Phase Coherent Detection Research Articles

A continuous‐wave reflection bridge ultrasonic spectrometer constructed from off‐the‐shelf components

A versatile continuous‐wave reflection bridge ultrasonic spectrometer has been developed. This spectrometer features simple tuning of the bridge element (commercial hybrid junction) for frequencies up to 100 MHz, a wide variation of applied acoustic power, simple computer control to lock frequency to any point of the acoustic response, diode and phase coherent detection, sensitivity equivalent to the best nuclear acoustic resonance spectrometers, and construction from off‐the‐shelf components. Use of the spectrometer as a cw spectrometer, sampled‐cw spectrometer, and NAR spectrometer will be discussed.

Intermolecular potentials from nmr data: C 2H 2, C 2H 2He, C 2H 2Ar, and C 2H 2N 2

Proton spin-lattice relaxation times were measured in C 2H 2 and mixtures of C 2H 2 with He, Ar, and N 2 as a function of density, composition, and temperature (300–600 K) in the region where T 1 ∞ ϱ with a 30 MHz spin-echo spectrometer using phase-coherent detection. The values of T 1 ϱ for infinite dilution of C 2H 2, ( T 1 ϱ ) C 2H 2−X , were found to be proportional to T − n where n = 1.42 for C 2H 2C 2H 2, 0.69 for C 2H 2He, 1.14 for C 2H 2Ar, and 1.36 for C 2H 2N 2. These data were used to obtain the parameters in the anisotropic intermolecular potential within the framework of Bloom-Oppenheim theory and assume that the correlation time of the spin-rotation interaction in a J state is equal to the lifetime of a molecule in that J state. Lennard-Jones (12-6) potential and modified Buckingham (exp-6) potential were used for the isotropic part. Relative anisotropy in the attractive r −6 term and the quadrupole moment of C 2H 2 molecule obtained from these two model potentials were compared with the calculated value from the polarizability data and the recommended value of C 2H 2, respectively.

Optical Correlation of Fresnel Images

A real-time optical correlator is described that uses an ultrasonic light modulator (ULM) in conjunction with an amplitude mask or replica of the signal to achieve linear, phase-coherent detection, without need for high quality optics. When the mask is placed in a Fresnel image plane of the ULM, optimum correlation and collinear heterodyning are achieved. Correlator response has been calculated and measured as a function of mask-ULM separation for a pulse modulated cw signal and a square wave modulated fm waveform. For essentially band-limited signals, the output is a pair of symmetrically time-shifted, matched filter responses, possessing negligible quadratic phase distortion.

High Efficiency Multiple Access Communications Through a Signal Processing Repeater

This paper describes a signal processing repeater which enhances the performance that can be obtained with random access discrete address (RADA) digital transmission links which operate in a multiple access mode. In general, RADA modulation techniques employ groups of pulses to transmit information. The pulses within a group normally have different carrier frequencies and are separated from each other by one or more time slots. The communication performance capability of a system which uses these techniques is a function of interference between the pulse groups of different users and background noise. A number of RADA techniques were proposed for the U. S. Army to provide communications within field army and divisions. For army application these had the advantages of rapid synchronization and ease of implementation, accompanied by the disadvantage of requiring large amounts of bandwidth. Significant bandwidth reductions can be obtained through the use of efficient data encoding-decoding techniques. Also, the downlink power requirements can be minimized through the use of an antipodal downlink signal. The processing repeater discussed in this paper can convert any number of randomly phased uplink signals to a single antipodal downlink signal which is amenable to phase coherent detection. The required downlink power is thereby minimized. Thus, the use of RADA techniques with efficient encodingdecoding and a processing repeater can provide a multiple access capability which efficiently uses bandwidth and downlink power.

Pulsed Nuclear Resonance Apparatus

The description of a complete pulsed nuclear resonance spectrometer suitable for experiments in solids is given. Its major features are the ability to operate at any frequency between 2 and 30 Mc, an rf pulse output power of 5 kW over most of this range, a receiving system recovery time of 5–10 μsec at 10 Mc, a sensitivity usually limited by thermal noise in the receiver coil, use of phase coherent detection, the availability of a wide variety of pulse sequences, and good long and short term stability of the entire system. Brief instructions for operating the apparatus are included. A method of using the ``boxcar'' integrator to record directly the steady-state absorption (χ″) or dispersion (χ′) is explained.

Automatic Recording Instrument for Measuring Optical Transfer Functions

An instrument is described which is capable of recording automatically both the amplitude and the phase of the O. T. F., as would be required by a test laboratory. A simplified version permits a continuous display on a C. R. T. and this is suitable for rapid workshop assessment of lens performance. The basic system consists of an object generator, an image analyser, and an electronic processing unit. By imaging part of a rotating radial grating on to a slit and then varying the orientation of the lines with respect to the slit it is possible to generate a periodic test pattern of constant contrast but in which the spatial frequency varies over a range of 100 to 1 and includes zero c/mm. The image of this object produced by the lens under test drifts at a constant rate over an analysing slit and photo-multiplier. A single low pass filter selects the fundamental of the photo-multiplier signal and this is proportional to the amplitude of O. T. F. of the lens under test. A reference optical system and photo-multiplier is used to measure phase and to provide a facility for null measurement by phase coherent detection.

Nuclear Relaxation in Aluminum

Nuclear spin-lattice relaxation times ${T}_{1}$ and phase memory times ${T}_{2}$ have been measured in 99.99% pure aluminum foil from 77\ifmmode^\circ\else\textdegree\fi{}K to the melting point by the spin-echo method. At high temperature the line narrows due to self-diffusion, as previously observed by Seymour. The temperature dependence of the narrowing gives an activation energy of self-diffusion ${E}_{D}$ of 1.4\ifmmode\pm\else\textpm\fi{}0.1 ev. From the measured "narrowing temperature" and Nowick's theoretical estimate of ${D}_{0}$, the same value of ${E}_{D}$ is found. As predicted theoretically, measurements show that ${T}_{1}$ arises from interaction with conduction electrons and is inversely proportional to temperature from 1\ifmmode^\circ\else\textdegree\fi{}K to 930\ifmmode^\circ\else\textdegree\fi{}K. At the highest temperatures, an unknown line broadening mechanism is found which is believed to represent quadrupole coupling to strain fields of dislocations. The measurements were made feasible only through the use of a phase-coherent detection system, a description of which is given.