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AMATEUR RADIO BY GARRY CRATT, VK2YBX Oscillators - which type is best for you? Last month, we discussed the operation of quartz crystals. Following on from that article, we discuss the different types of oscillators that can be used in amateur equipment, both crystal and free running types, and the advantages of using particular types. Without going into the detailed circuit theory behind oscillator design, there are two basic conditions that must be fulfilled in any design before oscillation can occur: (1). the amplifier around which the oscillator is based must have a gain of more than one; and (2). positive feedback must be applied between output and input. Fig.1 shows the block diagram of a basic oscillator. As can be seen, there is a feedback path from the active device output to its input, leading to ACTIVE DEVICE INPUT NETWORK -7 t ! the conclusion that the oscillator is essentially a feedback amplifier that supplies its own input. There are many types of oscillator designs, each with a particular advantage depending upon the application. In some applications (eg, the local oscillator in a superheterodyne receiver), it is necessary to use an oscillator design having good spectral purity and stable frequency characteristics. Other applications require a design that will be rich in harmonics; eg, a circuit requiring the use of an overtone crystal, Frequency stability is also often an important factor, as several stages of frequency multiplication may be used after the oscillator to reach the output frequency. There is also the added complication of temperature stability. Because temperature changes can affect the operating frequency, care must also be taken to maintain a stable temperature. 1 ~I __J Fig.1: block diagram of a basic oscillator. Note the feedback network from active device output to input. 86 SILICON CHIP Hartley oscillators Fig.2 shows the circuit of a Hartley oscillator. The frequency of oscillation is determined by L1 and Cl. The amount of feedback is determined by the ratio of the reactance of Lla and Cl (the volt- .---------ovcc RL C2 r----il_----, _ _ _ _ _ _O V0UT Fig.2: the Hartley oscillator. This type of oscillator exhibits a constant output voltage over its tuning range. age divider network within the tuned circuit), and the feedback energy is returned to the tuned circuit by the current passing through Llb and C3. This type of oscillator is most commonly used in VFO circuits due to its constant output level over the tuning range. However, this oscillator type has one major disadvantage and that is the high level of harmonics. As can be seen from Fig 2, the reactance of Ll increases with the output frequency. This means that harmonics of the fundamental frequency are developed in the tuned circuit and returned to the input, then being further c;tmplified before appearing, along with the fundamental, in the output. Thus, in order to use a Hartley oscillator sucessfully, a bandpass filter is required after the tuned circuit. Alternatively, a double tuned collector tank could be used. Colpitts oscillator The most common type of oscilla- ,---------ovcc ,------.----ovcc R1 C4 01 CJ CJ v~ o---------..J R2 .,. Fig.3: in the Colpitts oscillator, the emitter of the transistor is connected to the junction of two capacitors (Cl & C2). The main advantage of this configuration is low harmonic content in the output. tor using an inductive divider network is the Colpitts oscillator - see Fig.3. This circuit differs from the Hartley oscillator in that the emitter of the transistor is connected to the junction of two capacitors (Cl & CZ), instead of tapping the inductor. The voltage divider is formed by Ll & Cl. Typically, Cl is three times the value of CZ to ensure reliable oscillation. Changing the value of Ll or CZ will change the frequency, while changing the value of Cl will vary the output level. Colpitts oscillators are used because of their high spectral purity. Because the feedback voltage is developed across a capacitor, the reactance of which decreases at harmonic frequencies, the harmonic content is low. However, the main disadvantage of this type of oscillator is the difficulty in varying the output frequency without varying the output level. In fact, it is necessary to vary both capacitors, whilst maintaining the ratio between them. The Clapp oscillator is a variation of the Colpitts design, where a series variable capacitor has been added to the tank coil (Ll). This provides improved stability and allows tuning of the oscillator. If the value of the trimmer is low compared to the value of Cl & CZ, then the feedback ratio will be largely unaffected when the oscillator is tuned. Fig.4 shows the details. Crystal oscillators Although the previous designs can Fig.4: the Clapp oscillator is similar to the Colpitts configuration but employs a variable capacitor (VCl) in series with the tank coil (Ll). This provides improved stability and allows tuning. Quartz Crystal Oscillators C1 R1 .