US2204216A

US2204216A - Attenuator circuit

Info

Publication number: US2204216A
Application number: US200610A
Authority: US
Inventors: Daniel E Harnett; John F Farrington
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1938-04-07
Filing date: 1938-04-07
Publication date: 1940-06-11
Anticipated expiration: 1957-06-11

Description

2 Sheets-Sheet. 2

INVENTOR DANIEL E HARNETT,

ATTORNEY ATTENUATOR CIRCUIT Filed April 7, 1938 D. E. HARNETT ET AL.

June 11, 1940.

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Patented June 11, 1940 ATTENUATOR CIRCUIT Daniel E. Harnett, Tuckahoe, and John F.

Farrington,

Flushing,

N. Y., assignors to Hazeltine Corporation, a corporation of Delaware Application April 7, 1038, Serial No. 200,010

11 Claims.

This invention relates to modulated-carrier signal receivers and, more particularly, to an arrangement for reducing the maximum amplitude and the range of amplitude variations of the input to a repeated stage of such a receiver.

Under normal operating conditions, a modulated-carrier signal receiver is subject to the reception of signals of amplitudes varying within very wide limits. As a result, the input to the first repeater stage of the receiver normally has a correspondingly Wide range of variation. For weak signals it is desirable to maintain the transmission characteristics of the response of the preselector between the antenna and the first re- 15 peater stage of the receiver at a maximum, in order to raise the signals substantially above the noise level, thus to procure the maximum use .ful sensitivity of the receiver. However, with a high gain in thepreselector, asthe strength of desired received signals increases to such an extent that the swing of the grid of the first repeater stage includes a substantial nonlinear portion of the characteristic curve of the repeater, distortion of the desired signal-modulation envelope and cross modulation of the desired signal carrier by strong undesired signals may result. This is particularly true in the case of receivers provided with automatic amplification control by which strong signals cause the grid to be biased toward that portion of its characteristic which is most nonlinear. Undesired signals as well as desired signals which reach the input electrodes of the first repeater stage have the effect of increasing the grid swing to such an .3 extent that these disturbingefiects result. It is well known that such envelope distortion and cross, modulation are effects which cannot be filtered out by succeeding selective circuits.

In general, the preselector, that is, the selector circuit between the antenna and the input circuit of. the first vacuum-tube repeater in the signaltranslating channel, should be selective to pass a band of desired modulation frequencies sufiicient- 1y Wide to provide the desired fidelity of repro .;,-,,duction. Generally speaking, if an attempt is made to decrease the total desired and undesired signal input of the first repeater stage by discriminating against the undesired signals passed by the selector, as by adjusting its band width, the to fidelity of reproduction is impaired to an undesirable degree. Furthermore, this does not remove the envelope distortion of abnormally strong desired signals.

Variouscxpedients have heretofore been pro- 5 posed for automatically and adjustably attenuating the input to the first repeater stage of a modulated-carrier receiver in order to eliminate the disturbing effects described above. In cer' tain ofthese arrangement, adjustable impedance elements, such as vacuum. tubes, have been con- 5 nected in circuit with the preselector circuit of the receiver adjustably to damp such circuits, thereby adjustably to attenuate the input to the first repeater. This type of element, however, is usually inherently of high impedance, which re- 10 stricts its application to a parallel connection with other elements of the preselector circuit. As thus connected, such an adjustable impedance gener- ,ELIIY has an undesirable effect on the selectivity preselector circuits which aretunable over a wide range of frequencies, both. because it tends to restrict the tuning range and because its detuning effect may vary with adjustments of its attenuation effect.

.It is an object of the invention, therefore, to provide an improved, simple, and economical attenuator which may be easily adjusted to control the sensitivity of a modulated-carrier signal receiver in such manner as to eliminate the undesired effects described above.

It is a further object of the invention to provide an attenuator for adjustably attenuating the input to the first repeater stage of a modulated- 4n carrier signal receiver with freedom from undesired cross modulation of the desired signal carrier by strong undesired signals on nearby frequencies.

It is a further object of the invention to provide a preattenuator of the type described which secures the desired result without affecting the selectivity of a radio-frequency selector with which it is associated.

