VCXO Oscillators (Voltage-controlled crystal oscillators) are used in designs that need to control the output frequency via an external digital PLL. The VCXO frequency is pulled by the control voltage (Vc) applied. The function of this can be to maintain a fixed desired output frequency over time to counteract the ageing of the crystal that would otherwise cause a drift. The designer/engineer will need to know what the Pull Range is for the selected VCXO.
Total Pull Range is equal frequency shift from minimum to maximum control voltage; typically 50ppm to 200ppm max.
Another common parameter used when specifying VCXO is Absolute Pull Range or (APR) with VCXO's offering ±35ppm to 50pp min.
APR is defined as the minimum pull range after taking into account other aspects that affect the frequency, such as temperature, ageing, power supply variations, and load variations.
APR = Total Pull Range minus expected changes from temperature, ageing, power supply variations, and load variations.
A designer picks an APR that is expected to cover all drifts for the life of the product.
VCXO's designs fall into three types, those with a fundamental crystal, or a 3rd OT crystal, and those using an internal PLL to multiply the frequency and extend the output frequency.
Fundamental crystal types are limited to ∼80MHz, 3rd OT designs offer higher frequencies to ∼200MHz and the best phase noise/jitter performance. PLL types offer the broadest range of frequencies up to 2100MHz and can be utilized using programmable oscillators.
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