US2038358A - Superheterodyne receiver - Google Patents

Description

April 21`,A 1936. D. E. HARNETT SUPERHETERODYNE RECEIVER Filed may 31, 1934 com om. om. QN.- o. cm.-

OQO.

:con:

INVENTQR n @4N/EL .5. H1/FME 7 f ATTORNEYS www Patented Apr. 21, 1936 UNITED STATES SUPERHETERODYNE RECEIVER Daniel E. Harnett, Tuckahoe, N. Y., assignor to Hazeltine Corporation Application May 31, 1934, Serial No. 728,437

11 Claims.

This` invention relates vto the reception and selection of modulated carrier signals and has for its principal object-to improve the delity of reception. Y

A radio broadcast signal is ordinarily transmitted on a carrier wave having two sidebands of modulation frequencies. Under present broadcasting conditions, the different carrier frequencies are allocated at various positions throughout ir the broadcast range, usually 10 kilocycles apart,

and in many instances the sideband vfrequencies of one signal channel either overlap those of an adjacent Vsignal channel or else closely encroach upon them. VIn either case, it is difficult, when -tuning a radio broadcast receiver to a desired signal in one such channel, to eliminate interference due to signals in the adjacent channels, particularly when such interfering signals are received with a strength comparable to that of the 20. desired signal.

Operation without interference in such cases ordinarily requires that the selecting system shall pass a considerably narrowed band of frequencies so that passage of the interfering signals-is substantially prevented. But narrowing the selectedV band in this manner ordinarily impairs the fidelity of reception of the signals, since the sideband frequencies corresponding to the higher audio frequencies of modulation are suppressed. According to this invention, the fidelity of reception is improved by employing a frequency selecting system by which the selected band may be extended in one direction or the other. This has the effect of admitting a wider range of one of the sidebands. The direction in which the selected band should ordinarily be extended is that opposite from which a strong interfering signal on a nearby channel may happen to be present.

A feature of the invention is the use of two or more selecting systems tuned to select a relatively narrow band of frequencies, one of these systems being fixed and another being variable. The variable system is provided with means for shifting its resonant frequency somewhat in either direction.

Another feature is the use of double-tuned circuits wherein adjustable reactive elements are employed in each circuit to expand the selected band by any desired amount, in either direction, up to the limit of expansion.

The invention is very well adapted for use in the intermediate-frequency selecting circuits of superheterodyne receivers.

Referring to the drawing:

Fig. 1 illustrates a superheterodyne receiver embodying the invention; and

Fig. 2 shows graphically how the selected band is extended in one direction, in accordance with this invention.

The receiver of Fig. 1 is of a conventional superheterodyne type comprising an antenna IIJ and ground II; a radio-frequency amplifier I2; a modulator tube I3; an intermediate-frequency amplifier tube I4; a detector and audio-frequency amplier I5; and a loudspeaker I 6. The radiofrequency amplifier I2 and the detector and audio-frequency amplifier I5 are shown in generalizedv box form since their details are well understood in the art and constitute no part of this invention. The modulator tube I 3 is coupled to the radio-frequency amplifier I2 by a radiofrequency coupling system I'I tunable by a variable condenser I8. There is provided a local oscillator indicated generally by the box I9 having an loutput circuit 20 tunable by a variable condenser 2|. The output of the oscillator is coupled to a coil 22 which is located in the gridcathode circuit of the modulator tube I3.

In accordance with well-known modulator operation, modulation of the yradio-frequency signals by the local oscillations occurs at the tube I 3.

There consequently appears at the output of the modulator the intermediate-carrier frequency which is the difference between the signal-carrier frequency and the local oscillator frequency. The intermediate-carrier frequency has associated therewith the usual two sidebands of modulation which extend six or more kilocycles on either side of the intermediate-carrier frequency. To maintain the intermediate-carrier frequency substantially constant, the local oscillator frequency is varied by the condenser 2| by the same amount and in the same direction, that the resonant frequency of the radio-frequency selecting circuit is varied by the condenser I 8. The two condensers I8 and 2| are ordinarily operated by a single mechanical control device indicated generally by the dotted lines U.

