Delay device for use in transmission of oscillations

Nov. 25, 1941.

DELAY DEVICE FOR

w. s. PERCIVAL 2,263,902

USE IN TRANSMISSION OF OSCILLATIONS

Filed Feb. 2, 1939

<img file="US2263902A_D0001.tif" />

INVENTOR

WILLIAM SPEN£ERJ>ER.CIVAL

ATTORNEY

Patented Nov. 25, 1941

2,263,902

UNITED STATES PATENT OFFICE.

2,263,902

DELAY DEVICE FOR. USE IN TRANSMISSION OF OSCILLATIONS

William Spencer Percival, Ealing·, London, England, assignor to Electric &amp; Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain

Application February 2, 1939, Serial No. 254,171 In Great Britain February 8, 1938

Claims. (Cl. 178—44)

This invention, relates to. delay devices and is of general application in obtaining delay in the transmission of electric oscillations representing signal or controlling voltages.

High frequency signalling transmitting and 5 receiving systems sometimes include delay networks which are required for: example when the signals are passed through separate channels, the outputs of which are subsequently combined and the time of transmission through one 10 channel is different from that in the other. In order that the transmission times of the separate channels shall be equal, a delay network is placed in the channel having the shorter transmission time. 15

Again, in the interference suppression system described in the specification of British Patent #468,994, for “Improvements in and relating, to the reduction of interference in electrical signal transmission systems,” accepted July 8, 1937, an 20 auxiliary channel supplied with interfering, signals provides a controling voltage which serves to switch off the main signal channel and in order that the controlling voltage shall be applied to the main channel at the desired in- 2S stant, the delay in the auxiliary channel due to the selective and voltage generating circuits, is compensated in the main channel by the provision of a delay network capable of passing a band of signal frequencies. 30

In low frequency volume compression or expansion systems, it is necessary to generate a controlling voltage proportional to the compression effect introduced and in such systems delay in the auxiliary circuits provided, again has to be 35 compensated.

In the specification of U. S. Patent application Serial No. 228,911, filed Sept. 8, 1938, for “Signaling system,” by William S. Percival, now U. S. Pat. #2,219,936, patented Oct. 29, 1940, there 40 is described a wave filter system in which provision is made for selecting from a range of signal frequencies a frequency band of given width and subsequently including said selected band within said range, the output from a filter 45 employed to effect the selection of the smaller band being subtracted from said range. The whole of the frequencies in the range are in certain cases delayed before the subtraction is effected by a period equal to the time delay of the 50 filter. The delay networks employed may consist of filter sections including inductance and capacity in known combinations.

The object of the present invention is to provide apparatus for producing delay in the trans- 55 mission of high, frequency signals and according to the invention an electric, time delay device comprises a. quartz crystal for generating supersonic waves in a liquid through which said waves are transmitted and them detected by another quartz crystal, the delay in transmission: being determined by the distance through the liquid between the crystals. Water or kerosene have been found to be suitable liquids for the purpose of the invention. The crystals should: have the same or substantially, the: same resonant frequency and provision may be made to ensure an unimpeded path through the liquid between the crystals in order to reduce or avoid reflection of the supersonic waves.

The liquid container may be tubular in shape or narrow in one dimension and wide in another or wide in both dimensions. The effective length of the liquid container will, of' course, depend upon the amount of delay required. If any of the dimensions other than those of the length are not small compared with the length it is desirable to take steps to reduce reflection of the waves passing through the liquid; Such reflection can be reduced or eliminated by covering the walls of the liquid container with a suitable absorbent material such as wool or cork or alternatively the inside of the liquid container may have a number of narrow slots or holes formed therein so that the waves passing into the slots or holes are rapidly absorbed. Again a number of sheets of absorbent material may be hung in the liquid so as to leave clear a straight line between the quartz crystals.

In order to obtain a large delay in a comparatively small volume of liquid the container may be divided into a number of compartments, space being left alternatively at each end of the container so as to provide reflecting surfaces for the supersonic waves which are thus reflected into the adjacent compartments. Such a construction is shown diagrammatically in the accompanying drawing in which 1 represents a liquid container, 2 a tortuous space in the container in which kerosene or water is placed, quartz crystals 3 and 5 being arranged at opposite ends of the space 2. The approximate path of travel of the supersonic waves generated by the quartz crystal 3 to the crystal 4 is shown by the dotted line 5.

Assuming a velocity of the supersonic waves of 10⁵ cms./sec. a distance through the liquid of 10 cms. will give a delay of 100 micro seconds. The delay may, of course, be varied by adjusting

2,263,902 the position of one or other of the quartz crystals 3 and 4 along the length of the path 2.

The delay obtained by the use of the device described can be advantageously utilised for most purposes by employing a carrier wave of, for ex- 5 ample, 10 megacycles per second which carrier wave can be modulated by the signal it is desired to delay. The use of a modulated carrier wave is desirable owing to the relatively rapid change of attenuation with frequency and owing to the 10 resonant character of the quartz crystals. If a wider frequency band is desired than can be obtained by means of the natural damping of the quartz crystals by the liquid then the natural frequency of the two crystals can be “staggered” 15 to obtain a relatively flat response over a wide pass-band. The device described enables delays to be obtained of the order of several hundred micro seconds with a pass-band of one megacycle or more. 20

A particular application of the invention lies in the system described in the specification of co-pending U. S. Patent application Serial No. 228,911, now Patent No. 2,235,018, March 18, 1941, the device taking the place of the delay net- 25 works consisting of filter sections including inductance and capacity as indicated in that specification. The device can, however, replace any delay network when a long time delay covering a wide pass-band is required. In such cases it will 30 generally be desirable to include either a modulating and demodulating device respectively before and after the delay in order to change the frequency band to within a region of the order of, for example, 10 magacycles. Alternatively, 35 an oscillator may be used to heterodyne the signal to be delayed to the required frequency.