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Teach-In 2026 by Mike Tooley World of Wireless – An Introduction to Radio and Wireless Technology Series 12, part 3: transmitters and receivers I n the previous instalment, we presented an overview of RF circuits and their associated components. The discussion began with a simple VHF signal source, followed by an analysis of frequency-selective resonant circuits and RF filters. We then examined the application of quartz resonators as replacements for conventional L-C tuned circuits, highlighting their effectiveness in constructing filters with superior selectivity. Additionally, we provided a list of representative semiconductors suited for RF applications, several of which will feature in forthcoming practical exercises throughout this series. Our companion Hands-On project showcased an enhanced portable medium wave AM receiver. This month, our focus shifts to fundamental radio communication systems. We discuss the necessity for modulation and demodulation, and introduce the primary methods—AM and FM—following an initial overview of Morse code as a basic communication technique. TRANSMITTER Oscillator Our Hands-On projects involve building a crystal-controlled reference oscillator and a separate variable-frequency oscillator (VFO). These two handy modules will be used in conjunction with several of our subsequent Hands-On projects. A simple radio communication system Let’s start with a very simple radio communication system comprising a transmitter (TX) and receiver (RX) in which communication is achieved by simply switching (or ‘keying’) the radio frequency signal on and off (see Fig.3.1). Keying can be achieved by interrupting the supply to the power amplifier stage, or even the oscillator stage. However, keying the oscillator stage usually results in poor frequency stability, so it is normally applied within a driver stage operating at a modest power level. The receiver consists of nothing more than a radio-frequency amplifier (which provides gain and selectivity) followed by a detector and an audio amplifier. The detector stage mixes a locally generated radio-frequency signal produced by the beat frequency oscillator (BFO) with the incoming signal to produce a signal that lies within the audio frequency range (typically between 300Hz and 3.4kHz). As an example, assume that the incoming signal is at a frequency, fRF, of 100kHz and that the BFO is producing a signal, fBFO, at 99kHz. A signal at the difference between these two frequencies (1kHz), fAF, will appear at the output of the detector stage. This will then be amplified by the audio stage before being fed to the loudspeaker. The relationship between these three frequencies is fBFO = fRF ± fAF. Note that the BFO can operate above or below the incoming signal frequency by an amount equal to the desired beat frequency (the audible signal resulting from the ‘beating’ of the two frequencies). Morse code Morse code transmissions are Antenna Power amplifier 100kHz wave Antenna RECEIVER 100kHz Morse key RF amplifier Detector 1kHz AF amplifier Loudspeaker 99kHz Fig.3.1: a very simple radio communication system. 4 Beat frequency oscillator Practical Electronics | January | 2026 A B A C B D C E D F E G F H GI HJ KI LJ K M L M N O N P O Q P R Q S R T S U T V U W V X W Y X Z Y Z 1 6 2 7 1 6 3 8 2 7 4 9 3 8 5 0 4 9 Fig.3.2: Morse code. 5 referred to as continuous wave efficient. 0 often This makes them par(CW), but this term can be a little misleading as the wave is not actually continuous but periodically interrupted to send each character. Transmitters and receivers for CW operation are extremely simple but nevertheless can be very ticularly useful for disaster and emergency communication, or any situation that requires optimum use of low-power equipment. Signals are transmitted using the code invented by Samuel Morse (see Figs.3.2 & 3.3). C C C Fig.3.3: the letter “C” sent using an interrupted RF carrier wave. Morse Code may seem an outdated form of communication, but it is still used in several practical applications, including aeronautical beacons, amateur radio and emergency communications. Although speeds of around 1520 words per minute are typical, a skilled operator can send code using a straight key at more than 25 words per minute, and up to 40 words per minute using a paddle and electronic keyer. In addition, software is available that supports both sending and receiving