Physical research in space

The study of cosmic rays will have to discover many of the secrets of life on Earth. Cosmic rays are known to have been registered during flights made by automatic space stations. In case the scientists had studied them they would have investigated the process on the Sun. Cosmic rays are supposed to consist of the particles. One of method of the cosmic rays nature study is to investigate them at the stations situated in the mountains. Were there a special accelerator being put into operation in 1967 it produced particles with low energies.

To reach a new stage in the comprehensive study of cosmic rays, man created a powerful rocket to put the largest automatic space stations into orbit round the Earth. The main instrument to be installed in these stations is called the calorimeter. It is able to isolate particles of superhigh energies from the cosmic rays and to measure the energy of each particle.

A FEW WORDS ABOUT PENTODE

Pentode is a tube with five electrodes, a plate or anode, a cathode, a control grid, a screen and suppressor grid.

The cathode emits electrons, the plate collects them. The grid near the cathode is the control grid for controlling the electron flow. The screen grid shields the cathode from the plate helping the plate collect electrons. Moreover the screen grid reduces the capacity between the control grid and the plate. The grid nearest the anode is a suppressor grid. The latter repels secondary electrons and prevents bombardment of the screen grid. Having the negative potential the suppressor grid returns the secondary emission back to the plate and thus eliminates it in the tube.

Emission caused by bombardment of an electrode by electrons from the cathode is called secondary emission as the effect is secondary to the original cathode emission. The control grid is negatively biased. The plate current limitation is removed when a fifth electrode is placed within the tube between the screen and plate. It is usually connected to the cathode.

POWER AMPLIFIER

The final tube in an audio amplifier which is feeding audio frequencies into a loudspeaker mast be essentially a power amplifier. Its task is to deliver undistorted power to the loudspeaker, and not to develop any great amount of voltage amplification. The task of the previous amplifier stages is to build up the small output voltages of the detector so that the large voltages necessary to swing the grid of the power tube may be obtained. The A-C power in the plate circuits of tubes previous to the power stage is small; what is required is that each previous stage will give a maximum of voltage amplification without distortion. The fact that maximum power may not be developed in these plate circuits is not important. These tubes work at very high impedances in which it is not possible to generate much more power although it is possible to build up considerable voltages across them.

The a-c plate current of the last tube then must be large that in turn means that the curve of this tube must have a long and straight, line part.

DUAL-PURPOSE TUBES

Today's radio electronics and engineering are facing some problems those are an obstacle in the way of its further development. They are simplification of the radio tools and devices operation and designing the most reliable and efficient ones.

For reasons of economy and convenience several functions that would otherwise be accomplished by two or more tubes may be handled by a single multipurpose tube. Such a tube consists of the elements of two or more tubes all mounted within a single envelope, each unit acting independently of the others or it may be a combination that depends on its operation of some sort between the several elements.

An example of the first class is the twin triode that contains all the elements of two entirely distinct triodes, except that a single heater is used for both cathodes. It may be used in any circuit applications calling for two similar triodes. Another example is the tube that contains a diode, a triode and a pentode; but in this case the cathode, which is of the filamentary type, is common to all three. An added feature making for flexibility is that a filament tap is brought out, so that the tube may be operated at either 1.4 or 2.8 V for filament heating by using the two halves in parallel or series respectively.

The second class of multipurpose tubes can be illustrated by the 6A8, called a pentagrid converter.

FILTER CIRCUITS

The output waveforms are entirely satisfactory for many applications such as the operation of relays and battery charging. But they are not smooth and continuous enough to be useful for B-voltage supply of amplifiers and radio-receivers. Service of this sort requires that the supply voltage be practically pure d.c. with very little ripple superimposed upon it.

Smoothing of the rectified a.c. voltage is accomplished by the use of filter circuits composed of inductance and capacitance or resistance and capacitance.

Another form of filter circuit is known as the condenser-input filter, since the condenser is supplied directly by the rectifier. In operation the condenser is charged to the peak voltage available from the rectifier and this charge is withdrawn gradually by the load current. Fluctuations in current and voltage are smoothed out L (inductance) and condenser as in the choke-input filter. No further current is supplied by the rectifier until its voltage is again higher than that remaining on the condenser.

