US1990586A - Radio transmitter - Google Patents

Description

Feb. 12, 1935. v R, E I 1,990,585

RADIO TRANSMITTER Inventor: Robert? B. Don'we,

b (Al/66w v H i s Attovneg.

Feb. 12, 1935. B, DQME 1,990,586

'RADIO TRANSMITTER Filed April 28, 1932 2 Sheets-Sheet 2 f= Zero Tnvntort Robert B- Dome JQQMWOM H i s Aftorn ey- Patented Feb. 12, 1935 UNlTED STATES RADIO TRANSMITTER Robert B. Dome, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application April 2c, 1932, Serial No. 608,024

3 Claims.

My invention relates to radio transmitters and more particularly to transmitters of the high power shortwave type.

It has for one of its objects to provide means whereby currents, which as originally produced are extremely feeble and include frequencies extending over a very wide range, as for example, from frequencies relatively low in the audio range to frequencies relatively high in the radio range, may be faithfully transmitted by a high power short wave transmitter. Such currents are produced, for example, by the photo-electric cells employed in television apparatus. The currents flowing in these cells may include frequencies extending from about 20 cycles to a million or two million cycles. It is of course desirable to'transmit the widest possible range of these frequencies to the reproducing equipment. Difficult problems, however, are encountered in causing a radio transmitter, particularly one operating at ultra short wave lengths, and of high power rating, so to transmit currents having a range of frequencies greater than from 20 to 50,000 or 75,000 cycles that faithful reproduction of all of the signal currents in the frequency band is possible at a remote point.

Some realization of the problem encountered may be had from a consideration of the fact that a variation in frequency from 20 to 1,000,000

cycles, for example, amounts to a variation in frequency of 5,000,000 percent. To construct an amplifier to operate at frequencies which vary over such a range and to amplify faithfully to a high power level the entire range of frequencies has in the past been impracticable. Amplification of these currents at this frequency and varying over this frequency range to a high power level may be avoided in a great portion of the frequency range by modulating the short wave transmitter at a low power level and thereafter amplifying the modulation products to a high power level. At ultra high frequencies, as for example, at wave lengths less than meters, this does not avoid the difficulty arising in the construction of the amplifiers, for the reason that it is also impracticable at ultra short wave lengths to construct amplifiers which faithfully amplify the range of frequencies included in the modulation products.

-One of the objects of my invention is to provide a method and means whereby these difficulties are overcome.

In accordance with a further object of my invention a method and means are provided whereby the signal currents are converted'at a l w power level to an intermediate portion of the frequency spectrum where the entire frequency range may be faithfully and'practically amplified to the desired high power level. They are then amplified to the desired power level and reconverted at the high power level to the original portion of the frequency spectrum. The ultimate carrier wave comprising, of course, but a single frequency, may also be amplified to the desired high power level. At this high power level the carrier wave may be modulated with the high power reconverted signal currents and radiated from the antenna.

A further object of my invention is to provide means whereby the low power signal currents may be faithfully amplified to a level at which satisfactory conversion at low power to the intermediate portion of the frequency spectrum may be effected and whereby after reconversion the ultimate carrier wave maybe faithfully modu= lated thereby.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which Figs. 1 and.2 represent different embodiments of my-invention; Fig. 3 represents an equivalent circuit of a portion thereof; and Figs. 4 and 5 represent certain characteristics with reference to its operation.

Referring to the drawings, I have shown in 1 including a source of short wave high power radio frequency oscillations conventionally indicated at 2. This source may of course comprise a conventional short wave generator and series of amplifiers whereby the generated oscillations are raised to a high power level. The output from this source is supplied to the grids of a pair of push pull connected electron discharge devices 3 and 4 by which the high frequency oscillations are amplifiedand supplied to Fig. 1 thereof a conventional radio transmitter a radiating system 5. While I contemplate that to be televised with a spot of light. Currents are then produced in the photo-electric cells 6 maccordance with light falling thereon. These currents may include frequencies extending over the above-mentioned range of frequencies. They are extremely feeble as is of course well known in the art pertaining to photo-electric devices. It is necessary that they be amplified to such an extent that satisfactory modulation of the radio transmitter 1 in accordance with these currents may be effected. It is also necessarythat this amplification shall be effected in such a way that all of the frequencies within the desired range are amplified to an equal extent and phase displacement due to the operation of the amplifying means is wholly avoided. Unless the amplifying means be such that these results are effected then the transmitter is not modulated faithfully in accordance with the currents generated by the photo-electric cells and the picture which is produced by any remote reproducing equipment will not be a faithful reproduction of the object televised.

