US2483889A

US2483889A - Superheterodyne receiver with automatic frequency control

Info

Publication number: US2483889A
Application number: US661521A
Authority: US
Inventors: Groot Folkert Albert De
Original assignee: Hartford National Bank and Trust Co
Current assignee: Hartford National Bank and Trust Co
Priority date: 1941-08-16
Filing date: 1946-04-12
Publication date: 1949-10-04
Anticipated expiration: 1966-10-04
Also published as: FR53543E; GB630692A; US2483314A; DE869223C; DE889313C; NL70087C; NL65489C; BE446845A; FR885191A; FR53545E; CH231618A; DE878971C; GB616358A; GB632169A; CH256781A; US2595931A

Description

Oct. 4, 1949:

F A. DE GROOT 2,483,889

SUPERHETERODYNE RECEIVER WITH AUTOMATIC FREQUENCY CONTROL Filed April 12, 1946 FOLKERTALBERZ'DE 62001" IN VEN TOR.

ATTORNEY.

Patented Oct. 4, 1949 UNITED sTA r rem OFFICE SUPERHETERODYNE RECEIVER WITH AUTOMATIC FREQUENCY CONTROL Conn, as trustee Application April 12, 1946', S'erial'No. 661,521

In the Netherlands April 16, 1943 7 Claims.

The disclosure of U. S. Patent application Serial No. 661,523, filed April 12, 1946, relates to a superheterodyne receiver whose intermediatefrequency portion comprises a sharp filter for the selection of the carrier wave and wherein a control voltageforthe automatic frequency control is generated with the aid of a frequency-responsive-network to which areconnected one or more control rectifiers whilst exclusively owing to the influence exerted bythe said control voltage on the tuning of the sharp filter the intermediatefrequencycarrier wave is kept within the frequency range selected by this filter and the filter, which comprises one or more-oscillatory circuits with reduced damping, forms at the same time part of the frequency-responsive network.

The damping of the oscillatory circuits is generally reduced with theaid of a back-coupled discharge system or by means of-a discharge system wherein a negative resistance occurs between two electrodes.

By selection is construed herein to include the accentuation of the carrier wave with respect tothe side-band frequencies as well as the separation or suppression of the carrier wave.

With a receiver of this type it is possible, for

example, to. eliminate the serious distortion which frequently occurs due to. the fact that the carrier wave of the received signal is attenuated to ahigher extent than are the sidebands (selective fading effect). This is achieved by utilizing in the receiving channel a sharp filter which; accentuates the carrier wave with respect to the side-bands. influenced-by. the control: voltage of the-antimmatic frequency control in such manner that the intermediate-frequency carrier wave is; kept within thefrequency range selected by the filter.

In. orderv to. be able to vary the tuning ofthe sharp. filter over a comparativelywide range, use is preferably made of a filter which comprises one or more oscillatory circuits with reduced damping.

The present invention relates to one determined form of construction of' the above-de-- scribed superheterodyne receiver, viz. to a receiver provided with a sharp filter which com-- prises: at least one inductance coil provided with a ferromagnetic core (iron-cored coil) whose self-induction is influenced by the automatic frequency. control. in order to be able-to give the The tuning of this: filter is (Cl. 179--IE71)- cored coils in the above-described manner entails drawbacks since the circuit-arrangement either has. no or little filtering efiect or begins to oscillate. According to the invention, these drawbacks are avoided by incorporating the ironcored coil in an oscillatory circuit with reduced damping in such manner that variations in the quality factor, or Q, of this circuit, due to the effect of the automatic frequency control on the iron cored coil, have no harmful effect on the stability of the circuit with reduced damping or on: the selectivity of the filter.

For, due to the elfect of the automatic frequency control, it is not only the selfinduction but in general also the Q of the iron-cored coil and the: Q ofthecircuit that are modified. Since, however, inorder to ensure a satisfactory functioning of the filter, it is necessary to reduce the damping of the'circuit to a high extent, the Q of the circuit must not exhibit great variations; since even comparatively small variations of the Q exert a very great influence on the circuit and consequently on the functioning of the filter.

