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Now buy some more apparatus. You will need a grid condenser of about 0.0002 mf. The grid leaks to go with it you can make for yourself. I would use a piece of brown wrapping paper and two little metal eyelets. The eyelets can be punched into the paper. Between them coat the paper with carbon ink, or with lead pencil marks. A line about an inch long ought to serve nicely. You will probably wish to make several grid leaks to try. When you get satisfactory operation in receiving by the grid-condenser method the leak will probably be somewhere between a megohm and two megohms.
For this method you will not want a C-battery, but you will wish to operate the detector with about as high a voltage as the manufacturers will recommend for the plate circuit. In this way the incoming signal, which decreases the plate current, can produce the largest decrease. It is also possible to start with the grid slightly positive instead of being as negative as it is when connected to the negative terminal of the A-battery. There will then be possible a greater change in grid voltage. To do so connect the grid as in Fig. 115 to the positive terminal of the A-battery.
About this time I would shop around for two or three small double-pole double-throw switches. Those of the 5-ampere size will do. With these you can arrange to make comparisons between different methods of receiving. Suppose, for example, you connect the switches as shown in Fig. 113 so that by throwing them to the left you are using the audion and to the right the crystal as a detector. You can listen for a minute in one position and then switch and listen for a minute in the other position, and so on back and forth. That way you can tell whether or not you really are getting better results.
If you want a rough measure of how much better the audion is than the crystal you might see, while you are listening to the audion, how much you can rob the telephone receiver of its current and still hear as well as you do when you switch back to the crystal. The easiest way to do this is to put a variable resistance across the receiver as shown in Fig. 113. Adjust this resistance until the intensity of the signal when detected by the audion is the same as for the crystal. You adjust this variable resistance until it by-passes so much of the current, which formerly went through the receiver, that the "audibility" of the signal is reduced until it is the same as for the crystal detector. Carefully made resistances for such a purpose are sold under the name of "audibility meters." You can assemble a resistance which will do fairly well if you will buy a small rheostat which will give a resistance varying by steps of ten ohms from zero to one hundred ohms. At the same time you can buy four resistance spools of one hundred ohms each and perhaps one of 500 ohms. The spools need not be very expensive for you do not need carefully adjusted resistances. Assemble them so as to make a rheostat with a range of 0-1000 ohms by steps of 10 ohms. The cheapest way to mount is with Fahnestock clips as illustrated in Fig. 114. After a while, however, you will probably wish to mount them in a box with a rotary switch on top.
To study the effect of the grid condenser you can arrange switches so as to insert this condenser and its leak and at the same time to cut out the C-battery. Fig. 115 shows how. You can measure the gain in audibility at the same time.
After learning to use the audion as a detector, both by virtue of its curved characteristic and by the grid-condenser method, I would suggest studying the same tube as an amplifier. First I would learn to use it as an audio-frequency amplifier. Set up the crystal detector circuit. Use your audio-frequency transformer the other way around so as to step up to the grid. Put the telephone in the plate circuit. Choose your C-battery for amplification and not detection and try to receive.
You will get better results if you can afford another iron-core transformer. If you can, buy one which will work between the plate circuit of one vacuum tube and the grid circuit of another similar tube. Then you will have the right equipment when you come to make a two-stage audio-frequency amplifier. If you buy such a transformer use the other transformer between plate and telephones as you did before and insert the new one as shown in Fig. 116. This circuit also shows how you can connect the switches so as to see how much the audion is amplifying.
The next step is to use the audion as an amplifier of the radio-signal before its detection. Use the proper C-battery for an amplifier, as determined from the blue print of the tube characteristic. Connect the tube as shown in Fig. 117. You will see that in this circuit we are using a choke coil to keep the radio-frequency current out of the battery part of the plate circuit and a small condenser, another one of 0.002 mf., to keep the battery current from the crystal detector. You can see from the same figure how you can arrange the switches so as to find whether or not you are getting any gain from the amplifier.
Now you are ready to receive those C-W senders at 275 meters. You will need to wind another coil like the secondary coil you already have. Here is where you buy another condenser. You will need it later. If before you bought the 0.0005 size, this time buy the 0.001 size or vice versa. Wind also a small coil for a tickler. About 20 turns of 26 wire on a core of 3-1/2 in. diameter will do. Connect the tickler in the plate circuit of the audion. Connect to the grid your new coil and condenser and set the audion circuit so that it will induce a current in the secondary circuit which supplies the crystal. Fig. 118 shows the hook-up.
You will see that you are now supplying the crystal with current from two sources, namely the distant source of the incoming signals and the local oscillator which you have formed. The crystal will detect the "beat note" between these two currents.
To receive the 275 meters signals you will need to make several adjustments at the same time. In the first place I would set the tuning of the antenna circuit and of the crystal circuit about where you think right because of your knowledge of the settings for other wave lengths. Then I would get the local oscillator going. You can tell whether or not it is going if you suddenly increase or decrease the coupling between the tickler coil and the input circuit of the audion. If this motion is accompanied by a click in the receivers the tube is oscillating.
Now you must change the frequency at which it is oscillating by slowly changing the capacity in the tuned input circuit of the tube. Unless the antenna circuit is properly tuned to the 275 meter signal you will get no results. If it is, you will hear an intermittent musical note for some tune of your local oscillator. This note will have the duration of dots and dashes.
You will have to keep changing the tuning of your detector circuit and of the antenna. For each new setting very slowly swing the condenser plates in the oscillator circuit and see if you get a signal. It will probably be easier to use the "stand-by position," which I have described, with switch S open in the secondary circuit of Fig. 118. In that case you have only to tune your antenna to 275 meters and then you will pick up a note when your local oscillator is in tune. After you have done so you can tune the secondary circuit which supplies the crystal.
If you adopt this method you will want a close coupling between the antenna and the crystal circuit. You will always want a very weak coupling between the oscillator circuit and the detector circuit. You will also probably want a weaker coupling between tickler and tube input than you are at first inclined to believe will be enough. Patience and some skill in manipulation is always required for this sort of experiment.
