US2135561A - Wireless and like receiver - Google Patents

Description

Nov. 8, 1938.

w. H. CONNELL 2,135,561

WIRELESS AND LIKE RECEIVER I Filed March 14, 1936 LocnL osc ILLRTOR 8 4 F 3. 6 L n! I 2'7 1 u A47 2 g 14 ATTORNEY Patented Nov. 8, '1938 UNITED STATES WIRELESS AND LIKE RECEIVER William Horace Connell, Hillingdon, England, as-

signor to Electric & Musical Industries Limited,

ain

Middlesex, England, a company of Great Brit Application March 14, 1936, Serial No. 68,791 In Great Britain March 16, 1935 7 Claims.

The present invention relates to wireless and like receivers of the type in which there is provided automatic control of gain in accordance withfiuctuations in signal strength; the inven- 4: tion is particularly concerned with the prevention or reduction of certain forms of distortion due to the overloading of a signal-frequency amplifying stage of such a receiver. The term signal-frequency stage is intended to include not only a 1-0 carrier-frequency stage, but also an intermediate-frequency stage in the case of a superheterodyne receiver. One result of overloading in such a stage may be the production of a certain form of interference commonly known as cross-modul'5 lation; that is, the carrier of a desired station may become modulated with signals from a carrier of a station of a neighbouring frequency, with the result that signals from bothstations are heard simultaneously. Cross-modulation is chiefly due to the fact that the valves which are commonly used for signalfrequency amplification have anode current-grid potential characteristic curves which are of a form which may be described as consisting of two approximately rectilinear por- 25. tions joined by a portion of pronounced curvature. Such a curve is characterized by terms of the third and higher odd orders, and it is when these terms are present that cross-modulation occurs. i

30: Normally, it is arranged that the valve Works on one or other of these rectilinear portions, that is to say, thevalve is so biased that the applied signal does not swing the grid beyond the limits of the rectilinear portion on which the valve is 35 working. When the applied signal is of sufiicient amplitude to swing the grid on to the curved part of the characteristic, the conditions are those of overloading, and cross-modulation may occur. Thus if it is required to amplify a wanted signal 40 when there is also present an unwanted signal of greater amplitude, the unwanted signal overloading the valve in the sense that it causes it to work over the curved portion of its characteristic, it is found that cross-modulation occurs.

5' It will be clear that this interference will be prevented if the selectivity of the tuned circuit or circuits preceding the valve is made great enough to reduce the ratio of unwanted to wanted signal to a small figure, but this is often impracticable 50 both technically, and from the point of view of cost.

In known receivers in which automatic gain control is provided,a portion of the amplified signal is rectified and the D. C. component of the rectified signal is used to provide a grid bias for one or more signal frequency amplifying valves which generally are ofthe kind having characteristic curves of the form discussed above. In such a receiver, cross-modulation may occur when a weak signal which is in the neighbourhood of a 5 strong signal is being received; the reason for this is as follows. The selectivity of the receiver at that point where the automatic gain control potential is derived is usually very much greater than it is in the input circuits, and it follows that the bias voltage supplied by the automatic gain control rectifier will not be greatly affected by signals, however strong, separated from the wanted station by, say, more than 9 kcs. This must necessarily be the case if the receiver is to be sensitive enough to receive-very weak stations, for otherwise a strong adjacent signal (by providing a large automatic gain control potential) would cause the sensitivity of the whole receiver to be reduced. The weak station may produce only a very low output from the automatic gain control rectifier, and in these circumstances the amplifier valves work on the steep and short portion of their characteristic. Thus cross-modulation may occur very easily and has only been avoidable in the past by either reducing the overall sensitivity of the receiver or, as stated above, by increasing the input selectivity. The former method is obviously undesirable and the latter is often impracticable from a technical and economic point of view.

It will be understood that not only may crossmodulation occur due to overloading of the kind already defined, but it may also arise if the applied signal swings the grid of a signal-frequency amplifying valve into grid current, since the grid current-grid potential characteristic of the valve may comprise third order terms. This form of overloading may occur, in addition to the kind previously defined, in a receiver in which automatic gain control is provided, when receiving a signal having a signal of greater amplitude adjacent to it.

Distortion may also occur during the reception of very strong signals owing to the fact that the output of the automatic gain control rectifier is very often limited, and if a signal is received whose amplitude is greater than the maximum bias which can be supplied to the signal frequency valve by the automatic gain control rectifier, this valve will be overloaded and distortion will occur owing to rectification of the signal.

It is an object of the present invention to provide a wireless or like receiver in which distortion of the kinds discussed above is avoided or reduced.

The present invention accordingly provides a wireless or like receiver of the kind in which gain-control means are provided for controlling automatically the gain of a pairof radio-frequency amplifier stages whichare coupled in cascade by'the aid of frequency-selective coupling means, said gain-control meansbeing fed with radio-frequency oscillations from the second of said stages, wherein there are provided auxiliary gain-control means, associated with the coupling between said stages, for developingan auxiliary gain-control potential and applying said auxiliary potential to the first of said'stages. Since the two stages are coupled in cascade by the aid of frequency-selective couplinglmeans, the auxiliary gain-control potential -is developedat a point in the receiver where the selectivity is less than at the point where ranged.