In fact, if owing to the effect of the automatic frequency control on the iron-cored coil the Q of the circuit is improved, the latter approaches very soonthe point at which self-oscillation occurs. If, on the contrary, the Q of the circuit decreases, the selectivity of the filter may be reduced to such an extent that there is no longer any question of selection of the carrier wave, so that in both cases the working of the circuitarrangementdoes not appear to full advantage. By carrying the invention into effect, the abovementioned-drawbacks are avoided.

The iron-cored coil is preferably incorporated in the oscillatory circuit with reduced damping in such manner that within the tuning range of the filter the Q- of the said circuit is not or slightly modified.

If the oscillatory circuit with reduced damping comprises bothan air-coil and an iron-cored coil and if the reduction of the damping is brought about with the aid. of a back-coupled discharge system, this may be achieved by incorporating the whole or part of the iron-cored coil inthe back-coupling circuit of the discharge system by means of which the damping of l the circuit is reduced.

If in the oscillatory circuit with reduced damping use'is made of an iron-cored coil whose core material and/or whose dimensions have been. sochosen that upon variation of the selfinduction of this coil theseries resistance remains constant or substantially constant, we also ob-' tain a circuit whose Q does not vary or slightly varies.

It is in general advantageous negatively to couple back the damping-reducing discharge system, and this in such manner that the circuit in itself is not damped by the negative backcoupling. By taking this step it is avoided that in the case of a strong reduction of the damping of the filter there occurs self-oscillation due to variations of the quantities which determine the behaviour of the damping-reducing discharge system.

The invention will be explained more fully with reference to the accompanying drawing, which represents, by way of example, one embodiment thereof.

The drawing represents the: intermediate-frequency portion of a receiver to which the invention has been applied. An intermediate-frequency amplifying tube I is coupled to the control grid circuit of a discharge tube A through the intermediary of an oscillatory circuit 2, which is tuned to the intermediate-frequency, and of a filter 3, which is constituted by a circuit with reduced damping. The filter 3 is composed of a condenser 5, an iron-cored coil 6 and an air-core coil 7. The reduction of the damping of the circuit 3 is brought about owing to the fact that the circuit 3 is connected in three-point connection :1

to the discharge tube A; that is, circuit 3 is connected in regenerative feedback relationship to the tube 4. Moreover, the discharge tube l is in negative feedback relationship since a resistance 8 which is not shunted for the intermediatefrequency is connected in the cathode lead. The filter 3 is not damped by the presence of this resistance.

In the anode circuit of the discharge tube 4 is incorporated a circuit 9 which is tuned to the intermediate frequency and which is capacitatively coupled, via a condenser ID, to a diode-detector H' whose low-frequency output voltage is supplied to a low-frequency amplifier (not shown).

Furthermore, the circuit 2 has coupled to it a circuit [2 which is also tuned to the intermediatefrequency and to which may be connected two push-pull control rectifiers (diodes) l3 and It. The mid-point of the circuit l2 and the mid-point of the output resistance l5 of the two diodes l3 and Id are connected to a coupling coil i6 which is inductively coupled to the intermediate-frequency circuit 9. The circuits 1, 9 and I2 and the coupling coil i6 form a frequency-responsive network which, jointly with the control rectifiers l3 and I4 connected thereto, forms a device for generating a control voltage for automatic frequency control, which control voltage appears across the above-mentioned output resistance P5.

The control voltage is supplied to the control grid of a discharge tube H which is shown as a triode and whose anode current flows through a magnetizing coil [8 of a transformer [9 which comprises a core of high-frequency iron on which a magnetizing coil I8 and the previously described coil 6 are arranged. By varying the direct current flowing through this coil, it is possible to vary the inductance of the coil 6 and therefore the tuning of the sharp circuit 3 within narrow limits.

Due to the presence of the sharp circuit 3 the side-band frequencies of the intermediate-frequency signal are greatly attenuated with respect to the carrier wave, owing to which in the case of selective fading the danger of the production of an apparent over-modulation of the signal, which would be attended with a large non-linear distortion of the low-frequency signal, is avoided. The linear distortion caused by the attenuation of the side-band frequencies is suppressed by the proper choice of the frequency characteristic of the low-frequency amplifier.

