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Image source: https://unsplash.com/photos/aerial-photography-of-flowers-at-daytime-TRhGEGdw-YY Earrth Ra Ea Rad dio John Clarke’s Parrt 1 : w� Pa w�ispe isperrs of of the sk sky With this ‘natural radio receiver’, you can listen to solar and atmospheric disturbances, like storms or auroras. These create electromagnetic waves in the VLF (very low frequency) and ELF (extra low frequency) range. I ntriguing natural sounds such as whistlers, tweeks and the chorus can be heard using this simple receiver. Naturally produced electromagnetic waves are abundant throughout the world, and are there for the listening with the right equipment. Not only can you hear the sounds that are created in your local region, but even from other parts of the world! These low-­frequency electromagnetic waves, often created in the Earth’s atmosphere, are guided by the ionosphere that encircles the globe. The waves are reflected or refracted off the ionosphere, and can travel halfway around the world, all just waiting to be received and listened to. Our portable Earth Radio is powered by an internal or external battery and can receive most of the VLF and ELF frequencies covering the 3Hz to 30kHz bands. These are at the lower part of the electromagnetic spectrum, as shown in Fig.1. We only show frequencies on the spectrum down to 1Hz, although electromagnetic waves can be even lower in frequency than that. The electromagnetic spectrum covers a huge range of frequencies, including much higher frequencies such as broadcast radio waves (~1100MHz), microwaves (300MHz+), infrared (300GHz+) and visible light (400-790THz). The higher-frequency waves have more energy, which is why UV, X-rays and gamma rays can cause skin and cell damage. To pick up the naturally produced VLF and ELF signals, we use a loop antenna. Its advantage is that it is small enough to be portable despite the low wavelengths involved. Outputs on the Fig.1: electromagnetic waves span a huge range of frequencies, from 1Hz and below (ELF) up to 1026Hz (gamma rays). The range we’re interested in here is from 20Hz to 22kHz, within the ELF and VLF bands. 60 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.2: there are several ionised layers in the ionosphere, and they change between day and night. Without particles from the sun to keep them ionised, some layers disappear at night, while others move and/or become thinner. receiver include one for recording, with another suitable for listening via headphones or earphones. The Earth Radio receives signals due to atmospheric phenomena and also from the sun. The most significant atmospheric source is lightning. Lightning generates a wide range of electromagnetic waves, including light, radio waves, X-rays and gamma rays. The radio waves they produce extend into the VLF frequency range (3-30kHz) and some in the ELF range (3Hz-3kHz). Another source is geomagnetic storms. These are disturbances created by the coupling of the Earth’s magnetic field with the solar wind. The solar wind is produced by solar flares and other sun irregularities, such as coronal mass ejections. They can cause changes in the Earth’s magnetosphere (magnetic field), and as a result, induce currents into a receiving antenna. Apart from electromagnetic wave production, these events can also result in auroral (visible) displays. They are known as the Aurora Borealis (Northern Lights) in the northern hemisphere and Aurora Australis (Southern Lights) in the southern hemisphere. siliconchip.com.au Earth Radio Kit (SC7582, $55) this kit includes all non-optional parts, except for the case, battery, timber and cable/wire. Practical uses of the ELF and VLF electromagnetic bands are transmitters for communications with submarines and around the world (as described in our article on Underwater Communication in March 2023 – siliconchip. au/Article/15691). Other man-made sources of these low-frequency radio waves aren’t necessarily wanted. These include 50/60Hz mains hum and somewhat higher-frequency signals produced by electronic equipment like computers and electric motors. One of the main reasons that the signals from lightning and solar activity can be interesting is due to the ionosphere that surrounds the Earth. This layer of the atmosphere provides a waveguide for the VLF and ELF waves to travel within. They effectively bounce off the ionosphere and the Earth as they travel around the globe. For example, signals produced by lightning start out as a static noise. But by the time they are received, the waveforms may have morphed into different sounds such as tweeks, whistlers and the chorus. The