Oscillators

In a typical commercial broadcasting station the carrier frequency is generated by a carefully controlled quartz-crystal oscillator. The fundamental method of controlling frequencies in most radio work is by means of tank circuits, or tuned circuits. They have specific values of inductance and capacitance, and they are suitable for the production of alternating currents of a particular frequency and discourage the flow of currents of other frequencies.

In cases where the frequency must be extremely stable, however, a quartz crystal with a definite natural frequency of electrical oscillation is used to stabilize the oscillations. The oscillations are actually generated at low power by an electron tube and are amplified in a series of power amplifiers that act as buffers to prevent interaction of the oscillator with the other components of the transmitter, because such interaction would alter the frequency. The crystal is shaped accurately to the dimensions required to give the desired frequency, which may then be modified slightly by adding a condenser to the circuit to give the exact frequency desired. In a well-designed circuit, such an oscillator does not vary by more than one-hundredth of 1 percent in frequency. Mounting the crystal in a vacuum at constant temperature and stabilizing the supply voltages may produce a frequency stability approaching one-millionth of 1 percent. Crystal oscillators are most useful in the ranges termed very low frequency, low frequency, and medium frequency (VLF, LF, and MF). When frequencies higher than about 10 MHz must be generated, the master oscillator is designed to generate a medium frequency, which is then doubled as often as necessary in special electronic circuits. In cases where rigid frequency control is not required, tuned circuits may be used with conventional electron tubes to generate oscillations up to about 1000 MHz. And reflex klystrons are used to generate the higher frequencies up to 30,000 MHz. Magnetrons are substituted for klystrons when even larger amounts of power must be generated.

Modulation

This text gives very detailed information about the modulation of the carrier wave. The modulation of the carrier wave may carry impulses is performed either at low level or high level. In the former case the audio-frequency signal from the microphone is used to modulate the output of the oscillator. And the modulated carrier frequency is then amplified before it is passed to the antenna. In the latter case the radio-frequency oscillations and the audio-frequency signal are independently amplified, and modulation takes place immediately before the oscillations are passed to the antenna. The signal may be impressed on the carrier either by frequency modulation (FM) or amplitude modulation (AM).

The simplest form of modulation is keying, interrupting the carrier wave at intervals with a key or switch used to form the dots and dashes in continuous-wave radiotelegraphy.

The carrier wave may also be modulated by varying the amplitude, or strength, of the wave in accordance with the variations of frequency and intensity of a sound signal, such as a musical note. This form of modulation, AM, is used in many radiotelephony services including standard radio broadcasts. AM is also employed for carrier current telephony, in which the modulated carrier is transmitted by wire, and in the transmission of still pictures by wire or radio..

In FM the frequency of the carrier wave is varied within a fixed range at a rate corresponding to the frequency of a sound signal. This form of modulation, perfected in the 1930s, has the advantage of yielding signals relatively free from noise and interference arising from such sources as automobile-ignition systems and thunderstorms, which seriously affect AM signals. As a result, FM broadcasting is done on high-frequency bands (88 to 108 MHz), which are suitable for broad signals but have a limited reception range.

Carrier waves can also be modulated by varying the phase of the carrier in accordance with the amplitude of the signal. Phase modulation, however, has generally been limited to special equipment.

The development of the technique of transmitting continuous waves in short bursts or pulses of extremely high power introduced the possibility of yet another form of modulation, pulse-time modulation, in which the spacing of the pulses is varied in accordance with the signal.

The information carried by a modulated wave is restored to its original form by a reverse process called demodulation or detection. Radio waves broadcast at low and medium frequencies are amplitude modulated. At higher frequencies both AM and FM are in use; in present-day commercial television, for example, the sound may be carried by FM, while the picture is carried by AM. In the superhigh-frequency range (above the ultrahigh-frequency range), in which broader bandwidths are available, the picture also may be carried by FM. Experiments have also been conducted in which sound as well as pictures are transmitted digitally at these high frequencies. Such transmissions may some day replace current analog broadcasting techniques.

Antennas

The antenna of a transmitter should not be close to the transmitter itself. Commercial broadcasting at medium frequencies generally requires a very large antenna, which is best located at an isolated point far from cities, whereas the broadcasting studio is usually in the heart of the city. FM, television, and other very-high-frequency broadcasts must have very high antennas if appreciably long range is to be achieved, and it may not be convenient to locate such a high antenna near the broadcasting studio. In all such cases, the signals may be transmitted by wires. Ordinary telephone lines are satisfactory for most commercial radiobroadcasts; if high fidelity or very high frequencies are required, coaxial cables are used.