In accordance with my invention the signal currents are converted to a portion of the frequency spectrum in which the highest frequency to be amplified is but a very small percent greater than the lowest frequency to be amplified but where linear amplification of the entire range is practicable. After this conversion the currents in this relatively narrow range of frequencies may be satisfactorily amplified to the desired high power level by ordinary amplifiers.

For this purpose I provide a source of oscillations of a frequency within the mentioned intermediate portion of the frequency spectrum, such for example as 20,000 kilocycles. This source may comprise a conventional crystal controlled elec-. tron discharge oscillator 13, the output of which is amplified by amplifiers 14 and 15, in the output of the latter of which these oscillations are modulated by oscillations supplied from the photo-electric cells through amplifiers 10, 11, and 12. If we assume now that the currents generated by the photo-electric cells vary over a range of from 20 to 1,000,000 cycles then the lowest frequency in the lower side band of the modulation products appearing in the output of the amplifier 15 is 19,000 kilocycles, whereas the highest frequency produced in the upper side band is 21,000 kilocycles. These ferquencies vary from the carrier frequency by only five percent. Eventhis narrow range may be reduced on a percentage basis by employing a higher intermediate frequency carrier wave. Thus the desired range of frequencies when converted to this portion of the frequency spectrum may be very readily and faithfully amplified by amplifiers of ordinary construction. The modulated oscillations appearing in the output of the amplifier 15 are amplified by amplifiers 16 and 17 of conventional construction. The amplifier 17 is represented by a rec tangle and may include as many stages of amplification similar, for example, with amplifiers 15 and 16, as are necessary to amplify thedesired currents to the required power level. I

Prior to modulation of the transmitter 1, how

ever, it is necessary that these currents be reconverted to the original portion of the frequency spectrum. This is accomplished by electron discharge devices 18 and 19 the grids of which are connected in push pull relation to receive oscillations from the output of the amplifier 17. The anodes of these devices are connected in parallel through a circuit which includes a radio frequency choke coil 19, an iron core modulation reactor 20, and a source ofelectromotive force 21. The grids of these discharge devices are connected to the cathodes through opposite portions of an inductance 22 and a source of grid biasing potential 23. The discharge devices 18 and 19 are of course of high power rating comprising tubes of the type, which are commonly constructed with water cooled anodes. The source of potential 23, however, is such that the discharge devices are biased strongly negative thereby to cause them to act as giant detectors to demodulate the oscillations supplied to the grids. Thus in the output circuits of these discharge devices appear oscillations having the same frequency as those produced by the photo-electric cells, these oscillations having been amplified many fold. The anode circuit of the power amplifiers 3 and 4 of the radio transmitter are then modulated by the products of this demodulation by means of a circuit extending from the anodes of devices 18 and 19 to the anodes of devices 3 and 4 and including an inductance 24, a parallel combination of a voltage reducing resistance 25 and a bypass capacitor 26 and a. conductor 28 extending to the midpoint of the anode inductance 29 of the push pull power amplifier. In this way the low frequency electromotive forces developed across the modulation reactor 20 are supplied directly to the anodes of the push pull amplifier thereby varying the amplification of these amplifiers in accordance with the signal currents. The modulated radio frequency millations are then supplied to the antenna 5.

It will be noted that the currents generated in the photo-electric cells are amplified to a certain extent prior to modulation of the intermediate frequency carrier wave generated by the oscillator 13. This is necessary to practical modulation of the intermediate carrier wave even at a low power level. A special construction of the amplifiiers however, is necessary in order that the currents will be amplified equally over the entire range of frequencies and to avoid phase displacements which are likely to result. For this reason I have shown in detail the circuit construction between the anode of the discharge device 11 and the grid of the discharge device 12. It will of course be understood that any additional amplifiers represented by the rectangle 10 are of similar construction.

