Beat Frequency Technique Research Articles

An Experimental Investigation on a Liquid Fluidic Frequency Demodulating Circuit

This study deals with a frequency demodulating circuit using water as a working fluid. In constructing a liquid frequency demodulating circuit, some improvements on the dynamic characteristics of the fluidic element and the construction of the circuit were carried out to solve the problem which will occur as compared with the pneumatic one. Two methods by use of electro-viscous fluid. Another is a mechanical method with the use of a vibrator. In a liquid a-c fluidics, the upper limit of frequency response is an important factor. In this circuit, the phase discriminator showed a frequency response as large as 140 Hz. By using beat frequency technique, this frequency demodulating circuit was operated at about 700 Hz by the electrically controlled method and at about 400 Hz by the mechanical method.

Laser wavelength comparison by high resolution interferometry

High resolution interferometry has been used to determine the wavelength ratio between two molecularly stabilized He-Ne lasers, one locked to a methane absorption at 3.39 microm and the other locked to the k peak of (129)I(2) at 633 nm. An optical beat frequency technique gave fractional orders while a microwave sideband method yielded the integer parts. Conventional (third derivative) peak seeking servoes stabilized both laser and cavity lengths. Reproducibility of the electronic control system and optics was a few parts in 10(12), while systematic errors associated with curvature of the cavity mirrors limited the accuracy of the wavelength ratio measurement to 2 parts in 10(10). The measured wavelength ratio of the methane stabilized He-Ne laser at 3.39 microm [P(7) line, nu(3) band] to the (129)I(2) (k peak) stabilized He-Ne laser at 633 nm was 5.359 049 260 6 (0.000 2 ppm). This ratio agrees with that calculated from the (lower accuracy) results of earlier wavelength measurements made relative to the (86)Kr standard. Its higher accuracy thus permits a provisional extension of the frequency scale based on the cesium oscillator into the visible spectrum.

Heterodyne measurements of 12C 16O laser frequencies and improved Dunham coefficients

Earlier measurements of absolute frequencies of 12C 16O laser lines by heterodyning with second harmonics of a 12C 16O 2 laser have been extended to 15 more lines by use of the rare isotopes 12C 18O 2 and 13C 16O 2. Interband difference frequencies for 20 pairs of 12C 16O lines have also been measured by beat frequency techniques. These data together with 1438 other previously reported frequency measurements in the microwave and infrared regions have been analyzed to determine coefficients of a Dunham series E vJ = Σ m,nY mn(v + 1 2 ) m[J(J + 1)] n for the CO electronic ground state. Transition frequencies predicted from the derived coefficients are substantially more accurate than earlier tabulations, having estimated uncertainties of the order of 2 MHz in the strongest laser bands. The coefficient Y 31 has been determined for the first time.

Interfacial forces and the brillouin spectrum of the interfacial light-scattering power

Light scattered from a fluid surface has a spectrum that involves lines slightly displaced from the incident frequency. The detection of these lines requires resolution far beyond that of normal spectrometers. Laser heterodyne beat frequency techniques developed recently allow resolution of Brillouin lines down to the low audio-frequency region. We have developed a simple spectrometer that detects the Brillouin spectra that are associated with the interaction of light and random surface ripples. The theory and experimental techniques involved are discussed. The capillary ripple dispersion equations have been solved numerically in order to provide theoretical estimates of the frequency of the splitting and estimates of the line shape. A doublet in the photocurrent power spectrum was predicted and was observed with increasing wave number for the air-water interface. The Brillouin shift observed for the air-water interface obeyed the Kelvin dispersion relation. Applications of this new technique to the study of interfacial forces are discussed.