When observing the output of a direct digital synthesizer, which type of spectral content is typically present?

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The key idea is that a direct digital synthesizer produces a spectrum with discrete lines rather than a smooth broadband spread. The DDS generates its output by stepping through a sine table with a phase accumulator that advances at a fixed clock rate and then converts that digital value with a DAC. This creates a highly deterministic, periodic waveform, so its Fourier spectrum shows sharp spikes at specific frequencies—spurs—determined by the clock rate, the desired output frequency, and the word-length/quantization effects. Quantization and DAC nonlinearity introduce those spurs at predictable locations, and a reconstruction filter helps suppress out-of-band images but can’t eliminate the discrete spikes. So the typical spectral content you observe is spurs at discrete frequencies.