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High-speed digital-to-analog converters (DACs) play a crucial role in wireless and wireline communications, as well as in electronic test and measurement equipment. Modern high-speed DACs are no longer just simple conversion devices; they now integrate advanced digital signal processing (DSP) functions to enhance performance and flexibility. This article explores the key DSP techniques used in high-speed DACs, focusing on interpolation, zero-padding, and modulation.
One of the most common DSP functions in high-speed DACs is interpolation. It increases the sampling rate of the input signal, which helps reduce aliasing and improve the quality of the output. Interpolation is typically followed by a filter to remove unwanted images that appear due to the increased sampling rate. For example, the AD9772A, an older DAC model, includes x2 interpolation with zero-padding. There are two types of double interpolation: low-pass and high-pass. The low-pass 2x interpolation aligns the signal in the same direction, suppressing the image at fDATA - fFUND while generating a new image at 2*fDATA, which is easier to filter out.
In contrast, Qualcomm’s 2x interpolation uses an out-of-phase approach, where the fundamental signal is suppressed, and the image is preserved. This is particularly useful for intermediate frequency (IF) emissions, allowing the signal to be moved to an IF after DAC conversion at a lower interface rate.
Zero-padding is another important technique used in DACs. It involves inserting zeros between samples to increase the effective sampling rate. While this improves the flatness of the signal within the desired bandwidth, it can also weaken the suppression of image frequencies. As communication systems require wider bandwidths, the need for more sophisticated interpolation and filtering becomes even more critical.
With technological advancements, high-speed DACs now support higher interpolation ratios and include additional modulation and filtering features. The AD9779A, for instance, supports up to 8x interpolation and integrates complex modulation, enabling precise control over the signal frequency. It also includes a SINC-1 filter to compensate for the DAC's inherent frequency response roll-off, ensuring better in-band flatness.
The use of IQ (in-phase and quadrature) signals has become increasingly common in modern communication systems. IQ modulation offers two main advantages: wider signal bandwidth and improved image rejection. By using IQ, signals can be shifted in smaller steps, such as 1/8 of the DAC's clock frequency, providing greater flexibility in signal generation.
To further enhance the positioning of the IF, a Numerically Controlled Oscillator (NCO) was later introduced. This allows for even more freedom in selecting the IF frequency. The AD9154, a four-channel high-speed DAC, demonstrates this capability, offering advanced modulation and filtering options to meet the demands of complex communication systems.
August 22, 2025