摘要:

Exact expressions are given for the minimum detectable fractional change in the mean noise-input power to a radio receiver. These expressions involve only the pre-detector and post-detector filter responses and have been derived for receivers using either avth-power instantaneous detector (half- or full-wave type) or an idealvth-power envelope detector.Using these expressions an example, in which the pre-detector filter is a single tuned circuit and the post-detector filter is a simple rc integrator, is worked out in detail. By the use of some well justified approximations in treating this example the results are presented in simple form and show clearly the effect of pre- and post-detector-filter bandwidths, as well as the detector law, on the sensitivity of the receiver to changes in noise-input power. The results for this example are presented graphically.

DOI:10.1049/pi-4.1954.0011

出版商:IEE    

年代:1954

数据来源: IET

摘要:

A method of controlling the position and size of a television-type raster relative to arbitrary references by means of four servo mechanisms is described. One of the applications of such a system is that of a reading device for punched cards. A system for reading any one of a stack of cards without removing the selected card has been developed using the servo-controlled raster. The design of the servo mechanisms for this application is described in detail.

DOI:10.1049/pi-4.1954.0012

出版商:IEE    

年代:1954

数据来源: IET

摘要:

The uses of a precision termination (simulator) in pulse and bridge testing of disc-insulated coaxial cable are outlined and related to the accuracy of match required. A procedure to determine the average cable impedance law with sufficient exactness is described, and a known approximation is used both to illustrate this law and to discuss the departure of local cable impedances from the average. Thus the adjustment facilities necessary in a practical simulator are determined. Details are given of some theoretical networks having the required response, and the course of development from a first laboratory circuit to final prototype is indicated. The measured responses are discussed and the significance of various amendments to the circuit values to achieve the highest accuracy is explained. It is concluded that the design is well suited to the general requirements of pulse testing and to the accurate determination of end impedances to about 0.05 ohm. In bridge testing, similar accuracies in both real and imaginary parts may be obtained between 50 kc/s and 8 Mc/s.

DOI:10.1049/pi-4.1954.0013

出版商:IEE    

年代:1954

数据来源: IET

摘要:

Coaxial resistors used as reflection-free terminations of concentric transmission lines are usually enclosed in a jacket, the characteristic impedance of which is √3 times lower than the resistance.It is shown here that a much better performance can be obtained if this factor is about√4.5 provided additional means for compensating the reactive component of the resistor are used. The performance improvement due to an undercutting in the line and to a special compensating line is discussed in detail, taking also into account the discontinuity reactances in the resistor holder. Design methods and formulae are given, and their use is illustrated by an example.Voltage standing-wave ratios of 1.01 can be obtained with resistor lengths of, λ/8. The frequency range is several times greater than with a non-compensated “√3” jacket.

DOI:10.1049/pi-4.1954.0014

出版商:IEE    

年代:1954

数据来源: IET

摘要:

The paper describes experiments carried out at Cambridge on waves of frequency 30–65 kc/s reflected from the ionosphere at steep incidence. The phases and amplitudes of two linearly polarized components of the downcoming wave were measured with reference to the ground wave. The results for frequencies of 30, 43 and 65 kc/s are considered in turn, and compared with the results for 16 kc/s. The main conclusions were as follows:(a) The day-to-day variations of the downcoming wave were greater at the higher frequencies.(b) The change of height of reflection in passing from day to night was about the same for all the frequencies.(c) In summer, the amplitudes by day and by night were very different, except at 16 kc/s.(d) The polarization at all the frequencies was approximately circular, left-handed and constant.The height of reflection of waves of frequency 30 kc/s was determined by using the frequency-change method of Appleton and Barnett.

DOI:10.1049/pi-4.1954.0015

出版商:IEE    

年代:1954

数据来源: IET