US2132101A - Carrier wave system compensated automatic gain control - Google Patents

Description

CARRIER WAVE' SYSTEM COMPENSATED AUTOMATIC GAIN CONTROL Oct. 4, 1938.

I/YVE/YTOQ Ham/a; E EJ350151 ATTORNEY 60 40 Input Ravage z'iz Deaz'eds Patented Oct. 4, 1938 UNITED STATES CARRIER WAVE SYSTEM- COMP ENSATED AUTOMATIUGAIN CONTROL 2 Harold F. Elliott, Palo Alto, Calif.

. Application December 19, 1933, Serial No. 703,054

6 Claims.

My invention relates to carrier wave systems such as are used for communication by telegraphy, telephony, facsimile and television over wire and radio circuits and especially to systems embodying means for automatically controlling the output level at the receiving terminals.

One object of my invention is to extend the range of control of such systems. Another ob ject is to improve the precision of control. An-

ll) other object is to improve the stability of such systems. Still another object is to improve the quality of reproduction. These and other objectives will be described more fully in'the following description and appended claims.

Referring to the accompanying drawing in which like numerals of reference indicate similar parts throughout the several views:

Figure 1 is a schematic diagramilustrating one form of my invention;

Figure 2- is a circuit diagram of a carrier wave receiving system embodying my invention; and

Figure 3 gives curves showing the results obtained by my invention as compared with previous systems.

In the schematic diagram of a carrier. wave receiver embodying my invention, as shown in Fig. 1, the incoming carrier waves from wires or radio antenna are applied to the input terminals I, of a carrier frequency amplifier of one or more 30. stages. By Way of illustration, three carrier frequency amplifiers 2, 3 and 4 are shown, but the invention is not in any sense restricted to this number of amplifiers as more or less may be used. The amplified output, appearing at 5, is

applied in suitable proportion to ,a governor 6 which serves to rectify the carrier waves, with or without further amplification and selection, and to deliver governing potentials at I. A suitable proportion of these potentials is utilized through connections 8 for automatically governing or controlling the gain of, amplifiers 2, 3 and. 4.

This much of the system so far described is similar to systems embodying automatic gain or volume control now in use. The relation between input and output of such systems, when well designed, will be about as shown by curve B in Figure 3. A variation of 60 to 80 decibels in the level of the signals applied to the input terminals I, is held to a variation of 10.to 20 decibels at the output terminals. 5. If it is at tempted to hold the output. variation Within closer limits, the system becomes unstable, Stable governing action can only be'attained when there is a material slope, in the input-output curve. While such a variation in output is,not

objectionable for some purposes, there are other uses where it is desirable to have a much more nearly constant output, and one of the main objectives of my invention is the provision of means which accomplish this and also extend the'range of control while maintaining stability and high quality of reproduction.

In the form of my invention shown in Figure 1, the desideratum above-mentioned is carried out by a carrier frequency compensator 9, which 10' receives amplified and governed carrier frequency signals from 5 and governing potentials from 1, each in suitable proportion, through connections l0 and II. (Compensator 9 by preference takes the form of a carrier frequency amplifier of one it or more stages having provisions for a variation of gain which preferably just compensates for the variations in the level of the governed signals at 5. The output of compensator 9 at ter-' minals I2 is then constant, regardless of varia- 20- tions at terminals I or 5. The relation of the input at l to the output of the system as a whole at I2 is then as shown in curve A, Figure 3. The ouput from terminals I2 may be passed to a further amplifier or to a line, or an antenna, or 35 to a demodulator, or utilized in any desired manner. The amplifiers 2, 3 and 4 and compensator 9 should be of the type in which it is possible to vary the gain over a considerable range without introducing objectionable distortion. One way. of accomplishing this is to use so-called multi-mu electronic amplifier tubes which have a control characteristic free from abrupt changes in curvature and which can be used as controlled carrier frequency amplifiers over wide ranges as without introducing objectionable distortion.