using a computer. Fig.3.4 shows the immensely popular Fldigi application being used to decode a Morse code exchange between radio amateurs in the 40-metre (7MHz) band. It’s important to note that, unlike the ear of an experienced Morse operator, decoding software can often struggle in the presence of noise, fading and interference! Fig.3.4: the popular Fldigi software application being used to decode a conversation between radio amateurs in the 40m (7MHz) band. Practical Electronics | January | 2026 5 Modulation To convey information using a radio-frequency carrier, the signal information must be superimposed or ‘modulated’ onto the carrier wave. Modulation is the name given to the process of changing a particular property of the carrier wave in sympathy with the instantaneous voltage (or current) of the signal to be conveyed. The most common modulation methods are amplitude modulation (AM) and frequency modulation (FM). In the former case, the carrier amplitude (its peak voltage) varies according to the voltage, at any instant, of the modulating signal. In the latter case, the carrier frequency is varied in accordance with the voltage, at any instant, of the modulating signal. Fig.3.5 shows the effect of amplitude and frequency modulating a sinusoidal carrier (the modulating signal is in this case also sinusoidal). In practice, many more cycles of the RF carrier would occur in the span of one cycle of the modulating signal. The modulating of a carrier is undertaken by a modulator circuit. The input and output waveforms for amplitude and frequency modulator circuits are shown respectively in Figs.3.6(a) & 3.6(b). Common examples of the use of amplitude modulation (AM) include long-wave, medium-wave and shortwave broadcasting, as well as VHF aircraft communication. Common examples of the use of frequency modulation (FM) include FM broadcasting, radio microphones and lowcost handheld walkie-talkies. a modulated RF power amplifier stage. The inclusion of an amplifier between the RF oscillator and the modulated stage also helps to improve frequency stability. The low-level signal from the microphone is amplified using an AF amplifier before it is passed to an AF power amplifier. The output of the power amplifier is then fed Radio frequency carrier Modulating signal Amplitude modulated carrier (AM) Frequency modulated carrier (FM) AM transmitters Fig.3.8 shows the block diagram of a simple AM transmitter. An accurate and stable RF oscillator generates the radio-frequency carrier signal. The output of this stage is then amplified and passed to 6 Amplitude modulator Fig.3.5: note the differences between amplitude and frequency modulation. Carrier wave input AM carrier output Amplitude modulator Demodulation Demodulation is the reverse of modulation and is how the signal information is recovered from the modulated carrier. Demodulation is achieved by means of a demodulator (sometimes also called a ‘detector’). The output of a demodulator consists of a reconstructed version of the original signal information present at the input of the modulator stage at the transmitter. The input and output waveforms for amplitude and frequency demodulator circuits are shown in Figs.3.7(a) & 3.7(b), respectively. as the supply to the modulated RF power amplifier stage. Increasing and reducing the supply to this stage is instrumental in increasing and reducing the amplitude of its RF output signal. The modulated RF signal then passes through an antenna coupling unit. This matches the RF power amplifier’s output to the antenna Carrier wave input Modulating signal input AM carrier output (a) Amplitude modulation Modulating signal input (a) Amplitude modulation Frequency modulator Carrier wave input FM carrier output Frequency modulator Carrier wave input Modulating signal input FM carrier output Fig.3.6: a simplified way to look at the circuitry required for AM (upper) & FM (lower). (b) Frequency modulation Practical Electronics | January | 2026 Modulating signal input and helps to reduce the level of any high-level modulation, where the unwanted harmonic components Amplitudemodulating signal is applied to the that may be present. The AMdemodulator trans- final RF power amplifier stage. mitter shown in Fig.3.8 employs An alternative to high-level Modulating signal output AM carrier input Amplitude (a) Amplitude demodulation demodulator Modulating signal output AM carrier input (a) Amplitude demodulation