US2424961A - Demodulator for carrier telegraph signals - Google Patents

Description

A 4 E. P. BANCROFT ETAL 2,424,961 DEMODULATOR FOR CARRIER TELEGRAPH SIGNALS H Filed Dec. 4, 1944 2 Sheets-Sheet 1 1, 1 5 1 f Am 0 as INPUT Mfil l h T 44 IN V EN TORS GENT 1947. E. P. BANCROFT ETAL 2,424,961

DEMODULATOR FOR CARRIER TELEGRAPH SIGNALS Filed Dec. 4, 1944 2 Sheets-Sheet 2 ak/o mus INVENTORS z-mw/v I? MIVCAQFT JACOB L. JAIZOW 7 AGE/V2 Patented Aug. 5, rest UNETEFD STATE @FFME DEMODULATOR FOR CARRIER TELEGRAPH SIGNALS Application December 4, 1944, Serial No. 555,557

Claims.

This invention relates to demodulators and in particular to demodulators for carrier telegraph signals. More specifically the invention provides an improved method and circuit arrangement for controlling the bias of a carrier telegraph signal Wave.

In carrier telegraph transmission systems wherein a carrier current is modulated at a transmitter by a telegraph wave, means are provided to establish the desired signal bias, that is, the desired relative widths ofthe marking and spacing pulses, of the Wave. For most satisfactory operation the widths of the marking and spacing portions of the signal should be equal. However, as the signalling wave travels between the transmitter and the receiver it of necessity passes through various circuit elements such as transformers, filters, etc. The characteristics of these circuit elements are such that the modulated carrier wave, which is usually originally transmitted in the form of rectangular pulses, arrives at the receiver in a. form wherein the original square corners of the wave have been appreciably rounded oil" and the wave is otherwise changed so that it is far from being a true replica of the original wave. The characteristics of the transmission line itself may greatly influence the character of the received wave. Accordingly, the objects of our invention are To provide an improved demodulator for a carrier telegraph signal,

To provide an improved circuit arrangement for establishing the required signal bias in a carrier telegraph demodulator, and

To provide an improved circuit arrangement for maintaining a constant output in a carrier telegraph demodulator.

Other objects and features will become apparent as we proceed with the description of our invention as it is disclosed in the following specification and attached drawings in which:

Fig. 1 is a schematic circuit arrangement of one embodiment of our invention;

Figs. 2 and 3 are curves illustrating the principle of operation of the circuit arrangement of Fig. l; and

Fig. 4 is a schematic circuit arrangement showing a second embodiment of our invention.

Referring to Fig. 1 of the drawings, the reference character I represents a transformer having primary winding 2 and three secondary windings 3, 3 and 5. The winding 2 may be connected to the output of an amplifier (not shown) having any required number of stages. Preferably the circuits of the amplifier are equipped with automatic volume control means for maintaining the signal at a substantially uniform transmission level. The signal as it leaves the amplifier and appears in winding 2 has a form substantially as shown by the graph t. This graph shows the modulated carrier wave and its envelope and illustrates the manner in which the corners of the original Wave have been rounded off as it passes through the transmission system from the point of modulation to the coil 2.

Connected to the transformer secondary winding 3, t and 5 are three full wave rectifier cilcuits l, 8 and 9. The rectifier circuits are illustrated as voltage doublers but any type of half wave or full wave rectifiers could be employed. The output circuit of rectifier 7 consists of a load resistor Ml shunted by capacitors H and 12. The time constant of the output circuit as determined by the electrical constants of the resistor til and capacitors H and i2 is relatively short, the voltage wavedeveloped across the resistor it having a shape substantially that of the envelope of the carrier wave shown by graph t3, the ordinates of ie latter being proportional to the ordinates of graph 6. The carrier wave component of the signal is by-passed by the capacitors H and i2. The rectifying elements of rectifier "l are so poled that positive voltages with respect to ground are developed across the resistor It.

The rectifier ii is similar in all respects to rectifier l. The load resistor it, however, is so connected into the demodulator circuits that negative voltages with respect to ground are developed across the load resistor it. The voltage wave across resistor M has subtantially the form of the graph E5, the latter being substantially the negative of graph l3.