Before discussing the detailed circuits, it may be well to point out more specifically why the arrangements shown in Figure 1 make it, possible to obtain precise control over a wide range, an as illustrated by curve A in Figure 3, whereas previous methods have failed either to give satisfactory range or precise control or have introduced serious distortion or have been. unstable or had other defects. In the first place, (at the circuit arrangement of my invention makes it possible to proportion the carrier frequency amplifier and governor so that stability, con stancy, freedom from distortion and dependable performance are attained. Secondly the use vof 50' a compensator, operating at carrier frequency, after the main carrier frequency amplifier and governor, makes it possible .tocompensate for and correct the variations in the outputof the amplifier and governor without introducing ob- 5 jectionable distortion, or disturbing the stability or constancy of operation of the system. By placing acompensator at this point in the system, and by suitably proportioning it and the po-' tentials applied to it, it is possible to maintain very constant output from the system as a whole with high quality of reproduction, stability, and dependability. Normal variations in tube constants or potentials have little effect upon the operation. There 'are no critical adjustments and any desired operating characteristics within reason are attainable. Previous methods, on

the other hand, have failed to accomplish constant output by a wide margin or 'have been 7 unstable, or have introduced serious distortion or have been critical of adjustment or have Varied in performance with tube and potential changes. t

Figure 2 shows detailed circuits of a carrier frequency receiver, such as may be used for radio or wire line reception, embodying my invention. Carrier wave signals applied to the input terminals I3 are amplified in the carrier frequency amplifier tubes l4, I and I6, which are preferably of the so-called multi-mu or other type permitting a wide range of amplification control withoutintroducing objectionable distortion. The amplifier I4I5,I6, may be of the type in which the entire amplification function is carried out'at a singlecarrier frequency, or it may be of the type in which the frequency is converted to a lower or higher periodicity, as is done, forexample, in superheterodyne systemsv If the latter is used, tube I5 may, if desired, be a frequency converter tube and a suitableheterodyning frequency may be applied in any convenient manner, as, for example, through terminals I'I The transformers betweenstages and the connections for applying andcontrolling' the various operating potentials may be of any convenient and suitable form., These are not described in detail here since these matters ,are well understood, and this description may be simplified by confining it to the details that are new and novel.

In Figure 2,,Irhave shown the output of the last main carrier frequency amplifier stage I6, as feeding into an output transformer with tuned primary and secondary, I 8 and I9 but it will be understood that anyother suitable output termination 'may be employed. A connection 26' leads from 'a suitable point on secondary I9 to the input of governor 22. Another connection 2| leads from another suitable point on secondary I9 to the input of compensator 24. It will be noted that both governor 22 and compensator 24 are supplied with oarrier frequency signals from the output of amplifier I6- Other arrangements may be employed in carrying out the invention but the, particular arrangement shown has been found very effective and stable in operation as well as relatively simple in construction.

The governor comprises a carrier frequency amplifier tube 22, and a full wave rectifier 25, which serves to rectify the output of tube 22 and supply direct current governing potentials at potentiometer 26. Any other arrangement supplying suitable governing potentials may be'employed, but again, the circuits shown have been found to:be both simple and effective in practice. Suitable potentials from potentiometer 26 may be appllied through a suitable system of filters and connectionsTLto the gain control circuits of amplifiers I4, I5 and I6. Amplifiers I4, I5 and I6 together with governor 22-25 constitute a self-governing amplifier whose output at secondary I9 is held within the limits usually obtainable in amplifiers with well designed automatic gain control, as illustrated by curve B in Figure 3.

A suitable proportion of the controlled output from amplifiers I4I5I6 at secondary I9 may be applied through suitable connections 2| to the input of compensator 24. Also a suitable propor tion of the governing potentials at 26 maybe applied through appropriate filters and connections, as 23, to a gain control electrode or .othersuitable gain control means, in compensator 24. Compensator 24 should be a variable mu electronic broadcast receiver, by employing, standard multimu tubes at I4, I5, I6, 22 and-24, and a standard full wave rectifier at 25. Inthis particular re.- ceiver, the full rectified potential from 25 was applied to the gain control circuit 21, of; tubes, I4, I5 and I6 and about onehalf,of this potential was appliedrto the gain control circuit 23 of come pensator tube 24. Theinitial bias on tubes I4 and I6 was set byrrheostat 28 and rheostat 29 was .used for the same purpose on compensator tube 24. It was found thatthe output of com pensator .24, could be'made very constant overa.

very wide range of inputs atfI3 by suitably adjusting the initial bias at 29 and. properly pro-' portioning the. controhpotential taken from potentiometer 26.

iWhile I have shown compensator:24 asicomprising a single variable mu tube, it will be understood that additional tubes or even'entirely different means may be utilized if needed or desired. As :already outlined, theoutput of the compensator 24 may be passed-to a further amplifier, or to a'wire line, or to a demodulator or utilized in any desired manner. In the broad-i cast receiver shown in Figure 2, compensator 24 is shown as-provided with an output transformer having tuned primary and. secondary feeding into a full wave'diode rectifier 30. The output of the latter is shown feeding into. an audio frequency amplifier 32 througha potentiometer- 3|, with the output of the system appearing as audio fre-- quency signals at terminals33. Such an arrangement of a radio receiver is very advantageous since the-rectifier 30 or other detector operates at a very constant level which can be proportioned and adjusted for the'highest quality of reproduction and most effective operation. I 3

