A piece of quartz, or silicon dioxide, can be made to resonate at a frequency between a few tens of kilohertz and approximately 40 megahertz in an oscillator circuit, by careful control of the dimensions and ‘cut’ of that crystal. The ‘grown’ crystal blank has a structure similar to the grain in a piece of wood, and the direction of the sawing in relation to that grain, gives a characteristic that is consistent and repeatable for any number of finished parts. Sawn crystals are polished and partly coated in metal, to give electrical contacts, then mounted on leads and placed in a sealed container holding inert gas.

Common packages for frequencies above 1MHz are HC49/U and HC49/SMX, while for watch crystals, 2x6 and 3x8 cylinders predominate. The finished part is then tested for resonant frequency at 25°C and the two extremes of temperature it is designed to operate between. This gives two pass bands, a frequency tolerance at 25°C in terms of parts per million and another tolerance in parts per million over the operating temperature range.

The resonance may be described as series or parallel. If it is parallel, there is an associated load capacitance value for resonance at that frequency. By varying the value of the load capacitance, it is possible to change the output frequency of the oscillator containing the crystal and this is known as ‘pulling’. This is important in most wireless especially mobile phone applications. It is also used in Voltage Controlled Crystal Oscillators (VCXO), where a voltage variable tuning capacitance or ‘varactor’ diode is used to change the output frequency by +/- 200ppm for a change of +/- 1.5 volts.

Rather than design an oscillator that contains a crystal, many people use an off-the-shelf part in either a through hole or surface mount package. The most common type of ready-made oscillator is called a clock or Simple Package Crystal Oscillator (SPXO). This produces an output of ‘constant’ frequency that varies with the temperature of the crystal. The variation is between +/-10 and 100ppm over a range of at least 50°C. For applications where this is too much, there are Temperature Compensated Crystal Oscillators (TCXO) with a variation of +/- 2-10ppm. If this is still too much, for +/- 0.3-1ppm there are Digitally Compensated Crystal Oscillators (DCXO) and finally for +/-0.001-0.1ppm there are Oven Controlled Crystal Oscillators (OCXO). The pricing of each type of part is in proportion to the difficulty of manufacture.

A crystal based oscillator is smaller, more precise and better tolerant of changes in temperature, supply voltage, and output load than a resonant ‘tank’ circuit of inductors and capacitors.

An alternative to a crystal is a ceramic resonator. These parts are not as accurate as a crystal, but are significantly lower in cost. Typical tolerance is ±0.5%, or 5000 parts per million at 25°C with a frequency stability of ±0.3% over –20 to +80° C. The lower level of induced voltage at resonance means that a part may need to be characterised for use with a particular integrated circuit. This may mean special selections for resonant impedance, or non-standard values of load capacitance or feedback resistor in the recommended oscillator configuration. Poor selection of parts around the resonator may result in it oscillating at the wrong frequency, so it is best suited to cost sensitive applications where tolerance is a lesser issue.

A further topic for consideration is selection of filtering components. Filters are usually categorised as low pass, high pass, or band pass. We shall limit ourselves to band pass. There are monolithic crystal (MCF), ceramic or SAW filters. MCF parts are used in communications for intermediate frequency selection, so common frequencies are 10.7MHz, 16.9MHz, 21.4MHz, 45.0MHz, 70MHz, or 90MHz. Ceramic filters are used in consumer wireless, so an additional requirement to the above is 455kHz, and frequencies of 130MHz and above. SAW filters are designed for higher frequencies than these, and are usually targeted at the local oscillator radio frequency rather than one of the lower ‘down conversions’ of 10.7MHz - 90.00MHz. The market for SAW is RFI compliance or ‘image frequency’ attenuation. Most radio receivers operate by mixing an incoming signal with a slightly lower local oscillator frequency, This gives the two original frequencies, their sum and difference as four results. An incoming signal at the same distance below the local oscillator frequency as the target signal is above, will also create the correct difference frequency so it must be attenuated to an extent where it cannot interfere. The difference or intermediate frequency (IF) can then be amplified, the carrier frequency removed (demodulation) and the original signal restored.

Applications such as networking need a very stable clock source. Short-term clock variation is usually termed ‘phase jitter’. A common frequency is 155.52MHz. This can be derived by multiplying 31.104MHz by five, but may produce a result that has too much phase jitter. A typical figure is 15 picoseconds RMS maximum, in a period of 6.43 nanoseconds. If this needs to be minimised, the alternative is to use a fundamental crystal at 155.52MHz, much more expensive but giving a much lower phase jitter figure, of 1 picoseconds RMS max. This will reduce the number of data errors by a factor of roughly 20, very helpful in a large network where it is vital to minimise the number of re-transmissions of data packets.