High-Fidelity Systems

Fidelity is the equality of response of the receiver to various audio-frequency signals modulated on the carrier. Extremely high fidelity, which means a flat frequency response (equal amplification of all audio frequencies) over the entire audible range from about 20 Hz to 20 kHz, is extremely difficult to obtain. A high-fidelity system is no stronger than its weakest link, and the links include not only all the circuits in the receiver, but also the speaker, the acoustic properties of the room in which the speaker is located, and the transmitter to which the receiver is tuned. Most AM radio stations do not reproduce faithfully sounds below 100 Hz or above 5 kHz; FM stations generally have a frequency range of 50 Hz to 15 kHz, the upper limit being set by Federal Communications Commission regulations.

control grid - управляющая сетка

screen grid - экранирующая сетка

suppressor grid - защитная сетка

swing - размах

plate circuits - анодная цепь

impedance - полное сопротивление

filament - волокно, нить накала

in series - последовательно

pentagrid - пентагрид

ripple - пульсация

smoothing - сглаживание

choke-input - дроссельный вход

Тексты для студентов-заочников группы (“РТ” 2 к. 4с.)

GAS-FIELD TRIODE (THYRATRON)

There is no gas at all within conventional diodes, triodes, tetrodes and other tubes. But a gas-filled triode (thyratron) is a tube in which the amount of a gas is sufficient to determine the electrical characteristics of the valve when ionization takes place. In the gas-filled triode the control grid potential determines the plate voltage ionization occurs at. So the more negative the control grid as far as the cathode is concerned, the higher the plate voltage must be before ionization takes place. Ionization taking place, the plasma is produced. The plasma is referred to as a region containing substantially equal numbers of positive ions and electrons, so that the net charge in this region equals zero. If ionization takes place as a result of collision between positive ions and electrons, the arc is formed. The grid loses control over the arc as it is immersed in plasma and is separated from the arc by the positive ion sheath. Grid control can be reestablished when the plate potential is reduced to a value below that is necessary to maintain the arc. It's possible to extinguish the arc by lowering the plate voltage. Then the grid once more becomes the controlling factor and again determines when ionization will occur.

ELECTRONIC SCAN

Electronic movement of the radar beam is essential when the radar must operate in many modes simultaneously. Another principal reason for going to electronic scan is the high failure rate of mechanically scanned antennas.

Electronic scans have been used successfully in large ground radars like the Nike MAR and abroad large ships, as in the Hughes radars on the carrier. The emphasis therefore has switched to a search for simpler scanning systems for use on small ships, in aircraft and on the battlefield where the radar must be transported by motor vehicle or even by men on foot.

Both phase-phase scanning and phase-frequency scanning are being applied for these systems. For either type of scan better ferrite phase shifters are needed that have greater power handling ability, lower RF losses and better temperature stability, require less drive power and cost less. Since even a small phase-phase array uses several thousand-phase shifters, the cost factor is anything but negligible.

A very important development has been the appearance of digital latching phase shifters. These devices use ferrite elements in waveguides that can be switched from one saturation state to the other by a video pulse and remain in the new state between pulses.

ULTRA-THIN FILMS

Scientists at the Westinghouse Research Laboratories are "growing" materials in films only 10 atoms thick. It would take about 100 million of them, put one on the top of another, to make a pile of an inch high. A pile of 2,000 would equal the average wavelength of light. The films are made from both metals and insulating materials and are used in research on the physical properties of thin films, which are becoming of major importance for the fabrication of molecular electronic structures and other advanced micro-electronic devices.

The films are grown by depositing them from the evaporated material inside an ultra-high vacuum chamber developed especially for the purpose. The chamber creates pressures less than 10‾¹² of atmospheric pressure at sea level. This corresponds to the pressure in order in outer space at a distance of some 300 miles about the earth. At such pressures, contamination atoms of unknown composition deposit at a rate only one-millionth that of the material under study.

The vacuum chamber is a double-walled stainless steel vessel that is similar in appearance to a deep-sea diving bell. The space between the walls can be filled with liquid nitrogen to cool the walls to 320 degrees below zero Fahrenheit.