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.
Fractional Divider Tuning
Meaning
Digital mechanism for achieving high-resolution frequency control by alternating between integer division values in a clock synthesizer. Fractional divider tuning allows a phase-locked loop to generate an output frequency that is not a whole-number multiple of the reference clock. This technique is common in radio frequency synthesis where fine-grained channel spacing is required.
Accumulator Logic
Control of the divider occurs through a digital sigma-delta modulator. This modulator changes the divisor on a cycle-by-cycle basis to reach a desired average value. If a ratio of 10.5 is required, the divider alternates between 10 and 11 to achieve the target.
The resulting average frequency matches the setpoint while the phase error is managed by the loop filter.
Spectral Purity
Switching between different division ratios introduces phase noise into the signal. These spurs appear at offsets related to the fractional part of the division ratio. Designers use high-order modulators to push this noise to higher frequencies where it can be removed by the filter.
Reducing the loop bandwidth improves signal quality but slows down the speed at which the system can lock to a new frequency.
Resolution Step
Precision of the tuner is determined by the width of the fractional accumulator. A 24-bit accumulator provides sub-hertz precision even at gigahertz carrier frequencies. This level of control is necessary for Doppler compensation and fine-tuning in narrow-band communication systems.
The minimum frequency step is the reference frequency divided by the maximum value of the accumulator.