The anode circuit of the device 11 is connected to the grid of the device 12 through an air core inductance 30, a coupling capacitor 31, and a parallel combination-including resistance 32 and an iron core inductance 33. The condenser 31 is the conventional coupling capacitor which serves to isolate the grid of the discharge device 12 from the high unidirectional electromotive force which is'supplied to the plate of the discharge device 11 by the source 34. This condenser, for example, may be of about 4 microfarads.

In the higher portion of the frequency range which is to be amplified it is found that the capacity effects between the grid and cathode of the discharge device 12 and stray effects between the conductors and in the circuit connections generally, greatly impair the transmission of effectively to cut off at a frequency well within the range where a high degree of amplification is desired. It has been found, however, that this cutting offat the higher frequencies can be effectively avoided by the use of the inductance 30 having a value such that it resonates with the effective total capacitance between the grid and the cathode of the discharge device.

The effect of this inductance may best be understood by reference to Figs. 3 and 4. Fig. 3 represents the equivalent circuit included between the discharge devices 11 and 12. This equivalent circuit includes a source of electromotive force 34 which, of course, corresponds to the electromotive force generated in the anode circuit of the discharge device 11. This source of electromotive force may be considered as connected in a circuit including a resistance Rp,

which is the internal static resistance of the plate circuit of the discharge device 11, and a parallel combination of resistance R1; and capacitance C. The resistance R includes the resistance between the grid and the cathode of the discharge device 12 and comprises the resistance 35. The capacitance C is the capacitance between the grid and the cathode and that which is included in the circuit connections to these electrodes. This equivalent circuit, however, ignores the elements 31, 32, 33, and 40, except in so far as their stray capacity to ground effects the value C. With this exception these elements have little or no effect in the high frequency range. The impedance Z1 looking into the grid of the discharge device 12 as indicated in Fig.

3 may be expressed as follows: Z1=R1-iwx1.f

When R1 is the resistive component of the total input impedance, X1 is the capacitive component;

a=21|r times the frequency and The relation between the variables 21,. R1, and

X1 at the different frequencies may best be understood from a consideration of Fig. 4 in which this relation is shown vectorially. It will of course be understood that at zero frequency the capacitance has no effect and the impedance Z1 is purefresistance and may be represented 'by the vector R0; equal to the resistance of the path including'resistor 35. As the frequency increases theresistance 35 becomes less efiective and the capacitance qbecomes increasingly effective with the resultthat the resistive component R1 decreases and the reactive componentX1 increases. Thus, for example, at a certain low frequency the impedance Z1 may be represented by the vector A, this impedance being made up of a resistance component R and a reactance component X1 indicated in the diagram respectively as the horizontal and vertical sides of the triangle of which the vector A is the hypotenuse. At a still higher frequency this impedance Z1 may be. represented by the vector B comprising a smaller horizontal component R1 and a larger vertical component X1. At infinite frequency it will of course be understood that the impedance of the capacitance is zero and accordingly the resistance R0 is of no effect and the impedance Z1 is zero. It will .thus be seen that the locus of the various vectors Z1 drawn at different frequencies comprises for practical considerations, a semicircle D. The maximum reactive component of the impedance Z1 occurs at the point where the resistive and reactive components are equal.

Thus as the frequency increases to-a point where R1 equals Rn z the impedance Z1 becomes gradually more reactive until a maximum reactive component is reached.

The problem then is to insert an additional 5 corrective element in the circuit which so effects the total reactive component of the impedance into which the tube 11 works as to produce on the grid of tube 12 a voltage, which, at the highest frequencies to be amplified is equal to that produced at the frequencies low in the audio range. Since the reactive component of the impedance Z1 is capacitive the corrective element should be inductive and is indicated in the drawings at 30. 15

In setting up the circuit the only two known quantities are the internal resistance Rp of tube 11 and the input capacity to tube 12. The values of resistance 35 and inductance 30 are to be determined with reference to these quantities.