'I'heV intermediate-carrier frequency amplifier tube I4 is coupled to the output of the modulator tube I3 by an intermediate-frequency selecting system 23 of the double-tuned type. This selecting system comprises a primary coil 24 electromagnetically coupled to a secondary coil 25. The primary coil 24 has shunted across it a fixed condenser 26 .and a variable condenser'Z'I. The secondary coil 25 has shunted across it a fixed condenser 28 and a variable condenser 29. The two variable condensers 21 and 29 are operated simultaneously by a single mechanical control device indicated generally by U. The mechanical control device is arranged so that operation thereof varies the capacities of the two condensers 21 and 29 in the same direction.

Another intermediate-frequency selecting system 30 serves to couple the tube I4 to the detector and audio amplifier I5. This latter selecting system likewise comprises a primary coil 3l coupled to a secondary coil 32, these coils being tuned to the intermediate-carrier frequency by condensers 33 and 34, respectively, so that a relatively narrow band centered at the intermediate carrier frequency is selected. Coupling system 30, however, is not provided with variable condensers like condensers 21 and 29 of selecting system 23.

Referring to system 23, the primary and secondary circuits are each adjusted to tune to the intermediate-carrier frequency (frequently in the neighborhood of klocycles) whenv the mechanical control device U' is in an intermediate posiiton between the minimum and maximum limits of its range of operation. Preferably, the mid-range position of the device is chosen as that in which the system is so tuned. This position of U', then, is the neutral, or normal position in which the system is tuned to resonance at the intermediate-carrier frequency. In this neutral position the variable condensers 21 and 29 ,are adjusted to have capacity values intermediate their maximum and minimum values. These capacity values, in the neutral position, should preferably be such that the resonant frequency can be shifted from the intermediate carrier frequency in both directions by an equal amount.

The condensers 21 and 29 are so correlated by the device U that movement of U produces equal changes of the resonant frequencies of the primary and secondary circiuts of the coupling system 23. Hence, these primary land secondary circuits are always tuned to the same frequency throughout the range of operation of device U. In the convenient case where coils 24 and 25 are equal and condensers 26 and 28 are equal, condensers 21 and 29 may also be equal and made to have their capacities varied equally by movement of device U.

In the neutral or normal condition, the bandselecting characteristic of the selecting system 23 is the same as that of the succeeding selecting system 30, and is represented graphically by curve A in Fig. 2, which is a graph of relative output voltage on a logarithmic scale plotted against frequency on a linear scale. The intermediate carrier frequency is taken to be 175 klocycles. Since the systems 23 and 30 both have the characteristic A, the overall characteristic of the two systems is as represented by curve B, which is the square of curve A.

An inspection of curve B indicates that when the condensers 21 and 29 are in the neutral position the system is highly selective. If the selected band width be taken as that at which the relative output voltage is half the peak voltage, the band width passed is less than 4 klocycles wide, that is, less than 2 klocycles on each side of the carrier.

In accordance with the usual desired tuning operation the receiver is tuned with the main tuning control device U with the control device U set in the most selective, or neutral, position. After the desired signal is accurately tuned in, the device U' is adjusted to expand the band width the desired amount on the side away from the strongest interference.

When the control device U' is moved to decrease the capacity of condensers 21 and 29, the resonant frequency of the selecting system 23 is shifted upward with respect to the intermediatecarrier frequency. For an upward shift of 5 kilocycles in the resonant frequency of system 23 the characteristic shifts from that of curve A to that of curve C. The overall relative voltage output characteristic now takes the form of curve D which is the product of curves A and C. An inspection of curve D shows that the overall resonance peak has shifted about 21/2 klocycles, and that the band width has increased to about 6 kilocycles.

It is significant to note that the greatest shift has occurred at the high-frequency side of curve D, the low-frequency side having shifted upward very little. Consequently, most of the increase in band width has taken place in the region of the upper sideband. Hence, a much wider range of the upper frequency sideband is selected under the condition of curve D than under the condition of curve A, the selected portion of this sideband having increased from less Athan 2 klocycles to more than 5 klocycles. This means that the audio-frequency range has been correspondingly increased.

The width of one sideband, and hencethe audiofrequency range, may be further extended by increasing the detuning of system 23 from the intermediate carrier frequency. Curve E shows the characteristic when the system 23 is detuned by 12 klocycles from the intermediate carrier frequency. 'I'he greater output at the peak near 175 klocycles, relative to the peak near klocycles is due to the added selectivity of the radio frequency selecting system which is effective near frequencies corresponding to 175 klocycles. For the simplicity of explanation the effect of the radio frequency amplifier is not shown in curves A, B, C, and D.