In order to obtain the desired effect, it is necessary, however, that the tuning frequency of the circuit should always correspond within narrow limits to the frequency of the intermediate-frequency carrier wave. This correspondence is ensured by the automatic frequency control, for

; when there occurs a difference between the said frequencies there is set up across the resistance I5 a control voltage of suitable polarity which voltage is applied to the input circuit of tube 11, the anode current of which flows through coil l8 and thus modifies the inductance value of the coil 6 to such an extent that the tuning frequency of the circuit 3 approximately corresponds again to the carrier wave frequency.

Due to the effect of the automatic frequency control it is now not only the selfinduction but in general also the Q of the iron-cored coil 6 and of the circuit 3 which are modified. Besides, dependently on the kind of iron-cored coil, it may occur not only that the Q of the circuit 3 increases but also that the Q decreases at an increasing value of the selfinduction. These Q variations may be of such a value that the satisfactory func tioning of the receiver is endangered or even becomes quite impossible, for improvement of the Q may result in that the damping of the circuit is completely eliminated and that the circuit starts oscillating. This phenomenon readily occurs in practice since a comparatively slight improvement of the Q exerts a comparatively great influence upon the degree of the reduction of the damping of the circuit whose damping is already greatly reduced.

If, on the contrary, the Q gets worse the selectivity of the filter decreases, and this to such an I extent that a comparatively slight decrease of the Q of the circuit results in a comparatively large decrease of the selectivity of the filter. If, however, the filter does no longer function selectively, it is not only the carrier wave but also an appre- Z ciable portion of the side bands that is selected,

due to which a new distortion is introduced and the distortion due to selective fading is no longer suppressed.

According to the invention, for this reason the iron-cored coil 6 is incorporated in the oscillatory circuit with reduced damping in such manner that within the tuning range of the filter the Q of this circuit does not vary or slightly varies. Here this is achieved by incorporating part of the iron-cored coil in the back-coupling circuit of the discharge tube 4. The anode circuit of this tube is connected for this purpose to a tap on the iron-cored coil 6.

It is also possible, however, to incorporate the whole of the iron-cored coil in the anode circuit if in this case the discharge system 4 is connected to properly chosen taps on the air coil I.

If now, due to the effect of the automatic frequency control, the tuning of the filter 3 is modified, there does not longer arise the risk that the selectivity of the filter becomes excessively low or that the circuit starts oscillating.

What I claim is:

1 An amplifier circuit arrangement, particuto couple said resonant circuit and said filtercir cuit in cascade to the input circuit of said amplifier to' apply asignal voltage of frequency corresponding to said carrier and sideband frequencies tosaidamplifier, means to couple said filter circuit, tosaid amplifier to derive and apply to the inputcircuit of said amplifier a second'signal voltageof frequency corresponding to said carrier frequency to reduce the damping of said filter, means ,coupled to said resonant circuit and the output circuit of said amplifier to produce a frequency control voltage, means responsive to said control voltage to vary the magnetization of said ferromagnetic core to tune said filter sharply to an intermediate frequency corresponding to said carrier frequency,means coupling the output circuit of said amplifier to said filter circuit in regenerative feedback relationship to maintain variations in the quality factor brought about by variations in said control voltage at values at which the stability and selectivity of said filter are substantially constant, and means coupled to the output circuit of said amplifer to derive an output voltage comprising said first signal voltage and said second signal voltage and having carrier frequency components relative to sideband frequency components greater than the carrier frequency component of said first signal volt-, age.

2. An amplifier circuit arrangement, particularly for use in a receiver for receiving signal voltages of carrier and sideband frequencies, comprising a resonant circuit for translating a first signal voltage having frequency components corresponding to said carrier and saidlsideband frequencies, an amplifier having an input circuit and an output circuit, a filter circuit comprising an inductor having a magnetizable ferromagnetic core and being sharply tuned to a frequency corresponding to said carrier frequency, means to couple said resonant circuit and said filter circuit in cascade to the input circuit of said amplifier to apply a signal voltage of fre-- quency corresponding to saidcarrier and side band frequencies to said amplifier, means to couple said filter circuit to said amplifier to derive and apply to the input circuit of said amplifier a second signal voltage of frequency corresponding to said carrier frequency to reduce the dampingof said filter, means coupled to said resonant circuit and the output circuit of said amplifier to produce a frequency control voltage, means responsive to said control voltage to vary the mag netization of said ferromagnetic core to tune said filter sharply to an intermediate frequency corresponding to said carrier frequency, means coupling the output circuit of said amplifier to said,