The value of resistance 35, or R0, may readily be determined as follows:

The impedance Z1 is of course The resistance component of-this expression of Z1 is and the reactance component of Z1 is "(UCRQ 1+w C R At the critical frequency, 1. e. the highest frequency to be amplified R1=X1 as shown by Fig. 4. Therefore 1+w CR l+w C R Solving for R0 we find To determine the value of inductance of the corrective element 30 let us assume that at the critical frequency it has an unknown reactance X1. as indicated in Fig. 4.

The total impedance Z11 in which the tube 11 works is then The quantities .X and Z11 are also shown in Fig. 4.

The voltage E32 on the grid of tube 12 is the voltage E32 is For uniform amplification the -absolute value of Eu at the highest frequency to be amplified and at low frequencies should be equal.

That is,

This is a double valued function. But from the above relation since it gives a value Xr=0 at low frequencies.

NOW

Ro1/R where f is the highest frequency to be amplified. This equation expressed the general value of L for uniform amplification.

In a particular case we may arbitrarily make XL=X1 thereby to give tube 11 a unity power factor load. With this value XL to obtain uniform amplification the value of R0 as found in Equation 5 must be adjusted in accordance with Equation 16. That is to make X equal to zero R0 must equal 2R or Thus we have the following equations from which the circuit may be designed for uniform amplification, with unity power factor load on tube 11 R=JER, (21) Where a unity power factor load on tube 11 is not important, to obtain uniform amplification the circuit may set up in accordance with the following equations Piano-W (23) When Xx. equals X1 the added inductance 30 resonates with the input capacity to tube 12. In practice it has been found that with the inductance so chosen practically uniform amplifica tion may be obtained over the entire range extending from very low audio frequencies to relatively high radio frequencies as for example frequencies of the order of 1,000,000 cycles. Further, this value gives the maximum amplification over this range. This is apparent from Fig. 4 since when X1. equals Xi, Z11 is a minimum and identical to R1 and the ratio of Z1 to Zn is maximum thereby producing a maximum voltage on the grid of tube 12.

The reactance Xi. for maximum amplification at the different frequencies is shown by the curve E which is so drawn as to indicate that XL at the critical frequency equals X1. That is, the inductance 30 is in resonance with the effective input capacity to tube 12. At lower frequencies it will be observed that the inductive reactance X1. approximately balances the capacitive reactance X1. It has been found in practice that the sum of XL and X1 so closely approximates zero at frequencies lower than the critical frequency that the reactive component of the impedance Z1 inthis range of frequencies has substantially no effect upon grid voltage supplied to the tube 12, with the result that for practical actance at the highest frequency to be amplified the amplifier operates with substantially equal amplification over the entire range and cuts of! rapidly at frequencies above that range.

. It has been found that with R0 greater than at the critical frequency under unity power factor conditions the voltage supplied to the grid of device 12 increases at the high frequencies thereby increasing the amplification of the system. Similarly if R0 at the critical frequency be less than under unity power factor conditions then the voltage supplied to the grid of the device 12 is reduced at the high frequencies. While a uniform amplification characteristic is usually desirable, in some cases it is desirable to produce either a rising or falling amplification characteristic at the high frequencies thereby to compensate for an undesired amplification characteristic obtained in some other part of the equipment.

I 1,980,686 This may be effected by proper choice of R1.

Of course, the same difficulty is encountered in connection with-modulation of the power amplifier 3, 4 and for this reason the inductance 24 is employed. This inductance is constructed to exactly neutralize the capacitive component of the impedance which appears between the midpoint of coil 29 and the cathodes of devices 3 and 4. In this case the resistance R0 becomes the value where Eb is the average anode voltage supplied to the power amplifier and 1b is the average anode current.

At very low frequenciesthe capacitance of the coupling capacitor 31 between discharge devices 11 and 12 tends to shift the phase of the oscillations supplied to the grid of the discharge device 11; This shifting of the phase is of course intolerable since it seriously impairs the picture which is reproducedby the remote receiving apparatus. I For example. if a television receiver of the cathode ray type be employed the flying spot is either advanced or retarded from its true instantaneous position due to this phase displacement thereby greatly impairing the reproduced image. To avoid this effect the inductance33 shunted by a resistance 32 is included in the circuit, the inductive reactance of this combination having such a value that it resonates with the capacitance of condenser 31 at a frequency approximating the lowest frequency to be amplified. The purpose of resistance 32 is to prevent the inductance 33 from having an appreciable effect upon the amplification of the system at the higher frequencies.