It may often be desirable to increase the number of intermediate frequency tuned circuits. This may be done by utilizing two intermediate frequency amplifier tubes in tandem with three double tuned coupling systems like systems 23 and 30, that is six intermediate frequency tuned circuits. In the latter case, one of the systems may be designed like system 23 of Fig. 1 and a second of the systems may be designed to detune only one of the two tuned circuits thereof. In such a case, it will be convenient to use a threegang midget condenser having two like sections for the doubly adjustable system and one smaller capacity section for the coupling system having only one adjustable circuit. Such an arrangement would have the advantage of tending to flatten out the two peaks shown in curve E of Fig. 2, since the said smaller capacity section will not shift the resonance of its tuned circuit as far as the resonance of the doubly adjustable circuits are shifted. Hence the circuit of the said smaller capacity section will be resonant between these two peaks.

As an alternative arrangement, it would be possible to use like condenser sections but different inductance values. For example, the doubly adjustable system such as 23 could be constructed to have equal inductances, whereas the second coupling system having only one of the tuned circuits adjustable could have the inductance of the adjustable circuit somewhat smaller than the other inductance of the same system.

` A similar eifect may be obtained byv using siirv tunedA circuits as before, but only one inter-- mediate frequency amplifier tube. l'n such a case three tuned circuits may be used in each of the coupling systems corresponding to systems 23 and' Sli of Fig. l; or if desired, four tuned circuits might be used in one of the coupling systems and two tuned circuits in the other- Regardless of what particular arrangement of. tuned circuits is used it is highly desirable that at least three, and preferably four or more, tuned circuits b-e arranged in tandem i-n the intermediate frequency amplifier, in any manner familiar to those skilled in the art. It maybe stated as a general proposition that some, normally from one-third to three-fourths of the total number of tuned circuits, should be shifted in frequency by the device U".

From the foregoing it is seen that it is not essential that all of the adjustable circuits (or any two of them, for that matter) have equal rates of resonance shift. All that is required is thatv the resultant shift be suicient for the purpose herein set forth.

In a manner analogous to that described above, the lower sideband, instead of the upper sideband, could be extended, by moving the mechanical control device U in the opposite direction.

The particular sideband which is chosen for expansion should ordinarily be that which is most free from interfering signals. For example, if a strong interfering signal is present in the vicinity of the upper sideband, the lower sideband should be chosen for expansion.

Owing to the fact that the voltage output changes with the manipulation of device U' (e. g. curves B and D of Fig. 2) a system of automatic volume control should be employed if it is desired to produce a more uniform output. Any conventional automatic volume control system may be employed, such as that shown in H. A. Wheeler Patent 1,879,863. The automatic volume control voltage should be derived from a point in the system subsequent to the intermediate-frequency amplifier system.

The sources of direct voltage for placing the tube electrodes in an operative condition are not shown in detail, but are simply represented as B, C and Screen to indicate, respectively, the anode, the grid biasing and screen voltages. No source of cathode-heating current is indicated, since any well-known method of cathode-heating may be employed.

I claim:

l. In a superheterodyne receiver including means for modulating incoming signals to produce intermediate carrier-frequency signals, a selecting system for selecting a band of frequencies including the intermediate carrier frequency, and means for increasing the width of the selected band to select a greater portion of 'either of the side bands of modulation comprising means for shifting said selected band in either direction with respect to the carrier frequency by an amount which still includes said intermediate carrier frequency.

2. In a superheterodyne receiver including means for modulating incoming signals to produce intermediate carrier-frequency signals, a selecting system for selecting a band of frequencies including the intermediate carrier frequency, said selecting system being tuned to the intermediate carrier frequency, and means for increasing the width of the selected band to select a greater portion of either of the side bands of modulation comprising a variable reactive element associated with said system for shifting the selected band i-n eitherv direction relative to said intermediate carrier frequency by an amount Willich still includes said intermediate carrier frequency.

