US2185874A

US2185874A - Arrangement for amplitude modulation

Info

Publication number: US2185874A
Application number: US95519A
Authority: US
Inventors: Urtel Rudolf
Original assignee: Telefunken AG
Current assignee: Telefunken AG
Priority date: 1935-08-12
Filing date: 1936-08-12
Publication date: 1940-01-02
Anticipated expiration: 1957-01-02
Also published as: BE479302A

Description

ARRANGEMENT FOR AMPLITUDEMODULATION Filed Aug. l2, 1936 CARR/EP ATTORNEY Patented Jan. 2, 1940 ARRANGEMENT Foa AMPLITUDE MODULATION Rudolf Urtel, Berlin, Germany, assignor to Telefunken Gesellschaftl fr Drahtlose Telegraphie m. b. H., Berlin, Germany, a corporation of Germany ifilmlication August 12,

. 1936, serial No. y95,519

In Germany August 12, 1935 12 Claims.

The invention relates to an arrangement for amplitude modulation in which the value of the sum of two alternating currents of equall frequency is formed, and one or both partial currents are varied about an average value by one of several modulation voltages by means for inuencing the degree of amplification cfa controlled discharge tube. Such modulation ycircuits which may be designated as tube bridge circuits, were found in the past to be extremely unstable in operation. Therefore in practice, such tube bridges for modulation purposes have not found use, and other modulation arrangements have been resorted to. VThis invention overcomes the instability hitherto observed` in tube bridging circuits, which is due to the amplifier tubes proper, and which apparently has its origin in the fluctuations of the emission of the tube cathodes. when measured on the mean emission amperage of the cathode.

depends, is comparatively much more iniiuenced by the said fluctuations in the emission `than 'l the plate current, since the alternating plate-currents are substantially compensated. On the `basis of this experience, it is proposed in bridge circuits used for modulation purposes, that thel output voltages of the bridge be amplied, de-

tected, and modulation potentials filtered out,`

and to apply the direct potential obtained to the electrode of one or both controlled kdischarge tubes of the modulation arrangement which electrode influences the amplification of the alternating currents.

Several embodiments of the invention `are shown in the drawing wherein Fig. 1 shows a schematic circuit showing the means and method of stabilizing tube bridges;

Fig. 2 shows a graphical analysis of the circuit operation for explaining the invention;

Fig. 3 shows a modification, of one of the circuit elements of Fig. 1 useful for television;

Fig. 4 shows a block diagram of a modification These fluctuations are however small But in tube bridges they pro- I duce a disturbing phenomenon because of the fact that the difference current of the bridgeon which the modulation arrangement principallyV lel resonance circuit 8 tuned to the carrier frequency appliedto the primary winding of the transformer I2. Item Il' represents a high-frequency amplifier which amplies the entire band, and to which a detector` I9 is coupled by a transformer it. The detector circuit contains a yresistor 2B, a condenser 2l and afurther arrangement serving for filtering out the modulation potentials and consisting of a condenser 22 and a resistor 23.' The point P is connected to the.

control grid I of the tetrode II in seriesv with a compensation voltage source 24.

Thearrangement according toFig. 1 operates such that there will be applied to the control grid I of the tetrode Ill the modulation frequencies corresponding to the brilliancy of the picture point, and which may be derived for `instance from any picture scanning means. At'the two control grids 2 situated next to the anode, alterl hating potentials exist in opposite phase which.4

are taken from the two ends of the secondary winding of the transformer I2. Disregarding at .first the control action proposed in accordance with the invention, and thus assuming that the 4grid I oi tube II to be at constant potential `relative to the cathode whereby in this case the grid I is connected to the voltage divider connected to the battery I5, the arrangement operates in such way that owing to phase opposition of the potentials applied to the two grids- 2, l thealternating anode currents compensate each .other in the common plate circuit if the two grids l have thesame potentials. If however, for instance at grid I oftube Ill a higher positive potential appears than at grid I of tube II, theny in the common anode circuit, the Aalternating anode current supplied by the tube I predominates, so that at the parallel resonance circuit there appears a voltage having carrier frequency,

whose amplitude depends upon the potential dif?.r o

ference of the vtwo grids I.l The same action will .also take place for variation in the emission of the cathode of the one of the two tubes, the characteristic of that tube being displaced. The remaining circuit represented in Figure 1 now' operates in such manner that such fluctuations in the emission of the one tubeI is rendered ineffective by corresponding potential displacement of the grid Ici the tube II. Itwill'be appreciated that the fluctuations in tube emission takeI place relatively slowly compared with themodulation frequencies. In the high-frequency amplier I'I,-the voltage appearing at` the parallel resonance circuit will be amplified,` then detected by the arrangement I9 to 23 showing a rectifier and lter, and deprived of the modulation frequencies by suitable choice of constants for the condensers 2| and 22 and