Frequency demodulator FM carrier input Modulating signal output Fig.3.7: simplified amplitude demodulation (upper) & frequency demodulation (lower). Frequency (b) Frequency demodulation demodulator FM carrier input Microphone Modulating signal output (b) Frequency demodulation AF amplifier modulation of the carrier wave is shown in Fig.3.9. In this arrangement, the modulation is applied to a low-power RF amplifier stage. The amplitude-modulated signal is then further amplified by the final RF power amplifier stage. To prevent distortion of the modulated waveform, this final stage must operate in linear mode (the output waveform must be a faithful replica of the input waveform). Low-level modulation avoids the need for an AF power amplifier, but the RF power amplifier must operate in a linear mode. If this isn’t the case, the transmitted signal will be distorted and unwanted signal components will be generated. Fig.3.8: a block diagram of a simple AM transmitter employing high-level modulation. AF power amplifier Antenna RF oscillator RF amplifier Modulated RF power amplifier Antenna matching unit Earth Fig.3.9: an AM transmitter employing low-level modulation and a linear amplifier. Microphone AF amplifier Antenna RF oscillator Modulated RF amplifier Linear RF power amplifier Antenna matching unit Ground Practical Electronics | January | 2026 7 Fig.3.10: a block diagram of a simple FM transmitter. Antenna Modulated RF oscillator RF amplifier RF power amplifier Antenna matching unit Earth Microphone AF amplifier FM transmitters Fig.3.10 shows the block diagram of a simple FM transmitter. Once again, an accurate and stable RF oscillator generates the radio frequency carrier signal. As with the AM transmitter, the output of this stage is amplified and passed to an RF power amplifier stage. Here again, the inclusion of an amplifier between the RF oscillator and the RF power stage helps to improve frequency stability. The low-level signal from the microphone is amplified using an AF amplifier before it is passed to a variable reactance element (eg, a variable capacitance diode) within the RF oscillator tuned circuit. The application of the AF signal to the variable reactance element causes the frequency of the RF oscillator to increase and decrease in sympathy with the AF signal. As with the AM transmitter, the final RF signal from the power amplifier is passed through an antenna coupling unit that matches the antenna to the RF power amplifier and may also help reduce the level of any unwanted harmonic components. TRF receivers Tuned radio frequency (TRF) receivers provide a means of receiving local signals using minimal circuitry, and as such, were among the earliest receivers developed. The simplified block diagram of a TRF receiver is shown in Fig.3.11. The signal from the antenna is applied to an RF amplifier stage. This stage provides a moderate amount of gain at the signal frequency. It also provides selectivity by incorporating one or more tuned circuits at the signal frequency. This helps the receiver reject signals that may be present on adjacent channels. The output of the RF amplifier stage is applied to the demodulator. This stage recovers the audio frequency signal from the modulated RF signal. The demodulator stage may also incorporate a tuned circuit to further improve the selectivity of the receiver. The output of the demodulator stage is fed to the input of the AF amplifier stage. This stage increases the level of the audio signal from the demodulator so it is sufficient to drive a loudspeaker. Unfortunately, TRF receivers have several limitations regarding sensitivity and selectivity, making this type of radio receiver generally unsuitable for commercial radio equipment. Superheterodyne receivers Superhet receivers provide both improved sensitivity (the ability to receive weak signals) and improved selectivity (the ability to discriminate signals on adjacent channels) compared to TRF receivers. Superhet receivers are based on the supersonic heterodyne principle, where the wanted input signal is converted to a fixed intermediate frequency (IF) at which most of the gain and selectivity is applied. The intermediate frequency chosen is generally 455kHz or 1.6MHz for AM receivers and 10.7MHz for communications and FM receivers. The simplified block diagram of a simple superhet receiver is shown in Fig.3.12. The signal from the antenna is applied to an RF amplifier