In accordance with the invention, there is 5 provided in a modulated-carrier signal receiver including an input circuit and a vacuum tube having an input circuit in the signal-translating channel offthe receiver and comprising the firstsignal-tra-nslating tube of the receiver, an attenuator comprising a first feed-forward coupling circuit for coupling the input circuit of the receiver to the input circuit of the vacuum tube. The attenuator also includes a second feed-forward coupling circuit, substantially nonfrequency-selective over the operating range, and including a repeater and terminal circuits individually coupled to the input circuit of the receiver and to the input circuit of the vacuum tube, the second coupling circuit and its terminal circuits providing a system substantially nonfrequency-selective over the sideband range of the received signals. The second coupling circuit and its coupling to the input circuit of the tube comprises means for developing therein potentials opposite in phase to those developed therein by the first coupling circuit. Means are provided for controlling the repeating ratio of the second coupling circuit in accordance with the amplitude of the received signals.

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects thereof, will best be understood by reference to the specification taken in connection with the accompanying drawings in which Figs. 1. and 2 are circuit diagrams, partially schematic, of different forms of the attenuation control as applied to a radio broadcast receiver of the superheterodyne type.

Referring to Fig. 1 of the drawings, there is shown a circuit diagram, partially schematic, of a complete modulated-carrier signal receiver of the superheterodyne type employing the inventionv This receiver comprises, in cascade, an antenna ground circuit I ll, a preattenuating circult II, a radio-frequency amplifier I2, a frequency changer or oscillator-modulator I31, an intermediate-frequency amplifier I4 of one or more stages, a detector and A. V. C. supply I5, an audio-frequency amplifier I6 of one or more stages, and a sound reproducer I1. Automatic amplification control is secured in a well-known manner by applying a unidirectional voltage derived from the A. V. C. supply I5 over conductor I5 to the control electrodes of oscillator-modulator I3 and of one or more of the tubes included in the intermediate-frequency amplifier I4, as shown in the drawings. Suitable operating potentials are supplied to the tubes of the receiver from sources indicated as +56 and +B.

Neglecting for the present the operation of the preattenuating means of the invention, the apparatus just described constitutes, in general, a conventional superheterodyne radio receiver, the operation of which is well understood in the art; In brief, signals intercepted by antenna circuit I ll are translated to the input circuit of radio-frequency amplifier I2 through the preattenuating circuit II, are amplified in radiofrequency amplifier I2, and converted to an intermediate-frequency signal in oscillator-modulator I3. The intermediate-frequency signals are amplified in intermediate-frequency amplifier I4 and are delivered to detector I5 wherein the audio-frequency signals and the A. V. C.

biasing potentials are derived. These signals are,

in turn, amplified by audio-frequency amplifier I5 and supplied to loud-speaker I1 for reproduction. The automatic amplification control bias is effective to maintain the signal input to detector I5 within relatively narrow limits for a Wide range of received signal intensities.

Referring now more particularly to the preattenuating system I! for controlling the input to radio-frequency amplifier I2, the arrangement comprises, in general, a normal feed-forward coupling circuit between antenna circuit Ill and the input electrodes of radio-frequency amplifier I2, and an auxiliary feed-forward coupling circuit in parallel therewith and phased oppositely to the normal coupling circuit so that voltages of opposite potential are applied to the input circuit of amplifier I2 by the two parallel couplings. In order to equalize the gain between the antenna and the, amplifier I2 over the tuning range, the antenna circuit is tuned near the lowfrequency end of the tuning range by inductance 2I and the antenna capacitance, and resistors I8 and I9 are added to damp the resonance characteristic of this circuit. The normal coupling circuit between the antenna circuit III and the input electrodes of amplifier I2 is completed through a tuned circuit comprising an inductance 22 and a tuning condenser 23, inductance 22 being inductively coupled to inductance 2|. The auxiliary coupling circuit between the antenna circuit Ill and amplifier I2 is substantially nonfrequency-selective over the sideband range of received signals and comprises a control repeater tube 24 having input electrodes coupled across resistor I9 through condenser 25 of low reactance, and an output circuit comprising an inductance 26 inductively coupled to inductance 22 with a polarity such that the coupling of the auxiliary circuit is opposite in phase to that of the normal coupling circuit. The output circuit of i'adimfrequency amplifier I2 is coupled to the input circuit of oscillator-modulator I3 through a transformer comprising windings 21, 28.

