|Monday, 09 October 2006|
Oscillators depend on the use of regenerative feedback from the output of the oscillator back to its input. A crystal oscillator oscillates at its resonant frequency, which is determined primarily by the physical dimensions and orientation of the quartz crystal. A quartz crystal acts as a stable mechanical resonator, which by its piezoelectric characteristics and high Q value can determine the frequency generated in an oscillator circuit. An important property of a quartz crystal oscillator is that it has a very high quality factor or Q. This means that the frequency of oscillation is very tightly controlled and varies very little. Precise clocks for electronic systems are often generated by a crystal oscillator that is coupled to a crystal blank. The crystal oscillator applies a voltage difference across the crystal blank, causing the crystal blank to vibrate at a desired frequency. The crystal oscillator can then amplify and buffer this oscillating signal from the crystal blank to output a clock. Electronic oscillator circuits are typically associated with these piezoelectric devices that set up a basic oscillation, and the piezoelectric device is then used to predicably control the frequency at which the oscillator circuit will be resonant. To produce a base frequency of oscillation, an electronic oscillator circuit is employed that typically include a piezoelectric device. Piezoelectric devices can be designed to mechanically resonate at very precise and repeatable frequencies, and this mechanical resonance is translated by the device to an electrical signal. Electrodes are formed upon the surface of the piezoelectric device which enables the application or pick-up of an electric field across some part of the piezoelectric device. The mechanical properties of the piezoelectric device are obviously important to the proper functioning of the oscillator, and to accurate frequency control. Quartz crystal resonant frequencies are temperature dependent. The output frequency of quartz crystals experience frequency shifts that are caused by temperature changes in the quartz element. When used in an oscillator circuit, the quartz crystal can cause the oscillator output frequency to shift as the quartz crystal's temperature changes. The output signal of a quartz crystal oscillator can be kept steady over temperature by using circuits that sense temperature and which generate an appropriate corrective signal.
Crystal oscillators that include such electrical components are referred to as temperature compensated crystal oscillators (TCXO). A temperature compensated crystal oscillator generally includes a temperature compensation circuit which generates a temperature variable output voltage which is applied to a frequency control input to stabilize the oscillator output frequency over temperature. The temperature dependency is defined by a temperature characteristic or characteristic information individually associated to an oscillator crystal and is calculated or measured during the crystal manufacture. A typical quartz temperature compensated crystal oscillator utilizes several components including a piezoelectric element, an integrated circuit, capacitors, inductors, resistors, etc. These frequency control devices are commonly found in electronic communication devices such as cellular phones, pagers, radios and wireless data devices. In such devices, temperature-compensated crystal oscillators are used as, for example, the reference frequency sources of PLL (phase locked loop) circuits which output the communication frequency signal. Generally, electronic communication devices of different performance levels require functional and performance differences in their temperature compensated crystal oscillators. Temperature controlled crystal oscillators are typically constructed in the form of a crystal and a controlling chip. Within the controlling chip, a set of switchable capacitors and a feedback amplifier form a tank circuit that oscillates at a frequency determined by the amount of capacitance switched into the tank circuit. A temperature sensor is typically provided within the chip for sensing a temperature near the crystal. Based upon the temperature sensed, a controller switches capacitors into and out of the tank circuit based upon performance criteria of the tank circuit. In general, a temperature compensating circuit of the TCXO comprises a temperature detection circuit employing resistance variation of a thermistor, a controlled voltage generating circuit for controlling a voltage according to the resistance variation, and a frequency adjusting circuit for adjusting frequency by capacitance regulation according to the controlled voltage.
A quartz-crystal oscillator is formed as an oscillator unit in which a quartz-crystal element and an IC (an integrated circuit) chip which constitutes an oscillator circuit containing therein the quartz-crystal element are accommodated together in a common container. The most sensitive component in a crystal oscillator is the piezoelectric element. Typically, this element is independently sealed in a hermetic package. The purpose of encapsulation is to passivate the integrated circuit (IC) die of the oscillator from the effects of the environment. The main frequency and frequency stability determining element in crystal oscillators is the crystal resonator. Trimming the crystal's resonant frequency can be achieved by selective metal plating the crystal faces. In general, a quartz crystal resonator is housed in a quartz crystal unit and a quartz crystal oscillator comprises the quartz crystal unit. Surface-mount crystal oscillators, and in particular, surface-mount temperature-compensated crystal oscillators (TCXOs) feature light weight, compact size, and an oscillation frequency having superior stability. Surface-mount crystal oscillators can be divided between two types according to construction of receptacle: monolithic type and bonded type. In a monolithic-type crystal oscillator, a receptacle is employed in which are formed as a monolithic unit: a crystal unit housing which houses the crystal unit, and an element housing which houses circuit elements in which components such as an oscillation circuit are formed. In a bonded-type crystal oscillator, a first receptacle portion which includes a crystal unit housing and a second receptacle portion which includes an element housing are formed separately and then bonded together to construct the crystal oscillator. In the telecommunication field, a crystal oscillator necessarily has an adjustable frequency in a specific range in order to maintain synchronization among a number of signals or to synchronize a system clock with a transmitting carrier wave. Typically, a variable capacitive element is provided to allow absolute adjustment of the final frequency of the oscillator. This capacitive element is generally either in the form of a trim capacitor or an analog varactor controlled by an applied DC voltage. In many applications, especially those in which the oscillator is being used for timing purposes, drift is extremely undesirable. For this reason, oscillators are often made to operate within an oven, so that variations in the ambient temperature can be avoided. A typical oven controlled crystal oscillator (OCXO) consists of a crystal based oscillator and temperature control system and support circuitry surrounded by a layer of thermal insulation enclosed in a sealed metal can.
A crystal oscillator employing a quartz oscillator is an essential component used to generate an oscillation frequency for controlling transmission and reception of signals between mobile communication terminals. Crystal oscillator circuits using crystal have a number of advantages in actual application since crystals show high frequency stability and stable temperature characteristic as well as excellent processability. A crystal oscillator is generally used in various forms such as a frequency generator, a frequency modulator and a frequency converter. Crystal oscillators are used in many applications to provide an accurate and stable frequency reference such as a clock signal. Oscillator circuits are used with digital electronic circuits for a variety of uses including generation of a clock signal for synchronizing, pacing and coordinating the operations of the digital electronic circuit. Digital devices require precise system timing, a function provided by oscillators and similar timing sources. Telecommunication and data transmission systems, which have analog and digital components, likewise rely on oscillators for modulation, demodulation, system clocking, and other functions. Multiband RF transceivers such as the type utilized in cellular communication systems typically use crystal resonator oscillators. Temperature-compensated crystal oscillators are used as a reference frequency source in mobile communication devices such as cellular phone terminals because they are capable of compensating for the frequency vs. temperature characteristics thereof due to the crystal unit for increased frequency stability.