DSP CALIBRATION FOR ACCURATE TIME WAVEFORM RECONSTRUCTION

DSP waveform synthesizers cause signal distortion because of inherent limitations in their ability to construct continuous lime waveforms from discrete samples. Often this distortion is minimized by increasing the sampling rate of the synthesizer, thereby decreasing the processing bandwidth. This paper presentsa software-based, calibration routine for correcting the generated waveform. This technique uses a software equalizer to estimate and correct for the system response. The equalizer is created using a recursive least-squares (ULS) algorithm and has the form of a transversal, FIR filter. Thisfilter can then be used toprewarp the input waveform sequence. The results of this calibration method are presented in the final section. Tntroduction The problem addressed in this paper is accurate time-waveform generation. The actual output of a synthesizer is a time waveform transformed from a sequence of digital samples. Here, transformation should be understood as an inherent limitation on waveform reconstruction which can be described for all synthesizers. Specifically, this paper will not treat harmonic distortion or noise effects. In fact, this paper will show that the inherent limitations of waveform synthesizers can be successfully modelled as linear effects. There are three important types of transformation which will be discussed: sinex+m-x distortion, spectral images and group delay . Often these problems are solved by simply increasing the synthesizer sampling rate. This paper will present a new DSP-based technique for directly calibrating these sources of distortion without increasing the sampling rate. Paper 36.3 986 This technique is intended for multitone and complex waveform testing. The issues addressed by this paper must also be addressedin singletone testing, but there the solution is simpler. For singletone testing it is only necessary to generate calibration factors for gain and phase at a small number of frequencies, whereas the technique described here attempts to calibrate across an entire frequency band. iVaveform Synthesizers and their Limitations