Research on Key Technologies of Sub-Picosecond TDC Based on Surface Acoustic Wave Encoder Excitation

This paper presents several key technologies of sub-picosecond time-to-digital converter (TDC) based on surface acoustic wave dispersive delay line (SAWDDL) excitation for reducing the noise of the excitation and improving the signal-to-noise ratio of the output response of SAWDDL. Firstly, we redesign a new excitation circuit by which a low-voltage modulated pulse is generated to reduce the timebase short-term jitter of the excitation and improve the signal-to-noise ratio of the output response which is a chirp signal generated by SAWDDL. Additionally, a denoising algorithm based the fractional fourier transform (FRFT) is performed on a finite number of samples to effectively remove the noise in the chirp signal generated by SAWDDL. the FRFT can be regarded as the expansion of the signal on a set of orthogonal chirp bases. Therefore, we can use the method similar to frequency-domain band-pass filtering in the fractional Fourier transform domain to filter out most of the noise mixed into the chirp signal. By using these key technologies, we have achieved the single-shot time interval measurement precision of nearly 0.4ps rms, which is better than that of about 1ps rms without using these technologies.