Research of metrological capabilities of digital (DDS- and Trueform-) generators

One of the fundamental improvements of the measurement standard of frequency deviation of frequency-modulated oscillations was the replacement of analog frequency-modulated generators used in DETU 09-03-95 with digital ones based on the principle of direct digital synthesis (DDS) and its next version of Trueform technology. These generators have wider ranges of frequency deviation and modulation frequencies than analog ones, but nonlinear distortion laws of the frequency modulation are not standardized. The subject of the article is the development of methods and research of these generators and frequency-modulated signals generated by them for nonlinear distortions, accompanying amplitude modulation, frequency noise. Three methods were used in the study: direct measurement; “combination frequencies” and “frequency shift”. The experiment was performed using several measurement methods, which allowed to estimate very small values of nonlinear distortion. Methods were developed and experimental estimations of concomitant amplitude modulation of DDS-generators (in frequency modulation mode), as well as their frequency noise level, were performed. An experimental evaluation of the capabilities of the analog-digital demodulator of the R&S FSL6 spectrum analyzer with the K7 option was performed, its high linearity was shown, its capabilities with respect to the frequency ranges of carrier oscillations and frequency deviation were evaluated. The research results provide a basis for the method of calibration of DDS-generators and demodulator of spectrum analyzers with the K7 option in those ranges where their parameters are not normalized (at direct current up to 8 MHz and F up to 500 kHz).