Voltage controlled oscillators are commonly used where frequency or phase must be matched to an external clock. As an example, when data is transmitted on a wire, an optical fiber, or a wireless link, it is encoded at a rate set by the transmitter's clock. The data must be decoded at the receiver at exactly the same rate and at the right phase, or it will be corrupted. To do this, the receiver clock must be matched to the transmitter clock.
This matching is done by the receiver, which compares the embedded clock information in the data stream with its own clock. Depending on the difference, the receiver then fine tunes, or 'pulls' its clock as needed. This pulling is done by generating an analog voltage that is fed into the VCXO device. In quartz VCXOs, the voltage is applied to varactor diodes that load the quartz crystal. Varying this voltage varies the capacitance on these diodes and tunes the frequency.
This method of fine-tuning a frequency in a quartz VCXO has problems. One is that varactors are non-linear, so the oscillator's voltage to frequency function is non-linear and varies from part to part. This makes it harder to build a good control circuit. Secondly, the varactors and quartz crystals have limited pull range such that the performance would degrade when pulling is done in excess of 200 PPM (Parts per million). A third problem is that due to the inherence characteristics of quartz, frequency hops appear as temperature varies, thus causing receivers to lose lock and therefore, loss of data.
None of these are problems for Voltage Controlled MEMS Oscillators (MEMS VCXOs) as they do not pull a resonator, but instead, use Phase Locked Loops (PLLs) to vary the output frequency. Since the analog control voltage is digitized, the translation of voltage to frequency is very linear, unlike Quartz. Finally, MEMS resonators do not have activity dips so there are no frequency jumps.
With their performance, flexibility, higher pull range and excellent stability, Voltage Controlled MEMS Oscillators offer a sustainable advantage over quartz VCXOs.