filter circuit in regenerative feedback relationship to maintain variations in the quality factor brought about by variations in said control voltage at values at which the stability and selectivity of said filter are substantially constant throughout the effective range of said filter, and meanscoupled to the output circuit of said amplifiento: derive an output voltage comprising 75, saidcarrier frequency, means couplingthe output;

said-first signal voltage and-said second signal voltage and having carrier frequency components relative to sideband frequency components greater than the carrier frequency component first inductor and a second inductor havinga magnetizable ferromagnetic core connected in series, means to couple said resonant circuit and said filter circuit in cascade to the input circuit ofsaid amplifier to apply a signal voltage of frequency corresponding to said carrier and sideband frequencies to said amplifier, means to cou-. ple said filtercircuit to said amplifier to derive and apply to the input circuit of said amplifiera second signal voltage of frequency correspond-;-

ing to'sa-id carrier frequency to reduce the damping of said filter, means coupledto said resonant circuit and the output circuit of said amplifier to produce a frequencycontrol voltage, means responsive to said controlvoltage to vary the :mag

netization of said ferromagnetic coreto tune said filter sharply to an intermediate frequency corresponding to said carrier frequency, means cou-- pling the output circuit ofsaid amplifier to'the second inductor of said filter circuit in regenerative feedback relationship to maintain variationsin the quality factor brought about by variations in said control voltage at values at which the stability and selectivity of said filter are-substantially constant over the effective range oi said filter, and means coupled to the output circuit of said amplifier to derive an output volt-- age comprising said first signal voltage and said second signal voltage and having carrier frequency components relative to sideband frequency components greater than the carrier fre quency component of said first signal voltage;

4. An amplifier circuit arrangement, particularly for use in a receiver for receiving signal voltages of a carrier and sideband frequenciea'comprising a resonant circuit for translating a first signal voltage having frequency components correspondingto said carrier and said sideband fre-, quencies, an amplifier having an input circuit and.v

an output circuit, a filter circuit sharply tuned to a frequency corresponding to said carrier frequency, saidfilter circuit comprising a'first inductor and a econd inductor having a magnetizable ferromagnetic core connected in series, meansto couplesaid resonant circuit and said filter circuit in cascadeto the inputcircuit of said amplifier to apply a signal voltage of frequency corresponding to said carrier and sideband frequencies to said amplifier, means to couple said filter circuit to said amplifier to derive and apply to the input circuit of said amplifier a second si nal voltage of frequency corresponding to said carrier frequency to reduce the damping of said filter, means coupled to said resonant circuit and the output circuit of said amplifier to produce a frequency control voltage, means responsive to said control voltage to vary the magnetization of a said ferromagnetic core to tune said filter sharply to an intermediate frequency corresponding .to

circuit of said amplifier to the second inductor of said filter circuit in regenerative feedback relationship to maintain variations in the quality factor brought about by variations in said control voltage at values at which the stability and selectivity of said filter are substantially constant and said ferromagnetic core having dimensional values and being constituted by material having magnetic values at which the series resistance of said filter circuit remains constant over the effective range of said filter, and means coupled to'the-output circuit of said amplifier to derive an output voltage comprising said first signal voltage and said second signal voltage and having carrier frequency components relative to sideband frequency components greater than the carrier frequency component of said first signal voltage.