original static sound is altered by the pathways they take before being received. Australia's electronics magazine Changes in the ionosphere between night and day, and the strengthening of ionospheric regions due to solar flares and coronal mass ejection events, also contribute to variations in the sounds received. So the ionosphere has a large impact on the sounds produced. The ionosphere The ionosphere (Fig.2) is the region of the atmosphere where the gases are ionised (split up into positively and negatively charged particles) by solar and cosmic radiation. It ranges from 70-1000km above the Earth’s surface, and is generally considered as being made up of three regions: D, E, and F. The F region splits into two layers (F1 and F2) during the daylight hours, but merges into a single layer during the night. Ionisation is strongest in the upper F region, and weakest in the lower D region; the latter basically exists only during daylight hours. During daylight hours, VLF and ELF signals generally pass through the D region and are refracted by (or reflect off) the E region, leading to a weakened signal. The D region is stronger during a solar flare event and acts as December 2025  61 a waveguide for VLF and ELF signals. This is because the wavelength of these signals is a significant fraction of the height of the D region. For example, a 20kHz electromagnetic wave has a wavelength of 15km, while a 2kHz wave has a 150km wavelength. With a strong D region in the ionosphere, these signals refract off it, and less loss is experienced, since they no longer pass through the D region to refract in the E region. This generally leads to a sudden increase in received signal, called ‘sudden ionospheric disturbance’ (SID). Some ELF and VLF signals manage to exit the ionosphere, where they will follow the magnetic field lines of the magnetosphere. They can reach 10,000km or more above the Earth before re-entering at a different location. There are several types of emissions possible, which are characterised as static, tweeks, whistlers, the chorus and hiss. Spectrograms Signals from our Earth Radio or from recorded sources can be visualised with spectrograph software. Three examples of audio files and spectrograms are shown in Figs.3, 4 & 5. Figs.3 & 4 are spectrograms of the audio files at www.spaceweather.com/glossary/ inspire.html, while Fig.5 was captured using our Earth Radio. Fig.5 consists mainly of close-by lightning statics. The horizontal red line at just under 20kHz is from the VLF radio station in Exmouth, Western Australia, under the call sign of NWC. It transmits on a 19.8kHz carrier. The expanded view of the NWC radio station signal in Fig.5(a) shows how the encoding uses a variation of frequency-shift keying (FSK) modulation called minimum-shift keying. Here, there is a 50Hz change between a ‘0’ and a ‘1’. Static Lightning strike statics (sometimes called sferics, short for “atmospherics”) are the signals from lightning that most people will be familiar with. This is the sound you will hear on an AM radio during an electrical storm – constant crackling and popping. Static signals are from nearby lightning strikes, within about 1,000km. They are seen on a spectrogram (with frequency on the vertical axis and time on the horizontal axis) as vertical lines. This indicates that all frequency components in the signal arrive at the same time. Tweeks Tweeks are lightning-caused electromagnetic emissions that have travelled around 2000km or more within the waveguide between the Earth and the ionosphere. The ionosphere varies in its properties throughout its thickness, The Earth Radio can run off an internal 9V battery or external 12V source. 62 Silicon Chip Australia's electronics magazine so higher-frequency components travel faster than others and thus will be received sooner than the others. Fig.3 shows a spectrogram of both static and tweeks. The tweeks are characterised by frequencies around 2kHz being delayed compared to other frequencies. These sounds are reminiscent of the Australian bell miner bird call. This spectrogram was produced using the Raven Lite 2 spectrograph software. The top waveform is the approximately 14-second-long audio signal. The lower spectrogram shows how its frequency components change over time. The tweeks have a vertical line at high frequencies, but at lower frequencies, they curve off to the right a little at around 2kHz. This indicates that the lower frequencies arrive later at the receiver compared to other frequencies. This results in a somewhat musical twang quality to the sound. Whistlers Like tweeks, whistlers have a musical quality. This is due to