stage. As with the TRF receiver, this stage provides a moderate amount of gain at the signal frequency. The stage also provides selectivity by incorporating one or more tuned circuits at the signal frequency. The output of the RF amplifier stage is applied to the mixer stage. This stage combines the RF signal with the signal derived from the local oscillator (LO) stage to produce a signal at the intermediate frequency (IF). It is worth noting that the output signal produced by the mixer contains several components, including the sum and difference of the signal and local oscillator frequencies as well as the original signals plus harmonic components. The wanted signal (ie, at the intermediate frequency) passes (usually via a bandpass filter—see Part 2) to the IF amplifier stage. This stage provides amplification as well as a high degree of selectivity. The output of the IF amplifier stage is fed to the demodulator stage. As with the TRF receiver, this stage is used to recover the audio-frequency signal from the modulated RF signal. Finally, the AF signal from the demodulator stage is fed to the AF amplifier. As before, this stage increases the level of the audio signal from the demodulator so that it is sufficient to drive a loudspeaker. To cope with a wide variation in input signal amplitude, superhet receivers invariably incorporate some form of automatic gain control (AGC). In most circuits, the DC level Antenna RF amplifier Fig.3.11: a block diagram of a simple TRF radio receiver. 8 Demodulator AF amplifier Loudspeaker Practical Electronics | January | 2026 Antenna RF amplifier Mixer IF amplifier Demodulator AF amplifier Loudspeaker Fig.3.12: the block diagram of a simple superhet radio receiver. AGC Local oscillator from the AM demodulator is used to control the gain of the IF and RF amplifier stages. As the signal level increases, the DC level from the demodulator stage increases, reducing the gain of both the RF and IF amplifiers. This provides a similar audio output level for both weak and strong stations. Frequency conversion The superhet receiver’s intermediate frequency is the difference +V between the signal frequency and the local oscillator frequency. The required local oscillator frequency can be calculated from the relationship fLO = fRF ± fIF. In most cases, to simplify tuning arrangements and reduce the impact of ‘image’ channels, the local oscillator (LO) usually operates above the signal frequency, ie, fLO = fRF + fIF. So, for example, a superhet receiver with a 1.6MHz IF tuned to receive a signal at 5.5MHz will Fig.3.13: some common crystal oscillator configurations. +V Output Output 0V 0V (b) Colpitts oscillator (a) Pierce oscillator +V +V Output Output 0V 0V (c) Miller oscillator (d) Franklin oscillator 0V Fig.3.14: adding a trimmer capacitor for precise adjustment of oscillator frequency. 0V 0V (a) Parallel connected capacitor 0V (b) Series connected inductor (a) Parallel connected capacitor (b) Series connected inductor Practical Electronics | January | 2026 operate with an LO at (5.5MHz + 1.6MHz) = 7.1MHz. As another example, a VHF Band II FM receiver with a 10.7MHz IF covering the broadcast band of 88MHz to 108MHz would require a local oscillator tuning over a range extending from 98.7MHz to 118.7MHz. As well as the IF signal, many other frequency components will be present at the output of the mixer stage. These will need to be removed so that only the required IF signal is passed on to the IF amplifier stage. This can be achieved using an appropriately designed filter (usually a bandpass type) to achieve the desired selectivity. This topic will be explored in a future Hands-On project. Hands-On: An accurate reference oscillator The first of this month’s HandsOn projects features a useful quartz crystal-controlled reference oscillator. This module will provide you with a highly stable frequency source that can be used with several of our future Hands-On projects. Four basic crystal oscillator configurations are shown in Fig.3.13. For simplicity, a junction gate fieldeffect transistor (JFET) has been used as the active device. Where a bipolar junction transistor (BJT) is employed, additional bias components will be needed. The Pierce oscillator in Fig.3.13(a) is a fundamental-mode oscillator with feedback between the drain and gate and the internal capacitance of the device. The Colpitts circuit shown in Fig.3.13(b) is another popular and reliable fundamental-mode oscillator, and the inductor in the source is often replaced by a resistor. The Miller oscillator in Fig.3.13(c) is suitable for fundamental as well as overtone operation, depending on the resonant frequency of the L-C drain load. To achieve a precise output frequency, there’s often a need to trim the frequency of oscillation. This is usually achieved by a small-value series preset capacitor connected in parallel or a preset inductor connected in series with the crystal (see Fig.3.14). 