Analysis o fspurious signals notes for sdr

Sdr notes

1. Analysis of Spurious Signals

2. The purpose of the output filters of the DDS system is to interpolate the discrete time signal and minimize spurious outputs. Filtering, however, cannot completely remove the distortion due to phase and amplitude truncation and other non-linear distortion. A model for these sources of distortion is shown in Figure 4.5 and uses noise injected at particular points of an ideal DDS system, Pi(n), to model the imperfections. The three sources of error include the following.

3. 1. Amplitude truncation arises since a limited number of bits is used to represent the synthesized signal and is usually not the major source of the errors in a DDS system . 2. Phase truncation is due to truncating the number of bits used to address the ROM locations and is a major source of error. A number of techniques have been developed to mitigate this effect . 3. The resolution of the DAC, that is, the finite number of quantization levels for the DAC, introduces distortion, and for periodic signals, this distortion consists of harmonic spurious frequency component . The DAC resolution can be improved by oversampling but will lead to the need for a clock several times faster than the minimum . However, the DAC's number of quantization levels still imposes a fundamental limit on its accuracy.

4. Phase truncation arises when only the more significant accumulator bits are used to reduce the amount of ROM required. Phase truncation is one of the main causes of spurious signals in the output. The location of the spurs due to phase truncation alone occurs at integral multiples of frequency f given by