the longer propagation delay of their lower-­ frequency components compared to the higher frequencies. This leads to different frequency components of the signal becoming offset in time. It is the interaction of the signal with both the ionosphere and magnetosphere that causes the longer time delay for the different frequency components. Whistler signals travel along the magnetic field lines of the Earth, and can go to the opposite side of the Earth before returning. Since the path along magnetic field lines is very long (as much as three Earth diameters), the time delay differences are large, and the signal begins as a high-pitched tone, reducing to a lower pitch over time. It is likened to that of a falling bomb. Each whistler sound can last for as long as a couple of seconds. They are seen as long descending arcs on a spectrogram in the Fig.4 spectrogram. Again made using Raven Lite 2, the waveform shows whistlers intermixed with static. Each static signal is visible as a vertical line covering most of the frequency spectrum. Whistlers have a more musical quality than the tweeks in Fig.2 due to the propagation differences of more frequency components of the signal. siliconchip.com.au Different frequency components of the signal become offset over time. Note how the lower the frequency, the longer the time delay. There is about two seconds between the arrival of frequencies above 10kHz before the 1kHz component of that signal is received! Chorus Two types of ‘choruses’ can occasionally be heard: the dawn chorus and the auroral chorus. The dawn chorus is best listened to at sunrise, and can resemble crickets or a chorus of birds, or it may sound like dogs barking or squawks from flocks of birds. It comprises a range of overlapping sounds. The signals on a spectrogram show quick-rising arcs of less than a second each in duration. Its presence is dependent upon geomagnetic activity, such as the emission of a solar flare from the sun. The auroral chorus is generated within the aurora, and can be heard in areas close to where the aurora occurs. It is strongest during periods of high geomagnetic activity. A recording of a “VLF auroral chorus” (plus other natural radio sounds) is at www.youtube. com/MindOverMatter55/videos Fig.3: a spectrogram of an atmospheric recording that includes ‘tweeks’, sounds produced by the static from lightning crashes travelling around the globe, with the higher-frequency components travelling faster and thus arriving earlier. Source: www.spaceweather.com/glossary/inspire.html Hiss These sounds are typically emitted via the aurora and are high-pitched sounding. Hiss can also originate in the magnetosphere. When to listen Natural Radio signals can be heard at any time but are most prevalent before dawn. Tweeks are most common at night, and the chorus can be heard within several hours of sunrise. The results are usually better when there is strong geomagnetic activity. Sferics can be heard constantly at any time. If you want to find out the best times to be listening, there are websites such as www.abelian.org/vlf/index. php?page=live where natural radio events and solar and sunspot activity are logged. For events that occur within Australia, there is a page at www.facebook. com/groups/1953353338413426/ plus alerts and warnings for solar and geomagnetic activity and auroras on the Bureau of Meteorology website (Australia; www.sws.bom.gov.au/Space_ Weather). siliconchip.com.au Fig.4: this shows whistlers, which are similar to tweeks, but they have travelled further. As a result, their component frequencies are more spread out over time. Fig.5: some lightning statics captured with our Earth Radio prototype, along with the 19.8kHz signal from NWC in Exmouth, WA. The zoomed-in view shows its FSK/MSK modulation scheme, with the carrier varying by 50Hz to indicate different bits. Fig.6: this block diagram shows the stages of the radio. Current from the loop antenna is converted directly into a voltage signal, then higher frequencies and 50/60Hz hum are removed. It is further amplified and can be listened to using headphones/earphones or recorded with an audio recording device. Other countries should have similar indicators of space weather conditions. Natural radio receivers There are several well-documented receivers that are designed to receive the VLF and ELF electromagnetic bands. These include the interactive NASA space physics ionosphere radio experiments (Inspire) VLF receiver (https://theinspireproject.org). This is available as a kit of parts and is meant to ‘inspire’ school and university students to take an interest in and study science, technology, engineering