When you have completed this experiment in heterodyne receiving, using a local oscillator, you are ready to try the regenerative circuit. This has been illustrated in Fig. 92 of Letter 18 and needs no further description. You will have the advantage when you come to this of knowing very closely the proper settings of the antenna circuit and the secondary tuned circuit. You will need then only to adjust the coupling of the tickler and make finer adjustments in your tuning.
After you have completed this series of experiments you will be something of an adept at radio and are in a position to plan your final set. For this set you will need to purchase certain parts complete from reputable dealers because many of the circuits which I have described are patented and should not be used except as rights to use are obtained by the purchase of licensed apparatus which embodies the patented circuits. Knowing how radio receivers operate and why, you are now in a good condition to discuss with dealers the relative merits and costs of receiving sets.
Before you actually buy a completed set you may want to increase the range of frequency over which you are carrying out your experiments. To receive at longer wave-lengths you will need to increase the inductance of your antenna so that it will be tuned to a lower frequency. This is usually called "loading" and can be done by inserting a coil in the antenna. To obtain smaller wave-lengths decrease the effective capacity of the antenna circuit by putting another condenser in series with the antenna. Usually, therefore, one connects into his antenna circuit both a condenser and a loading coil. By using a variable condenser the effective capacity of the antenna system may be easily changed. The result is that this series condenser method becomes the easiest method of tuning and the slide wire tuner is not needed. Fig. 119 shows the circuit.
For quite a range of wave-lengths we may use the same loading coil and tune the antenna circuit entirely by this series condenser. For some other range of wave-lengths we shall then need a different loading coil. In a well-designed set the wave-length ranges overlap. The calculation of the size of loading coil is quite easy but requires more arithmetic than I care to impose on you at present. I shall therefore merely give you illustrations based on the assumption that your antenna has a capacity of 0.0001 or of 0.0002 mf. and that the condensers which you have bought are 0.0005 and 0.001 for their maxima.
In Table I there is given, for each of several values of the inductance of the primary coil, the shortest and the longest wave-lengths which you can expect to receive. The table is in two parts, the first for an antenna of capacity 0.0001 mf. and the second for one of 0.0002 mf. Yours will be somewhere between these two limits. The shortest wave-length depends upon the antenna and not upon the condenser which you use in series with it for tuning. It also depends upon how much inductance there is in the coil which you have in the antenna circuit. The table gives values of inductance in the first column, and of minimum wave-length in the second. The third column shows what is the greatest wave-length you may expect if you use a tuning condenser of 0.0005 mf.; and the fourth column the slightly large wave-length which is possible with the larger condenser.
TABLE I
Part 1. (For antenna of 0.0001 mf.)
Inductance in Shortest wave-length Longest wave-length in meters mil-henries. in meters. with 0.0005 mf. with 0.001 mf.
0.10 103 169 179 0.20 146 238 253 0.40 207 337 358 0.85 300 490 515 1.80 400 700 760 2.00 420 750 800 4.00 600 1080 1130 5.00 660 1200 1260 10.00 900 1700 1790 30.00 1600 2900 3100
Part 2. (For antenna of 0.0002 mf.)
0.10 169 225 240 0.16 210 285 305 0.20 240 320 340 0.25 270 355 380 0.40 340 450 480 0.60 420 550 590 0.80 480 630 680 1.20 585 775 840 1.80 720 950 1020 3.00 930 1220 1320 5.00 1200 1600 1700 8.00 1500 2000 2150 12.00 1850 2400 2650 16.00 2150 2800 3050
From Table I you can find how much inductance you will need in the primary circuit. A certain amount you will need to couple the antenna and the secondary circuit. The coil which you wound at the beginning of your experiments will do well for that. Anything more in the way of inductance, which the antenna circuit requires to give a desired wave-length, you may consider as loading. In Table II are some data as to winding coils on straight cores to obtain various values of inductance. Your 26 s. s. c. wire will wind about 54 turns to the inch. I have assumed that you will have this number of turns per inch on your coils and calculated the inductance which you should get for various numbers of total turns. The first part of the table is for a core of 3.5 inches in diameter and the second part for one of 5 inches. The first column gives the inductance in mil-henries. The second gives number of turns. The third and fourth are merely for convenience and give the approximate length in inches of the coil and the approximate total length of wire which is required to wind it. I have allowed for bringing out taps. In other words 550 feet of the wire will wind a coil of 10.2 inches with an inductance of 8.00 mil-henries, and permit you to bring out taps at all the lower values of inductance which are given in the table.
Table II
Part 1. (For a core of 3.5 in. diam.)
Inductance in Number Length Feet of wire mil-henries. of turns. in inches. required. 0.10 25 0.46 25 0.16 34 0.63 36 0.20 39 0.72 42 0.25 44 0.81 49 0.40 58 1.07 63 0.60 75 1.38 80 0.80 92 1.70 100 0.85 96 1.78 104 1.00 108 2.00 118 1.20 123 2.28 133 1.80 164 3.03 176 2.00 180 3.33 190 3.00 242 4.48 250 4.00 304 5.62 310 5.00 366 6.77 370 8.00 550 10.20 550
Part 2. (For core of 5.0 in. diam.)
2.00 120 2.22 160 3.00 158 2.93 215 4.00 194 3.58 265 5.00 228 4.22 310 8.00 324 6.00 450 10.00 384 7.10 530 12.00 450 8.30 625
The coil which you wound at the beginning of your experiment had only 75 turns and was tapped so that you could, by manipulating the two switches of Fig. 112, get small variations in inductance. In Table III is given the values of the inductance which is controlled by the switches of that figure, the corresponding number of turns, and the wave-length to which the antenna should then be tuned. I am giving this for two values of antenna capacity, as I have done before. By the aid of these three tables you should have small difficulty in taking care of matters of tuning for all wave-lengths below about 3000 meters. If you want to get longer waves than that you had better buy a few banked-wound coils. These are coils in which the turns are wound over each other but in such a way as to avoid in large part the "capacity effects" which usually accompany such winding. You can try winding them for yourself but I doubt if the experience has much value until you have gone farther in the study of the mathematical theory of radio than this series of letters will carry you.