9 The circuit of our Hands-On reference oscillator module is shown in Fig.3.15. We have chosen the Pierce configuration, but added a second stage that acts as a unity-gain buffer to provide a relatively high degree of isolation of the load. This helps maintain amplitude and frequency stability when the load impedance changes. Enhancement-mode insulatedgate FET TR1 (a Mosfet) has a 10MHz quartz crystal connected between its gate and drain in the Pierce configuration, similar to Fig.3.13(a). Trimmer capacitor TC1 provides adjustment of the frequency, while TR2, another 2N7000 Mosfet, acts as a source follower. DC bias derived from the source of TR2 is fed back to the gate of TR1 via R1. A simple shunt zener stabiliser, D1, helps maintain a constant 6.8V supply to TR1 and TR2. The output amplitude is adjusted via a preset potentiometer, RV1. The component layout for the reference oscillator module (viewed from the top) is shown in Fig.3.16, while the corresponding track layout (viewed from below) is in Fig.3.17. The required track breaks can be made using a spot face cutter or small drill bit, and the links on the upper side of the boards are made using short lengths of tinned copper wire. The pin connections for the semiconductor devices are shown later in Fig.3.22. When completed, the stripboard module, DC jack connector and BNC output connector (with pigtail cable and male SMA connector) can be mounted into an ABS enclosure of your choice, as shown in Fig.3.18. Testing As always, it’s important to check the stripboard and internal wiring before applying power. When these checks are complete, apply power to the module and connect the BNC output to an oscilloscope or digital frequency meter. Check the output waveform for frequency (10MHz within a few Hz) and amplitude (about 2V peak-to-peak). Adjust TC1 for an output of exactly 10MHz. If you don’t have any RF test equipment, you can check the oscillator using an ordinary FM receiver. Connect a short length of wire to the BNC output (centre conductor only) and position the receiver close to the oscillator. Now tune the receiver to around 90MHz (or 100MHz). 10 Fig.3.15: the circuit of the crystal-controlled reference oscillator. R4 220Ω P1-2 R3 4.7kΩ TR2 2N7000 X1 10MHz D1 6V8 C4 10uF d g TR1 2N7000 g TC1 50pF + C3 100nF d s s P1-3 R1 1MΩ C2 100nF C1 100nF R2 470Ω RV1 500Ω SK1 Output P1-1 Figs.3.16 & 3.17: the reference oscillator module layout. + Parts List – Reference oscillator module 1 25 × 64mm piece of stripboard (9 × 24 holes) 1 3-pin male 0.1in/2.54mm header (P1) 1 female SMA PCB-mounting connector (SK1) 1 10MHz HC-49S quartz crystal (X1) Semiconductors 2 2N7000 Mosfets, TO-92 (TR1, TR2) 1 6.8V 300-500mW zener diode, DO-35 (D1) Capacitors 1 10µF 16V radial electrolytic (C4) 3 100nF 50V ceramic (C1-C3) 1 50pF miniature preset trimmer (TC1) Resistors (all ¼W axial, ±5% or better) 1 1MΩ (R1) 1 470Ω (R2) 1 4.7kΩ (R3) 1 220Ω (R4) 1 500Ω miniature preset potentiometer/trimpot (RV1) Off-board components 1 ABS enclosure (see text) 1 15cm bulkhead panel-mount BNC female to SMA male coaxial cable adaptor 1 panel-mount 5.5mm DC barrel jack connector 4 M3-tapped, 10mm-long brass or nylon hex spacers 8 M3 × 6mm panhead machined screws Practical Electronics | January | 2026 Fig.3.18: the internal assembly and wiring of the reference oscillator module. of the Colpitts oscillator configuration, and have once again added a second stage to act as a unity gain buffer. An LC oscillator is used, rather than a crystal oscillator, since the frequency needs to vary over a wide range. An NPN silicon bipolar junction transistor, TR1, is connected in Colpitts configuration with its operating frequency determined by L1 and VC1, similar to Fig.3.13(b). When the ninth (or tenth) harmonic is located, the receiver will go quiet as the strong unmodulated signal is received. If no output is detected, disconnect the power and recheck the module and off-board