and mathematics (STEM) subjects. The American Radio Relay League (ARRL) magazine QEX also has several articles and designs on VLF receivers; the January/February and the March/ April publications in 2010 are of interest. The first is entitled Radio Astronomy Projects by Jon Wallace and the second Amateur Radio Astronomy Projects; A Whistler Radio by Jon Wallace. These two publications are available at www.qsl.net/w/wb4kdi/AROL/ ARRL/QEX/ There is also Renato Romero’s home page at www.vlf.it with articles and designs exploring the ELF and VLF radio bands. Everyday Practical Electronics magazine (EPE) in the UK had an Atmospherics monitor that was a receiver for the VLF band in their April 2003 issue (www.pemag.au/ projects-legacy.html). One of the difficulties with VLF and ELF receivers is that the wavelengths are so long (15,000km for 20Hz and 15km for 20kHz). This makes using a ¼-wave whip antenna impractical; any antenna of any practical length will be so much shorter than this that it will only provide a low signal level. An alternative to a ¼-wave whip is the Marconi “T” antenna. One suitable for ELF to VLF waves comprises an 11m high antenna with a 45m beam at the top. However, this is still very large and is certainly not portable. See siliconchip.au/link/ac8q for more information on these antennas. We envisaged using a loop antenna that gave comparable reception to a Marconi ‘T’ antenna but without the huge size. It is based upon “AN EASY VLF LOOP, 200Hz-20kHz reception without transformers” by R. Romero & M. Bruno (see www.vlf.it/easyloop/_ easyloop.htm). Another advantage of a loop antenna is that it is directional, so interference noise, especially mains-derived noise, can sometimes be nulled out to a large extent. Radio design Natural radio VLF and ELF band receivers are rather unique because most of the frequency range of the VLF to ELF bands is within the audio frequency range of 20Hz-20kHz. This allows the signals to be directly heard by simply converting the electromagnetic waves to sound using headphones or earphones after the signal from the antenna has been amplified sufficiently. Radio receivers designed for the AM broadcast band (530-1700kHz) and higher frequencies are well above the audio band. In these cases, audio signals are used to modulate the high-­ frequency carrier. Modulation varies the carrier level for AM (amplitude modulation) or the frequency for FM (frequency modulation). The receiver demodulates the received signal to recover the audio. Receiver block diagram The loop antenna is held on a timber frame and is intended to be portable so you can find an ideal place to use it. Fig.6 shows the block diagram of our Earth Radio. The loop antenna receives Australia's electronics magazine siliconchip.com.au 64 Silicon Chip Parts List – Earth Radio Fig.7 shows the full circuit. It uses seven op amps in three packages (one single, one double and one quadruple). Some provide amplification, some active filtering, with another to drive headphones or an earphone via current-­boosting transistors. The receiver is powered either by a 9V battery or an external 12V DC supply. When a DC plug is inserted into barrel socket CON4, its internal switch disconnects the 9V battery’s negative terminal from circuit ground. Without the plug inserted, the 9V battery is 1 double-sided, plated-through PCB coded 06110251, 97 × 70mm 1 105 × 80 × 40mm Hammond RM2005LTBK, Multicomp MP004809 or RS Pro ABS translucent enclosure [RS 198-1379, Mouser 546-RM2005LTBK] 1 9V battery with matching snap (BAT1) 1 9V battery holder clip [Altronics S5050] 3 3.5mm stereo PCB mount jack sockets [Altronics P0092, Jaycar PS0133] (CON1-CON3) 1 2.1mm or 2.5mm inner diameter PCB-mount DC power socket (CON4) 2 SPDT right-angle PCB-mount sub-miniature toggle switches (S1, S2) [Altronics S1421] 2 8-pin DIL IC sockets 1 14-pin DIL IC socket 1 M3 × 5mm panhead or countersunk screw and nut 1 100mm-long cable tie 2 1mm PCB pins (optional) Potentiometers 3 100kW top-adjust 3296W style trimpot (VR1, VR2, VR8) 2 50kW top-adjust 3296W style trimpot (VR3, VR5) 1 2kW top-adjust 3296W style trimpot (VR4) 1 10kW top-adjust 3296W style trimpot (VR6) 1 10kW logarithmic taper 18-tooth spline 10mm horizontal PCB-mounting potentiometer (VR7) [Altronics R1935, Jaycar RP8756] 1 13mm knob for VR7 Semiconductors 1 OP07CP low-noise precision op amp, DIP-8 (IC1) [Jaycar ZL3974] 1 TL074 quad JFET-input op amp, DIP-14 (IC2) 1 TL072 dual JFET-input op amp, DIP-8 (IC3) 1 BC337 45V 0.8A NPN transistor, TO-92 (Q1) 1 BC327 45V 0.8A PNP transistor, TO-92 (Q2) 2 1N4148 75V 200mA silicon diodes, DO-35 (D1, D2) 1 1N5819 30V 1A schottky