Since the audio output of the system is substan: tially constant, some" visual means of indicating when the circuits are tuned to resonance with the incoming carrier is desirable. Such an indicator is shown at 34 in Figure 2 Indicator 34 may; comprise any desired or known instrumentality'. It will be apparent that'many variations may be made without" departing from the spirit of P the invention, as defined in the appended claims andthat details of wiring, filtering, tuningp'ro-Q visions, etc., maybe carried out in an almost'infinite variety ofWays. 3 The question may arise asto whethenth e method may be applied to the compensation of signals at audio frequency and this may be accomplished if the design of the compensator and the proportion of the signals and potentials applied. to it are such that the necessary range of control can be accomplished without introducing objectionable distortion. The necessary control range and other essential features are readily obtained when the compensator is operated at carrier frequencies as shown in the accompanying diagrams and this is therefore given as the preferred form of the invention.

With reference to the location of manual means for controlling the absolute output of a system of the type described, such control means should preferably, though not necessarily, follow the compensator 9 in Figure 1 or 24 in Figure 2. If the compensator is followed by a detector and audio amplifier, the manual control may be on either side of the detector, or may be in the audio amplifier. If the compensator feeds to a line, the control may be either side of the compensator and may take any suitable form. The preferred location is after the compensator or after the detector.

Having thus described my invention, I claim:

1. A superheterodyne radio receiver comprising a radio frequency amplifier with tuned circuits and a tube having gain control means, a frequency converter having tuned circuits and a tube having gain control means, an intermediate frequency amplifier having tuned circuits and a tube having gain control means, a governor comprising an amplifier and a diode rectifier interconnected by a tuned circuit, a circuit connecting the input of the governor and the output of the intermediate frequency amplifier, a circuit connecting the output of the rectifier and the gain control means in the radio frequency amplifier, frequency converter and intermediate frequency amplifier, an intermediate frequency compensator having input and output circuits and gain control means wherein the gain characteristic inversely and proportionately matches and offsets the input-output characteristic of the radio frequency amplifier-frequency converter-intermediate frequency amplifier and governor combination, a circuit connecting the input circuit of the compensator and the output of the intermediate frequency amplifier, a circuit with potential proportioning means connecting the gain control means of the compensator and the output of the rectifier, a detector with input and output circuits, a circuit connecting the input of the detector with the output of the compensator, an audio amplifier with input and output circuits, a circuit connecting the input to the audio amplifier and the output of the detector and a volume control regulating the input to the audio amplifier.

2. A radio receiver comprising a carrier frequency amplifier having a plurality of multi-mu amplifier tubes, input and output circuits and tuning means, a governor comprising an amplifier followed by a diode rectifier, input and output circuits and tuning means, a resistor with tapping means in the output of the rectifier, a connection between the output of the carrier frequency amplifier and the input of the governor, connections between the resistor and gain control electrodes in the multi-mu tubes, a compensator comprising a tube having a variable gain characteristic which is the complement and inversely proportionate to the input-output characteristic of the carrier frequency amplifiergovernor combination, input, output and gain control circuits for the compensator, a connection from the compensator input to the output of the carrier frequency amplifier, a connection from the compensator gain control circuits to the resistor, a detector with input and output circuits, a connection from the detector input to the compensator output, an audio amplifier with input and output circuits, and a circuit including volume control means interconnecting the output of the detector and the input of the audio amplifier.

3. In combination, a main carrier frequency amplifier with input and output circuits and gain control means, a governor comprising a carrier frequency amplifier followed by a diode fier, connections from the rectifier output to the gain control means, a carrier frequency compensator comprising input and output circuits and a variable gain electronic tube having in combination with the potentials applied thereto an input-output characteristic whose slope is inversely proportionate and complementary to the slope of the input-output characteristic of the main carrier frequency amplifier and governor, connections from the compensator input to the main carrier frequency amplifier output, connections from the rectifier output to gain control means in the compensator, a constant level detector connected to the output of the compensator and an audio frequency amplifier connected through a volume control to the output of the detector.

4. A superheterodyne receiver in accordance with claim 1 with the addition of potentially operated tuning indicating means.

5. A radio receiver in accordance with claim 2 with the addition of potentially operated tuning indicating means.

6. The combination as set forth in claim 3 with the addition of resonance indicating means.

HAROLD F. ELLIOTT.

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