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Crystal-Controlled Oscillators Quartz crystals can be used in place of tuned LC circuits in RF oscillators, as long as it isn t necessary to change the frequency often. Crystal oscillators offer 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. 24-16. The crystal frequency can be varied somewhat (by about 0.1 percent, or 1 part in 1000) by means of an inductor or capacitor in parallel with the crystal. But the frequency is determined mainly by the thickness of the quartz wafer, and by the angle at which it is cut from the original quartz sample. 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 crystal ovens. In this environment, crystals maintain their frequency so well that they are sometimes used as frequency standards against which other oscillators are calibrated. The Phase-Locked Loop One type of oscillator that combines the flexibility of a VFO with the stability of a crystal oscillator is known as a phase-locked loop (PLL). This makes use of a circuit called a frequency synthesizer. The heart of the PLL is a VCO. The output of this oscillator passes through a programmable multiplier/divider, a digital circuit that divides and/or multiplies the VCO frequency by integral (whole-number) values chosen by the operator. As a result, the output frequency can be any rationalnumber multiple of the crystal frequency. A well-designed PLL circuit can be tuned in small digital increments over a wide range of frequencies. The output frequency of the multiplier/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 multiplier/divider is exactly on the reference oscillator frequency, the two signals are in phase, and the output of the phase comparator is 0 V dc. If the VCO frequency begins to drift, the output frequency of the multiplier/divider will drift, too (although at a different rate). But even a frequency change of less than 1 Hz causes the phase comparator to produce a dc error voltage. This error voltage is either pos-
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24-16 A Pierce oscillator
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circuit using an N-channel JFET.
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24-17 Block diagram of a
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itive or negative, depending on whether the VCO has drifted higher or lower in frequency. The error voltage is applied to a varactor, 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 value. It is a loop circuit that locks the VCO onto a particular frequency by means of phase sensing, hence the term phase-locked loop (PLL). The key to the stability of the PLL lies in the fact that the reference oscillator is crystalcontrolled. A block diagram of a PLL circuit is shown in Fig. 24-17. When you hear that a radio receiver, transmitter, or transceiver is synthesized, it usually means that the frequency is determined by a PLL. The stability of a synthesizer can be enhanced by using an amplified signal from the shortwave time-and-frequency broadcast station WWV at 2.5, 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.
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