TABLE III Circuit of Fig. 112 Number Inductance in Wave length with antenna of of turns. mil-henries. 0.0001 mf. 0.0002 mf. 14 0.04 120 170 20 0.07 160 220 28 0.12 210 290 36 0.18 250 360 44 0.25 300 420 56 0.38 370 520 75 0.60 460 650
In the secondary circuit there is only one capacity, that of the variable condenser. If it has a range of values from about 0.00005 mf. to 0.0005 mf. your coil of 60 turns and 0.42 mf. permits a range of wave-lengths from 270 to 860 m. Using half the coil the range is 150 to 480 m. With the larger condenser the ranges are respectively 270 to 1220 and 270 to 670. For longer wave-lengths load with inductance. Four times the inductance will tune to double these wave-lengths.
[Footnote 11: If you can afford to buy, or if you can borrow, ammeters and voltmeters of the proper range you should take the characteristic yourself.]
LETTER 22
HIGH-POWERED RADIO-TELEPHONE TRANSMITTERS
MY DEAR EXPERIMENTER:
This letter is to summarize the operations which must be performed in radio-telephone transmission and reception; and also to describe the circuit of an important commercial system.
To transmit speech by radio three operations are necessary. First, there must be generated a high-frequency alternating current; second, this current must be modulated, that is, varied in intensity in accordance with the human voice; and third, the modulated current must be supplied to an antenna. For efficient operation, of course, the antenna must be tuned to the frequency which is to be transmitted. There is also a fourth operation which is usually performed and that is amplification. Wherever the electrical effect is smaller than desired, or required for satisfactory transmission, vacuum tubes are used as amplifiers. Of this I shall give you an illustration later.
Three operations are also essential in receiving. First, an antenna must be so arranged and tuned as to receive energy from the distant transmitting station. There is then in the receiving antenna a current similar in wave form to that in the transmitting antenna. Second, the speech significance of this current must be detected, that is, the modulated current must be demodulated. A current is then obtained which has the same wave form as the human voice which was the cause of the modulation at the distant station. The third operation is performed by a telephone receiver which makes the molecules of air in its neighborhood move back and forth in accordance with the detected current. As you already know a fourth operation may be carried on by amplifiers which give on their output sides currents of greater strength but of the same forms as they receive at their input terminals.
In transmitting and in receiving equipment two or more of these operations may be performed by the same vacuum tube as you will remember from our discussion of the regenerative circuit for receiving. For example, also, in any receiving set the vacuum tube which detects is usually amplifying. In the regenerative circuit for receiving continuous waves by the heterodyne method the vacuum tube functions as a generator of high-frequency current and as a detector of the variations in current which occur because the locally-generated current does not keep in step with that generated at the transmitting station.
Another example of a vacuum tube performing simultaneously two different functions is illustrated in Fig. 120 which shows a simple radio-telephone transmitter. The single tube performs in itself both the generation of the radio-frequency current and its modulation in accordance with the output of the carbon-button transmitter. This audion is in a feed-back circuit, the oscillation frequency of which depends upon the condenser _C_ and the inductance _L_. The voice drives the diaphragm of the transmitter and thus varies the resistance of the carbon button. This varies the current from the battery, _B_{a}_, through the primary, _T_{1}_, of the transformer _T_. The result is a varying voltage applied to the grid by the secondary _T_{2}_. The oscillating current in the plate circuit of the audion varies accordingly because it is dependent upon the grid voltage. The condenser _C_{r}_ offers a low impedance to the radio-frequency current to which the winding _T_{2}_ of audio-frequency transformer offers too much.
In this case the tube is both generator and "modulator." In some cases these operations are separately performed by different tubes. This was true of the transmitting set used in 1915 when the engineers of the Bell Telephone System talked by radio from Arlington, near Washington, D. C., to Paris and Honolulu. I shall not draw out completely the circuit of their apparatus but I shall describe it by using little squares to represent the parts responsible for each of the several operations.
First there was a vacuum tube oscillator which generated a small current of the desired frequency. Then there was a telephone transmitter which made variations in a direct-current flowing through the primary of a transformer. The e. m. f. from the secondary of this transformer and the e. m. f. from the radio-frequency oscillator were both impressed upon the grid of an audion which acted as a modulator. The output of this audion was a radio-frequency current modulated by the voice. The output was amplified by a two-stage audion amplifier and supplied through a coupling coil to the large antenna of the U. S. Navy Station at Arlington. Fig. 121 shows the system.
The audion amplifiers each consisted of a number of tubes operating in parallel. When tubes are operated in parallel they are connected as shown in Fig. 122 so that the same e. m. f. is impressed on all the grids and the same plate-battery voltage on all the plates. As the grids vary in voltage there is a corresponding variation of current in the plate circuit of each tube. The total change of the current in the plate-battery circuit is, then, the sum of the changes in all the plate-filament circuits of the tubes. This scheme of connections gives a result equivalent to that of a single tube with a correspondingly larger plate and filament.
Parallel connection is necessary because a single tube would be overheated in delivering to the antenna the desired amount of power. You remember that when the audion is operated as an amplifier the resistance to which it supplies current is made equal to its own internal resistance of _R_{p}_. That means that there is in the plate circuit just as much resistance inside the tube as outside. Hence there is the same amount of work done each second in forcing the current through the tube as through the antenna circuit, if that is what the tube supplies. "Work per second" is power; the plate battery is spending energy in the tube at the same rate as it is supplying it to the antenna where it is useful for radiation.
All the energy expended in the tube appears as heat. It is due to the blows which the electrons strike against the plate when they are drawn across from the filament. These impacts set into more rapid motion the molecules of the plate; and the temperature of the tube rises. There is a limit to the amount the temperature can rise without destroying the tube. For that reason the heat produced inside it must not exceed a certain limit depending upon the design of the tube and the method of cooling it as it is operated. In the Arlington experiments, which I mentioned a moment ago, the tubes were cooled by blowing air on them from fans.
