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.     

GaAs m.e.s.f.e.t. prepared by organometallic chemical vapour deposition

GaAs m.e.s.f.e.t.s with gate dimensions of 1.5 ?m × 300 ?m were fabricated in the epitaxial layers grown by organometallic chemical vapour deposition (o.m.c.v.d.) technique. The average saturation velocity in the channel was deduced to be 1.3 × 107 cm/s and is equal to that of epitaxial layers grown by AsCl3 chemical vapour deposition (c.v.d.). The velocity degraded region was confined to within about 350 ? of the interface. A gain of 10 dB and a noise figure of 3 dB with an associated gain of 5.5 dB at 8 GHz were measured.     

Detection and partial correction of isolated errors from the received data in a 1st-order d.p.c.m. codec

A detection and partial-correction (d.p.c.) system located at the receiver in a 1st-order d.p.c.m. system enables isolated transmission errors that would cause gross distortion in the recovered signal of a conventional d.p.c.m. decoder to be corrected from the received data, providing that the input signal does not exhibit a rapid change in its slope during a sampling period. The d.p.c. system is disabled when this occurs.     

Digital Bandlimiting Filters for P.C.M. Transmission Systems

Bandlimiting filtering is required at the input to p.c.m. transmission and switching systems to prevent aliasing. The required filtering can be performed by a combination of analog and digital filters or by analog filters alone. This paper discusses a combination of a simple analog filter and digital filter suitable for Large Scale Integration. Both recursive and transversal digital filters are considered each in a variety of realizations. The realizations include two filters which exploit the companded nature of the p.c.m. code by using logarithmic signal processing to simplify multiplication. Comparisons are made between the realizations and the most suitable types for Large Scale Integration are found to be a recursive look-up table filter and a transversal logarithmic filter.     

Spectra of biternary digital f.m.

Spectra of digital f.m. have been published recently for general n-level transmission. However, biternary signals, as used in p.c.m. systems, are not covered by the published formulas and require a special treatment. Spectrum density and spectrum distribution are derived for biternary transmission, numerical curves are presented for certain values of f.m. deviation, and the results are compared with ordinary binary transmission.     

Radar m.t.i.: a comparison of 2nd-order digital-filter realisations when the stopband is very narrow

Design of recursive digital m.t.i. filters becomes difficult when the cutoff frequency is as low as 1/100 p.r.f., because the filter may have large internal gain, generate high noise levels and have a response which is sensitive to parameter errors. The results of a study of 2nd-order sections are described and two realisations which minimise the word length are presented.     

Growth of Ga0.47In0.53As on InP by low-pressure m.o. c.v.d.

The first heteroepitaxy is reported of Ga0.47In0.53As on InP by low-pressure metalorganic chemical vapour deposition (m.o. c.v.d.). Triethylindium (t.e.i.) and triethylgallium (t.e.g) were used as group III sources, and arsine as the group V source. These Ga0.47In0.53As epilayers exhibit high electron mobility (7.74 cm2 V?1 s?1 for a total impurity concentration of 2.5 × 1016 cm?3 at 300 K) and a photoluminescence efficiency comparable to l.p.e. layers. The lattice mismatch is only ?a0/a0 ? 4 × 10?4. Our mobility measurements indicate that m.o. c.v.d. Ga0.47In0.53As is a promising material for microwave device applications.     

Substrate conduction in GaAs m.e.s.f.e.t.s

Substrate currents in a gateless GaAs m.e.s.f.e.t. have been measured at d.c, 0.9 MHz and 2 GHz. The results are consistent with the assumption of substrate conduction at high frequencies and active-layer conduction alone at low frequencies.     

Stability of p.c.m.-oscillator control systems

An extension of existing techniques allows a proof of the stability of two classes of linear oscillator control system for use in integrated p.c.m. systems. Stability is shown to depend on the response of various filters used in the system in a manner which should be easily realisable in a practical network.     