Rectifier 23 is also illustrated as a voltage doubler and its circuits are similar to those of rectifiers l and 9. The main difference is that the output circuit consists of resistors 16 and I? and capacitors i8, i9 and 28 having electrical constants such that the time constant of the circult is much longer than the time constants of the load circuits of rectifiers l and 9. Capacitors l3, l9 and 28 are sufficiently large to bypass not only the carrier component of the signalling wave but also the telegraph component, the latter being the envelope of the signal, The rectifying elements of rectifier 8 are so poled that the resistors it and H, which carry only the direct current component of the signalling wave, are poled as illustrated in Fig. 1. Resistors it, it and I 6 are connected in series in a manner such that the polarity of the voltage developed in resistor it is opposed to the voltages developed across resistors 16 and M for reasons which will be described later.

Amplifier tubes 2| and 22, known as a marking tube and a spacing tube, respectively, are connected to the three rectifier load circuits as follows. Control grid 23 of tube 2| is so connected to resistor it! that any desired portion of the voltage developed across said resistor may be applied to the grid. The cathode 24 of the tube 2| is connected to ground and to a central position on resistor l6. In series between the oathode, the more negative portion of resistor IS, a portion of resistor l0, and the grid 23, is a resistor 25. The latter is a voltage dropping resistor. When the combined voltages developed across the aforesaid portions of resistors l6 and H! become positive and tend to cause the flow of grid current, the latter, in flowing through resistor 25, produces a voltage drop which tends to make constant the resulting voltage between cathode 24 and grid 23. This of course tends to make the plate current of the tube constant. Capacitor 26, connected between the grid 23 and cathode 24, together with resistor 25 acts as a filter to still further remove any high frequency variations in the signalling current.

Cathode 21 and grid 28 of amplifier tube 22 are connected to the most positive portion of resistor IS a portion of resistor l4, and resistor 29 to form the input circuit to the amplifier tube 22. Resistor 29 and capacitor 30 function as a voltage dropping resistor and filter element as do the similar elements connected in the input circuit of tube 2 l.

Anodes 3| and 32 are connected to windings 33 and 34 in the output circuits of tubes 2| and 22, respectively. A battery or other source of potential 35 is a power supply for the plate circuits of both tubes. Resistors 36 and 3! in series with windings 33 and 34, respectively, are merely low resistance shunts around the ammeter 38. The latter instrument is employed when reading the current flow in coils 33 and 34. If desired, amplifiers 2| and 22 may be tetrodes or pentodes in place of the triodes as shown.

Reference character 39 represents a polar relay the armature 40 of which is attracted to contact 4| or 42 depending upon which winding, 33 or 34, is carrying current and acting to attract the armature. Terminals 43, 44 and 45 of the relay may be connected with the operating winding or windings of a sounder, of a relay, or of a printer, for example. A source of current would, of course, be inserted in series with the relay and the operating windings.

The manner in which the circuit arrangement of Fig. 1 may be adjusted for best operation may be understood by referring to Figs. 2 and 3. These figures illustrate the grid voltage-plate current characteristics of the amplifier tubes and the manner in which the tubes are biased to reproduce a signal having an equal marking and spacing bias.

Referring to Fig. 2, curve 46 represents the usual grid voltage-plate current characteristic of the marking tube 2 I. An amplifier tube is chosen which preferably has a sharp cut-off bias as illustrated at point 41' on the curve. The negative grid voltagefrom the point 4'! to the point is termed the cut-off bias. The point 48 where the curve 46 intersects the plate current coordinate represents the required current (for some suitable value of the potential of plate battery 35) for operating relay 39.

Referring to Fig. 3 the curve 49 represents the grid voltage-plate current characteristic of the spacing tube 22. The cut-off bias for this tube is represented by the point 56 on the curve 49 and the point 51 indicates the required current flow in winding 34 for reversing the armature 49.

A telegraph signal is considered most satisfactory when it has zero signal bias, i. e., the average times during which either marking current flows or spacing current flows are equal. Referring to Figs. 2 and 3 it will be seen that either marking or spacing currents begin to fiow as soon as the grid voltages exceed the cut-01f biases of the amplifiers. Therefore, the grid voltage curve 52 of Fig. 2, which represents the envelope of the rectified telegraph signal, may be so positioned with respect to the cut-off bias that the time during which the grid voltage is greater than the cut-oil bias and the time when the grid voltage is less than cut-01f bias are equal. This is illustrated in Fig. 2 by the fact that the distances 53 and 54 are equal. Still referring to Fig. 2, curve 55 represents the plate current flowing in winding 33. In order that this curve may have substantially straight sides and a fiat top portion, the

, slope of the grid voltage curve 52 should be substantially straight between the cut-off bias and the point of zero bias. As the grid voltage swings above zero bias grid current flows but as explained above the flow of grid current through resistor 25 produces a voltage drop in said resistor and maintains the output current substantially constant as shown in curve 55.