We can find the power expended in the plate circuit of a tube by multiplying the number of volts in its battery by the number of amperes which flows. Suppose the battery is 250 volts and the current 0.02 amperes, then the power is 5 watts. The "watt" is the unit for measuring power. Tubes are rated by the number of watts which can be safely expended in them. You might ask, when you buy an audion, what is a safe rating for it. The question will not be an important one, however, unless you are to set up a transmitting set since a detector is usually operated with such small plate-voltage as not to have expended in it an amount of power dangerous to its life.
In recent transmitting sets the tubes are used in parallel for the reasons I have just told, but a different method of modulation is used. The generation of the radio-frequency current is by large-powered tubes which are operated with high voltages in their plate circuits. The output of these oscillators is supplied to the antenna. The intensity of the oscillations of the current in these tubes is controlled by changing the voltage applied in their plate circuits. You can see from Fig. 123 that if the plate voltage is changed the strength of the alternating current is changed accordingly. It is the method used in changing the voltage which is particularly interesting.
The high voltages which are used in the plate circuits of these high-powered audions are obtained from generators instead of batteries. You remember from Letter 20 that an e. m. f. is induced in a coil when the coil and a magnet are suddenly changed in their positions, one being turned with reference to the other. A generator is a machine for turning a coil so that a magnet is always inducing an e. m. f. in it. It is formed by an armature carrying coils and by strong electromagnets. The machine can be driven by a steam or gas engine, by a water wheel, or by an electric motor. Generators are designed either to give steady streams of electrons, that is for d-c currents, or to act as alternators.
Suppose we have, as shown in Fig. 124, a d-c generator supplying current to a vacuum tube oscillator. The current from the generator passes through an iron-cored choke coil, marked L{a} in the figure. Between this coil and the plate circuit we connect across the line a telephone transmitter. To make a system which will work efficiently we shall have to suppose that this transmitter has a high resistance, say about the same as the internal resistance, R{p}, of the tube and also that it can carry as large a current.
Of the current which comes from the generator about one-half goes to the tube and the rest to the transmitter. If the resistance of the transmitter is increased it can't take as much current. The coil, _L_{a}_, however, because of its inductance, tends to keep the same amount of current flowing through itself. For just an instant then the current in _L_{a}_ keeps steady even though the transmitter doesn't take its share. The result is more current for the oscillating tube. On the other hand if the transmitter takes more current, because its resistance is decreased, the choke coil, _L_{a}_, will momentarily tend to keep the current steady so that what the transmitter takes must be at the expense of the oscillating tube.
That's one way of looking at what happens. We know, however, from Fig. 123 that to get an increase in the amplitude of the current in the oscillating tube we must apply an increased voltage to its plate circuit. That is what really happens when the transmitter increases in resistance and so doesn't take its full share of the current. The reason is this: When the transmitter resistance is increased the current in the transmitter decreases. Just for a moment it looks as though the current in L{a} is going to decrease. That's the way it looks to the electrons; and you know what electrons do in an inductive circuit when they think they shall have to stop. They induce each other to keep on for a moment. For a moment they act just as if there was some extra e. m. f. which was acting to keep them going. We say, therefore, that there is an extra e. m. f., and we call this an e. m. f. of self-induction. All this time there has been active on the plate circuit of the tube the e. m. f. of the generator. To this there is added at the instant when the transmitter resistance increases, the e. m. f. of self-induction in the coil, L{a} and so the total e. m. f. applied to the tube is momentarily increased. This increased e. m. f., of course, results in an increased amplitude for the alternating current which the oscillator is supplying to the transmitting antenna.
When the transmitter resistance is decreased, and a larger current should flow through the choke coil, the electrons are asked to speed up in going through the coil. At first they object and during that instant they express their objection by an e. m. f. of self-induction which opposes the generator voltage. For an instant, then, the voltage of the oscillating tube is lowered and its alternating-current output is smaller.
For the purpose of bringing about such threatened changes in current, and hence such e. m. f.'s of self-induction, the carbon transmitter is not suitable because it has too small a resistance and too small a current carrying ability. The plate circuit of a vacuum tube will serve admirably. You know from the audion characteristic that without changing the plate voltage we can, by applying a voltage to the grid, change the current through the plate circuit. Now if it was a wire resistance with which we were dealing and we should be able to obtain a change in current without changing the voltage acting on this wire we would say that we had changed the resistance. We can say, therefore, that the internal resistance of the plate circuit of a vacuum tube can be changed by what we do to the grid.
In Fig. 125 I have substituted the plate circuit of an audion for the transmitter of Fig. 124 and arranged to vary its resistance by changing the potential of the grid. This we do by impressing upon the grid the e. m. f. developed in the secondary of a transformer, to the primary of which is connected a battery and a carbon transmitter. The current through the primary varies in accordance with the sounds spoken into the transmitter. And for all the reasons which we have just finished studying there are similar variations in the output current of the oscillating tube in the transmitting set of Fig. 125.
In this latter figure you will notice a small air-core coil, _L_{R}_, between the oscillator and the modulator tube. This coil has a small inductance but it is enough to offer a large impedance to radio-frequency currents. The result is, it does not let the alternating currents of the oscillating tube flow into the modulator. These currents are confined to their own circuit, where they are useful in establishing similar currents in the antenna. On the other hand, the coil _L_{R}_ doesn't seriously impede low-frequency currents and therefore it does not prevent variations in the current which are at audio-frequency. It does not interfere with the changes in current which accompany the variations in the resistance of the plate circuit of the modulator. That is, it has too little impedance to act like _L_{a}_ and so it permits the modulator to vary the output of the oscillator.
The oscillating circuit of Fig. 125 includes part of the antenna. It differs also from the others I have shown in the manner in which grid and plate circuits are coupled. I'll explain by Fig. 126.
