Voltage Controlled Crystal Oscillators (VCXOs) are ubiquitous in timing systems.However, a new player has recently entered the scene that has designers curious. Is it worth making the switch to Numerically Controlled Oscillators (NCOs) in your projects?
VCXOs
VCXOs are used for extremely fine adjustment of frequencies. A typical tuning range may be only a few hundred ppm (parts per million) over an input voltage range of 0 to 3.3V. This type of tuning is useful when an output frequency must match an external reference very precisely, such as when a VXCO is used as a clock generator for synchronization.
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This method of clock tuning is usually easy to implement, as the control voltage is often already an inherent part of the circuit and can be used to “tug” the oscillating crystal’s frequency up or down as needed. A crystal oscillator can even have voltage control implemented discretely later in a design cycle using voltage-controlled capacitors, standard capacitors, and resistors so long as the designer knows the precise parameters of the crystal. The amount that the frequency can be tugged or pulled depends on the resistance and loading capacitance of the crystal and even the package. This design typically combines a crystal, tuning circuitry, and a PLL to generate the final clock outputs to be used in a circuit.
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NCOs
Working with numerically controlled oscillators is quite a bit more complicated than putting varicaps around your crystal. NCOs use a microcontroller to read the speed of the rising edge of a clock pulse to determine the actual frequency, then match that value against a look up table to generate the numerical values for the sine wave that most closely matches that frequency. The output of an NCO is a digital value that can be run through a DAC to generate a very accurate clock frequency based on what the crystal was trying to achieve. Depending on your design, this may mean you can use a cheaper and less accurate crystal while getting a cleaner clock signal on the other end of your NCO. Companies like Microsemi that see the value in clocks with jitter below 0.7ps RMS, especially for applications like 10G PHYs, create single chips that combine the MCU and DAC to simply accept any messy clock signal and output a crisper version.
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This takes the load off the customer, as the lookup table is pre-programmed and the complete chip is already well-defined. Many designers have clung to bulky and expensive VCXO solutions because they are still easier than rolling your own NCO solution, even in applications that cannot tolerate much jitter. Now that these chips are established and coming down in price, you may just be out of excuses to tug around a wandering oscillator frequency rather than use an NCO.