Optimised hybrid unidigit p.c.m. system

The coding efficiency of unidigit p.c.m. systems has been improved by using a secondary feedback loop, and the gain/frequency characteristic of the coder has been equalised. The overall signal/noise characteristic of this hybrid system is found to be better than those of all other unidigit systems; it is inferior to the conventional p.c.m. system only for bit rates higher than 60 × 103 bit/s.     

ZOGMOST: an improved m.o.s.t. for c.c.d. peripheral circuits

An improved formation for m.o.s. transistors fabricated with the `shadow-etch? c.c.d. process is reported. The narrow self-aligned gaps produced by the process are used to provide an additional screen gate, which allows the gate-drain overlap capacitance to be minimised, thereby improving the frequency characteristics of linear and digital m.o.s. circuits. Depending on the gain of the stage involved, an improvement in the frequency performance of up to 5 times is expected, and practical results for the modified m.o.s.t. agree with this prediction.     

New n-channel m.o.s.f.e.t.s

By using plasma c.v.d. and lift-off, an n-channel m.o.s.f.e.t. with effective channel length of 0.4 ?m has been fabricated. Its main fabrication processes and obtained electrical characteristics are described.     

S.A.W. bandpass filter with ?50 dB sidelobes using unweighted i.d.t.s

The use of a resonantly loaded i.d.t. (r.l.i.d.t.) as an s.a.w. track changer in bandpass filter design is described. A filter with 3dB fractional bandwidth (f.b.w.) of 11.5% and ?50 dB sidelobes is realised using unweighted input/output i.d.t.s of 5? finger pairs each and a 7? finger pair r.l.i.d.t. as the s.a.w. track changer. The scheme is expected to be useful in realising low-shape-factor low-sidelobe bandpass filters using relatively short i.d.t.s.     

Microwave InxGa1?xAsyP1?y/InP f.e.t

Microwave performance of InxGa1?xAsyP1?y quaternary alloy metal-semiconductor field effect transistors (m.e.s.f.e.t.s) is reported. Liquid-phase epitaxy (l.p.e.) using a step cooling technique has been employed to grow submicrometre quaternary layers having room-temperature bandgaps of 0.9, 1.0, 1.05, 1.15 and 1.2eV. F.E.T.s having gate dimensions of 1×200 ?m and a channel length of 5 ?m have been fabricated using 1.15 and 1.2 eV quaternary alloys. A maximum d.c. transconductance of about 10 mS and a maximum available gain of about 7 dB at 10 GHz have been obtained.     

n-channel formation on semi-insulating InP surface by m.i.s.f.e.t.

InP m.i.s.f.e.t.s using Fe-doped semi-insulating material surface have been fabricated. The devices, composed of sulphur-diffused n-type source and drain and c.v.d. Al2O3 gate insulator, exhibited n-channel normally-off behaviour and a source drain capacitance two orders of magnitude smaller than that of p-InP m.i.s.f.e.t.s with the same dimensions.     

X and Ku band GaAs m.e.s.f.e.t.

A GaAs Schottky-barrier f.e.t. (m.e.s.f.e.t.) with a 1 ?m gate has been built, which is suitable for X band and Ku band applications. The unilateral power gain over the X band is above 9 dB and the noise figure is only 5 dB at 10 GHz.     

Normally-off m.e.s.f.e.t. with fast switching behaviour

Normally-off m.c.s.f.e.t.s, fabricated on an insulating GaAs substrate with a vapour-phase-grown double layer (n?/n), showed a d.c. characteristic with a good saturation range and a switching response of 35 ps risetime.     

Two-phase m.n.o.s. charge-coupled device

A 2-phase charge-coupled device (c.c.d.) is proposed. It makes use of charge storage in an m.n.o.s. structure to define in the silicon substrate the asymmetrical potential wells required for unidirectional charge flow. The fabrication and operation of the device are described. The structure permits substantial simplification of the fabrication method as compared with other c.c.d.s.     

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.