The effect of this combination is best, illus trated in Fig. 5 where I have shown the curve F showing the relation betweenthe capacitive reactance of the condenser 31 plotted as ordinates and frequency plotted as abscissa. It will be seen that at zero frequency the reactance is infinite and that it rapidly decreases in accordance with the curve as the frequency increases. By the curve G I have shown the relation between the inductive reactance of the combination 32, 33 and frequency. It will be seen that at zero frequency this reactance is zero and that it rapidly rises to a certain value where the resistance 32 becomes effective to shunt the reactance. At the higher frequencies this resistance becomes increasingly effective with the result that the inductive reactance reaches a maximum value and then drops off in accordance with the curve G. The total reactance of the condenser 31 and the combination 32, 33 is thus the sum of the reactances represented by the curves G and F and is shown by the curve H. It will be observed that at a certain frequency ii the inductive reactance is equal to the capacitive reactance, At the higher frequencies there is a slight inductive reactance. It has been found, however, that this reactance may be made so small over the range where the capacitance of condenser 31 is effective as to pre vent any appreciable shift in phase of the oscillations supplied to the grid of device 12.

To obtain even better operation the combination 32, 33 may be shuntedby a condenser 40 this condenser having such a value that it is substantially ineffective except at very high frequencies. This condenser then acts as an additional shunt to the inductance 33 thereby further tending to reduce the impedance of the combination 32, 33,

40 to zero at the highest frequency to be amplified. In this way increased amplification is obtained particularly in the extremely high portion of the frequency range.

If it is desired to amplify current of from 20 cycles up, for example, the frequency f1 may be of about 24 or 25 cycles. Then, at frequencies below about 20 cycles the total. reactance of the circuit increases extremely rapidly thereby ren-- dering the amplifier inoperative at the extremely low frequencies.

It will be readily seen from Fig. 4 that with the circuit constants of the amplifiers 11 and 12 chosen as described the system is operative to produce uniform amplification over a practical maximum range of frequencies. If the inductance 30 has a value such that it resonates with the effective input capacity of device 12 at a frequency greater than the frequency at which uniform amplification will not be had over the entire range of lower frequencies. Instead the system would operate more nearly in the nature ofa tuned amplifier having selectivity at particular high frequencies.

In Fig. 2 I have shown a system in accordance with my invention adapted for reception and retransmission of signals of the type herein referred to. Such systems are necessary particularly when short wave lengths of the order of 5 meters are employed since waves of extremely short length behave in the nature of light waves and rebroadcasting is necessary where such waves cannot travel in a straight line between the transmitting and receiving points.

The receiving equipment comprises an antenna 36 whereby thereceived oscillations are supplied to the grid of a detector 37. Heterodyne oscillations are also supplied to the grid of the detector 37 from a heterodyne source 38. In the output circuit of the detector 37 a frequency is produced which may, for example, be of about 20,000 kilocycles or any other suitable frequency where amplification may be readily effected. The remainder of the system is then the same as has been described in connection with Fig. 1 and comprises amplifiers 16 and 17, detectors 18 and 19, and the conventional radio transmitter 1.

While I have shown particular embodiments of my invention it will of course be understood that I do not wish to be limited thereto since many modifications may be made both in the circuit arrangement and in the instrumentalities employed. I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention. I

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a source of oscillations having a wide ;range of frequencies including frequencies in the audio range and frequencies relatively high in the radio range, an electron discharge device, operating connections therefor including connections whereby said high frequency oscillations are supplied to the electrodes quency oscillations are supplied to said discharge device said inductance having such a value relative to the resistances of the input circuit to said discharge device that all frequencies within said range are supplied to said lectrodes with substantially equal transmission e ilciency. I

2. In combination, a source of oscillations having a wide range of frequencies including frequencies in the audio range and Frequencies relatively high in the radio range, an electron discharge device, operating connections therefor including connections whereby said high frequency oscillations are supplied to the electrodes of said discharge device, said range of frequencies being such that in the higher portion of said frequency range capacity effects between electrodes of said discharge device and in said operating connections reduce the efliciency of transmission of said oscillations to said electrodes, and an inductance included in said connections whereby said high frequency oscillations are supplied to said discharge device, said inductance havin such a value that it resonates with said capacity effects at substantially the highest frequency in said range, and the input resistance to said discharge device being adjusted to equal /2 times the resistance of the source of said oscillations.

