Abstract: A printhead (1) for a continuous ink jet printer has a single pair of nozzles (3,3'). One or more oscillators for vibrating the ink (3,3') may be used to break up the twin streams (4,4') of droplets issuing from the nozzles. A pair (5) of charging gates is provided for applying, selectively and respectively, a charge to droplets in the two streams (4,4'). A pair of deflector means (7) apply respective electric fields across the streams (4,4') to deflect the droplets individually in accordance with the charge thereon. A gutter (10) or pair of gutters (11,11') receive droplets from the respective streams if they are not charged. The arrangement is such that the two streams of droplets are deflected in opposite directions so as to be printed.

Abstract: A building construction system provides a building which contains four separate but connected self-standing units each with its own roof and four walls. A member in the shape of a cross extends vertically above the roofs and horizontally beyond the outside walls of adjacent units. The member has two planar arms intersecting each other at right angles along a central vertical axis, thus providing four 90 degree quadrants, each of which contains a different one of the four units.

Abstract: In order to obtain greater flexibility in the design of a linear-phase electrical filter comprising an acoustic surface wave device including a pair of apodized transducers intercoupled by means of a multi-strip coupler, for example to obtain more equal contributions by the two transducers to the gain versus frequency characteristic of the filter, the transducers are apodized differently but have mutually complementary non-linear phase characteristics. First a transfer characteristic corresponding to the collective contribution to the desired overall gain versus frequency characteristic of the filter, is derived in terms of the z-transform and the roots or the reciprocals of the roots of the resulting polynomial in z.sup.-1 are found. These roots or reciprocals are then divided into two groups so that at least one pair of non-identical roots or reciprocals which have equal ratios between their real and imaginary parts and the product of the moduli of which is equal to unity are included one in each group.

Abstract: A surface acoustic wave bandpass filter consists of input and output interdigital transducers (IP, OP) with an interposed track changing multistrip array (MSA). A conventional broadband array (MSA) is divided into two groups of strips with respect to a center of symmetry (C) separated by different distances along the input track (T1) and the output track (T2) such that the array (MSA) suppresses a predetermined frequency. If the input and output transducers (IP, OP) both have a double electrode configuration, electrode separation .lambda.o/4, providing a fundamental passband at a corresponding frequency f.sub.o and an equal amplitude passband at the third harmonic 3f.sub.o, then a separation difference of .lambda.o/6 between the groups of the array (MSA) in the two tracks (T1, T2) can suppress the third harmonic passband in the filter output. The two groups of the multistrip array may be further sub-divided to suppress more than one frequency (FIG. 5).

Abstract: In order to make optimum use of the available transmission channels in an information transmission system, such as a trunked mobile ratio transmission system, where a number of uncoordinated users may wish to signal to a central control point in addition to using the channels for another purpose, the central control point monitors the channels and, when a channel is or becomes free, sends a command signal on that channel. When a user who is waiting to signal receives the command signal he chooses one of a predetermined number of immediately following time slots in an effectively random manner and sends his request in that slot in digital message form. The central control point sends an acknowledgement in the next time slot if and only if it receives the request in an error-free manner. If a user does not receive such an acknowledgement in response to the transmission of such a request he tries again when he receives the next command.

Abstract: In order to form a first-come first-served queue in an information transmission system employing a common transmission channel when several transmitters of the system try to gain access to the channel when it is already occupied, without employing a further transmission channel to a point at which an account is kept of the order in which the various transmitters request access, each transmitter which tries to gain access in these circumstances is caused to delay after the channel subsequently free before it actually accesses the channel, this delay being inversely proportional to the time the corresponding transmitter has waited. Thus the transmitter which has waited the longest gains the channel first and causes the other waiting transmitters to continue to wait.

Abstract: In an acoustic wave device having a pair of spaced apart reflectors forming a resonant cavity, each reflector consists of an array of parallel reflective elements. At least one of the reflectors is weighted not by simple withdrawal of reflecting elements from a nominal uniform array but instead by substituting reflective elements of the uniform array with elements providing both net-zero reflection at required positions and also at least partial compensation for the velocity perturbation which would be produced by such simple withdrawal. Such weighting may be used to suppress the side lobes of the reflector response and hence improve the stopband performance of the acoustic wave device and, for example, a coupled resonator filter in which that device is incorporated.

Abstract: An acoustic wave resonator device comprises a substrate carrying a pair of spaced apart reflectors forming a resonant cavity that supports only three adjacent distinct resonant mode frequencies. An interdigital transducer is disposed on the substrate between the reflectors and includes an array of electrodes symmetrically arranged relative to a central point of the cavity to form a plurality of electrode pairs with the electrodes of each pair equally spaced on opposite sides of the central point. The central point of the cavity coincides with a node or an anti-node of the standing wave. If the two electrodes of each electrode pair are connected to the same terminal then the transducer has zero net coupling with one set of alternate modes of the cavity, whereas if said two electrodes are connected to opposite terminals then the transducer has zero net coupling with the other set of alternate modes.

Abstract: An acoustic wave resonator device includes two pairs of coupled spaced apart reflectors which define two resonant cavities. The two cavities are each provided with two interdigital transducers which provide intercavity coupling. Additional reactance coupling between the transducers in one cavity combines with the equivalent cavity reactance to provide series or parallel resonance at a selected pole frequency.

Abstract: An acoustic surface wave device employs a substrate structurally orientated to enhance the propagation of acoustic surface skimming bulk waves. Multiple reflections from opposed end faces of the substrate establish an acoustic surface skimming bulk standing wave at a predetermined frequency. An interdigital transducer couples to the standing wave. A plurality of devices may be intercoupled to provide a bandpass filter.

Abstract: The input and output transducers associated with an acoustic surface wave resonator, or with respective acoustically coupled cavities, are split into staggered pairs. Furthermore they are located with respect to the standing wave pattern(s) and connected to input and output terminals in such manner as to suppress the response of the output transducer to travelling waves received directly from the input transducer and reduce the stopband level of the device associated with that response.

Abstract: An electrode structure within the cavity of an acoustic surface wave resonator, e.g. an interdigital transducer or a multistrip coupler, has two sections separated by an odd integral multiple of a quarter wavelength of the standing wave in the cavity and located with respect to the center of the cavity so that all the electrodes are midway between a node and an adjacent antinode of the standing wave. Distortion of the resonator response due to reflection by the electrodes of the transducer or coupler is suppressed by this arrangement.

Abstract: A surface acoustic wave band-pass filter comprising a piezoelectric substrate having on one surface first and second SAW resonators tuned to the same frequency and coupled together. An input transducer is coupled to the first resonator and an output transducer is coupled to the second resonator. Each resonator is preferably formed by two spaced apart aligned grating reflectors and the two resonators are preferably coupled by an array of discrete parallel metallic strips extending between the two cavity resonators.