diode, DO-41 (D3) 1 3mm LED (LED1) Capacitors 4 470μF 16V PC electrolytic 3 100μF 16V PC electrolytic 1 22μF 16V PC electrolytic 1 2.2μF 16V PC electrolytic 1 1μF 16V PC electrolytic 3 100nF 63/100V MKT polyester 4 47nF 63/100V MKT polyester (with closely matched values; see text) 1 33nF 63/100V MKT polyester 2 1.5nF 63/100V MKT polyester 1 1nF 63/100V MKT polyester 1 15pF NP0/C0G ceramic Resistors (all ¼W ±1% axial unless noted) 1 100kW 2 30kW 2 4.7kW 2 1kW 1 150W 1 82kW 7 10kW 1 1.5kW 1 620W 2 1W ½W (±5% OK) Parts for loop antenna 4 20 × 12 × 2400mm dressed all-round timber batten (hardwood for outdoor use) 1 8mm diameter, 1.2m-long timber dowel 1 3.5mm stereo jack line plug 1 3m length of twin-core shielded audio cable 3 0.63mm-diameter, 36m-long enamelled copper wire spools (105m total length) [Altronics W0406, Jaycar WW4018] OR 2 0.5mm-diameter, 57m-long enamelled copper wire spools (114m total length) • [Altronics W0405, Jaycar WW4016] 1 50mm length of 1mm diameter heatshrink tubing 1 wire clamp or cable tie (see text) • will likely result in reduced performance compared to 0.63mm-diameter wire siliconchip.com.au Australia's electronics magazine low-frequency electromagnetic waves, and IC1 converts the current from the antenna to a voltage. The signal is amplified using a low-noise operational amplifier chip (op amp). It also includes signal roll-off above 22kHz. This removes signals that could otherwise overload the receiver. The signal is then sent to a low-pass filter to further remove signals above 22kHz (as the first filter is not perfect) before passing through a notch filter. This notch filter removes the mains frequency and prevents mains hum from dominating the received signal. The notch can be set to attenuate either 50Hz or 60Hz, depending on the local mains frequency. Switch S2 selects the signal from before or after the notch filter. You can also minimise mains hum by adjusting the orientation of the loop antenna. This may not be successful in built-up areas, where there are sources of mains radiation in almost every direction. A further gain stage based on IC3 provides more gain, up to 51 times, set using VR5. The recording output signal level can be adjusted with VR6, while the volume to the headphone amplifier is adjusted with VR7. Circuit details December 2025  65 connected to and powers the circuit. Diode D3 prevents reverse polarity connection current flow, while power is switched via toggle switch S2. LED1 lights via the 1.5kW series resistor when power is on. A half-supply rail (around 4.5V with a 9V battery) is used to bias signals so that they can swing symmetrically about this reference before clipping to the supply rails. This half-supply rail is derived by two 10kW resistors across the main supply, with the junction decoupled to ground with a 100μF capacitor. Op amp IC2d buffers this half-­ supply voltage, and this buffered reference (+4.5Vb) goes to non-inverting pin 3 of op amp IC1 via a 4.7kW resistor. IC1 is a low-noise op amp that is used to convert the alternating current in the loop antenna to a voltage. The loop antenna is AC-coupled by a 470μF capacitor at one end to the non-inverting input of IC1, with the other end connecting directly to the inverting input (pin 2). The signal level is set by the 4.7kW feedback resistance connected between the op amp output at pin 6 to the inverting input (pin 2), in conjunction with the current generated in the loop. The op amp output level adjusts so that the inverting input voltage matches the non-inverting input. The op amp input offset voltage and input offset current will affect how close they are. Both are low due to IC1 being a precision op amp. A 1nF capacitor between pins 2 and 3 of IC1 shunts high-frequency noise, while the 1.5nF capacitor across the 4.7kW feedback resistor provides a high-frequency roll off at 22.6kHz. A roll-off is also inherent in the op amp itself, as it has limited bandwidth beyond audio frequencies, so is not capable of providing a signal output at AM broadcast frequencies. This also applies, to a lesser extent, to other op amps used in the circuit. The OP07 has a typical noise specification of 9.6nV/√Hz. The noise current specification for this op amp is also very low, typically below 1.7pA/√Hz. For this design, having a low noise current is important since we are amplifying the loop antenna current rather than the voltage, and we don’t want noise to swamp the signal. The 470μF capacitor that AC-­ couples the antenna loop to IC1 prevents large DC shifts in the op amp output when the loop antenna is moved or when the loop wires move in wind. These small antenna movements can