3. In combination, a carrier wave amplifier, comprising an electron discharge device having an anode, a cathode, and a grid, operating connections therefor including means to supply a carrier wave to be amplified to said grid, a source of signal oscillations, and connections from said source to said cathode and anode whereby said carrier wave is modulated with said signal oscillations, said signal oscillations including frequencies extending over a wide range such that capacity effects between the electrodes of said discharge device and in said operating connections prevent efficient transmission of signal oscillations in the high frequency portion of said range to said cathode and anode, and an inductance included in said connections from said source having such a value relative to the resistances and capacity of the circuit that all frequencies within said range are supplied to said cathode and anode with substantially equal transmission efficiency.

4. In combination, a pair of electron discharge devices connected in cascade for successive amplification of signal currents having a wide range of frequencies, an inductance connected between the anode of one of said devices and the grid of the. other device having such a value that it resonates with the effective capacity of the input circuit to said other discharge device at the highest frequency to be amplified, and a resistance connected between the grid and cathode of said second device having a value at said highest frequency equal in magnitude to the input circuit capacity reactance of said second tube.

5. In combination, a pair of electron discharge devices, each of said devices having an anode, a cathode, and a grid, a source of oscillations connected between the grid and cathode of one of said discharge devices, a resistance connected between the grid and cathode of the other of said discharge devices, and an inductance connected between the anode of said one of said devices and the grid of the other device having such value that it resonates with the effective capacity between the grid and cathode of said other device at the frequency where the effective resistance between the grid and cathode of said other device is equal to one half of the value of said resistance connected between the grid and cathode at zero frequency.

6. In combination, an electron discharge device having a cathode and a grid, 9. source of oscillations having desired frequencies extending over a wide range, connections between said source and said cathode and grid including a coupling capacitor, and a pair of inductive impedances all connected in series, one of said impedances having a value such that it resonates with the capacity of said coupling capacitor at a frequency low in said range and the other of said impedances having such a value that it resonates with the effective capacity between said grid and cathode at a frequency high in said range.

7. In combination, an electron discharge device, having a grid circuit and an anode circuit, said grid circuit including av source of oscillations to be repeated to said anode circuit and including a wide range of frequencies, and a coupling capacitor having one electrode connected to said source and another electrode connected to said grid whereby said source is coupled to the grid of said discharge device, and an inductive impedance in said circuit resonant with the capacitance of said coupling capacitor at a frequency low in said range said impedance being connected in series between said source and said grid.

8. In combination, an electron discharge device, having a grid circuit and an anode circuit, said grid circuit including a source of oscillations to be repeated to said anode circuit and including a wide range of frequencies a coupling capacitor whereby said source is coupled to the grid of said discharge device, an inductance connected in series with said source and said grid to neutralize the reactance of said coupling capacitor at low frequencies, and means substantially to prevent said inductance from having an appreciable eilect in the high frequency portion of said range.

9. In combination, a pair of electron discharge devices, each of said discharge devices having an anode circuit and a grid circuit, a source of oscillations having frequencies both low in the audio range and relatively high in the radio range connected in the grid circuit of one of said devices, a connection between the anode circuit of said one of said devices and the grid circuit of the other device including a coupling capacitor, an iron core inductance and an air core inductance all in series, said inductances having such values relative to the capacitances of the circuit and connections including the grid of said second discharge device that said oscillations are repeated to the anode circuit of said second discharge device with substantially equal fidelity throughout said frequency range.

ROBERT B. DOME.

Certificate of Correction Patent No. 1,990,586. February 12, 1935. I

v ROBERT B. DOME It is herebyicertified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Page 3, second column, lines 70 to 74, strike out the formula and Insert mstead ll o and page 6,' second column,vljne 41, claim 8, after frequencies insert a comma; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 19th day of March, A. D. 1935.

[SEAL] I LESLIE FRAZER,

Acting Commissioner of Patents.

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