4 Common pitfalls in frequency planning

Despite its benefits, it is possible to have an improper frequency plan, which can lead to issues that degrade the ADC’s performance. One common challenge is that the Nyquist zone overlaps. Poorly planned input signals may fall at the boundaries of Nyquist zones, leading to aliasing effects that reduce system performance. To prevent this, signals must be allocated within appropriate frequency bands to maintain spectral integrity with the Nyquist zone under consideration.

Clock spur contamination is another frequent issue, especially in systems using low-quality clocking devices or suboptimal clock distribution. These spurious signals, modulated onto the ADC spectrum, can severely impact sensitive applications by providing a known offset spur. Careful design of the clocking infrastructure, including the use of higher-quality clocking solutions, helps to mitigate these effects. Another possible method is to digitally filter the data with a high-order band rejection filter at this offset frequency – although if implemented incorrectly, any wanted signal will be removed along with the spur.

Another challenge to overcome is the correction of tightly modulated third-order intermodulation distortion spurs. These spurs will almost always fall within the passband and are often the spur limiting the spurious-free dynamic range. In the event of a very high decimation factor, it might be possible that these tones land within the attenuation band. This is unlikely for most multitone systems, however, which inherently require a larger instantaneous bandwidth than single-tone systems and thus cannot incorporate such a large decimation filter.

Finally, you must navigate the trade-off between bandwidth and dynamic range. While decimation can suppress spurious components and harmonics, it comes at the cost of reduced instantaneous bandwidth. Balancing these trade-offs is essential to achieving the best performance for specific application requirements.