Low and infralow frequency phase calibrator

Abstract

In the first are analog methods [i] based on application of frequency synthesizers. The phase shifts are here given at high frequency and are carried over into the low and infralow frequency domains by conversion. These methods have certain advantages, but the apparatus in which they are realized is complex and awkward, and moreover, the content of high harmonic components in the signal being synthesized is significant. In the second group are digital methods [2, 3] based on utilization of digital generators whose output signal is approximated by functions being modeled by using digital-to-analo g converters. The singularity of these emthods is the application of frequency dividers that possess high accuracy in the infralow frequency band, and low-frequency filters that operate on the shallow section of the phase--frequency characteristic, to filter out the high harmonic components. However, high placement of the phase shift regulation is achieved by increasing the input clock pulse frequency tenfold at each place, which will result in narrowing the frequency range of the phase calibrator. A method is examined below [4] based on shaping three sinusoidal step voltages, one of which Ui1(a) is the reference while the phase shifts of the others Ui2(a) and Ui3(~) vary in such a manner that the phase difference between them remains constant. These voltages are summed with weight coefficients governing the lowest place of the phase shift. The block diagram of the apparatus, the phase calibrator realizing the method, is presented in the sketch. It contains a tunable clock pulse generator (CPG), a control unit (CU) for the highest places of the phase shift, a reference phase channel (RFC), two variable phase channels (VFCI, VFC2), a code-phase shift transducer (CFT), a control module (CM), and lowfrequency filters (FI and F2) to extract the first harmonics of the output voltages. The reference and variable phase channels are identical circuits and contain a reversible counter (Co), a reference voltage source (RVS) connected to the analog input of the digital--analog converter (DAC) at whose output the sinusoidal step voltage is shaped