How does the spurious free dynamic range of a DAC affect the spectral purity of a transmitted signal?
DAC SFDR
DAC output spectrum contains: the desired signal, harmonics of the signal, Nyquist images at n×f_clock ± f_signal, and the sinc roll-off envelope. The sinc response attenuates the output by sinc(π×f/f_clock), causing 3.9 dB loss at the Nyquist frequency. Digital pre-compensation (inverse sinc filter) corrects this roll-off for signals occupying a large fraction of the Nyquist band.
To improve SFDR: use higher DAC resolution (each additional bit improves theoretical SFDR by 6 dB), operate the DAC in a higher Nyquist zone where spurs may fall outside the band of interest, use digital pre-distortion to cancel DAC nonlinearities, and choose a DAC with built-in calibration for INL/DNL correction.
Frequently Asked Questions
How does SFDR affect EVM?
DAC spurs that fall within the signal bandwidth degrade EVM. For 64-QAM: EVM < -25 dB is required, so in-band SFDR must be > 30 dBc. For 256-QAM: EVM < -32 dB, requiring in-band SFDR > 37 dBc. Out-of-band spurs must meet the transmit spectrum emission mask.
What is the Nyquist image?
For a DAC running at f_clock producing a signal at f_signal: images appear at n×f_clock ± f_signal for all integers n. The first image at f_clock - f_signal is typically the strongest and must be attenuated by a reconstruction filter (typically 40-60 dB rejection required).