US2201325A - Radio receiver circuit arrangement - Google Patents

Description

May 21, 1940.

B. D. H. TELLEGEN ET AL RADIO RECEIVER CIRCUIT ARRANGEMENT Filed Dec. 15 1938 1 1 .1

. 2 Sheets-Sheet 1 I. $620 LEAH/Z run/N6 'lND/C'ATO/Z j:

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INVENTORS B D H. TELLEGEN A. V N WEEL BY 4 ATTORNEY May 21, 1940'. B. D. H. TELLEGEN El AL 2,201,325

I RADIO RECEIVER CIRCUIT ARRANGEMENT Filed D80. 15, 1938 2 Sheets-Sheet 2 Fig.3

ATTORNEY Patented May 21, 1940 UNITED .FSTATES PATENT OFFICE 2,201,325 RADIO aeomven omcmr- ARRANGEMENT Bernardus Dominicus Hubertus Tellegen and poration of Delaware I Application December 15, 1938, Serial No. 245,831

In the Netherlands June 4, 1938 6 Claims.

The invention relates, to a radio receiver cir- I cuit'arrangement, in which means are applied to suppress interferences occurring within one of wave in respect to at least the frequencies of the non-suppressed side-band which correspond to the modulation frequencies giving rise to the greatest modulation-depths.

This has the efiect that the modulation-depth, atleast that of the modulation frequencies giving rise to the greatest modulation depths, be-

comes lower, thus permitting the distortionoccurring with linear detection to be maintained within reasonable limits. The circuit arrangement may comprise, for example, an electric filter with a resonance curve which shows a comparatively sharp peak at the frequency corresponding to the carrier-wave of the received signal, which has a steep flank on one side of said peak and which extends on the other side al mcst horizontally over at least that part of the side-band to be transmitted which corresponds to the modulation frequencies giving rise to the greatest modulation depths. The electric filter is preferably constituted by a number .of oscilvlatory circuits with different damping, the circuit with the lowest damping being coupled to one or more of the other circuits in such manner that thefrequency, at which said peak occurs in the resonance curve of the electric filter, is equal to the natural frequency of the circuit with the lowest damping.

According to a suitable form of construction of the invention said filter is constituted by four oscillatory circuits forming part of the receiving cascade, two of said circuits forming a band=pass filter with an approximately critical coupling which are tuned to a frequency approximately in the middle of the side-band to be transmitted, the two other circuits having a different damping which is lower for the two circuits than the damping of the former circuits, which are weakly coupled to each other, which are separated from said band-pass filter by at least one electron tube.

which are tuned successively to frequencies on both sides of the coupling frequencies of the band-pass filter, the tuning frequency of the cir;

cuit with the. lowest damping corresponding to the carrier-wavefrequency of the received signal.

It may be: advisable in certain cases to conduct the intermediate frequency produced in a mixing tube, to a second mixing tube, in which a second and preferably lower intermediate frequency is produced, whilst at least the two circuits with the lowest damping are connected to the output of the second mixing tube.

If a second mixing tube is used, there is, in, addition, the possibility of adjusting the oscillatory frequency conducted to the mixing tube or excited in it at two different values which are such that at will each of the side-bands of the received signal may be suppressed.

The circuit with the lowest damping is reierably used as the last circuit, a tuning indicator being connected to this circuit and the voltage for the automatic volume control being derived from the circuit preceding the last.

The invention will be more clearly understood with reference to the accompanying drawings showing, by way of example, an embodiment thereof.

Fig. 1 shows a resonance curve of the filter to 1 be used, and the curves from which said rest)- nance curve is obtained by multiplication.

Fig. 2 is a form of construction of the circuit arrangement according to the invention.

Fig. 3 shows another embodiment of the cirt'; cuit arrangement.