3. In asuperheterodyne receiver including means for modulating incoming signals to produce intermediate carrier-frequency signals, a selecting-l system for selecting a band of frequencies including the intermediate carrier frequency, saidA4 selecting system comprising a primary circuit and a secondary circuit coupled together, eachH of said circuits being tuned to the intermediate carrier frequency and having associated therewith means f'or increasing the width of the selected band to select a greater portion of either of the side bands of modulation comprising a variable reactive element to detune said circuits i-n the same direction and in either direction, from the intermediate frequency.

4. In a superhe-terodyne receiver including meansfor modulating incoming signals to produce intermediate carrier-frequency signals, a selecting system for selecting a band of frequencies including the intermediate carrier frequency, said selecting system comprising a primary ceiland a secondary coil coupled together, eachof said coilsV being tuned by capacity to the intermediate carrier frequency, and means for increasing the Width of the selected band to select a greater portion of either of the side bands of modulation comprising a variable condenser comprising a part of said capacity and associated with each of said coils, and a mechanical control device for simultaneously operating said variable condensers to Vary the capacity thereof in the same direction, whereby said coils may each be detuned from said intermediate carrier frequency, in the same direction.

5. In a superheterodyne receiver including means for modulating incoming signals to produce intermediate carrier-frequency signals, a selecting system for selecting a band of frequencies including the intermediate carrier frequency, said selecting system comprising a primary coil and a secondary coil coupled together, each of said coils being tuned by capacity to the intermediate carrier frequency, and means for increasing the Width of the selected band to select a greater portion of either of the side bands of modulation comprising a Variable condenser included as a part of the capacity across each of said coils and a mechanical control device attached to said variable condensers, which when moved produces equal capacity variations in the same direction, whereby said system may be detuned from said intermediate carrier frequency in either direction.

6. In a modulated carrier frequency signaling arrangement, a sideband selecting system comprising a plurality of circuits each normally tuned to the carrier frequency, a portion only of said circuits including adjustable reactance elements, and means for operating said reactance elements to shift the resonant frequencies of the circuits which include said reactance elements, relative to the resonant frequencies of other of said circuits, whereby the selected band may be expanded in either one direction or the other to select a Vgreater width of one of the sidebands of modulation.

7. In a modulated carrier frequency signaling arrangement, a carrier frequency amplier having a rst pair of coupled tuned circuits at its input and a second pair of coupled tunedcircuits at its output, said pairs of circuits being normally tuned to a desired carrier frequency, said first pair having an adjustable reactance element in each of its circuits, mechanical control means for operating said reactance elements in the same direction to shift the resonant frequency of said first pair relative to the resonant frequency of said second pair, whereby the selected band may be expanded in either direction to select a greater Width of one of the sidebands of modulation.

8. In a superheterodyne receiver, an intermediate carrier frequency amplifier comprising a plurality of selecting circuits each sharply tuned to said intermediate carrier frequency, and means for slightly shifting in the same direction, the resonant frequency of some of said circuits in either direction relative to the resonant frequency of other of said circuits, whereby the selected band may be expanded in one direction or the other to select a greater Width of one of the intermediate frequency sdebands.

9. In a modulated carrier-frequency signaling arrangement, a band-selecting system comprising a plurality of circuits each normally tuned to the carrier frequency, and means for shifting the resonant frequencies of a portion only of said circuits relative to the resonant frequencies of other of said circuits, whereby the selected band may be expanded in one direction or the other to select a greater Width of one of the side bands of modulation.

10. In a modulated carrier-frequency signaling arrangement, a band-selecting system comprising a plurality of reactively coupled circuits each normally tuned to the carrier frequency, and means for shifting the resonant frequencies of a portion only of said circuits relative to the resonant frequencies of other of said circuits, Whereby the selected band may be expanded at one side of said carrier frequency to select a greater portion of the frequencies of modulation at said side of the carrier.

11. In a superheterodyne receiver including means for modulating incoming signals to produce intermediate carrier-frequency signals, a selecting system for selecting a band of frequencies including the intermediate carrier frequency, and means for increasing the Width of the selected band to select a greater portion of the modulation frequencies at one side of said intermediate carrier frequency comprising means for shifting said selected band with respect to said intermediate carrier frequency in the direction of said side by an amount which still includes said intermediate carrier frequency.

DANIEL E. HARNE'I'I.

Images