Hardware RTC Compensation Latency versus Sensor Mass Lag in Rapid Thermal Gradients
RTC compensation latency during thermal transitions causes clock frequency drift, requiring wider radio receive guard windows that drain battery reserves.
Switched Capacitor Array
Meaning
Network of capacitors and solid-state switches used to adjust the load capacitance of an oscillator circuit. A switched capacitor array provides a programmable method for fine-tuning the resonant frequency of a quartz crystal or an integrated tank circuit. This component is typically integrated directly into a clock generator or a radio transceiver chip.
Digital Tuning
Switches in the array are controlled by a digital word from a configuration register. Closing a switch adds a specific capacitor to the total load, while opening it removes it. Binary weighting of the capacitor values allows for a wide range of adjustment with a small number of control bits.
An 8-bit array provides 256 distinct capacitance levels.
Pulling Range
Frequency shift produced by the array depends on the motional parameters of the crystal. Higher load capacitance generally lowers the frequency of oscillation. The relationship is non-linear, so the tuning sensitivity is higher at low capacitance values.
Designers must ensure the array provides enough range to cover all manufacturing and environmental variations.
Parasitic Effect
Resistance and capacitance of the switches themselves can degrade the quality factor of the oscillator. High-quality switches are designed with low on-resistance to minimize losses. Careful layout of the array on the silicon die reduces unwanted coupling between the tuning circuit and other sensitive signals.
The array is often calibrated at the factory to account for process variations in the silicon.