The present invention 'further provides wire;-

less or like receiver of the kind in which gaincontrol means are provided for controlling auto,- matically the gain of a :plurality of radio-frequency amplifier stages each of which comprises a thermionic valve, said stages. being. fed with oscillations corresponding ,toa desiredsignalfrom an input circuit. and being coupled-in cascade by the aid of frequency-selective coupling means, in which said gain-control means operate to develop a gain-control bias which is applied to bias the control grid of each of said valves so as to-reduce the gain of said stagesv as the strength of said desired signal increasea'and vice versa, and in.

which said again-control means are fed with radio-frequency oscillations from the output of the last of said stages, characterized. in that there are providedauxiliary means-for ensuring that the value of the ain-control bias applied to the first of said valves, that is, the valve adjacent said input circuit, is always sufiicient to ensure that overloading in said first valve is substantially completely avoided. I p

In a preferred arrangement, the auxiliary means are so arranged that, when said receiver is tuned for the reception of a desired-signal the frequency of which is closely adjacent the frequency of a relatively much stronger station, the gain-control bias; due to said gain-control means, which is applied to said first valve is ;supplemented by an additional gain-control bias derived from said auxiliary means. 7

The invention will now be described,-by way example, with reference to the accompanying drawing, in which v V v Fig. 1 isa schematic diagram of a receiver. according to the invention,

Fig. 2. illustrates a-modification cf the arrangestage comprising a tetrode valve 4 adapted to be fed from a suitable aerial system 5 through a tunable resonant circuit '6, a thermionic frequency changing stage 7 associated with a 'tunable local oscillator 8, two intermediate frequency ampli- -fy ing stages 9 and In each comprising a thermionic amplifying va'lve, a-second detector H, and

a modulation frequency amplifying and output stage l2 feeding a reproducer I 3. g The valve 4 is coupled to frequency-changer l by means of a 75 tunab1e resonant circuit [1, and: stage 1 .is

the gain-control means are arcoupled to stage 9 through two mutually-coupled tuned circuits [8 and I9; two pairs of similar circuits 26, 2| and 22, 23 serve to couple stages 9 and I0, and I and II.

Means for deriving an automatic gain control potential difference are associated with the second detector stage I I. These means may take any known or suitable form; they preferably comprise a rectifier fed with radio-frequency oscillations and are preferably so arranged that automatic gain control does not come into operation until a signal of greater than a predetermined threshold strength is tuned in. Suitable means are well known to those versed in the art, and since the nature thereof is not an essential feature of this invention, they are not illustrated in Fig. 1. The automatic gain control potential difference is applied by means of lead l4 tothe signal frequency amplifying stage, the frequency changer I and the intermediate frequency amplifying stages 9 and II) in well-known manner,

valve of the so-called variable-mu type, the gain control potential difference being established be-' tween the control grid and cathode of each valve.

The anode circuit of the signal-frequency am.- plifying valve 4 comprises a radio-frequency choke coil 15 acting as a coupling impedance; the anode of this valve is connected through a blocking condenser Hi to the anode of a twoelectrode auxiliary rectifier such as a copper-copper oxide rectifier 24, the cathode of the auxiliary rectifier being connected to earth. A load resistance 25 is connected in shunt with the rectifier 24 to complete the direct current path for the current in the rectifier. a p I A variable tapping point in the load resistance 25 is connected to the lead 14 from the automatic gain control circuit to the controlgrid of the first amplifying valve. Thus there is set up at the tapping point, relative to earth, a. potential dependent upon the rectified output of the valve 4, and hence upon the peak amplitude of the signal voltage applied to that valve.

The. selectivity of the circuits preceding the auxiliary rectifier 24 is much less than the selectivity of the circuits preceding the automaticv fraction of the potential difference set up across the load resistance is establishedbetween the grid and cathode of the valve 4, overloading of the first amplifyingstage of the receiver may be prevented.

In the lead l4'from the automatic gain LCOIl- H trol means, at a point before it is joinedby the lead from the tapping'point in the load resistance 25 of the auxiliary rectifier 24, is connected a two-electrode blocking rectifier 26such as a copper-copper oxide rectifier, having its anodeconnected to the tapping point, and. its cathode con nected to a point in the automatic gain control circuit through the gain control potential lead M. A decoupling resistance 2! of a small value is preferably connected in rectifier;

series with the blocking:

' Thefunction of the blocking rectifier 26 as, follows: when, the automatic gain, control, potential on lead I4 is less than that due to the rectified output of the first amplifying-valves,

the blocking rectifier '26 passes no current and isolates the automatic gain control circuits from' the control grid of the valve 4. By this means the sensitivity of the remainder of'the receiver will not be reduced even though a large local automatic gain control bias is applied to the grid of the signal'frequency amplifier valve 4.

When, however, the gain control potential on lead l4 exceeds the potential due to the rectified common to Fig. 1 are given the same references.