In Fig. 1 the dotted curve I represents an approximation of the desired resonance curve. The frequency is stated in kilocycles oil resonance (detuning) in respect to the carrier-Wave, the 1 place of which is denoted by O. The resonance curve extends substantially horizontally, but shows a strong peak at the frequency which corresponds to the carrier-wave of-the received signaLthus causing said freqnuency to be trans,-

mitted by the filter with approximately twice the strength of the other frequencies in the band. Accordingly the carrier-wave is accentuated in respect to the transmitted side-band with the result that the modulation depth becomes lower. This involves a lower distortion in case of the usual linear detection. The dotted resonance curve represented in Fig. 1 may be obtained by making use of four tuned oscillatory circuits.

rangement in Fig. 2 said four oscillatory circuits form two band-filters separated by an electron tube, each comprising two of these circuits.

When two oscillatory circuits with equal natural irequency are ,coupled to each other, the

According to the embodiment of the circuit arband-pass filter thus formed has a resonance curve which may be supposed to be produced from the multiplication .of two resonance curves situated with their peaks on both sides of the original natural frequency. The frequencies, at which the peaks of said resonance curves are situated, may be called the coupling frequencies of the band-pass filter.

In Fig. l the references 3 and 4 represent the above-mentioned imaginary resonance curves of one of the band-pass filters represented in Fig. 2.

Also if two coupled oscillatory circuits do not have the same natural frequencies, the resonance curve of the formed band-pass filter may be supposed to be produced by multiplication of two resonance curves situated with their peaks on both sides of the frequency band surrounded by the two natural frequencies.

The curves 2 and 5 are the imaginary resonance curves of the other band-pass filter in Fig. 2. The dotted resonance curve I is produced if the four imaginary resonance curves 2, 3, 4 and 5 are multiplied by one another. The multiplication has the effect that the peak in the curve I does not coincide with the peak of the curve 2, but falls somewhat inwardly. As mentioned above, it is possible to make the coupling between the circuits with the lowest damping such I that the frequency, at which the peak in the curve I occurs, coincides with the natural frequency of the circuit with the lowest damping. When the voltage across the circuit preceding the last is maintained constant by means of the auto matic volume control, the voltage across the last circuit with the lowest damping varies as afunction of the frequency according to the resonance curve of said circuit, so that an accurate tuning of the receiver is possible by means of a tuning indicator connected to said last circuit.

In Fig. 2 the reference 6 represents the first mixing tube, the first control grid of which has the local oscillator voltage and the fourth control grid of which has the high-frequency or signal voltage fed to it. The oscillator frequency is determined by the oscillatory circuit I, the tuning condenser of which is coupled mechanically to the tuning condensers of the high-freqency circuit 8 which is fed by the antenna 9. The anode circuit of the mixing tube 6 comprises the circuit II of a band-pass filter I3 constituted by the circuits II and I2. The circuit I2 is inserted in the grid circuit of an amplifier tube I3. The anode circuit of said amplifiertube comprises a circuit I4 coupled capacitatively to the circuit I6 through an adjustable condenser I5. A series-connection of a diode I'I-3Il and a parallel-connection of a condenser I8 and a resistance I9 are connected in parallel with a part of the circuit I6. The low-frequency signal voltage is derived from said resistance by means of a sliding contact 20 and is conducted to a low-frequency amplifier not shown in the drawing through a conductor 2!.

After converting the high-frequency signal in the first mixing tube 6 into an intermediate freof the received signal. This is done by connecting the fixed condensers 22, 23 and 24 in parallel to their respective circuits by means of three switches 22, 23', 24' mechanically coupled to one another. Thus the side-band which one wishes to suppress may be selected by hand.

In order to realise a low damping of the nected to a tapping on the circuit I4 through a condenser 29 and ischarged with a resistance 3|. A control voltage for the automatic volume control is taken from the end of the resistance 3I not connected to earth through a filter constituted by a resistance 32 and a condenser 33 and is conducted to the control grids of the tubes tobe controlled through a conductor 34.

In Figl3 in which another form of construction I is represented, the same numeral references are maintained for the corresponding parts.

The high-frequency voltage supplied by the antenna 9 is converted by the mixing tube 6 into an intermediate frequency voltage which is conducted to a second mixing tube 27 through a band-pass filter 26. This second mixing tube converts the first intermediate frequency voltage into a voltage of lower intermediate frequency. The anode circuit of the mixing tube 21 comprises the circuit I I of the band-pass filter which is com stituted by the circuits II and I2 and istuned to said low intermediate frequency. After being amplified by the amplifying tube I3, the voltage is conducted to the capacitatively coupled circuits I4 and I6. This part of the circuit arrangement corresponds in principle entirely to the part of the circuit-arrangement in Fig. 2 following the mixing tube 6 so that it need not again be described in detail.