--------0vcc C2 .,. Fig.5: quartz crystal oscillators are used when good stability & high Q are required. This Hartley oscillator uses crystal Xl as the active resonant element. .,. Fig.6: a crystal controlled Colpitts oscillator. VCl allows some adjustment of the output frequency. be used in many applications, a-quartz ing the inductance of the crystal and crystal oscillator is inevitably used Cl, while the energy to maintain oscillation is returned via CZ. where good stability and a high Qare required. Figs.5 & 6 show how a crysIn addition to fixed frequency ostal can be used as the active resonant cillators, circuits are available to alelement in both Hartley and Colpitts oscillators. If an addi,-------vvcc tional degree of stability is required, the Pierce oscillator design shown in Fig. 7 can be used. VC1 CJ The conditions to sustain osl---0 OUTPUT cillation are present only at the frequency which makes the crystal behave as an inductive reactance. Hence the crystal replaces the inductor in the circuit. Variable capacitor VCl allows a small adjustment to be niade to the output frequency. The circuit operation is identiFig.7: a crystal-controlled Pierce cal to that of the Colpitts oscillaoscillator. It operates in exactly the same tor. The feedback voltage is deway as the Colpitts oscillator but offers improved stability. veloped by the network comprisFEBRUARY1991 87 .-------0+12V RFCrr+gv 100pF 1mH x1I' ~ - - --- - - 22pF 12MHz c, 100k .00 1 D1 .01 1N4148 22pF .,. Fig.8: these two variable frequency oscillator (VXO) circuits can be run at 12MHz and subsequently multiplied to 144MHz for use in a 2-metre transmitter. ,-----------------<J+9-12V 47k 01 OUTPUT VC1 220pF .,. Fig.9 this modified Colpitts oscillator uses diode switching to select between three different crystals (Xl, X2 & X3). 270U MWA120 2xHOT CARRIER 4.lk oUTPuT(J') 50U 'I L1 1uH L2 1uH 5-60pF+ Fig.10: experimental oscillator circuit based on the Motorola MWA-120 MIC (microwave integrated circuit). low crystals to be "pulled" in frequency, allowing as much as 8kHz to l0kHz of frequency change to be achieved. These circuits are called 88 SILICON CHIP variable frequency crystal oscillators or VXOs. In a 2-metre transmitter, if a crystal oscillator followed by several stages of frequency multip lication was used, it can be seen that at least 100kHz of frequency adjustment would be available at the output frequency. Two circuits that can be used at 12MHz (and subsequently multiplied to 144MHz) are shown in Fig.8. A buffer stage should always be used after a VXO to eliminate frequency pulling. VXOs are a compromise between cost (simplicity) and full band coverage. However, if only a small segment of an amateur band is of interest, such circuits can be used. In addition, a scheme to switch crystals covering segments of the band could be implemented as shown in Fig.9. Here, a Colpitts oscillator has been modified to allow diode switching of a bank-of crystals. A superior approach is to use a VFO circuit, some of which are capable of operation over several megahertz at VHF or UHF. Without going into design detail, VFOs for HF tend to use large plate variable capacitors and rely on mechanical stability and vernier gearing to give good stability and adequate frequency coverage. These units often only need to cover 500kHz, as the output can be mixed up to any HF band. VFOs designed specifically for VHF/UHF work tend to be PC board mounted and varicap tuned, relying on multiturn potentiometers or optocoupled controls for stability and frequency coverage. Also of interest to amateurs working at VHF is the advent of MICs (microwave integrated circuits), which these days form part of many RF systems. The most common MIC is the cascade amplifier, often designed with 50-ohm input and output impedances. Provided a resonant net- work with a phase shift of 180° at the desired operating frequency and some form of current limiting external to the MIC are used, output levels of around 6dBm are achievable. Fig.10 shows the experimental circuit used, based on a Motorola MWA-120. Finally, Fig.11 shows a series of circuits using an MMIC, illustrating the simplicity of oscillator design using these devices. 56-27DpF! (FUNDAMENTAL) Fig.11 (a) Further reading (1). Oscillator Design Handbook; (2). ARRL Handbook; (3). VHF/UHF Manual - G.R. Jessop, G6JP; (4). Es- Fig.11 (C) Fig.11: these three circuits illustrate how simple oscillator design can be using MMICs. They include a basic 70MHz oscillator (a), a 210MHz 3rd overtone oscillator (b ), & a 330MHz fundamental oscillator (c ). IC1 uPC1651G One of the most comprehensive publications on RF oscillator design has recently been released by Cardiff Publishing Company, 6300 S. Syracuse Way, Suite 650 Englewood, Colorado 80111. Called "Oscillator Design Handbook", it is available from the publishers for around $20. References IC1 uPC1651G IC1 uPC1651G L1 f56pF .07uH.,. (3RD OVERTONE) Fig.11 (b) sentials of Communication Electronics - Slurzberg/Osterheld. Footnote: The MWA-110/120 & MWA-310/320/330 MICs should be available from Motorola stockists (VSI Electronics, etc). The uPC1651 MMIC is available from Dick Smith Electronics. SC PC INSTRUMENTATION FOR THE 90s Australia's widest range (over 10,000 items) of PC/XT/AT/'386 based engineering/industrial and scientific hardware and software. 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