FREQ-DDC-FILTER-CALC

This Excel calculator provides system designers a way to simplify the design and debugging of direct RF-sampling receivers. It offers three functions: frequency planning, analog filtering, and decimation filter spur location.

In the concept phase, a frequency-planning tool enables fine tuning of both the ADC sampling rate and input-frequency location in order to optimize the spurious-free dynamic range (SFDR) during a blocking event. Some designs have flexibility on both; others, such as a L-band receiver or a wireless infrastructure base station, deal with fixed-frequency bands and only offer sampling-rate tuning.

The external RF filter response is highly dependent on both the system SFDR targets and the SFDR performance of the ADC itself; an analog filter tool can help there during the design phase.

A decimation filter spur locator tool helps in situations where unwanted spurs in the fast Fourier transform (FFT) spectrum occur during system bring-up. When the ADC outputs the entire Nyquist zone, it is simple to determine the original frequency, but not necessarily the source of the spur. However, once digital decimation with complex mixing comes into play and only a partial FFT spectrum (into which the spurs are folded) is available, this simple tool maps original spurs of the ADC to their new locations.

• Frequency planning
• Analog filtering
• Decimation filter spur location