In order to position the grid voltage curve in suitable relation to the cut-off bias a certain fixed bias may be placed on the grid 23 of the amplifier. This fixed bias is obtained by the rectification of the telegraph carrier wave by rectifier 8 and appears across the resistors 16 and I1 constituting the load circuit of the rectifier. By a suitable adjustment the resistor I1 and the proper positioning of variable contact on the resistor i6 the desired negative bias may be placed on the amplifier 2|, this bias being represented by the distance 056. It is desirable that the amplitude of the signal wave be such that a reasonably straight portion of voltage curve 52 exists between the point 0 and the cut-off bias 41.

Referring to Fig. 3, it is desirable that the time during which the voltage values of curve 51 are greater than the cut-off bias be equal to the time that the voltage values are less than the cut-01f bias. 'It is understood that the curve 51 represents a curve whose ordinates are proportional to the envelope of the rectified signal. To secure these equalities of time a positive bias equal to the distance 9-58 is placed on the grid 28, the bias being obtained from the voltage developed across that portion of the resistor I6 which is between the variable contact on resistor l6 and the resistor IT. This biasing voltage is developed, as described above, from the rectified signal and varies with the signal intensity. The amplitude of the grid voltage curve 51 should be such that a substantially straight portion exists between the point 6 and the cut-off bias 56. This results in plate current flow represented by the curve 59, the latter being substantially the same as that of curve 55. The currents represented by curves 55 and 59 flow alternately in windings 33 and 34 to operate the relay 39.

In the above description we have emphasized that it is prefable that the telegraph wave operate with a zero signal bias and have described a method of obtaining this result by so biasing the grids of the amplifier tubes that while one tube, for example, the marking tube, is drawing current, the other or spacing tube is drawing no current, and vice versa. The described method was by way of example and for simplicity of description only since one skilled in the art will appreciate that the same end result, in so far as obtaining zero signal bias is concerned, may be obtained even though both the marking and spacing tubes are drawing current at the same time. This condition may be brought about by changing the biases (shown in Fig. 2 for example) so that the equal portions 53 and 54 of curve 52 will not occur at the cut-off bias d? but at some other bias between point 8 and the point 41. This will result in the lengths of the marking pulses 5.5 being somewhat greater than the lengths of the spaces between them. A similar change in the biases for the spacing impulses shown in Fig. 3 would also be required, resulting in the lengths of the spacing pulses 59 being increased a like amount. Under these conditions current will flow simultaneously for a short period of time in both windings 33 and 34 but they exert a diiferential attraction on the armature 65) and the net result in so far as zero signal bias is concerned is substantially the same as when the windings 33 and 34 do not carry current simultaneously. Under most conditions the method of operation wherein both coils carry current is to be preferred since it produces somewhat steeper slopes to the curves representing the marking and spacing impulses.

In Fig. 4 we have illustrated another embodiment of our invention. The circuit arrangement shown in this embodiment is substantially the same as that shown in Fig. 1 except that the rectifiers and the biasing voltages developed thereby have, in a sense, been reversed from their positions as shown in Fig. 1. For example, rectifier 8 has been replaced by two rectifiers 6i) and 6|. Rectifier 6E3 develops a negative normal voltage across load resistor 62 and rectifier 6i develops a positive normal voltage across resistor 63. Positive and negative marking and spacing biasing voltages are developed across resistors 64 and 65, respectively, by rectifier 66. The voltages across resistors 62 and 6d are applied to the grid 23 of marking amplifier 2| and biasing voltages across resistors 63 and B5 are applied to the grid 28 of spacing amplifier 22. All of these voltages may be varied as represented by the arrowheads on the various resistors. The relay which is operated by the marking and spacing amplifiers is not shown in Fig. 4 since it may be identical with that of Fig. 1. The rectifiers in Fig. 4 have not been explained in detail since their operation is the same as that shown in Fig. 1. It is to be understood, of course, that rectifiers 60' and 6| have long time-constant load circuits in order that the voltages developed thereacross will be proportional to the direct current component of the signals. The load circuit of rectifier 66 has a relatively short time-constant so that voltages developed across resistors 64 and 65 will follow the envelope of the telegraph signal.