In order to provide a bias for control tube 24 which varies in accordance with received signal intensities, thereby to control the repeating ratio of the auxiliary coupling circuit, there is provided an amplifying and rectifying channel including tubes 2!! and 3D. Amplifier tube 29 is coupled to the radio-frequency channel of the sistors 20 and 39 being provided to aid in obtaining such characteristics. The diode 35, 36 is provided with a load

circuit comprising resistors

31, 38, 39 across which there is developed a unidirectional voltage which varies in magnitude in accordance with the signal input to amplifier 29. Theportion of this voltage developed across resistor 38 is applied negatively to the grid of amplifier I2 through a filter, comprising series resistors l0 and shunt condenser II, to vary the amplification thereof inversely in accordance 1 with the amplitude of received signals, in a conventional manner. A second portion of the control-bias voltage, developed across resistor 31, is applied positivelythrough a filter comprising series resistors 42 and shunt condensers 42' to the control grid of control tube 24. In the absence of a biasing potential derived from resistor 31, control tube 24 is biased by means of battery 4-3 to a very low value of transconductance or beyond cutoff. There is preferably provided a limiting circuit for tube 24 comprising the

diode electrodes

44, 45 of tube 38 biased by a cathodebias battery 45 and connected in shunt to resistor 31, across which the control bias for tube 24 is developed, through one of resistors 42,

In considering the operation of the system just described, it will be assumed that, in the absence of received signals or for signals of a very low amplitude, the tube 24 is biased to a low value of transconductance or beyond i cutofi so that the auxiliary coupling circuit of the invention is practically inoperative, the entire coupling between the antenna circuit l0 and the input electrodes of radio-frequency amplifier l2 being that of the normal coupling circuit comprising doubletuned transformer 2|, 22. However, as the amplitude of the received'signals increases, there is an appreciable input to amplifier 29, derived from the signal-translating channel of the receiver through the transformer 3|, 32. The output voltage of amplifier 29 is rectified by diode 35, 36 to develop bias potentials across

resistors

37 and 38 which vary. in accordance with the amplitude of received signals. The negative-bias voltage across resistor 38 is applied through resistors 48 to the control grid of amplifier l2 to decrease its amplification as the intensity of received signals increases. At the same time, the voltage developed across resistor 31 and applied positively to the control grid of control tube 24 through the circuit comprising resistors 42 serves to increase the transconductance of the control tube 24. Inasmuch as the coupling due to the normal coupling circuit and that due to the auxiliary coupling circuit are opposite in phase, an increase in the transconductance of tube 24 serves to decrease the resultant coupling between the antenna circuit In and the input circuit of vacuum tube l2; that is, to attenuate the signal input to the first repeater of the receiver. When the voltage developed across resistor 31 reaches a predetermined amplitude, the

limiter diode

44, 45 becomes conductive, effectively grounding the grid of control tube 24 through battery 45' and maintaining its transconductance constant with further increases in received signal amplitude. This limiting circuit thus prevents the coupling of the auxiliary circuit from over-balancing that of the normal coupling. between antenna circuit l0 and the input of radio-frequency amplifier l2 forreceived signal amplitudes of unusually large values and maintaining the resultant coupling above some predetermined lower limit. It will be understood that the selector circuits in the main signal-translating channel of the receiver following the winding 3| are more selective than those preceding the'winding 3| so that the major portion of the selectivity of the receiver is provided by the circuits in the signal channel succeeding the point to which the attenuation control circuit is coupled; furthermore, the auxiliary coupling circuit comprising the control repeater tube 24 and its terminal circuits coupled to the antenna circuit 10 and the input circuit of the first signal-translating tube i2 is substantially nonfrequency-selective over the sideband range of desired received signals and preferably, also, with respect to adjacent undesired signals. Hence, undesired signals as well as desired signals are included in the input to amplifier 28 and to control repeater tube 24.