The transmitting set which I have just described involves many of the principles of the most modern sets. If you understand its operation you can probably reason out for yourself any of the other sets of which you will hear from time to time.
LETTER 23
AMPLIFICATION AT INTERMEDIATE FREQUENCIES
DEAR SON:
In the matter of receiving I have already covered all the important principles. There is one more system, however, which you will need to know. This is spoken of either as the "super-heterodyne" or as the "intermediate-frequency amplification" method of reception.
The system has two important advantages. First, it permits sharper tuning and so reduces interference from other radio signals. Second, it permits more amplification of the incoming signal than is usually practicable.
First as to amplification: We have seen that amplification can be accomplished either by amplifying the radio-frequency current before detection or by amplifying the audio-frequency current which results from detection. There are practical limitations to the amount of amplification which can be obtained in either case. An efficient multi-stage amplifier for radio-frequencies is difficult to build because of what we call "capacity effects."
Consider for example the portion of circuit shown in Fig. 127. The wires a and b act like small plates of condensers. What we really have, is a lot of tiny condensers which I have shown in the figure by the light dotted-lines. If the wires are transmitting high-frequency currents these condensers offer tiny waiting-rooms where the electrons can run in and out without having to go on to the grid of the next tube. There are other difficulties in high-frequency amplifiers. This one of capacity effects between parallel wires is enough for the present. It is perhaps the most interesting because it is always more or less troublesome whenever a pair of wires is used to transmit an alternating current.
In the case of a multi-stage amplifier of audio-frequency current there is always the possibility of the amplification of any small variations in current which may naturally occur in the action of the batteries. There are always small variations in the currents from batteries, due to impurities in the materials of the plates, air bubbles, and other causes. Ordinarily we don't observe these changes because they are too small to make an audible sound in the telephone receivers. Suppose, however, that they take place in the battery of the first tube of a series of amplifiers. Any tiny change of current is amplified many times and results in a troublesome noise in the telephone receiver which is connected to the last tube.
In both types of amplifiers there is, of course, always the chance that the output circuit of one tube may be coupled to and induce some effect in the input circuit of one of the earlier tubes of the series. This will be amplified and result in a greater induction. In other words, in a circuit where there is large amplification, there is always the difficulty of avoiding a feed-back of energy from one tube to another so that the entire group acts like an oscillating circuit, that is "regeneratively." Much of this difficulty can be avoided after experience.
If a multi-stage amplifier is to be built for a current which does not have too high a frequency the "capacity effects" and the other difficulties due to high-frequency need not be seriously troublesome. If the frequency is not too high, but is still well above the audible limit, the noises due to variations in battery currents need not bother for they are of quite low frequency. Currents from 20,000 to 60,000 cycles a second are, therefore, the most satisfactory to amplify.
Suppose, however, one wishes to amplify the signals from a radio-broadcasting station. The wave-length is 360 meters and the frequency is about 834,000 cycles a second. The system of intermediate-frequency amplification solves the difficulty and we shall see how it does so.
At the receiving station a local oscillator is used. This generates a frequency which is about 30,000 cycles less than that of the incoming signal. Both currents are impressed on the grid of a detector. The result is, in the output of the detector, a current which has a frequency of 30,000 cycles a second. The intensity of this detected current depends upon the intensity of the incoming signal. The "beat note" current of 30,000 cycles varies, therefore, in accordance with the voice which is modulating at the distant sending station. The speech significance is now hidden in a current of a frequency intermediate between radio and audio. This current may be amplified many times and then supplied to the grid of a detector which obtains from it a current of audio-frequency which has a speech significance. In Fig. 128 I have indicated the several operations.
We can now see why this method permits sharper tuning. The whole idea of tuning, of course, is to arrange that the incoming signal shall cause the largest possible current and at the same time to provide that any signals at other wave-lengths shall cause only negligible currents. What we want a receiving set to do is to distinguish between two signals which differ slightly in wave-length and to respond to only one of them.
Suppose we set up a tuned circuit formed by a coil and a condenser and try it out for various frequencies of signals. You know how it will respond from our discussion in connection with the tuning curve of Fig. 51 of Letter 13. We might find from a number of such tests that the best we can expect any tuned circuit to do is to discriminate between signals which differ about ten percent in frequency, that is, to receive well the desired signal and to fail practically entirely to receive a signal of a frequency either ten percent higher or the same amount lower.
For example, if the signal is at 30,000 cycles a tuned circuit might be expected to discriminate against an interfering signal of 33,000. If the signal is at 300,000 cycles a tuned circuit might discriminate against an interfering signal of 330,000 cycles, but an interference at 303,000 cycles would be very troublesome indeed. It couldn't be "tuned out" at all.
Now suppose that the desired signal is at 300,000 cycles and that there is interference at 303,000 cycles. We provide a local oscillator of 270,000 cycles a second, receive by this "super-heterodyne" method which I have just described, and so obtain an intermediate frequency. In the output of the first detector we have then a current of 300,000—270,000 or 30,000 cycles due to the desired signal and also a current of 303,000—270,000 or 33,000 cycles due to the interference. Both these currents we can supply to another tuned circuit which is tuned for 30,000 cycles a second. It can receive the desired signal but it can discriminate against the interference because now the latter is ten percent "off the tune" of the signal.
You see the question is not one of how far apart two signals are in number of cycles per second. The question always is: How large in percent is the difference between the two frequencies? The matter of separating two effects of different frequencies is a question of the "interval" between the frequencies. To find the interval between two frequencies we divide one by the other. You can see that if the quotient is larger than 1.1 or smaller than 0.9 the frequencies differ by ten percent or more. The higher the frequency the larger the number of cycles which is represented by a given size of interval.
While I am writing of frequency intervals I want to tell you one thing more of importance. You remember that in human speech there may enter, and be necessary, any frequency between about 200 and 2000 cycles a second. That we might call the range of the necessary notes in the voice. Whenever we want a good reproduction of the voice we must reproduce all the frequencies in this range.
