A crystal oscillator is said to be an electrical oscillator circuit that utilizes the mechanical resonance of a vibrating crystal made up of piezoelectric material to establish an electrical signal with an exact frequency. This frequency is found to be useful in providing a stable clock signal in digital integrated circuits, keeping a track of time (in quartz wristwatches), and to settle frequencies for radio receiver and transmitters. The most widely recognized kind of piezoelectric resonator utilized is the quartz crystals, so oscillator circuits consolidating them got known as crystal oscillators, but other piezoelectric materials including polycrystalline ceramics are utilized in comparable circuits.

A crystal oscillator, particularly that utilizes quartz crystal, works by mutilating the crystal by an electric field when a potential difference is applied to the electrode. This property is known as inverse piezoelectricity or electrostriction. As soon as the electric field is taken off, the quartz that is oscillating at a precise frequency returns to its previous shape and generates an electric field. This can even set up a voltage. The consequence is that this quartz crystal starts to act like an RLC circuit but with a much higher value of Q.

Quartz crystals are produced for frequencies ranging from a few tens of Kilo Hertz to hundreds of MegaHertz. It is estimated that more than 2 Billion crystals are produced per year. Most of them have found their use in consumer devices like clocks, computers, wristwatches, cellphones, and radios. These crystals are also used inside measuring and texting equipment like oscilloscopes, signal generators, counters, etc.

Operation

A crystal is defined as that solid where all its constituent atoms, ions, or molecules are packed in regular order, repeated pattern in all its 3 dimensions.

Whenever a crystal made up of quartz is cut and mounted properly, it very well may be made to contort in an electric field by applying a voltage to a terminal close or on the crystal. As said earlier, this property is known as Electrostriction. When the field is removed, it sets up an electric field as it gets back in its initial shape. Thus, it behaves like an RLC circuit, made up of a capacitor, resistor, and an inductor.

One more advantage of quartz is that its size and elastic constants change in such a manner that the frequency reliance on temperature can be extremely low. The specific characteristic depends on two things: the angle at which the quartz is cut (with respect to its crystallographic axes) and the mode of vibration. As a result, the resonant frequency of the plate (depending on size) does not vary much. This conveys that a quartz filter, oscillator, or a clock remains accurate. For basic applications, the quartz oscillator is mounted in a temperature-controlled holder known as Crystal Oven. It can also be mounted on shock absorbers to forestall irritation by outer mechanical vibrations.

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A temperature-controlled chamber utilized in maintaining the quartz crystal present in the electronic crystal oscillators (at constant temperature) is known as a crystal oven. Its main aim is to cease any changes in the frequency because of varieties in encompassing temperature. An oscillator of this category is known as Oven-Controlled Crystal Oscillator (OCXO; Here “XO” is an old short form for “crystal oscillator”). This kind of oscillator gets the greatest frequency stability possible using a crystal. They are used generally to manage the frequency of cellular base stations, radio transmitters, equipment for military communications as well as in measuring frequency precisely.

Description

Quartz crystals have found its wide use in electronic oscillators to manage the frequency generated precisely. The frequency of vibration of a quartz crystal resonator depends completely on its dimensions. When there is a change in temperature, it leads to the thermal expansion of the resonator. This results in the change of frequency of the signal produced. The coefficient of thermal expansion is very low in quartz, still, the change in temperature is the major cause of the difference in frequency in the oscillator.

The oven comes with the crystal and some electrical heating elements. It is also thermally insulated. Other electrical equipments present inside the circuit are likewise helpless against temperature float, thus, the whole oscillator circuit is enclosed inside the oven. A thermistor temperature sensor in a closed-loop control circuit is utilized to make sure that the oven is kept at a desired precise temperature as well as to manage the power of the heater. The oven is said to start its work above ambient temperature and because of it the oscillator generally requires a warm-up time after the power is supplied to reach its operating temperature. The frequency will not have the full rated stability during this warm up time.

The operating temperature chosen for the oven is that at which the slope of Crystal’s Frequency Vs. The temperature curve is Zero. SC-cut (Stress-Compensated) or AT- crystals are utilized. The SC-cut has a wide range of temperatures over which the near-zero temperature coefficient is accomplished and along these lines lessens warmup time. Instead of resistance heating elements, power transistors are used for the heaters. The power output of these transistors is not proportional to the square of the current but only to the current. This linearizes the gain of the control loop.

One of the temperatures which is common for a crystal is 75 degree Celsius. But depending on the setup it may vary between 30 to 80 degrees Celsius.

Most standard commercial crystals are indicated to an ecological temperature of 0 to 70 degrees Celsius and the industrial ones are generally specified to -40 to +85 degrees Celsius.

This was all we have regarding OCXO Oscillators. Hope you liked reading it. Brands that manufacture them are ABRACON and ECS Inc. Do give a check to these manufacturers using the links as provided. 

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