25-Watt uhf Transmitter

Application	Mission Time	Reliability, Probability of N Failure during Mission	MeanTime to Failure (MTTF)
Aircraft Missile Satellite A Satellite B	8 hr,without maintenance 1,75 hr 1 month 1 yr	0,92 0,99 0,96 0,96	100 hr 175 hr 25 months (18,000 hr) 25 yr (216,000 hr)

TEXT 17

Reliability of electronic systems

With the growth in complexity of electronic systems, perhaps the most crucial problem to be faced, and one particularly critical in military applications, is that of reliability.

The probability that any complete electronic system will function as intended is found by multiplying together the probabilities of the individual components and connections making up the system. Indeed, with the present state of development of reliable electronic devices, the soldered connections in a system—which outnumber the components many times over — are probably more of a collective hazard than are the components themselves.

As a simple numerical example, suppose a system is composed of only seven components and connections, each having a probability of survival of 90 per cent. Connected together, these seven probabilities give an average probability of survival for the entire system of less than 50 per cent (0.9×0.9×0.9×0.9×0.9×0.9×0.9=0.48).

Of course, in practice, the individual probabilities will be higher than 90 per cent. But when the calculation is extended to the thousands of components and tens of thousands of soldered joints composing the electronic systems of modern aircraft or missiles, it is obvious that these individual probabilities must be extremely high if the entire system is to have any real chance of carrying out its intended function. Some idea of the magnitude of the problem can be gained from this comparison: the B-17 or B-29 of World War II used some 2,000 individual electronic components; today, the B-58 requires nearly 100,000 in systems of much greater complexity.

In recent years, therefore, a great deal of attention has been given to this problem of increasing the reliability of electronic components and circuit construction. However, the probability formula shows that a reduction in the number of components and connections raises the over-all probability of system survival much faster than does improvement in the probabilities of the individual parts of the system. For this reason, modern electronics research has as one of its major objectives a reduction in the number of components and connections required to perform a given function. In this respect, molecular electronics is particularly attractive. If a given function is performed within a single, solid block of material, interconnections are eliminated completely.

Text 18 propagation of light

Velocity of Light.– Light is a transverse wave motion. It travels through empty space, as well as through such transparent substances as glass, air, and water. Its velocity, which is 186,000 miles per sec, is so great that in 1 sec it would travel more than seven times around the earth at the equator. Light travels from the sun to the earth in a little over 8 min, but it requires 4 years for light to travel from the nearest star to the earth. If the North Star were obliterated, the earth would continue to receive light from it about 44 years.

Frequency and Wave Length. – The relation between frequency, velocity, and wave length is the same for light waves as it is for sound waves. Waves of yellow light have been found to have a wave length equal to about 0.000059 cm. The wave length of light is often expressed in angstrom units. One angstrom unit = 10^-8 cm.

Fig. 32. Rectilinear propagation of light.

Rectilinear Propagation of Light.– Under ordinary circumstances light travels in straight lines and does not appreciably bend around objects. That light travels in straight lines may be shown by placing a candle or other source of light behind a screen having in it a small hole (Fig. 32). In front of this screen AB are placed two screens CD and GF, each with a small hole at the center. When these screens are so adjusted that the eye E can see the source of light S distinctly, it will be found that the straight line joining S and E passes through the holes in the screens. This shows that light from S to E comes in a straight line.

Sources of Light.– The sun is the chief source of light and heat, but there are many artificial sources. Any body when heated to a sufficient high temperature becomes a source of light.

As the temperature of a body is raised, the body emits invisible radiation. When it becomes red-hot, visible radiations begin to be emitted. The higher the temperature, the greater is the amount of both heat and light waves that are emitted, but the percentage of visible radiations becomes larger and larger as the temperature of the source of radiations is increased. For this reason, the modern tungsten lamp is much more efficient than the old carbon incandescent lamp. Tungsten has a very high melting point, and when it is surrounded by nitrogen or when it is in a vacuum, it can be heated to a high temperature and its efficiency thus made large.