The antenna attenuator arrangement of Fig. 2 is similar in principle of operation to that of Fig. 1, corresponding elements being identified by similar reference numerals. The arrangement of Fig. 2 includes an antenna-ground circuit comprising an inductance 46 inductively coupled to an inductance 4'! of a signal selector circuit which includes in series tuning condenser 49 and resistor 48. A second signal selector circuit, comprising a series-connected inductance 41, tuning condenser 5|, and resistor 50, is coupled to the input electrodes of a radio-frequency amplifier 52 having output electrodes coupled to modulator l3. In this embodiment of the invention, the normal signal-translating channel of the receiver comprises

selector circuits

41, 48, 43 and 41', 50, 5| coupled through resistor 52 and extended portions of windings 41 and 41', effectively constituting tertiary windings. The auxiliary control channel, having a coupling effect opposite to that of the normal coupling circuit, comprises a vacuum tube 53 having an input electrode coupled to

selector

41, 48, 49 through a condenser 54 of low impedance at signal frequencies and an output circuit, including a highfrequency choke coil 56, coupled to selector circuit 41', 50, 5| through a condenser 51. For developing a control bias for control tube 53, there is provided an amplifier 58 having input electrodes coupled directly to the input circuit of radio-frequency amplifier 52 through the coupling condenser 59. Coupled to the output electrodes of amplifier 58 through a

broadband transformer

62, 63 is a diode 64 having a load resistance 65 across which is developed a controlbias potential variable in accordance with received signal intensities. The voltage developed across the resistor 65 is applied through a filter circuit comprising series resistors 66 and shunt condenser 61 to the control grid of tube 53 to vary the transconductance thereof in accordance with the amplitude of the input to radio-frequency amplifier 52. Battery 64' provides an initial large bias potential for the grid of control tube 53 to make it inoperative in the absence of signal-derived bias potentials across resistor 65. The auxiliary diode 68, 69 of tube 58 serves to limit the control voltage applied to control tube 53 over

filter circuit

66, 61. tential which varies in accordance with that utilized in conventional A. V. C. circuits is developed across a load resistor 10 by means of auxiliary diode 68, H of tube 58 and is applied negatively through a circuit comprising resistor 12 to the control grid of radio-frequency amplifier 52 automatically to control its amplification. A cathode-biasing resistor 55, by-passed by condenser 55', is provided for tube 53 and a cathode-biasing resistor 68, by-passed by condenser Si, is provided for tube 58.

It is believed that the operation of the circuit of Fig.2 will be apparent from the description given with respect to the circuit of Fig. 1. The normal feed-forward coupling path between the antenna and the input electrodes of radio-frequency amplifier tube 52 comprises tuned circuits ll, 48, 49, and 41', 50, 5| coupled by the tertiary portions of coils 41 and 41' and the resistor 52'. The auxiliary parallel feed-forward coupling of opposite phase comprises vacuum tube 53, the transconductance of which is controlled by means of the voltage derived from the input circuit of radio-frequency amplifier 52 by means of amplifier 58 and rectifier 64. The diode 68, 68 serves to limit the transconductance of tube 53 of Fig. 2 in the same manner that

diode

44, 45

of Fig. 1 limits the transconductance of tube 24.

. While there have been described what are at present considered to be the preferred embodipr" 'lllg an input circuit in the signal-translating rnents of the invention, it Will be obvious to those skilled in the art that various changes and modications may be made thereinwithout departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

i. In a modulated-carrier signal receiver comprising an input circuit and a vacuum tube having an input circuit in the signabtranslating channel of the receiver and comprising the first signahtranslating tube of the receiver, an attenuator comprising a first feed-forward coupling circuit for coupling said input circuit of said receiver to said input circuit of said vacuum tube, a second l'ccddorivard coupling circuit. substantially nonlrequencyselective over the tunins; range i the receiver and including a repeater and terminal circuits individually cou pled to said input circuit of said receiver and to said input circuit of said vacuum tube, said second coupling circuit and its terminal. circuits providing a system substantially nonirecuencwselective over the sideband range of received signals, said second coupling circuit and its coupling to the input circuit of said tube comprising means for developing therein potentials opposite in phase to those developed therein by said first coupling circuit, and means for controlling the repeating ratio of said second coupiling circuit in accordance With the amplitude of received signals.

