US1905946A - Automatic gain control - Google Patents

Description

April 25, 1933. A MATHIEU ET AL 1,905,946

AUTOMAT I 0 GAIN CONTROL Filed April 1, 1932 2 Sheets-Sheet l HFA \FA D LFA INVENTOR GASTON A. MATHIEU ATTORNEY GERAL A ISTED BY W April 1933- G. A. MATHIEU ET AL AUTOMATIC GAIN-CONTROL- Filed April l, 1932 2 Sheets-Sheet 2 AQG c I L LUV Patented, Apr. 25, 1933 UNITED STATES PATENT OFFICE GASTON ADELIN MATHIEU, 0] LONDON, AND GERALD ARTHUR ISTED, OF ESSEX, ENG- LAND, assrenons we wear RADIO CORPORATION OF AMERICA, A CORPORATION OF DELA-.

AUTOMATIC GAIN CONTROL Application filed April 1, 1932, Serial No. 602,524, and in Great Britain March 28, 1831.

This invention relates to automatic gain control devices for use in high frequency receivers and more particularly to automatic ain control devices for use in radio receivers or the purpose of minimizing or eliminating the eifectsof fading.

As is well known one important difiiculty met with in radio reception and more particularly in radio reception on short waves i. e. on waves of the order of 80 or meters or less, is that known as fading, and many attempts have been made to overcome this difiiculty by providing means for automatically varying the amplification gain in the receiver with the intention of causing the sigpal finally indicated, or recorded, to be of su stantially constant intensity notwithstanding variations in the received signal strength obtained upon the receiver aerial.

The principal ob]ect of'the present invention is to provide an improved automatic gain control device of simple arrangement and construction, and which shall be capable of varying the gain of a sensitive and powerful multistage receiver between very wide limits of variations and substantially without introducing distortion. Modern highly sensitive radio receivers are capable of giving very large amplification of the order of from 50 to 80 decibels when working on a wave length of the order of 60 to 80 meters, and it will readily be appreciated that it is not a simple matter to provide automatic means for varying the amplification gain of such a receiver to an extent commensurate with the maximum possible gain thereof.

According to this invention a high frequency r ceiver comprising a plurality of thermionic amplifier members in cascade, is provided with means, operated automatically in accordance with the stren th of the rectified output from a detector included in said receiver, for varying the gain of thermionic amplifier members in said receiver, the arrangement being characterized by the feature that the control operates first to vary the gain of one of the amplifier members, and then, if the rectified output still departs from a predetermined value, to vary the gain of a second amplifier member in addition, and

then of a third amplifier member, and so on.

Preferably the control is obtained by utilizing voltage drop set up by the rectified output of the detector to vary the bias upon the control electrodes (control grids in the case of triodes or control grids or screening rids in the case of screened grid valves) 0 the valves (which may include said detector) .Whose gain is to be controlled and the required successive operation upon the plurality of amplifier members is obtained by providin such normal bias upon the am 11- fier 111cm ers which are to be controll in succession that a larger variation in grid bias is required substantially to modif the output of a given amplifier yalve, t an is required substantially to modify the output of an amplifier valve whose gain is required to be controlled to compensate for a lesser degree of fading.

Preferably also the circuit from which the control potentials is obtained, consists of a simple diode circuit in parallel with the normal receiver detector provided for separating out signal modulation.

Although no limited to its application thereto, the invention is particularl well adapted for use in connection with highly sensitive radio receivers of the frequency changing type, and will be described with reference to its application to a radio receiver consisting of a high frequency am 'lifier, frequency changer, an intermediate requency amplifier, a second detector and an audio frequency'amplifier. In certain cases a receiver of this kind may comprise a second frequency changer and a second intermediate frequency amplifier (operating at a difi'erent.frequency from the first intermediate frequency amplifier) preceding the last detector i. e. that which is provided to separate out the modulation frequencies.

The invention is illustrated in the accompanying diagrammatic and schematic drawings wherein Fig. 1 schematically shows a superheterodyne receiver embodying the invention,

Fig. 2 shows the essential circuits of the receiver of Fig. 1,

cally a superheterodyne receiver embodying the invention A is a receiving aerial; HFA- a high frequency amlplifier; FC a frequency changer; LO a ocal oscillator; IFA. an intermedlate frequency amplifier; D a modulation separating detector and LFA a low frequency amplifier, all these parts being of ordinary well known general arrangement and construction. AGC is an automatic gain control unit and the leads marked 1, 2 3 and 4 are so marked to identify them wit leads shown in Fig. 2 which illustrates the circuits of the detector D and automatic gain control unit AGO.

Referring to Fig. 2 the input circuit of the valve V (the valve of the detector D) is coupled to the preceding IFA stage by a transformer TR The grid of the valve V is connected through a condenser C to the grid of a diode, or, as shown, a triode V connected as a diode, the cathode of the valve V being connected'directly to earth at one end and being earthed at the other through a condenser C The plate of the valve V is connected to the grid, and, through an indicating instrument M and a high frequency blocking impedance R as a choke coil, to a bias battery as shown; and finally through a second resistance R to earth.

The grid circuits for the audio frequency detector valve V the intermediate frequency amplifier (see Fig. 1), the frequency changer valve (see Fig. 1), and the high frequenc amplifier (see Fig. 1) are completed throng leads 4, 3, 2 and 1 respectively to tapping points upon a bias battery E,,, the positive terminal of which is connected to one end of resistance R as illustrated. The impedances R and R may be variable if desired, and it Will be noted that the latter resistance is tapped upon the battery E in order to obtain a required positive voltage for the valve V This voltage may or may not be necessary according to the nature and type of valve V The valve V is preferably a socalled' high power detector valve, having a low impedance output and a fairly low amplification factor, thus permitting a large negative biasing of its grid, and avoiding all possibilities of grid current rectification.

