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Crystal-controlled oscillators
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Quartz crystals can be used in place of tuned LC circuits in RF oscillators, if it isn t necessary to change the frequency often. Crystal oscillators offer excellent frequency stability far superior to that of LC-tuned VFOs. There are several ways that crystals can be connected in bipolar or FET circuits to get oscillation. One common circuit is the Pierce oscillator. An N-channel JFET and quartz crystal are connected in a Pierce configuration as shown in the schematic diagram of Fig. 25-6. The crystal frequency can be varied somewhat (by about 0.1 percent) by means of an inductor or capacitor in parallel with the crystal. But the frequency is determined mainly by the thickness of the crystal, and by the angle at which it is cut from the quartz rock. Crystals change in frequency as the temperature changes. But they are far more stable than LC circuits, most of the time. Some crystal oscillators are housed in temperature-controlled chambers called ovens. They maintain their frequency so well that
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The voltage-controlled oscillator 465
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25-6 A JFET Pierce oscillator.
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they are often used as frequency standards, against which other oscillators are calibrated. The accuracy can be within a few Hertz at working frequencies of several megahertz.
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The voltage-controlled oscillator
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The frequency of a VFO can be adjusted via a varactor diode in the tuned LC circuit. Recall that a varactor, also called a varicap, is a semiconductor diode that works as a variable capacitor when it is reverse-biased. The capacitance depends on the reverse-bias voltage. The greater this voltage, the lower the value of the capacitance. The Hartley and Clapp oscillator circuits lend themselves well to varactor-diode frequency control. The varactor is placed in series or parallel with the tuning capacitor, and is isolated for dc by blocking capacitors. The schematic diagram of Fig. 25-7 shows an example of how a varactor can be connected in a tuned circuit. The resulting oscillator is called a voltage-controlled oscillator (VCO).
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25-7 Connection of a varactor in a tuned LC circuit.
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466 Oscillators Why control the frequency of an oscillator in this way It is commonly done in modern communications equipment; there must be a reason. In fact there are several good reasons why varactor control is better than the use of mechanically variable capacitors or inductors. But it all comes down to basically one thing: Varactors are cheaper. They re also less bulky than mechanically variable capacitors and inductors. Nowadays, many frequency readouts are digital. You look at a numeric display instead of interpolating a dial scale. Digital control is often done by a microcomputer. You program the operating frequency by pressing a sequence of buttons, rather than by rotating a knob. The microcomputer might set the frequency via a synchro on the shaft of a variable capacitor or inductor. But that would be unwieldy. It would also be ridiculous, a Rube Goldberg contraption. A varactor can control the frequency without all that nonsense.
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The PLL frequency synthesizer
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One type of oscillator that combines the flexibility of a VFO with the stability of a crystal oscillator is known as a PLL frequency synthesizer. This scheme is extensively used in modern digital radio transmitters and receivers. The output of a VCO is passed through a programmable divider, a digital circuit that divides the VCO frequency by any of hundreds or even thousands of numerical values chosen by the operator. The output frequency of the programmable divider is locked, by means of a phase comparator, to the signal from a crystal-controlled reference oscillator. As long as the output from the programmable divider is exactly on the reference oscillator frequency, the two signals are in phase, and the output of the phase comparator is zero volts dc. If the VCO frequency begins to drift, the output frequency of the programmable divider will drift, too (although at a different rate). But even the tiniest frequency change a fraction of 1 Hz causes the phase comparator to produce a dc error voltage. This error voltage is either positive or negative, depending on whether the VCO has drifted higher or lower in frequency. The error voltage is applied to a varactor in the VCO, causing the VCO frequency to change in a direction opposite to that of the drift. This forms a dc feedback circuit that maintains the VCO frequency at a precise multiple of the reference-oscillator frequency, that multiple having been chosen by the programmable divider. It is a loop circuit that locks the VCO onto a precise frequency, by means of phase sensing, hence the term phase-locked loop (PLL). The key to the stability of the PLL frequency synthesizer lies in the fact that the reference oscillator is crystal-controlled. A block diagram of such a synthesizer is shown in Fig. 25-8. When you hear that a radio receiver, transmitter, or transceiver is synthesized, it usually means that the frequency is determined by a PLL frequency synthesizer. The stability of a synthesizer can be enhanced by using an amplified signal from the National Bureau of Standards, transmitted on shortwave by WWV at 5, 10, or 15 MHz, directly as the reference oscillator. These signals are frequency-exact to a minuscule fraction of 1 Hz, because they are controlled by atomic clocks. Most people don t need precision of this caliber, so you won t see consumer devices like ham radios and shortwave receivers with primary-standard PLL frequency synthesis. But it is employed by some corporations and government agencies, such as the military.
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