In Fig. 2, the rectifier 24 of Fig. 1 is replaced by an auxiliary diode anode 28 co-operating with the cathode of valve 4; the diode anode 28 is connected to the main anode of valve 4 through blocking condenser l6, and to the cathode of valve 4 through load resistance 25. A tapping point on the latter is connected to automatic gain-control potential lead [4 in the manner described with reference to Fig. 1.

In order to prevent a signal frequency amplifying valve of the receiver such as the valve 4 from being overloaded by a signal which produces a signal voltage at the grid of that valve which is greater than the potential that the automatic gain control means are capable of furnishing in the circumstances, the arrangement illustrated in Fig. 3 may be adopted; this figure illustrates a modification of the signal-frequency amplifying stage of Fig, 1, like parts in the two figures being given the same references. Referring to Fig. 3, a two electrode rectifier 29 such as a copper-copper oxide rectifier is connected between the control grid and cathode of the valve 4, the anode of the rectifier 29 being connected to the grid, and the cathode thereof to the cathode of the valve 4. The rectifier 29 breaks down and passes current when the control grid tends to become positive relative to the cathode, and this current fiows through the grid leak resistance 30 and biasing resistance 3|, the potential difference thus set up being applied between the control grid and cathode of the valve 4 in such a sense as to make the grid more negative relative to the cathode. By this means, it can be arranged that the applied signal never causes the control grid to become positive relative to the cathode, and distortion due to this cause is thus avoided. It is to be noted that in Fig. 3, the gain-control bias from lead I4 is applied to the low-potential end of input circuit 6, a decoupling condenser 32 being connected between the low-potential end of circuit 6 and earth.

The two-electrode rectifiers referred to in the above description need not necessarily be of the copper-copper oxide type, but may if desired be of any other suitable kind such as thermionic diodes. The invention is, of course, not limited in its application to superheterodyne receivers, and many modifications of the invention within the scope of the appended claims will occur to those versed in the art.

I claim:

1. A radio receiver comprising an input circuit, a first radio-frequency amplifying stage, coupling means between said input circuit and said first stage, a second radio-frequency amplifying stage, frequency-selective coupling means for coupling said stages in cascade, gain-control means for controlling automatically the gain of said stages, means for feeding radio-frequency oscillations from the output of said second stage to said gain-control means, auxiliary gain-control means including a rectifier for developing an auxiliary gain-control potential, means for applying said potential to said first stage for controlling the gain thereof and means for feeding radio-frequency oscillations to said auxiliary gain-control means from a point in said frequency-selective coupling means at which the selectivity is less than at the output of said second stage, and the selectivity at the output of the second stage being sufficiently greater than that at said point to prevent response of said first control means to relatively strong adjacent channel signals.

2. A radio receiver comprising an input circuit for receiving oscillations corresponding to a desired signal, a plurality of radio-frequency amplifying stages each comprising a thermionic valve, frequency selective means for coupling said stages in cascade, coupling means between said input circuit and the first of said stages, gain-control means for developing a gain-control bias, means for feeding radio-frequency oscillations from the output of the last of said stages to said gain-control means, means for applying said gain-control bias to the control grid of each of said valves so as to decrease the gain of said stages as the strength of said desired signal increases, and vice versa, an auxiliary rectifier coupled to the first of said stages for the rectification of radio frequency oscillations and generating an additional gain-control bias which is of a relatively large value when said desired signal is of a frequency which is closely adjacent the frequency of a relatively much stronger station, and means for applying said additional gain-control bias to the control grid of the first of said valves.

3. A receiver according to claim 2, in which there is a conductive coupling between said gaincontrol means and the control grids of said valves, characterized in that means are provided for preventing said additional gain-control bias from reducing the gain of any of said valves which is more remote from said input circuit than is said first valve.

4. A receiver according to claim 2, in which there is a conductive coupling between said gaincontrol means and the control grids of said valves, characterized in that a unidirectionally conducting device is connected in said conductive coupling for preventing said additional gaincontrol bias from reducing the gain of any of said valves which is more remote from said input circuit than is said first valve.

5. In combination with a signal transmission tube followed by a network of high selectivity, an automatic gain control means coupled to the network output and arranged to decrease the tube gain with signal amplitude increase, a second gain control means coupled to the network input and arranged to decrease the tube gain with increase of received signal amplitude, and additional means, common to both said control means for preventing the first control means from acting on said tube when the control action of the second control means is at a predetermined level.

6. In combination with a signal transmission tube followed by a network of high selectivity, an automatic gain control means coupled to the network output and arranged to decrease the tube gain with signal amplitude increase, a second gain control means coupled to the network input and arranged to decrease the tube gain with increase of received signal amplitude, additional means, common to both said control means for preventing the first control means from acting on said tube when the control action of the second control means is at a predetermined level, said network including cascaded signal transmission tubes, said first control means additionally regulating the gain of the cascaded tubes, and said common means preventing the second control means from acting on said cascaded tubes at said control level.

7. In combination with a signal transmission tube followed by a network of high selectivity, an automatic gain control means coupled to the network output and arranged to decrease the tube gain with signal amplitude increase, a second gain control means coupled to the network input and arranged to decrease the tube gain with increase of received signal amplitude, additional means, common to both said control means for

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