Crystal Oscillators and Circuits
Bill Sheets K2MQJ Rudolf F Graf KA2CWL
It is often required to produce a signal whose frequency or pulse rate is very stable and exactly known. This is important in any application where anything to do with time or exact measurement is crucial. It is relatively simple to make an oscillator that produces some sort of a signal, but another matter to produce oneof relatively precise frequency and stability. AM radio stations must have a carrier frequency accurate within 10Hz of its assigned frequency, which may be from 530 to 1710 kHz. SSB radio systems used in the HF range (2-30 MHz) must be within 50 Hz of channel frequency for acceptable voice quality, and within 10 Hz for best results. Some digital modes used in weak signal communication may requirefrequency stability of less than 1 Hz within a period of several minutes. The carrier frequency must be known to fractions of a hertz in some cases. An ordinary quartz watch must have an oscillator accurate to better than a few parts per million. One part per million will result in an error of slightly less than one half second a day, which would be about 3 minutes a year. This might not soundlike much, but an error of 10 parts per million would result in an error of about a half an hour per year. A clock such as this would need resetting about once a month, and more often if you are the punctual type. A programmed VCR with a clock this far off could miss the recording of part of a TV show. Narrow band SSB communications at VHF and UHF frequencies still need 50 Hz frequency accuracy. At440 MHz, this is slightly more than 0.1 part per million. Ordinary L-C oscillators using conventional inductors and capacitors can achieve typically 0.01 to 0.1 percent frequency stability, about 100 to 1000 Hz at 1 MHz. This is OK for AM and FM broadcast receiver applications and in other low-end analog receivers not requiring high tuning accuracy. By careful design and component selection, andwith rugged mechanical construction, .01 to 0.001%, or even better (.0005%) stability can be achieved. The better figures will undoubtedly employ temperature compensation components and regulated power supplies, together with environmental control (good ventilation and ambient temperature regulation) and “battleship” mechanical construction. This has been done in some communications receivers usedby the military and commercial HF communication receivers built in the 1950-1965 era, before the widespread use of digital frequency synthesis. But these receivers were extremely expensive, large, and heavy. Many modern consumer grade AM, FM, and shortwave receivers employing crystal controlled digital frequency synthesis will do as well or better from a frequency stability standpoint. Anoscillator is basically an amplifier and a frequency selective feedback network (Fig 1). When, at a particular frequency, the loop gain is unity or more, and the total phaseshift at this frequency is zero, or some multiple of 360 degrees, the condition for oscillation is satisfied, and the circuit will produce a periodic waveform of this frequency. This is usually a sine wave, or square wave, buttriangles, impulses, or other waveforms can be produced. In fact, several different waveforms often are simultaneously produced by the same circuit, at different points. It is also possible to have several frequencies produced as well, although this is generally undesirable. In an oscillator, the feedback network determines the frequency and stability of the generated signal. Frequency is of course thenumber of cycles per unit time produced and is generally specified in Hz, kHz (1000 Hz), MHz (1 million Hz), or even GHz (1 billion Hz). Stability is another matter. What we are trying to express is how much the oscillator frequency will change in a certain amount of time. The key here is the length of time. Long term stability is generally expressed in frequency drift (delta F or ∆F) per unit time...
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