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A voltge controlled crystal oscillator (VCXO) is an electronic oscillator whose frequency of oscillation is controlled by the externally applied input voltage. The applied voltage determines the instantaneous frequency of oscillation. The basic circuit of VCXO comprises of a standard crystal oscillator along with an electronic means of tuning the frequency, generally achieved by the means of diodes.
Typically, frequency of oscillation varies from 30ppm/volt to 50ppm/volt. Commony, VCXO’s lies in this range of frequency. The frequency cannot be puuled too far because as the capacitance increases, the performance activity of the crystal reduces. They are widely employed in broadcast and telecommunication equipments. They are used to design signals for clock generation. They are generally used in narrow band phase locked phase to provide frequency variations.
Parametres Frequency: This is generally specified in MHz for frequencies over 1.0MHz or in kHz below this. It needs to be specified to the correct number of decimal places to enable the manufacturer to be able to determine the required frequency needed.
Output: VCXOs are required for a number of applications. Some may be sued to drive different forms of logic, whereas others may be required for analogue applications. Therefore, the output requirements are an important element of the overall specification.
Frequency stability: The basic frequency stability of VCXO’s is generally specified for room temperature, 20°C and with the voltage control point set to its centre value. The frequency is specified in terms of parts per million, ppm. Standard stability specifications are typically ±25ppm, ±50ppm and ±100ppm. Supply voltage: It is necessary to specify the supply voltage to ensure that it operates within the unit for which it is intended. Ideally it should be able to operate outside the expected tolerance of the supply so that any slight mismatch does not cause an issue.
Operating temperature range: Most units have a temperature operating range over which they will function. Accordingly it is necessary to specify a range for the VCXO. There are three main ranges:
0 – 70° C – Often referred to as a commercial temperature range. It is normally satisfactory for operation within office or laboratory environments.
-10 – + 70°C – Industrial range which is needed where temperature ranges may fluctuate more widely.
-40 – +85°C – this is nearing the military range (normally -40 to +125°C) and is required where much larger temperature ranges are needed.
For example for equipment that may be used externally.
Pullability: The pullability is the frequency range over which the VCXO can be pulled for a given change in control voltage. It is specified in terms of parts per million, ppm, for a given voltage. As is expected, large pullability figures give a larger tuning range, but VCXOs will smaller levels of pullability offer greater stability and lower phase noise.
Package: It is necessary to specify the package as VCXOs come in many forms and various options may be available through hole mount to surface mount. Apart from that options such as tape and reel are often available for large scale manufacture using automated manufacturing machinery.
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