wiring. Table 3.1 shows the test voltages obtained from our prototype. The output of TR1 is directly connected to the gate of the source follower, TR2, which helps isolate the frequency determining components from the load connected to SK1. A low-cost three-terminal voltage regulator, IC1, maintains a constant 9V supply to TR1 and TR2. As before, the output amplitude is adjusted via a preset potentiometer, RV1. The component layout for the variable frequency oscillator module Hands-On: A variable frequency oscillator (VFO) Variable frequency oscillators (VFO) are found in RF equipment in applications where continuous variable tuning is required. The circuit of our Hands-On VFO is shown in Fig.3.19. Here, we are making use Fig.3.20: the component layout for the variable frequency oscillator module. Out Fig.3.19: the circuit of the variable-frequency oscillator. TR1 BC548 R2 C1 1nF 220kΩ b P1-2 VC1 270pF (see text) TR2 2N7000 e C2 1nF L1 100µH P1-1 C5 10µF c + In P2-2 + C6 10µF Com. d g R1 220kΩ C3 1nF IC1 78L09 s R3 2.2kΩ RV1 500Ω P2-3 C4 100nF SK1 Output P2-1 Practical Electronics | January | 2026 11 Table 3.1 – Expected voltages (Ref. Osc.) Device TR1 TR2 Device 1 2 3 BC548 78L09 1 source collector out 2 gate base com 3 drain emitter in D1 6.8V + TR1 TR2 Fig.3.22: the pin connections for the semiconductors used in both Hands-On projects. Fig.3.23: the internal assembly and wiring of the VFO module. (viewed from the top) is shown in Fig.3.20, while the corresponding track layout (viewed from below) is in Fig.3.21. As usual, the required track breaks can be made using a spot face cutter or small drill bit, and the links on the upper side of the boards are made using short lengths of tinned copper wire. The pin connections for the semiconductor devices are shown in Fig.3.22. When completed, the stripboard module, on/off switch, DC jack connector and BNC output connector (with pigtail cable and male SMA connector) can be mounted into another small ABS enclosure of your choice (Fig.3.23) or retained for use with next month’s Hands-On project. Testing Once again, it’s important to check the stripboard and internal wiring before applying power. When these checks are complete, apply 12V to the DC jack, switch it on, set RV1 to mid-position and connect the BNC output to an oscilloscope or digital frequency meter. Observe the output waveform as VC1 is varied over its full range. The frequency should be adjustable over a range extending from about 680kHz (with VC1 full anticlockwise) to 910kHz (with VC1 fully clockwise). Note that the VFO tuning range will depend on the component used for VC1 (smaller 12 Voltage D 2.8V G 1.3V S 0.5V D 6.8V G 2.8V S 0.8V Table 3.2 – Expected voltages (VFO) Fig.3.21: the track layout for the variable frequency oscillator module (underside view). 2N7000 Pin IC1 Pin Voltage C 9.1V B 2.8V E 4.9V D 9.1V G 4.9V S 3.1V IN 12V COMMON 0V OUT 9.1V values will produce a correspondingly smaller VFO tuning range). Finally, with RV1 set to maximum (fully clockwise), check that the output is about 2V peak-to-peak over the full VFO tuning range. If you don’t have any RF test equipment, the VFO can be tested with the aid of an ordinary medium-­ wave AM receiver (including either of the AM receivers that we described in previous parts of this Teach-In series). Connect a short length of wire to the BNC output (centre conductor only) and place the receiver within about two metres (about six feet) of the VFO. With VC1 set to its mid position, tune the receiver between about 700kHz and 900kHz. When the VFO signal is located, the receiver will go quiet as the strong unmodulated signal is received. Next, locate a strong MW broadcast station in the same range (in the UK you can use BBC Radio 5 Live on 693kHz and 909kHz for this test). Having located the broadcast signal, rotate VC1 until you hear a loud whistle (beat note) as the VFO is tuned across the broadcast signal. This neatly illustrates the BFO effect that we discussed earlier! If no output is detected, switch it off and disconnect the power before rechecking the module and offboard wiring and, in particular, the connections to VC1. If you are still experiencing problems, Table 3.2 provides the test voltages obtained from our prototype. Practical Electronics | January | 2026 Parts List – Variable frequency oscillator 1 25 × 64mm piece of stripboard (9 × 24 holes) 1 2-pin male 0.1in/2.54mm header (P1) 1 3-pin male 0.1in/2.54mm