Suppose we have a radio-current of 100,000 cycles modulated by the frequencies in the voice range. We find in the output of our transmitting set not only a current of 100,000 cycles but currents in two other ranges of frequencies. One of these is above the signal frequency and extends from 100,200 to 102,000 cycles. The other is the same amount below and extends from 98,000 to 99,800 cycles. We say there is an upper and a lower "band of frequencies."
All these currents are in the complex wave which comes from the radio-transmitter. For this statement you will have to take my word until you can handle the form of mathematics known as "trigonometry." When we receive at the distant station we receive not only currents of the signal frequency but also currents whose frequencies lie in these "side-bands."
No matter what radio-frequency we may use we must transmit and receive side-bands of this range if we use the apparatus I have described in the past letters. You can see what that means. Suppose we transmit at a radio-frequency of 50,000 cycles and modulate that with speech. We shall really need all the range from 48,000 cycles to 52,000 cycles for one telephone message. On the other hand if we modulated a 500,000 cycle wave by speech the side-bands are from 498,000 to 499,800 and 500,200 to 502,000 cycles. If we transmit at 50,000 cycles, that is, at 6000 meters, we really need all the range between 5770 meters and 6250 meters, as you can see by the frequencies of the side-bands. At 100,000 cycles we need only the range of wave-lengths between 2940 m. and 3060 m. If the radio-frequency is 500,000 cycles we need a still smaller range of wave-lengths to transmit the necessary side-bands. Then the range is from 598 m. to 603 m.
In the case of the transmission of speech by radio we are interested in having no interference from other signals which are within 2000 cycles of the frequency of our radio-current no matter what their wave-lengths may be. The part of the wave-length range which must be kept clear from interfering signals becomes smaller the higher the frequency which is being modulated.
You can see that very few telephone messages can be sent in the long-wave-length part of the radio range and many more, although not very many after all, in the short wave-length part of the radio range. You can also see why it is desirable to keep amateurs in the short wave-length part of the range where more of them can transmit simultaneously without interfering with each other or with commercial radio stations.
There is another reason, too, for keeping amateurs to the shortest wave-lengths. Transmission of radio signals over short distances is best accomplished by short wave-lengths but over long distances by the longer wave-lengths. For trans-oceanic work the very longest wave-lengths are best. The "long-haul" stations, therefore, work in the frequency range immediately above 10,000 cycles a second and transmit with wave lengths of 30,000 m. and shorter.
LETTER 24
BY WIRE AND BY RADIO
DEAR BOY:
The simplest wire telephone-circuit is formed by a transmitter, a receiver, a battery, and the connecting wire. If two persons are to carry on a conversation each must have this amount of equipment. The apparatus might be arranged as in Fig. 129. This set-up, however, requires four wires between the two stations and you know the telephone company uses only two wires. Let us find the principle upon which its system operates because it is the solution of many different problems including that of wire-to-radio connections.
Imagine four wire resistances connected together to form a square as in Fig. 130. Suppose there are two pairs of equal resistances, namely R{1} and R{2}, and Z{1} and Z{2}. If we connect a generator, G, between the junctions a and b there will be two separate streams of electrons, one through the R-side and the other through the Z-side of the circuit. These streams, of course, will not be of the same size for the larger stream will flow through the side which offers the smaller resistance.
Half the e. m. f. between a and b is used up in sending the stream half the distance. Half is used between a and the points c and d, and the other half between c and d and the other end. It doesn't make any difference whether we follow the stream from a to c or from a to d, it takes half the e. m. f. to keep this stream going. Points c and d, therefore, are in the same condition of being "half-way electrically" from a to b. The result is that there can be no current through any wire which we connect between c and d.
Suppose, therefore, that we connect a telephone receiver between c and d. No current flows in it and no sound is emitted by it. Now suppose the resistance of Z{2} is that of a telephone line which stretches from one telephone station to another. Suppose also that Z{1} is a telephone line exactly like Z{2} except that it doesn't go anywhere at all because it is all shut up in a little box. We'll call Z{1} an artificial telephone line. We ought to call it, as little children would say, a "make-believe" telephone line. It doesn't fool us but it does fool the electrons for they can't tell the difference between the real line Z{2} and the artificial line Z{1}. We can make a very good artificial line by using a condenser and a resistance. The condenser introduces something of the capacity effects which I told you were always present in a circuit formed by a pair of wires.
At the other telephone station let us duplicate this apparatus, using the same real line in both cases. Instead of just any generator of an alternating e. m. f. let us use a telephone transmitter. We connect the transmitter through a transformer. The system then looks like that of Fig. 131. When some one talks at station 1 there is no current through his receiver because it is connected to c and d, while the e. m. f. of the transmitter is applied to a and b. The transmitter sets up two electron streams between a and b, and the stream which flows through the Z-side of the square goes out to station 2. At this station the electrons have three paths between d and b. I have marked these by arrows and you see that one of them is through the receiver. The current which is started by the transmitter at station 1 will therefore operate the receiver at station 2 but not at its own station. Of course station 2 can talk to 1 in the same way.
The actual set-up used by the telephone company is a little different from that which I have shown because it uses a single common battery at a central office between two subscribers. The general principle, however, is the same.
It won't make any difference if we use equal inductance coils, instead of the R-resistances, and connect the transmitter to them inductively as shown in Fig. 132. So far as that is concerned we can also use a transformer between the receiver and the points c and d, as shown in the same figure.
We are now ready to put in radio equipment at station 2. In place of the telephone receiver at station 2 we connect a radio transmitter. Then whatever a person at station 1 says goes by wire to 2 and on out by radio. In place of the telephone transmitter at station 2 we connect a radio receiver. Whatever that receives by radio is detected and goes by wire to the listener at station 1. In Fig. 133 I have shown the equipment of station 2. There you have the connections for wire to radio and vice versa.