otherwise generate very low-frequency signal due to movement within the Earth’s magnetic field; enough to result in signal clipping unless prevented by the low-frequency roll-off of the capacitor. The output of IC1 is fed to IC2c, which provides the active part of the 22kHz low-pass filter. It attenuates signals above 22kHz so that higher-­ frequency signals do not swamp the wanted VLF and ELF waves. The filter is a third-order multiple-feedback arrangement. The filter components were chosen to produce a steep roll-off above 22kHz, but this is at the sacrifice of having a small amount of ripple in the passband, below 22kHz. This is called a Chebyshev filter. The ripple in the design is minimal, though, at a maximum of just ±2dB. For our design, we obtain an overall 85dB per decade roll-off due from IC1 and the 22kHz low-pass filter combination. The third-order filter itself provides a much steeper roll-off compared to a first- or second-order filter. Designing these filters is made easier using the filter design tools from siliconchip.au/link/ac8r Following the low-pass filter is the active Twin-T filter used to notch out and severely attenuate the mains frequency, based on IC2a & IC2b. This can be tuned to 50Hz or 60Hz to match Fig.7: the full circuit of the Earth Radio, which is laid out similarly to the block diagram (Fig.6). IC1 is a low-noise precision op amp, IC2 is a quad JFET-input general-purpose op amp and IC3 is a similar dual op amp. 66 Silicon Chip Australia's electronics magazine siliconchip.com.au the mains frequency in your location. The twin-T comprises two T sections, with one half being VR1, VR2 and the two parallel 47nF capacitors. The other half consists of the two series-connected 47nF capacitors and VR3. The notch frequency in Hz for the first tee is 1 ÷ (π × [VR1 + VR2] × 47nF). VR1 and VR2 are set to the same value: 68.1kW for a 50Hz notch, or 56.2kW for a 60Hz notch. For the second tee, the frequency is 1 ÷ (4π × VR3 × 47nF), so VR3 is set to 34kW for the 50Hz notch, or 28.1kW for 60Hz. Typical 47nF capacitors are rated at ±5% or even ±10%, so unless you buy special ±1% (or better) 47nF capacitors, they need to be chosen so that the values are all within 1% of each other. More on selecting them later. If the average is above or below 47nF, VR1, VR2 and VR3 can be adjusted to set the notch to the correct frequency. The main thing is that they are all close in value. You can find the Twin-T filter calculations are at siliconchip.au/link/ac8s The depth of the notch filter is adjustable using VR4. It can set the notch so that the rejection level is deep, with VR4 clockwise for a value of 220W. It is less deep with VR4 adjusted fully anti-clockwise (2kW). The PCB is relatively compact but still easy to assemble. We provide this adjustment since it is easier to adjust the notch frequency when the notch is not too deep. Once the frequency is set, increasing the notch depth will reduce the 50Hz or 60Hz hum further. This will also narrow the notch so that frequencies on either side of the notch frequency will be less affected by attenuation. Switch S1 switches the notch in or out. In the ‘out’ position, S1 selects the signal directly from the output of the 22kHz filter at pin 8 of IC2c. In the ‘in’ position, the signal is selected from the notch filter output at the output of IC2a. The frequency response is shown in Fig.8, giving a high-frequency rolloff at around 85dB/decade, with the 50Hz notch around 80dB down. More gain IC3a provides more signal gain, which is set using VR5. The maximum gain of 51 times is with VR5 set at its maximum resistance. The amplifier’s input signal is already biased at half supply by its source; the 22μF capacitor coupling the 1kW resistor from pin 2 to ground charges to the average DC voltage of pin 3 and adds a low-­ frequency roll-off at 7.2Hz. The output from IC3a is also AC-coupled to prevent DC current flowing in VR6 and VR7. This prevents DC voltage shifts when making adjustments with these potentiometers. VR6 sets the recording level at the 3.5mm output jack socket, CON2. VR7 sets the volume of the signal level Earth Radio Kit (SC7582, $55): includes the PCB and everything that mounts on it, plus the antenna jack plug. The case, battery, power supply and antenna parts are not included. siliconchip.com.au Australia's electronics magazine December 2025  67 applied to headphone amplifier IC3b. The wiper of VR7 is AC-coupled to IC3b’s non-inverting input and biased to half supply via a 100kW resistor connecting to IC2d’s output. The bias voltage from IC2d is additionally low-pass filtered by a 1kW resistor and 100μF capacitor to prevent