage and higher overall voltage gain. This technique is known as “bootstrapping” 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 Celebrating 30 Years 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 draws no current at all. This may not seem important because the speaker will normally always be connected. But if you try to monitor the amplifier without the speaker connected or plug in a bare jack socket into CON2, 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 that connects across the 9V supply. This relatively large capacitor may seem unnecessary. But since the circuit can be powered from a 9V battery as well as a DC plugpack, it is a requirement. That’s because as the battery ages, its internal impedance rises and so it is less able to deliver the relatively high current pulses demanded by the amplifier and the result is more distortion from the amplifier. 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. Note that the speaker signal goes via the integral switch in headphone socket CON2, so that if headphones/an earphone is plugged in, the speaker is automatically disconnected. Note also that the tip and ring connections are wired in parallel, so you will get audio from both sides of stereo headphones/earbuds, even though the AM radio output is mono. Finally, indicator LED1 shows when the circuit is switched on, via power switch S1 and reverse battery protection diode D3. D3, is another schottky diode, which means that its very low forward voltage will result in minimum loss from the battery, while still protecting against accidental polarity reversal. While you can’t permanently fit a 9V battery in the holder the wrong way around, you can certainly make accidental contact the wrong way around. In the next article, we will show you how to assemble your Super-7 AM Radio, including its custom-made case and hand-span dial. We will also describe the alignment procedure. November 2017  51 What is “AM” radio? When radio stations first began broadcasting in Australia (and for many decades after), they all used the amplitude modulation (AM) system, predominantly using the “broadcast band” which covers 531kHz to 1.602MHz. The other transmitting system, FM, or frequency modulation only commenced in Australia in the 1970s and uses a higher frequency band, from roughly 88 to 108MHz. And more recently, the digital system, DAB+, transmitting on a range of frequencies around 200MHz, has started mainly in capital cities. Apart from the difference in frequencies, trying to listen to AM with an FM receiver (or vice versa) will not be successful. The same applies to DAB+ on any other receiver. AM transmission AM is relatively simple: it involves transmitting a signal with a fixed frequency (known as the radio frequency [RF] carrier) but its amplitude (power) is modulated, or varied, by the voltage level of an audio signal such as from a microphone or music being played. The receiver is tuned to the carrier frequency and once it picks it up, it’s “demodulated” to produce a voltage that’s proportional to the signal amplitude. The resulting signal is then amplified and fed to the radio’s loudspeaker. The “state of the art” analog approach for receiving an AM signal is superheterodyne (or “superhet”) principle, invented by Edwin Armstrong in 1918. The first commercial AM superheterodyne 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 worldwide. Prior to the superheterodyne, radios were either crystal sets or used the tuned radio frequency (TRF) principle, of which there are a number of variations. In a TRF receiver, all amplification up to the detector (demodulator) takes place at the frequency of the incoming signal. The superheterodyne radio brought with it two major advantages over previous circuits. The first was greatly increased gain. This was a big boost compared to TRF tuners which were strictly limited as far as maximum gain was concerned when using valves (or “vacuum tubes”). Any attempt to increase the gain over this limit would cause the circuit to oscillate, resulting in a loud squeal. 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 that would otherwise tend to blitz half or more of the tuning dial. Finally, the superheterodyne receiver brought with it the possibility of automatic volume control (AVC), also known as automatic gain control (AGC), although this did not become a feature until around 1930. AGC did away with the need for manual gain controls and meant that all stations came in with roughly the same loudness, in spite of the fact that some stations may be very strong and some very weak. Since the advent of the superhet, there have been relatively few changes to the basic circuit configuration until the advent of software-defined radios (SDRs), although the components used have changed radically over time. Originally, valves ruled but now transistors are used or even a single integrated circuit with just a few external components. So if you decide to build this AM superhet receiver, you will be building a circuit configuration which has been around for over 90 years but one which is still just as relevant today. 52 Silicon Chip Parts list – Super-7 AM Radio Receiver 1 double-sided PCB coded 06111171, 313 x 142.5mm 1 set of laser-cut acrylic case and dial pieces (SILICON CHIP Online Shop Cat SC4464) 1 AM radio coil pack (Jaycar LF-1050) (T2-T5) 1 mini tuning gang capacitor (Jaycar RV-5728) (VC1-VC4) 1 ferrite rod with coil (Jaycar LF-1020) (T1) 1 100mm (4-inch) 4- or 8-ohm loudspeaker (Jaycar AS3008) 1 DPDT push-on/push-off switch (Altronics S 1510) (S1) 1 round knob for switch S1 (Altronics H 6651) 1 16mm 10kΩ logarithmic taper potentiometer with 6.35mm D-shaft (Jaycar RP7610, Altronics R2253) (VR1) 1 knob to suit VR1 1 2.1 or 2.5mm inner diameter DC socket (Altronics P 0621A, P 0620, Jaycar PS-0519, PS-0520) (CON1) 1 6.35mm stereo switched jack socket (Altronics P 0073, Jaycar PS-0190) (CON2) 1 9V DC 250mA (or higher current) plugpack and/or 9V battery 1 9V PCB battery holder (Altronics S 5048, Jaycar PH-9235) 12 PC stakes 8 M3 tapped 25mm spacers 8 M3 flat washers 8 M3 x 10mm machine screws 4 M3 x 15mm Nylon or Polycarbonate machine screws 4 100mm cable ties 3 No.4 x 6mm self-tapping screws 4 M3 x 15mm machine screws and nuts (for mounting speaker) 1 150mm length of medium-duty hookup wire Optional knob to suit the dial (Jaycar HK7010/HK7011) Semiconductors 4 BC547 NPN transistors (Q1-Q4) 1 BC327 PNP transistor (Q5) 1 BD139 NPN transistor (Q6) 1 BD140 PNP transistor (Q7) 1 BAT46 schottky diode (D1) 1 1N4148 diode (D2) 1 1N5819 schottky diode (D3) 1 3mm high brightness blue LED (LED1) Capacitors 2 470µF 16V PC electrolytic 1 47µF 16V PC electrolytic 4 10µF 16V PC electrolytic 3 100nF ceramic 5 22nF MKT polyester 1 10nF MKT polyester 1 4.7nF MKT polyester Resistors (0.25W, 1% [^5% carbon OK]) 1 1.2MΩ^ 1 1MΩ 1 820kΩ 1 56kΩ 1 47kΩ 1 39kΩ 1 27kΩ 1 22kΩ 1 12kΩ 1 10kΩ 1 4.7kΩ 2 3.3kΩ 1 2.2kΩ 2 1kΩ 1 470Ω 2 100Ω 1 200Ω miniature horizontal trimpot (VR2) Celebrating 30 Years SC siliconchip.com.au