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 one of 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
require frequency 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 sound like 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. At 440 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, and with 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 used by 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.
An oscillator 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, but triangles, 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 the number 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 or specified time interval. Long term drift is caused by component aging due to electrical,
thermal, physical, and chemical changes in components over a relatively long (100 hours or more) time
period. This is generally, but not always, permanent. This is generally compensated for by readjustment
ofcircuit parameters, either manually or automatically. Short-term stability is usually caused by component
changes due to circuit heating, warmup, temperature fluctuations, and instability of components, both