2 In a modulated-carrier signal receiver c0m- Jg an input circuit and a vacuum tube havchannel of said receiver and comprising the first signal translating tube of the receiver, an attcnuator comprising a first feed-forward coupling circuit for coupling said input circuit of said receiver to said input circuit of said vacuum tube, a second feed-forward coupling circuit comprising terminal circuits and a vacuumtube signal repeater, said terminal circuits individually being coupled to said input circuit of said receiver and said input circuit of said tube, said second coupling circuit being substantially ,nonfrequency-selective and being poled to provide a coupling opposite in phase to that of said first coupling circuit, said second coupling circuit and its terminal circuits providing a system substantially nonfrequency-selective over the sideband range of received signals, and means for controlling the transconductance of said repeater in accordance with the amplitude of received signals.

3. In a modulated-carrier signal receiver comprising an input circuit, a plurality of vacuum tubes the first one of which has an input circuit, and a plurality of signal selectors in the signaltranslating channel of said receiver, an attenuator comprising a first feed-forward coupling circuit comprising one of said signal selectors for coupling said input circuit of said receiver to i said input circuit of said first one of said vacuum tubes, said one of said selectors being less irequency-selective than others of said selectors, a second feed-forward coupling circuit comprising terminal circuits and comprising a vacuum-tube signal repeater, said terminal circuits individually being coupled to said input circuit of said receiver and said input circuit of said one of said tubes, said second coupling circuit being poled to provide a coupling opposite in phase to that of said first coupling circuit and being substantially nonfrequency-selective, said second coupling circuit and its terminal circuits providing a system which is substantially nonfrequency-selective over the sideband frequency range of received signals, and means for controlling the transconduotance of said signal repeater in accordance With the amplitude of the input to said one of said vacuum tubes.

4. In a modulated-carrier signal receiver for operation over a given frequency range comprising an input circuit and a first vacuum tube having an input circuit in. the signal-translating channel of said receiver, an attenuator comprising a first feed-forward coupling circuit for coupling said input circuit of said receiver to said inputcircuit of said vacuum tube, a second feedforward coupling circuit comprising terminal circuits and comprising a vacuum-tube signal repeater, said terminal circuits individually being coupled to said input circuit of said receiver and said input circuit of said tube, said second coupling circuit being substantially nonfrequencyselective and being poled to provide a coupling opposite in phase to that of said first coupling circuit, said second coupling circuit and its terminal circuits being substantially nonfrequencyselective over the sideband frequency range of received signals, and means controlling the transconductance of said signal repeater in accordance with the amplitude of received signals. 5. In a modulated-carrier signal receiver comprising an antenna, a plurality of signal selectors, and a plurality of vacuum tubes having input circuits in the signal-translating channel of said receiver, an attenuator comprising a first leed-forward coupling circuit comprising one of said signal selectors which is less selective than others of said signal selectors for coupling said antenna to the input circuit of the first of said vacuum tubes, a second 'feed-forvvard coupling circuit comprising a vacuum-tube signal repeater and terminal circuits individually coupled to said antenna and said input circuit of said first tube, said second coupling circuit being poled to provide a coupling opposite in phase to that of said first coupling circuit, said second coupling a vacuum-tube amplifier coupled to said signal translating channel at a point preceding said others of said signal selectors, and means for deriving from the output of said amplifier a control-bias potential for said signal repeater variable in accordance with the amplitude of received.

signals. a

6. In a modulated-carrier signal receiver com-- prising an antenna and a plurality of vacuum tubes having input circuits in the signal-translating channel of said receiver, an attenuator comprising a first feed-forward coupling circuit for coupling said antenna to the input circuit of the first of said vacuum tubes, a second feed forward coupling circuit comprising a vacuumtube signal repeater and terminal circuits individually coupled to said antenna and said input circuit of said first tube, said second coupling circuit being poled to provide a coupling opposite in phase to that of said first coupling circuit, said second coupling circuit and its terminal circuits providing a system substantially nonfrequency-selective over the sideband frequency range of received signals, an auxiliary amplifier coupled to the input circuit of said first tube,

rectifying means coupled to said amplifier for deriving a bias potential varying in accordance with the amplitude of the desired and undesired signal input to said first tube, and means for utilizing said bias potential to control the transconductance of said vacuum-tube signal repeater.