One of the most interesting developments of recent years is that of "wired wireless" or "carrier-current telephony" over wires. Suppose that instead of broadcasting from the antenna at station 2 we arrange to have its radio transmitter supply current to a wire circuit. We use this same pair of wires for receiving from the distant station. We can do this if we treat the radio transmitter and receiver exactly like the telephone instruments of Fig. 132 and connect them to a square of resistances. One of these resistances is, of course, the line between the stations. I have shown the general arrangement in Fig. 134.
You see what the square of resistances, or "bridge" really does for us. It lets us use a single pair of wires for messages whether they are coming or going. It does that because it lets us connect a transmitter and also a receiver to a single pair of wires in such a way that the transmitter can't affect the receiver. Whatever the transmitter sends out goes along the wires to the distant receiver but doesn't affect the receiver at the sending station. This bridge permits this whether the transmitter and receiver are radio instruments or are the ordinary telephone instruments.
By its aid we may send a modulated high-frequency current over a pair of wires and receive from the same pair of wires the high-frequency current which is generated and modulated at the distant end of the line. It lets us send and receive over the same pair of wires the same sort of a modulated current as we would supply to an antenna in radio-telephone transmitting. It is the same sort of a current but it need not be anywhere near as large because we aren't broadcasting; we are sending directly to the station of the other party to our conversation.
If we duplicate the apparatus we can use the same pair of wires for another telephone conversation without interfering with the first. Of course, we have to use a different frequency of alternating current for each of the two conversations. We can send these two different modulated high-frequency currents over the same pair of wires and separate them by tuning at the distant end just as well as we do in radio. I won't sketch out for you the tuned circuits by which this separation is made. It's enough to give you the idea.
In that way, a single pair of wires can be used for transmitting, simultaneously and without any interference, several different telephone conversations. It takes very much less power than would radio transmission and the conversations are secret. The ordinary telephone conversation can go on at the same time without any interference with those which are being carried by the modulations in high-frequency currents. A total of five conversations over the same pair of wires is the present practice.
This method is used between many of the large cities of the U. S. because it lets one pair of wires do the work of five. That means a saving, for copper wire costs money. Of course, all the special apparatus also costs money. You can see, therefore, that this method wouldn't be economical between cities very close together because all that is saved by not having to buy so much wire is spent in building special apparatus and in taking care of it afterwards. For long lines, however, by not having to buy five times as much wire, the Bell Company saves more than it costs to build and maintain the extra special apparatus.
I implied a moment ago why this system is called a "carrier-current" system; it is because "the high-frequency currents carry in their modulations the speech significance." Sometimes it is called a system of "multiplex" telephony because it permits more than one message at a time.
This same general principle is also applied to the making of a multiplex system of telegraphy. In the multiplex telephone system we pictured transmitting and receiving sets very much like radio-telephone sets. If instead of transmitting speech each transmitter was operated as a C-W transmitter then it would transmit telegraph messages. In the same frequency range there can be more telegraph systems operated simultaneously without interfering with each other, for you remember how many cycles each radio-telephone message requires. For that reason the multiplex telegraph system which operates by carrier-currents permits as many as ten different telegraph messages simultaneously.
You remember that I told you how capacity effects rob the distant end of a pair of wires of the alternating current which is being sent to them. That is always true but the effect is not very great unless the frequency of the alternating current is high. It's enough, however, so that every few hundred miles it is necessary to connect into the circuit an audion amplifier. This is true of carrier currents especially, but also true of the voice-frequency currents of ordinary telephony. The latter, however, are not weakened, that is, "attenuated," as much and consequently do not need to be amplified as much to give good intelligibility at the distant receiver.
In a telephone circuit over such a long distance as from New York City to San Francisco it is usual to insert amplifiers at about a dozen points along the route. Of course, these amplifiers must work for transmission in either direction, amplifying speech on its way to San Francisco or in the opposite direction. At each of the amplifying stations, or "repeater stations," as they are usually called, two vacuum tube amplifiers are used, one for each direction. To connect these with the line so that each may work in the right direction there are used two of the bridges or resistance squares. You can see from the sketch of Fig. 135 how an alternating current from the east will be amplified and sent on to the west, or vice versa.
There are a large number of such repeater stations in the United States along the important telephone routes. In Fig. 136 I am showing you the location of those along the route of the famous "transcontinental telephone-circuit." This shows also a radio-telephone connection between the coast of California and Catalina Island. Conversations have been held between this island and a ship in the Atlantic Ocean, as shown in the sketch. The conversation was made possible by the use of the vacuum tube and the bridge circuit. Part of the way it was by wire and part by radio. Wire and radio tie nicely together because both operate on the same general principles and use much of the same apparatus.