header (P2) 1 female SMA PCB-mounting connector (SK1) 1 100µH axial RF inductor (L1) Semiconductors 1 BC548 30V 100mA NPN bipolar junction transistor, TO-92 (TR1) 1 2N7000 Mosfet, TO-92 (TR2) 1 78L09 9V linear voltage regulator, TO-92 (IC1) Capacitors 2 10µF 16V radial electrolytic (C5, C6) 1 100nF 50V ceramic (C4) 3 1nF 50V ceramic (C1, C2, C3) 1 270pF miniature solid-dielectric variable (VC1) Resistors (all ¼W axial, ±5% or better) 2 220kΩ (R1, R2) 1 2.2kΩ (R3) 1 500Ω miniature preset potentiometer/trimpot (RV1) Off-board components 1 ABS enclosure (see text) 1 SPST miniature toggle switch 1 15cm bulkhead panel-mount BNC female to SMA male coaxial cable adaptor 1 panel-mount 5.5mm DC barrel jack connector 4 M3-tapped, 10mm-long brass or nylon hex spacers 8 M3 × 6mm panhead machined screws Using software simulation Simulation software can be invaluable in most areas of RF design. As an example, we used software modelling to test our VFO module, comparing the simulated results with those obtained from real measurements. Fig.3.24 shows the module simulated using the popular (and freely available) Tina-TI SPICE package from Texas Instruments (available from https://www.ti.com/ tool/TINA-TI). It’s worth comparing Tina’s virtual oscilloscope display in Fig.3.24 with that obtained using a real digital storage scope (Fig.3.25). The two waveform displays are virtually identical, illustrating the value of software modelling techniques for this simple RF application. Component values and active devices can be very easily changed, and the results observed using Tina’s handy range of virtual instruments. Output spectrum and harmonics If you take a close look at Fig.3.25, you should notice that the output of our VFO is not perfectly sinusoidal. This distortion results from nonlinearity, and it indicates the presence of harmonic components in the output from the VFO. These can be viewed and analysed using Fast Fourier Transform (FFT) techniques provided by the DSO software. Fig.3.25: the output waveform from the VFO module displayed on a low-cost digital storage oscilloscope (DSO). Fig.3.24: SPICE modelling our Hands-On VFO module using Tina-TI. Practical Electronics | January | 2026 13 Fig.3.26: the output frequency spectrum from the VFO module displayed using FFT techniques (note the relative levels of the various harmonic components). The FFT-derived spectrum of the VFO output is shown in Fig.3.26. We’ve indicated the order of each of the most significant harmonics, as well as their relative level (shown in decibels, dB) below the fundamental. Notice how, in general, the amplitude of the harmonic components decreases with the order of the harmonic. Note how the second harmonic is at double the frequency of the fundamental, the third harmonic is at triple the frequency and so on. You can see harmonics up to the 9th here; in general, they decay in amplitude as they increase in frequency, although not always. It’s important to note that harmonics present in the output of a VFO (or any other oscillator for that matter) are generally undesirable. In a receiver, they can cause unwanted mixing products. In a transmitter, they can result in the emission of signals that may cause interference to other services. In practice, we would either need to improve the oscillator design or to apply subsequent filtering to ensure that any harmonic components are reduced to a negligible level. Coming up! In Part 4 of this Teach-In series, we will be delving into the exciting world of software-defined radio (SDR). We will explain how this technology is used to perform radio signal processing tasks that have traditionally been managed by hardware and show how you can use a low-cost SDR together with a PC to receive an immense variety of signals over a frequency range extending from HF (3-30MHz) to UHF (into the GHz range). Our Hands-On project next month will put this month’s RF oscillator modules to good use as the basis for a complete amplitudePE modulated signal source. JTAG Connector Plugs Directly into PCB!! No Header! No Brainer! Our patented range of Plug-of-Nails™ spring-pin cables plug directly into a tiny footprint of pads and locating holes in your PCB, eliminating the need for a mating header. Save Cost & Space on Every PCB!! Solutions for: PIC . dsPIC . ARM . MSP430 . Atmel . Generic JTAG . Altera Xilinx . BDM . C2000 . SPY-BI-WIRE . SPI / IIC . 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