7. In a modulated-carrier signal receiver comprising an antenna and a plurality of vacuum tubes having input circuits in the signal-translating channel of said receiver, an attenuator comprising a first feed-forward coupling circuit for coupling said antenna to the input circuit of the first of said vacuum tubes, a second feedforward coupling circuit comprising a vacuumtube signal repeater and terminalcircuits individually coupled to said antenna and said input circuit of said first tube, said vacuum-tube sig nal repeater being normally biased beyond its cutoff point, said second coupling circuit being poled to provide a coupling opposite in phase to that of said first coupling circuit, said second coupling circuit and its terminal circuits providing a system substantially nonfrequency-selective over the sideband frequency range of received signals, means responsive to the input to said first tube for developing a bias potential varying in accordance with the amplitude of the signal input thereto, and means for applying said potential positively to said vacuum-tube signal repeater to vary its transconductance.

8. A modulated-carrier signal receiver comprising an antenna, a plurality of vacuum-tube amplifiers having input circuits in] the signaltranslating channel of the receiver, a first feed forward coupling circuit for coupling said antenna to the input circuit of the first of said vacuum tubes, a second feed-forward coupling circuit comprising a vacuum-tube signal repeater comprising terminal circuits individually coupled to said antenna and said input circuit of said first tube, said second coupling circuit being poled to provide a coupling opposite in phase to that of said first coupling circuit, said second coupling circuit and its terminal circuits providing a system substantially nonfrequency-selective over the sideband frequency range of received signals, means responsive to the signal input to said first tube for developing a bias potential varying in accordance with the amplitude of the input to said first tube, means for applying at least a portion of said potential positively to said vacuum-tube signal repeater to vary its transconductance, and means for applying at least a portion of said potential negatively to one or more of said vacuum-tube amplifiers as an automatic amplification control bias.

9. In a modulated-carrier signal receiver comprising an input circuit and a plurality of vacuum tubes having input circuits in the signaltranslating channel of said receiver, an attenuator comprising parallel feed-forward coupling circuits between said input circuit of said receiver and said input circuit of the first of said vacuum tubes, one of said parallel coupling paths comprising a vacuum-tube signal repeater and terminal circuits, said one of said paths being poled to provide a coupling opposite in phase and lesser in magnitude than that of said first coupling circuit, said one of said coupling paths and its terminal circuits providing a system substantially nonfrequency-selective over the range of sideband frequencies of received signals, and means for controlling the transconductance of said repeater in accordance with the amplitude of received signals.

10. In a modulated-carrier signal receiver comprising an input circuit and a vacuum tube having an input circuit in the signal channel of said receiver and comprising the first signaltranslating tube of the receiver, an attenuator comprising a first feed-forward coupling circuit for coupling said input circuit of said receiver to said input circuit of said vacuum tube, a second feed-forward coupling circuit comprising a vacuum-tube signal repeater and terminal circuits individually coupled to said input circuit of said receiver and said input circuit of said tube, said second coupling circuit being poled to provide a coupling opposite in phase to that of said first coupling circuit, said second coupling circuit and its terminal circuits providing a system substantially nonfrequency-selective over the sideband frequency range of received signals, means for controlling the transconductance of said vacuum-tube repeater in accordance with the amplitude of received signals, and means for limiting the action of said control means so that said transconductance is not increased beyond a predetermined value.

11. In a modulated-carrier signal receiver comprising an input circuit and a vacuum tube having an input circuit in the signal channel of said receiver and comprising the first signaltranslating tube of the receiver, an attenuator comprising a first feed-forward coupling circuit for coupling said input circuit of said receiver ond feed-forward coupling circuit comprising a uacuum-tube signal repeater comprising terfier means for controlling the transconductance of said signal repeater in accordance with the amplitude of received signals, and means operative to by-pass said rectifier means at a predetermined output thereof, thereby to limit the action of said control means so that said transconductance is not increased beyond a predetermined value.

, DANIEL E. HARNE'IT.

JOHN F. EARRING-TON.