The circuits of our invention are designed primarily for closed key operation, that is, when no signals are being sent a steady or unmodulated carrier wave voltage is applied to the input circuit. Under these conditions the current flowing in winding 33 is constant and equal to the maximum value shown on curve 55. The current flowing in winding 34 is zero during this time.

Although we have illustrated only two embodimerits of our invention it will be understood that other embodiments thereof will occur to those skilled in the art. It will be understood that these embodiments are given by Way of example only and not as limiting the scope of our invention as set forth in the objects and the following claims.

What is claimed is:

1. In a demodulator for carrier telegraph waves, the combination of a carrier wave source, a first rectifier connected to said source, a first load circuit having a long time-constant connected to said first rectifier, the rectified output of said first rectifier developing in said first load circuit a normal biasing potential of a given-polarity; a second rectifier connected to said source, a second load circuit havin a short time-constant connected to said second rectifier, the rectified output of said second rectifier developing in said second load circuit a marking biasing potential of opposite polarity to that of said given polarity, a third rectifier connected to said source, a third load circuit connected to the output of said third rectifier, an amplifier having grid and cathode input electrodes, means for serially connecting said first and second load circuits to said grid input electrodes, and means for connecting said third load circuit to the cathode electrodes.

2. A demodulator for carrier telegraph Waves in accordance with claim 1 wherein the means for serially connecting one of said load circuits to one of said input electrodes includes a variable means for selecting a predetermined portion of one of said load circuits included in said series connection.

3. A demodulator for carrier telegraph waves in accordance with claim 1 wherein the means for serially connecting said second load circuit to one of the grid input electrodes includes a variable means for'selecting a predetermined portion of said second load circuit included in said series connection.

4. A demodulator for carrier telegraph 'waves in accordance with claim 1 wherein the means for serially connecting one of said load circuits to one of said input electrodes includes a resistor for limiting the potential which is applied between said input electrodes.

5. A demodulator comprising two amplifiers, each having input and output electrodes, a modulated carrier wave source, rectifier means for rectifying said carrier wave, said rectifier means having a plurality of outlet connections, each including a load resistor, means associated with two of said load resistors for supplying therefrom negative biasing and positive biasing potentials to the input electrodes of one of said amplifiers, and means associated with a third load resistor and one of said two resistors for supplying therefrom positive biasing and negative biasing potentials for the other of said amplifiers.

6. A demodulator according to claim 5 wherein the two means associated with said load resistors includes two adjustable contacts, one for each plifier, and a third contact common to both amplifiers.

7. A demodulator according to claim 5 wherein the two means associated with said load resistors includes two adjustable contacts, one for each amplifier, and a third contact common to both amplifiers, and the load resistor common to said amplifiers having an adjustable resistor connected between one part thereof and said rectifier means.

8. A demodulator according to claim 5 wherein the two means associated with said load resistors includes two adjustable contacts, one for each amplifier, one connection common to said amplifiers and said one of said two resistors, and adjustable contacts between said one of said two resistors and the other resistors.

9. A carrier telegraph demodulator comprising a source of carrier signal input, said carrier signal having a signal bias other than zero, a marking amplifier having input and output electrodes, a spacing amplifier having input and output electrodes, a first rectifier, normal biasin means for said amplifiers, said means being connected to said first rectifier and providing biasing voltages of opposite sign, two signal rectifiers, one of said signal rectifiers providing'a positive biasing voltage for said marking amplifier, the other signal rectifier providing a negative biasing voltage for said spacing amplifier, the wave form of said marking and spacing biasin voltages being substantially proportional to the rectified envelopes of said carrier signal, load circuits connected to the output circuits of said amplifiers, and means for selecting a predetermined value for REFERENCES CITED ihe following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,925,838 Kunc Sept. 5, 1933 2,101,256 Jones Dec. '7, 1937 FOREIGN PATENTS Number Country Date 327,770 France 1938 97,233 Sweden 1939

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