Feasibility of using digital filters to interconnect p.c.m. and f.d.m. transmission systems

A time-division multiplex (t.d.m.) and a frequency-division multiple (f.d.m.) system may be interconnected by means of gates and lowpass filters. The use of digital filters (d.f.s) for this purpose appears attractive when the t.d.m. system uses pulse-code modulation (p.c.m.), because digital-analogue and analogue-digital conversion are unnecessary at the interface between the d.f. and the p.c.m. system. The problems involved in applying d.f.s in this application are discussed. It is concluded that the interconnection of p.c.m. systems with f.d.m. systems using quite low carrier frequencies requires the development of digital circuits operating at several hundred megahertz.     

New V-groove double-diffused m.o.s. (v.d.m.o.s.)

A new V-groove double-diffused m.o.s. (v.d.m.o s.) is proposed which combines the V-groove technology and the double-diffused m.o.s. (d.m.o.s.) technology. The fabrication processes arc qualitatively described. The gate is located on the vertical V-shaped surface, and the effective channel length is controlled by the vertical-diffusion process of the double-diffusion step. The v.d.m.o.s. is expected to have a faster speed and higher production yield than the ordinary d.m.o.s.     

Pitch synchronous 1st-order linear d.p.c.m. system

A system called p.s.f.o.l.d. is described which exploits the correlation between successive pitch periods of a speech signal. This system is a differential one and can employ various types of encoders. We describe a p.s.f.o.l.d. system using a 1st-order d.p.c.m. encoder and show that for a speech utterance this system has a peak signal/noise ratio which is 6 dB larger, and has an increase in dynamic range of 13 dB, compared with a 1st-order d.p.c.m. codec.     

Meander-channel c.c.d. (m.c.c.d.) recursive filter

A modified input-weighting concept for a c.c.d. recursive filter is proposed; it has been implemented using an m.c.c.d. with a complementary input scheme which provides net weighting coefficients. This improved structure requires neither operational amplifiers nor resistors. The operation and performance of the fabricated 2nd-order bandpass filter are presented.     

Meander channel c.c.d. (m.c.c.d.) transversal filter

A simple and compact m.c.c.d. transversal filter is presented. The device features (a) an input-weight technique to achieve a large output voltage, (b) a new `split-input-gate? weighting to provide accurate filter coefficients, and (c) a simplified electrode structure by using the m.c.c.d. as a summing register.     

V-DH laser: a laser with a V-shaped active region grown by metalorganic c.v.d.

A single mode c.w. AlxGa1?xAs/GaAs laser was made by metalorganic c.v.d. A double heterostructure with a V-shaped active region and a junction stripe geometry was grown on a groove-etched substrate. The minimum c.w. threshold current and the maximum differential quantum efficiency obtained were 15 mA and 65% (250 ?m length cavity). The laser's beam shape was nearly circular, with an aspect ratio from 1.3 to 1.4.     

Boron segregation data for d.m.o.s. devices

A simple formula is given for the extent of boron depletion from the surface channel region of a d.m.o.s. f.e.t. during fabrication.     

Noninteger sampling to subcarrier frequency ratios in d.p.c.m. of PAL signals

A simple predictor having three coefficients is described for d.p.c.m. encoding of composite PAL colour signals. Noninteger ratios of sampling frequency fs to subcarrier frequency fsc can be selected. For a colour-bar signal, it is shown that the s.n.r. of the d.p.c.m. system using fs/fsc = 2.5 is approximately the same as when third-previous sample prediction is used, i.e. when fs/fsc = 3.0.     

Low-Loss Two-Dimensional GaAs Epitaxial Waveguides at 10.6-/spl mu/m Wavelength

The successful fabrication of low-loss two-dimensional GaAs epitaxial waveguides by chemical etching for use in integrated optics at 10.6 /spl mu/m is reported. Selective excitation of specific E/sub pq//sup y/ modes was observed by placing the prism at specific angles in the horizontal plane. Loss measurements showed no increase in attenuationfor lower order E/sub pq//sup y/ modes (as compared to corresponding one-dimensional waveguide modes) when the guide width is 50 /spl mu/m. As the guide width is reduced, there is a significant increase in attenuation as p increases.     

Pulsed 1.55 μm transmission mode microchip laser usingsemiconductor modulator

A semiconductor saturable absorber modulator is used, for the first time, in a passively Q-switched microchip laser in transmitting mode. The laser has a wavelength of 1.55 μm in a single longitudinal TEM₀₀ mode beam and has pulse characteristics that compare favourably with previously reported data on comparable devices     

0.34 THz折叠波导行波管设计及流通管实验

以0.34 THz折叠波导行波管为研究对象,分析了慢波结构的色散特性、耦合阻抗、冷损耗特性和工作模式等,并按优化后折叠波导慢波结构的要求设计电子光学系统,进行流通管实验,得到电子注通过率大于80%的实验结果。最后对输入输出结构进行优化设计,满足中心频率为0.345 THz,带宽大于10 GHz,输出功率大于20 mW的0.34 THz折叠波导行波管设计要求。     

