| 摘要 |
A direct digital frequency synthesizer is provided that solves the problems of spurious spectra and spectral enveloping. The output of the D/A converter is gated such that a signal to be filtered is connected to circuit ground during an initial noisy portion of the D/A conversion cycle and is connected to the output terminals of the D/A converter during a later portion of the D/A conversion cycle. The spurious spectral components attributable to noisy operation of the D/A converter may therefore be largely eliminated. Furtherore, gating the output signal in this manner produces a similar effect as a decimating filter, shifting the enveloping function such that the first null occurs at a higher frequency, for example 2fc instead of fc. At the Nyquist frequency therefore, the level of the output signal is down less than one dB compared to the output signal at the lowest frequency, producing a largely flat spectral response over the frequency range of interest.
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