resistors

20 and 23. The potential appearing at point AP thus Varies in accordance with the variable emission of the two tubes. This potential is applied, through a compensation voltage source 24, to the grid I of tube II, land hence influences the amplification of this tube such that the fluctuation of the input potential of the ampliiier II produced by variation in the emission will be compensated almost entirely. Under these conditions changes 'of the grid I potential in the tube I serves to produce a modulated carrier wave which is proportional to the diierence between the anode current of the tubes I and II. It will be noted in this respect that when the bridge is balanced, the carrier wave produces plate currents in phase opposition in the two tubes IIl and II, butinasmuch as the anodes of the two tubes are connected together, the resultant current flowing through the .tuned circuit 8 is zero since when the plate currentof the tube `Ill increases, the plate current of the tube II decreases in equal amount so that the sum remains constant. When the grid I of the tube i0 has applied modulating potentials thereto, the balance is destroyed and if the modulating potentials are of positive polarity, the plate current of the tube IU increases so that the total increase of the plate current of the tube I0 is greater than the total change in an opposite sense of the plate current of tube II, and consequently, there will appear a Voltage across the tuned circuit 8 which is proportional to the voltage applied to the grid I of tube lil. This instantaneous change in Voltage, however, is not effective to rebalance tube II .since the filter constants of the elements 20-23 are chosen to suppress high frequency components.

The functioning of the circuit according to Fig. 1 can be explained graphically by means of the tube characteristics shown in Fig. 2. In this gure, there is shown in relation to the voltage ci at the grid I of tube Il, the input voltage e2 of the high-frequency amplifier il. The bridge may be completely balanced at the value e1 at the grid of tube I8. Therefore, the carrier-frequency potential at the parallel resonance circuit is zero. For lower, or higher values of c1 the equilibrium of the bridge is disturbed, so that a carrier frequency appears at the parallel resonance circuit having the amplitude shown as ordinate of the line course a, b, c. At another emission current of the one of the two tubes other values of e1 are required to establish the equilibrium of the bridge, namely the values e1 and 81 respectively, and correspondingly the line course for 'the bridge is a, b, c, and a", b", 0', respectively. In assuming that the Voltage at grid I of tube I i would have the value El continuously,

at a Variation of the emission of the one tube, the carrier frequency amplitude at the input of the amplifier Il would vary, i. e. the voltage ez would Vary between the values e2 and 62', if however as proposed in accordance with the invention, a potential is applied to the grid I of tube II, consisting of the compensation voltage ek and of the voltage es at point P, then only an unlike lower fluctuation of the voltage c2 occurs. This can be readily seen when marking the voltage e2 at the* abscissa in Figure 2, and observing that the cotangent of angle corresponds to the v ratio between the voltage e3 and the voltage e2.

Now the balance point of the bridge i. e. the

purpose of .the so-called auto-modulation.v

working point on the bridge line can be adjusted in a simple manner by varying the value of the compensation voltage ek. Figure 2 shows that at a decrease of the value ek, the working point moves downwards on the right hand branch of the bridge characteristic, andy moves4 upwards with an increase-of ek.

When applying the described modulation arrangement to a television transmitter, it will be desirable to apply to the grid i of tube I0 only a modulation potential which corresponds to the alternating component of the picture brilliancy, and to Vary the compensation potential ek in accordance with the average picture brilliancy. This can be accomplished by inserting in place oi" the compensation voltage source 2li in Figure l, the circuit shown in Figure Herein, item. 25 designates a photocell on which the entire picture to be transmitted is projected, item 26 is` a D. C. Voltage source, and 2l is a resistor. Thus the photocell 2.5 integrates the brilliancy values of the entire image, and the voltage drop through resistor 2T thus corresponds to the average picture brilliancy.

A bridge arrangement of the type described alsoaffords the transmission of other signals in an especially simple manner than those corresponding to the picture brilliancy, such as for instance line and picture alternation signals. If the line and picture alternation pulses are to be sentout by means of a shorter or longercomplete interruption of the carrier wave, it is only necessary to take care that for the transmission of completely black picture points, the bridge arrangement .is not modulated to the value @2:02-

and for sending out these special additional signals such as synchronizing pulses, it is only necessary to. disconnect from the bridge the high frequency feed.

This may be accomplished for instance by means of the circuit shown in Figure 4. Herein, the tube bridge isdesignated by 28, the carrier wave generator is represented by 29, and the keying. stage is designated by 30, and may consist for-instance of a hexode which has the keyed .potential applied to one of its control grids.

Item` 3| is a generator for the synchronizing pulses, and 32, 33 are saw tooth generators of a cathode ray picture scanning means 34. The currents furnished bythis generator are fed across an amplier 35 into the grid i of tube I0 in the bridge in Figure 1. To simplify the illustration, the control arrangement for the continuous adjustment of the working point of the bridge is omitted in Figure 4.