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INDEX
A-battery for tubes, 42
Accumulator, 29
Acid, action of hydrogen in, 7
Air, constitution of, 10
Ammeter, alternating current, 206; calibration of, 53; construction of, 205
Ampere, 49, 54
Amplification, 182; one stage of, 185
Amplitude of vibration, 155
Antenna current variation, 141
Arlington tests, 233
Artificial telephone line, 252
Atom, conception of, 6; nucleus of, 10; neutral, 34
Atomic number, 13
Atoms, difference between, 12; kinds of, 6, 10; motion of, 35
Attenuation of current in wires, 259
Audibility meter, 218
Audio-frequency amplifier, 185; limitations of, 185
Audion, 35, 40, 42
Audion, amplifier, 182; detector, theory of, 126; modulator, 232; oscillator, theory of, 89; frequency control of, 99
B-battery for tubes, 43; effect upon characteristic, 128
Banked wound coils, 228
Battery, construction of gravity, 16; dry, 27; reversible or storage, 29
Band of frequencies, 249
Beat note, detection of, 221, 245
Bell system, Arlington transmitter, 249
Blocking of tube, reason for, 171
Blue vitriol, 16
Bridge circuit, 255
Bureau of Standards, 50
C-battery for tubes, 46, 166; variation of, 75; for detection, 66
Calibration of a receiver, 214
Capacity, effect upon frequency, 100; measurement of, 104; unit of, 104; variable, 107
Capacity effects, 243; elimination of, 228
Carrier current, modulation of, 146; telephony, 255
Characteristic, of vacuum tube, 68, 74; effect of B-battery upon, 128; how to plot a, 70
Characteristic curve of transformer, 64
Chemistry, 8
Choke coils, 210, 221
Circuit, A, B, C, 187; coupled, 115; defined, 43; oscillating, 113; plate, 45; short, 30; tune of a, 117
Condenser, defined, 77; charging current of, 78; discharge current of, 80; impedance of, 135; theory of, 78; tuning, 224
Common battery system, 254
Connection for wire to radio, 254
Continuous waves, 86
Copper, atomic number of, 13
Copper sulphate, in solution, 21
Crystals, atomic structure, 147
Crystal detectors, 146; characteristic of, 148; circuit of, 150; theory of, 147
Current, transient, 114; radio, 144
Cycle, 94, 97
Damped oscillations, 114
Demodulation, 231
Detection, explained, 146
Detectors, audion, 126; crystal, 146
Direct currents, 205
Dissociation, 22
Distortion, of wave form, 163
Dry battery, 27
Earth, atomic constitution, 11
Effective value, of ampere, 207; of volt, 207
Efficiency, of regenerative circuit, 182
Electrical charge, 22
Electricity, current of, 15, 16
Electrodes, of vacuum tube, 41; definition of, 41
Electrolyte, definition of, 34
Electrons, properties of, 4; planetary, 10, 12; rate of flow, 48; vapor of, 39; wandering of, 14
Electron streams, laws of attraction, 200
E. M. F., 59; alternating, 76; of self-induction, 238
Energy, expended in tube, 235; of electrons, 113; radiation of, 125
Ether, 88
Feed-back circuit, 182
Frequency, 98, 158; effect upon pitch, 133; interval, 247; natural, 117; of voice, 163
Fundamental note, of string, 157
Gravity battery, theory of, 23
Grid, action of, 47; condenser, 169; current, 173; leak, 171; leak, construction, 172, 216; of audion, 41
Harmonics, 160
Helium, properties of, 9
Henry, 83
Heterodyne, 181
Hot-wire ammeter, 51
Human voice, mechanism of, 152
Hydrogen, action of in acid, 7; atom of, 7
Impedance, of coil, 136; of condenser, 136; of transformer, 195; effect of iron core upon, 207; matching of, 196
Intermediate-frequency amplification, 242
Inductance, defined, 83; effect upon frequency, 100; impedance of, 135; mutual, 109; of coils, 101; self, 83; table of values, 227; unit of, 83; variable, 108
Induction, principle of, 208
Inducto-meter, 109
Input circuit, 187
Interference, 249
Internal resistance, 191
Ion, definition of, 19; positive and negative, 20, 21
Ionization, 20
Larynx, 153
Laws of attraction, 204
Loading coil, 224
Loop antenna, 198
Magnet, pole of, 203; of soft iron, 205; of steel, 205
Magnetism, 202
Matter, constitution of, 5
Megohm, 172
Microfarad, 104
Mil-ampere, 71
Mil-henry, 83
Modulation, 145, 230, 237, 239
Molecule, kinds of, 6; motion of, 35
[Greek: mu], 190
Multiplex telegraphy, 258; telephony, 258
Mutual inductance, 109; variation of, 110
Natural frequency, 161
Nitrogen, 10
Nucleus of atom, 10, 12
Ohm, defined, 64
Organ pipe, 160
Oscillations, 87; damped, 114; to start, 114; intensity of, 236; natural frequency of, 117
Output circuit, 187
Overtones, 159
Oxygen, percentage in air, 10
Phase, 180
Plate, of an audion, 41
Plunger type of instrument, 205
Polarity of a coil, 204
Power, defined, 234; electrical unit of, 235
Proton, properties of, 4
Radio current, modulation of, 145
Radio-frequency amplification, 243; limitations, 243
Radio-frequency amplifier, 186, 198
Radio station connected to land line, 254
Rating of tubes, 235
Reception, essential operations in, 235
Regenerative circuit, 176; frequency of, 179
Repeater stations, 261
Resistance, measurement of, 64; non-inductive, 103; square, 251
Resonance, 161
Resonance curve, 117
Retard coils, 210
Salt, atomic construction of, 17; crystal structure, 147; molecule in solution, 19; percentage in sea water, 11
Saturation, 38
Sea water, atomic constitution of, 11
Self-inductance, 83; unit of, 83
Side bands, 248; relation to wave lengths, 249
Silicon, percentage in earth, 11
Sodium chloride, in solution, 19
Sound, production of, 152
Speech, to transmit by radio, 230
Speed of light, 122
Standard cell, 58
Storage battery, 28, 30
Sulphuric acid, 22
Super-heterodyne, 242; advantages of, 242
Telephone receiver, 130; theory of, 131
Telephone transmitter, 142
Telephony, by wire, 253
Tickler coil, 182
Transcontinental telephone line, 261
Transmission, essential operations in, 230
Transmitter, Arlington, 233; continuous wave, 94, 119; for high power, 233
Transformer, 185; step-up, 193
Tubes, connected in parallel, 234
Tuning, curve, 117; sharp, 214; with series condenser, 224
Undamped waves (see continuous waves), 86
Vacuum tube, 35, 40; characteristics of, 67; construction of, 205; modulator, 239; three-electrode, 41; two-electrode, 42
Variometer, 108
Vibrating string, study of, 154
Vocal cords, 153
Voice frequencies, 163
Volt, definition of, 57; measurement of, 61
Voltmeter, calibration of, 62; construction of, 205
Watt, 235
Wave form, 182
Wave length, relation to frequency, 98, 122; defined, 122
Wire, inductance of, 104
Wire, movement of electrons in, 14; emission of electrons from, 37
Wire telephony, 253
Wired wireless, 255; advantages of, 257
X-rays, 147
Zero coupling, 177
Zinc, electrode for battery, 23
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