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Oscilent VCXO crystal oscillator

A VCXO is Crystal Oscillator, only the output frequency can be adjusted by varying the external control voltage across a varactor diode within the oscillator circuit. The change in frequency versus control voltage is otherwise referred to as pullability. To achieve this adjustment, a VCXO utilizes a variable capacitor (varactor diode) that, when used in series with an internal Quartz Crystal, combined with a change in control voltage, causes the diode capacitance to change. This capacitance change results in a change in the internal Crystal load capacitance thereby resulting in a change in frequency. VCXO's are typically used in phase locked loops (PLLs) for data acquisition or signal regeneration applications, and their inherent capabilities are sometimes referred to as Frequency Modulation.

Specifying an Oscilent VCXO:
Generally, VCXO's are manufactured according to industry standard guidelines pertaining to such variables as package types, output types, input voltages and currents, rise/fall times, and duty cycles. Although these parameters may vary slightly between manufacturers, and review of these parameters should be done when crossing between manufacturers, one can generally assume that these parameters will remain reasonably constant. When selecting a VCXO product, careful consideration should be given to the following variables for which parameter selections are made when specifying an Oscilent Part Number:

a. Package Type: Will the package be Through Hole or Surface Mount, and are there size constraints with either.

b. Output: Usually one of TTL, HCMOS, Compatible (simultaneous TTL and HCMOS compatible), or ECL Output.

c. Frequency

d. Supply Voltage: Most VCXO products are designed to operate at either 5.0V, or 3.3V, although other options are available. The Oscilent Specification assumes this standard. For other Supply Voltage options, please contact Oscilent for assistance


Frequency Stability:
Over the Operating Temperature Range, an amount of total deviation (in ppm) acceptable for the application. The VCXOFrequency Stability includes the initial adjustment (with the control pin set to center trim voltage of 2.5V) at 25 degrees Celsius, the Tolerance over Operating Temperature Range, the Load, and the effects of Supply Voltage variation, unless otherwise defined on the Oscilent data sheet. Normally there are three (3) industry standard options (100ppm, 50ppm, and 25ppm), however tighter parameters are possible for most Oscilent VCXO products.


Operating Temperature:
Generally, the industry standards for VCXO Operating Temperature Ranges are 0-+70 degrees Celsius (considered commercial temperature), and -40-+85 degree Celsius (considered industrial temperature) These common ranges make up approximately 80% of all products available in the market. For common uses in military and automotive applications, some VCXO products can be designed for more extreme conditions. However, the higher the Operating Temperature specified, the higher the cost of the device. Special consideration should be given in an attempt to avoid "over-specifying" the required Operating Temperature. Although, in general, a VCXO will continue operation outside their normal temperature range with a degradation in stability, damage to the VCXO will result if the temperatures reached are excessive.


Duty Cycle:
Also referred to as Symmetry, the Duty Cycle measures the time that the output waveform is in a logic high state, expressed as a percentage. A perfectly symmetrical output waveform would be in the logic high state exactly 50% of the time, and in a logic low state the other 50% of the time producing an oscillator Duty Cycle specification of 50%/50%. However, when specifying this parameter in an oscillator (VCXO), a tolerance for this parameter must be specified. Typically, this tolerance is specified at +/-10% creating a Duty Cycle parameter of 40%/60% (+/- 10% from the 50% reference) Industry standards allow for specifying a tighter tolerance Duty Cycle, usually allowing for a 5% maximum deviation (expressed as 45%/55%) These standards allow for a selection of either 40/60% or 45/55% Duty Cycle in the Oscilent Part Number Guide.

Pulling Range/Frequency Deviation:

The Pulling Range is the amount of Frequency Deviation realized by a corresponding change in voltage. The Pulling Range of a VCXO is determined by the motional capacitance characteristics of the Quartz Crystal, among other less critical factors. The Voltage Control function typically utilizes Pin 1 of both Through Hole and Surface Mount packages. The deviation is normally expressed in Parts Per Million ranging from 15-200ppm MIN. on most Oscilent devices.


Linearity:
When plotting the Output frequency versus Control Voltage, Linearity (expressed as a percentage) refers the maximum allowable deviation away from the best straight line drawn through the tuning curve. For example, a VCXO with a 1kHz tuning range, and a 10% Linearity parameter, would allow for an excursion of 100Hz away from the best straight line.

Manufacturer/supplier's information

Company name:		Oscilent Corportation [ Homepage ]
Business address:		18195 East McDurmott Street Building D Irvine . CA USA
Zip/postal code:		92614
Telephone number:		1.949.252.0522
Fax number:		1.949.252.0599
Contact name:		Sales [ Email ]

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Product groups: crystal oscillator

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