MAGICTRAC,a novel method for conversion of whispering-gallery modes into a free-space Gaussian-like beam

A novel device, MAGICTRAC, is described for efficient conversion at millimeter wavelengths of the TE _m,n whispering-gallery mode into a linearly polarized, free-space Gaussian-like beam. MAGICTRAC uses a mode-converting waveguide taper and three mirror optics, one of which incorporates a twist reflector to linearly polarize the output beam. An example design is presented for the TE_15,2 mode at 140 GHz with a calculated efficiency of 96%. Related possible applications include (1) installation of the MAGICTRAC within the vacuum envelope of a gyrotron to separarate the spent e-beam from the generated rf, (2) generation of a whispering-gallery mode by injection of a Gaussian-like beam into the output end, and (3) conversion of TE _m,n modes into TE_0n modes for low-loss transmission in smooth-wall waveguide.     

Injection-locked IMPATT oscillators applied to f.d.m. microwave transmission

The noise, locking bandwidth and gain characteristics of the injection-locked IMPATT amplifier are discussed and related to the requirement for f.d.m. telephone transmission over microwave circuits. It is shown that the unloaded noise produced by the IMPATT diode limits its application to low-gain operation in medium-capacity radio links, although the gain obtainable can be improved by employing multistage amplifiers.     

Selective excitation of high-order modes in circular waveguides

The excitation of very high-order modes in circular waveguides has been performed in a cavity with a connected up-taper with a geometry similar to those used in gyrotrons. A Gaussian beam was coupled to the cavity which was made translucent by an array of holes. With the help of a special optics, the amplitude as well as the phase distribution of the beam was matched to the mode to be excited in the resonant cavity. By simple rotation of one mirror to adjust the phase distribution together with the change of frequency to match the resonance condition, a large number of modes could be produced with one experimental set-up. Field measurements in the output waveguide show a high mode purity of the radiation and confirm the calculations. The method can be used for cold tests of electrodynamic systems operating with these modes, e.g. quasi-optical converters for gyrotrons.     

Upper bound on signal/quantising-noise ratio for television d.p.c.m. systems

Consideration is given to an upper bound on signal/quantising-noise ratio for television d.p.c.m. systems. The 7.2 dB gain for entropy coding assumes ? entropy of the bit stream of the quantiser output. The calculations are based on Laplacian signals, because television signals at d.p.c.m. quantiser inputs are approximately Laplacian.     

E.I.R.P. allocation in single-destination f.d.m.a. satellite communication systems

The optimum allocation of downlink e.i.r.p. in a satellite communication system utilising single-destination f.d.m.a. carriers and a downlink multiple-beam antenna (m.b.a.) is considered. The optimum gain setting for the m.b.a. is determined and it is shown that this setting is independent of the level of the intermodulation and front-end noise generated by the satellite. The optimum allocation is expressed in terms of simple formulae which can be used for adaptive as well as static allocation of satellite power and antenna gain.     

Rays, Beams, and Modes Pertaining to the Excitation of Dielectric Waveguides

The two-dimensional problem of excitation of an inhomogeneous dielectric layer by a Gaussian beam is considered, with emphasis on useful representations that treat the field either in terms of multiple reflections or in terms of guided modes. A recently developed method is employed whereby the beam fields are generated from line source fields by assigning a complex value to the source coordinates. When applied to the asymptotic solution for the line source field, this procedure furnishes a simple and quantitative relation between line-source-excited ray optics and paraxial beam optics. It also clarifies the role of lateral ray and beam shifts for reflection at a boundary with incidence-angle-dependent reflection coefficient, especially when multiply reflected fields are converted into modal form. Results are given for beams which are reflected at both boundaries, reflected at one boundary and refracted before reaching the other boundary, and trapped by refraction without reaching either boundary. In the first case, conversion to modal form is more convenient at large distances whereas in the latter case, paraxial beam tracking is preferable.     

Adaptive statistical encoding of d.p.c.m.-coded videotelephone signals

Adaptive statistical encoding of d.p.c.m.-coded video-telephone signals is considered. The need for an adaptive statistical encoder to operate on short-term batch statistics is explained and the effect on the bit rate of limiting the number of codes employed by the statistical encoder is reported.     

Analogue c.c.d. correlator using monolithic m.o.s.t. multipliers

A 32-stage analogue correlator has been hybridised by using a multitapped c.c.d. delay line and m.o.s.t. multipliers fabricated on the same silicon chip. A multiple-port sample?hold system has been adopted for storage of the reference signal, thereby allowing it to be refreshed continuously. The viability of a design for a fully analogue monolithic c.c.d. correlator is established.     

Possible use of pilot signals to indicate delay distortion in operating f.m.-f.d.m. radio systems

Pilot tones normally used in f.m.-f.d.m. transmission allow propagation distortion to be estimated on each path of an operating system. The phases of the 1st-order sidebands relative to the carrier provide a finite-difference approximation to the gradient of group delay with frequency.