The valve V can be as shown connected to act as a. diode by connecting the grid and the plate together, or it could, of course, be replaced by a diode valve. This valve is far from being critical and on many occasions it will be found quite suitable to use the grid and filament of a triode to act as a diode leaving the "plate free. CH and CH are high frequency blocking chokes; C is a bypass high frequency condenser, and C C and C are low frequency by-pass condensers. Chokes CH and CH are low frequency blocking chokes. Transformer TR is the low frequency output transformer for detector valve V and M an indicating instrument.

The position of the tapping points on the battery E are so chosen that as the received signals vary from very Weak to very strong the reduction of the total gain of the receiver as a whole is obtained first by modifying only the output of the audio frequency detector, then by modifying also the out at from the intermediate frequency ampli er,

then by modifying also the output of the' frequency changer valve (modulating valve), and finally, for very strong signals, by modifying also the high frequency amplifier so that for the strongest of signals all the various circuits of the receiver are controlled to reduce their gain. This effect is obtained by adjusting the constant bias of the audio frequency detector, so that the said detector operates on a low portion of the plate characteristic curve, so that a very small variation of biasing will have a considerable effect on the detector output.

The constant bias for the other valves to be successively controlled are such that said valves operate about mean points on the plate characteristic curves, which are successively higher, and therefore successively increasing variations of bias are required to produce substantial variations of outputs. In a slight variation of the arrangement above described separate grid bias batteries are employed for the various grid circuits instead of a common grid bias battery, these batteries being connected at their positive ends to suitably chosen tapping points upon that resistance, one end of which is connected to earth and/or to the common cathode point.

Obviously the invention is applicable to almost any multiamplifier receiver, the feature of the said invention being the successive control first of one and then of increasing numbers of amplifying members incorporated in the receiver. Obviously also in a frequency changing circuit such as above described, it is not necessary that the various circuits should all be controlled in the particular order set forth. For example, as regards the frequency changing arrangements, it is possible so to arrange'the control that both the amplitude of the incoming signal, and that of the local heterodyne is reduced on the input grid circuit of the modulating valve. Alternatively, as regards the frequency changer, the control may be such that only the amplitude of the heterodyne is effected.

In a further modification shown in Fig. 3 the automatic gain control device is energized from the first detector or frequency changing valve FC instead of as shown in Fig. 1. The arrangement is the same as shown in Fig. 2 except for such obvious changes as are necessitated by the changed position of the control unit AGC.

Fig. 4 shows a further modification in which the invention is applied to control the frequency changer of a supersonic heterodyne receiver incorporating a frequency changer of the push-pull modulating type. With such a frequency changer the power transferred to the intermediate frequency amplifier may readily be made to be a direct function of the amplitude of the heterodyning wave simultaneously impressed upon the grid circuits of the two modulating valves. In Fig. 4, which shows the modulating valves and associated ap aratus only, the local 0scillator valve LOWis coupled through a coupling valve CV to the mid-point of the common grid inductance for the modulating valves VM and -VM The frequency changer stage comprising these valves, of course is followed by the intermediate frequency amplifier. The grid bias of the coupling valve CV is controlled through lead 3 in accordance with the output of the gain control rectifier V (see Fig. 2). The oscillatory circuit II in the output circuit of the valve CV is tuned to a harmonic of the frequency of the oscillator valve LOV 1. e., to a harmonic of the natural frequency of the oscillatory circuit I. It will be seen that in the arrangement shown in Fig. 4 the amplitude of the heterodyne wave applied to the valves VM V'M is controlled by the device AGC.

It is possible to obtain control of the frequency changer in a manner which results in control both of the signal and heterodyne waves by connecting the lead 2 (of Fig. 2) as indicated in broken lines in Fig. 4, i. e., merely by connecting the center point of the common grid circuit of the Valves VM VM to the biasing battery E (Fig. 2) instead of employing the coupling valve bias control arrangement shown in full lines in Fig. 4. The latter arrangement (that shown in full lines in Fig. 4) which results in control of the heterodyne wave only, is preferred although it is, of course, somewhat more expensive to construct.

While we have indicated and described several systems for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means llmited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of our invention as set forth in the appended claims.

What we claim is:

1. An automatic ain control arrangement for a superhetero yne receiver which includes a radio frequency amplifier, first detector, intermediate frequency amplifier and a second detector, said arrangement comprising a diode control tube having a cold electrode connected to an input electrode of said second detector for the impression of signal currents on said diode electrode, a control bias source connected in the circuit of said diode cold electrode, and a plurality of independent direct current connections from a plurality of points of increasing negative potential on said source to the gain control electrodes of said second detector, intermediate frequency amplifier and said radio frequency amplifier respectively, said points being so chosen that as the received signals vary from weak to strong the biases of said gain control electrodes are successively altered in the aforesaid order.

2. An automatic gain control arrangement for a superheterodyne receiver which includes a radio frequency amplifier, first detector, intermediate frequency amplifier and a second detector, said arrangement comprising a diode control tube having a cold electrode connected to an input electrode of said second detector for the impression of signal currents on said diode electrode, a control bias source connected in the circuit of said diode cold electrode, and a plurality of independent direct current connections from a plurality of points of increasing negative potential on said source to the gain control electrodes of said second detector, intermediate frequency amplifier, first detector, and said radio frequency amplifierv respectively, said points being so chosen that as the received signals vary from weak to strong the biases of said gain control electrodes are successively altered in the aforesaid order.

GASTON ADELIN MATHIEU. GERALD ARTHUR ISTED.

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