oscillation of the overall circuit. This can occur when the half-supply reference for the circuit varies with the signal level. Without the filtering, the half supply at IC1’s pin 3 input from the IC2d output could be modulated by the signal at VR7’s wiper, causing feedback and oscillation. IC3b works in conjunction with buffer transistors Q1 and Q2 to drive headphones or earphones. The output of IC3b drives Q2 directly, in emitter-­ follower mode, with Q1’s base set at approximately two diode voltage drops higher. Diodes D1 and D2 ensure that both transistors are conducting some current even with no signal by applying sufficient bias voltage to their base-emitter junctions. There is always a small voltage across the 1W resistors, which minimises crossover distortion during the period when output drive current hands over from one transistor to the next, as the signal passes the half-rail voltage. Feedback to the inverting input of IC3b also minimises distortion by correcting the signal output to match that of the signal applied to the non-inverting input. The presence of the 1W resistors also stabilises the quiescent current via local negative feedback. Current through the bias diodes is set by 10kW resistors in series with them from both supply rails. Trimpot VR8 is included to reduce this bias current should the total diode voltage be significantly higher than the sum of the transistor base-to-emitter voltages. This could otherwise cause high quiescent current and transistor overheating. VR8 is normally set to its maximum unless the quiescent current of Q1 & Q2 is too high. Adjusting VR8 for a lower resistance will bypass some of the diode current and reduce the resulting forward voltages. This can be used to account for differences between different batches of diodes and transistors. The output from the headphone amplifier, at the junction of the two 1W resistors, is AC-coupled to headphone socket CON3 using a 470μF capacitor to remove the half-supply voltage, preventing a direct current flow through the headphones. Another 470μF capacitor bypasses the power supply. Several 100nF capacitors and a 470μF capacitor also bypass the supply for the op amps throughout the circuit for stability, providing a low supply source impedance for each device. Selecting the 47nF capacitors As previously mentioned, the 47nF capacitors for the Twin-T filter need to be selected so their values are within 1% of each other. Typically, if you buy 5% plastic film capacitors on a bandolier (cardboard tape/belt), the adjacent components will have similar values. We found that four capacitors of the same marked value in a row weren’t within ±1% of the actual 47nF rating, but whatever value we measured for the first one, the other three would all measure within 1% of that. You may need to get more than four capacitors so that at least four will be of a similar value. That’s still a lot cheaper than purchasing 1% capacitors. If you have a capacitance meter, the values can be measured directly. Alternatively, if you have an oscilloscope or frequency meter, the capacitors can be tested using a standard astable oscillator made with a 555 or 7555 timer. The frequency of oscillation is related to the capacitance. Fig.9 shows the circuitry required. Using 10kW for RA and RB, the frequency of oscillation would be around 1023Hz (ie, just over 1kHz) for a 47nF capacitor. Note that the oscillator frequency doesn’t accurately tell us the capacitance value. However, if you select capacitors that give the same frequency to within 1%, the capacitor values will be within 1%. This means you need a spread of less than 10Hz for the configuration shown. The easiest method is to measure the frequency of all four capacitors and then subtract the lowest reading from the highest. If the number you get is no more than 10, you’ve found a set of capacitors that’s close enough. Otherwise, measure a fifth and then remove whichever value is the furthest from the others and repeat until you get a spread of no more than 10Hz. Next month The follow-up article next month will start with the PCB assembly instructions for the Earth Radio. After that, we’ll describe how to build the loop antenna, then testing the Earth Radio, followed by some advice on SC getting the best out of it. Fig.8: the frequency response of the Earth Radio, as determined by simulation. You can see the 50Hz notch and the high-frequency roll-off. Fig.9: if you don’t have an accurate capacitance meter, this simple circuit can be used to check how close a set of capacitors are in value, using a frequency meter. Australia's electronics magazine siliconchip.com.au 68 Silicon Chip