The reason of the second intermediate frequency transformation is that it is very difiicult to manufacture an oscillatory circuit of sufficiently low damping at an intermediate frequency I of 450 kilocycles. It is therefore necessary to change over to a lower intermediate frequency, say kilocycles or 60 kilocycles, which is possible by means of a second mixing tube. Said circuit arrangement is therefore only useful for receivers in which a high intermediate frequency is applied, unless one wishes to make use of an additional advantage involved by the use of a second mixing tube. In fact the conversion of the oscillator frequency by the second mixing tube makes it possible to suppress one side-band or the other without any additional changing-over of oscillatory circuits, so that the switch may be simpler and thecircuits may be constructed with a lower damping.

We claim: I

1. In a receiving circuit, means for suppressing one of the side-bands of the received signal and accentuating the carrier-wave in respect to the frequencies of the non-suppressed side-band, said means comprising an electric filter which is constituted by four oscillatory circuits which form part of the receiving cascade, two of said circuits forming a band-passfilter with approximately critical coupling and which are tuned to a frequency approximately in the middle of the transmitted side-band, the two other circuits being loosely coupled to each other and having a damping which is lower than the damping of the former circuits and being tuned successively to frequencies on both sides of the coupling frequencies of theiband-pass filter, the tuning frequency of the circuit with the lowest damping corresponding to the carrier-wave frequency of the received signal.

2. In a receiving circuit, means for suppressing one of the side-bands of the received signal and accentuating the carrier-wave in respect to the frequencies of the non-suppressed side-band, said means comprising a first pair of oscillatory circuits forming a band-pass filter with approximately critical coupling and which are tuned to a frequency approximately in the middle of the transmitted side-band, and a second pair of oscillatory circuits loosely coupled to each other and having a damping which is lower than the dampingof the first pair of circuits and being tuned sucessively to frequencies on both sides of the coupling frequencies of the band-pass filter, the tuning frequency of the circuit with the lowest damping corresponding to the carrier-wave frequency of the received signal.

3. In a receiving circuit, means for suppressing one of the side-bands of the received signal and accentuating the carrier-wave in respect to the frequencies of the non-suppressed side band, said means comprising an electron discharge device, a first pair of oscillatory circuits connected to the input of said discharge device and forming a band-pass filter with approximately critical coupling, said circuits being tuned to a frequency approximately in the middle of the transmitted side-band, a second pair of oscillatory circuits connected to the output of said discharge device,

, said circuits being loosely coupled to each other damping corresponding to the carrier wave frev quency of the received signal, a signal detector coupled to said last-mentioned circuit, and an automatic volume control rectifier connected to the other of said low damped circuit.

4. In a circuit of the superheterodyne type, an

intermediate frequency amplifier tube, a pair of means for simultaneously detuning each of the above coupled circuits except the one tuned to the carrier-wave so that the overall transmission characteristic of said circuits is such that one of the side-bands of the received signal is suppressed and the carrier-wave is accentuated to substantially twice the value of the transmitted side-band-frequencies.

5. In a circuit of the superheterodyne type, an intermediate frequency amplifier tube, a pair of critically-coupled circuits normally tuned to the operating intermediate frequency and connected to the input of said tube, a pair of loosely-coupled circuits of low damping connected to the output of said tube, the first of said loosely-coupled circuits being normally tuned to the operating intermediate frequency and the second of said circuits being tuned to the carrier-wave, and means for simultaneously detuning each of the above coupled circuits except the one tuned to the carrier-wave so that the overall transmission characteristic of said circuits is of a form having a sharp peak at the frequency corresponding to the carrier-wave of the received signal with a sharp cut-off on one side of the peak, the other side of the peak extending substantially horizontally over the width of one of the side-bands to be transmitted.

6. The combination as defined in clam 5, wherein the first of the loosely-coupled circuits is connected to an automatic volume control rectifier, and the second of said loosely-coupled circuits is connected to a signal detector.

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