The modulation arrangement proposed according to the invention may also be utilized for the By this is meant a modulation method in which for the purpose of saving transmitter energy, the high frequency amplitude is always adjusted to a value approximately proportionalV to the amplitude of the modulation voltage. Thus at a lower modulation voltage, the carrier frequency amp-litude is-small and vice versa. To this end the compensation voltage and hence, the carrier frequency amplitude isl to be rendered dependent upon the amplitude of the modulation voltage. A corresponding arrangement is shown in Figure 5. Herein, justas in Figure 4, item 28 representsA the bridge arrangement, and just as in Figure 1, item I1 is the high frequency amplier, and I9 to 23 represent the detector arrangement.` In addition Figure 5 containsa further detector 36 byl which the modulation voltagewill be detected,`

and whose output terminals are connected to a resistor 31 at which the compensation voltage appears.

What I claim is:

1. In a balanced bridge modulator, the method of stabilizing the balance or" the bridge against fortuitous unbalancing, which comprises producing energy in accordance with fortuitous unbalancing of the bridge, and balancing the bridge with the produced energy. l

2. In a balanced bridge modulator, the method of stabilizing the balance of the bridge against fortuitous unbalancing, which comprises deriving from the bridge carrier Wave energy modulated by both signalling energy and spurious energy, deriving energy from the last mentioned energy representative only of the spurious modulated energy, and balancing the bridge with the second mentioned derived energy.

3. In a balanced bridge modulator, the method of stabilizing the balance of the bridge against fortuitous unbalancing, which comprises .deriving from the bridge carrier wave energy modulated by both signalling energy and spurious energy, rectifying the derived carrier Wave energy, ltering out a component of the rectified energy representative only of the spurious energy, and balancing the bridge in accordance with the filtered component of energy.

4. A stabilized balanced bridge modulator comprising means for producing energy in accordance with fortuitous unbalancing of the bridge, and means for balancing the bridge with the produced energy.

5. A stabilized balanced bridge modulator comprising means for deriving from the bridge carrier Wave energy modulated by both signalling energy and spurious energy, means for deriving energy from the last mentioned energy representative only of the spurious modulated energy, and means for balancing the bridge with the second mentioned derived energy.

6. A stabilized balanced bridge modulator cornprising means for deriving from the bridge carrier wave energy modulated by both signalling energy and spurious energy, means for rectiiying the derived carrier wave energy, means for lter ing out a component of the rectified energy representative only of the spurious energy, and means for balancing the bridge in accordance with the filtered component of energy.

7. In a balanced bridge modulator, the method of stabilizing the balance of the bridge against fortuitous unbalancing, which comprises deriving from the bridge carrier wave energy modulated by both signalling energy and spurious energy, rectifying the derived carrier Wave energy, ltering out a component of the rectified energy representative only of the spurious energy, supplying a source of constant energy, and feeding the ltered component of energy in series with the supplied source of constant energy to the bridge to balance the bridge for the spurious energy.

8. A stabilized balanced bridge modulator comprising means for deriving from the bridge carrier Wave energy modulated by both signalling energy and spurious energy, means for rectifying the derived carrier Wave energy, means for iiltering out a component of the rectified energy representative only of the spurious energy, supplying a source of constant energy, and means for feeding. the filtered component of energy in series With the supplied source of constant energy to the bridge to balance the bridge for the spurious energy.

9. In a balanced bridge modulator, the method of modulating carrier Wave energy, which comprises the steps of adjusting the bridge to a predetermined balance, deriving energy from the bridge'representative only of spurious modulated energy, re-establishing the balance of the bridge in accordance with the derived energy, and photoelectrically altering the balance of the bridge in accordance With the integrated illumination of an optical representation to be transmitted.

10. A balanced bridge modulator comprising meanslfor adjusting the bridge to a predetermined balance, means for deriving energy from the bridge representative only of spurious modulated energy, means for re-establishing the balance of the bridge in accordance with the derived energy, and means for photoelectrically altering the balance of the bridge in accordance Withvthe integrated illumination of an optical representation to be transmitted.

11. The method of operating a balanced bridge modulator which comprises deriving from the bridge carrier Wave energy modulated by both signalling energy and spurious energy, deriving energy from the last mentioned energy representative only of the spurious modulated energy, balancing the bridge With the second mentioned derived energy, and unbalancing the bridge in accordance with the average value of the signalling energy.

12. A balanced bridge modulator comprising means for deriving from the bridge carrier Wave energy modulated by both signalling energy and spurious energy, means for deriving energy from the last mentioned energy representative only of the spurious modulated energy, means for balancing the bridge with the second mentioned derived energy, and means for unbalancing the bridge in accordance with the average value of the signalling energy.

RUDOLF URTEL.