5. An intermediate frequency amplifier circuit arrangement, particularly for use in a superheterodyne receiver for receiving signal voltages of carrier and sideband frequencies, comprising aresonant circuit for translating a first intermediate frequency signal voltage having frequency components corresponding to said carrier and said sideband frequencies, an electron discharge amplifier tube having a cathode, a control grid and an anode, a filter circuit comprising a first capacitor connected in parallel with a first inductor and a second inductor connected" in series, means to couple the first inductor of said filter circuit to said resonant circuit and between the control grid and cathode of said amplifier tube to apply a signal voltage of frequency corresponding to said carrier and sideband frequencies to said amplifier tube, means to couple said filter circuit to the cathode of said amplifier tube to apply to the control grid of said amplifier tube a second intermediate frequency signal voltage of frequency corresponding to said carrier frequency to reduce the damping of said filter circuit, means coupled to'the anode of said amplifier tube and to said resonant circuit to derive a frequency control voltage, a third inductor electromagnetically coupled by means of a ferromagnetic core to the second inductor of said filter circuit, means responsive to Said frequency control voltage to vary the current through said third inductor to 'vary the inductance of said second inductor to tune said filter sharply to an intermediate frequency corresponding to said carrier frequency, and means coupled to the anode of said amplifier tube to derive an output voltage comprising said first intermediate frequency signal voltage and said second intermediate frequency signal voltage and having carrier frequency components relative to sideband frequency components greater than the carrier frequency component of said first intermediate frequency signal voltage.

6. An intermediate frequency amplifier circuit arrangement, particularly for use in a superheterodyne receiver for receiving signal voltages of carrier and sideband frequencies, comprising a resonant circuit for translating a first intermediate frequency signal voltage having frequency components corresponding to said carrier and said sideband frequencies, a first electron discharge amplifier tube having a cathode, a control grid and an anode, a filter circuit comprising a first capacitor connected in parallel with a first inductor and a second inductor connected in series, means to couple the first into said carrier and sdieband frequencies to said" ductor electromagnetically coupled by means ofa ferromagnetic core to the second inductor of said filter circuit, a second electron discharge tube amplifier to apply said frequency control voltage to said third inductor automatically to vary the magnetization of said ferromagnetic core and to vary the inductance value of said second inductor to tune said filter sharply to an intermediate frequency corresponding to said carrier frequency, means coupling the anode circuit of said first electron discharge tube to said filter circuit in regenerative feedback relationship at a value at which the quality factor of said filter is substantially constant over the effective range of said filter, and means coupled to the anode of said first amplifier tube to derive an output voltage comprising said first intermediate frequency signal voltage and said second intermediate frequency signal voltage and having carrier frequency components relative to sideband frequency components greater than the carrier frequency component of said first intermediate frequency signal voltage.

7. An intermediate frequency amplifier circuit arrangement, particularly for use in a superheterodyne receiver for receiving signal voltages of carrier and sideband frequencies, comprising a resonant circuit for translating a first intermediate frequency signal voltage having frequency components corresponding to said carrier and said sideband frequencies, a first electron discharge amplifier tube having a cathode, a control grid and an anode, a filter circuit comprising a first capacitor connected in parallel with a first inductor and a second inductor connected in series, said second inductor having a tapping thereon and a ferromagnetic core, means to couple the first inductor of said filter circuit inductively to said resonant circuit and between the control grid and cathode of said first amplifier tube to apply a signal voltage of frequency corresponding to said carrier and sideband frequencies to said amplifier, means to couple said filter circuit to the cathode of said amplifier tube to apply to the control grid of said first amplifier tube a second intermediate frequency signal voltage of frequency corresponding to said carrier frequency to reduce the damping of said filter circuit, a negative feedback resistor interposed between the first inductor of said filter circuit and the cathode of said first amplifier tube, a frequency responsive detector circuit coupled to the anode of said amplifier tube and to said resonant circuit to derive a frequency control voltage, a third inductor electromagnetically coupled to the second inductor of said filter circuit by means of said ferromagnetic core, a second electron discharge tube amplifier to apply said frequency control voltage to said third inductor automatically to vary the magnetization of said ferromagnetic core and to vary the inductance value of said second inductor 9 to tune said filter sharply to an intermediate frequency corresponding to said carrier frequency, a second capacitor coupling the anode circuit of said first electron discharge tube to said tapping on the second inductor of said filter circuit to apply regenerative feedback to said filter at a value at which the quality factor of said filter is substantially constant over the effective range of said filter, and means coupled to the anode of said first amplifier tube to derive an output voltage comprising said first intermediate frequency, signal voltage and said second intermediate frequency signal voltage and having carrier frequency components relative to sideband frequency components greater than the carrier frequency component of said first intermediate frequency signal voltage.

FOLKERT ALBERT DE GROOT.

REFERENQES CETED The following references are of record in the file of this patent:

UNITED STATES PATENTS