US2245385A - Double heterodyne signal receiving system - Google Patents

Description

June 10, 1941- W. L. CARLSON 2,245,385

DOUBLE HETERODYNE SIGNAL RECEIVING SYSTEM Filed Feb. 29, 1940 2 sheets-sheet 1 1 noun "Il noun lx June 10, 1941. w. L. CARLSON DOUBLE HETERODYNE SIGNAL RECEIVING SYSTEM 2 Sheets-Sheet 2 Filed Feb. 29, 1940 Patented June 10, 1941 DOUBLE HETERODYNE SIGNAL RECEIVING SYSTEM Wendell L. Carlson, Haddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application February 29, 1940, Serial No. 321,539

(Cl. o-20) 7 Claims.

This invention relates to double heterodyne signal receiving systems, and has for its primary object to provide an improved receiving system of the character referred to wherein fundamental and harmonic beat frequency interference from the oscillator frequencies is minimized or prevented without extensive electrical shielding of the various circuits.

It is also an object of the present invention to provide, in a double heterodyne signal receiving system, an improved relation between the operating frequencies of the first and second oscillators Iand the harmonics thereof `with respect to the selected pass bands of the iirst and second intermediate frequency amplifiers so that a. minimum of variably tuned circuits are required to be adjusted when tuning the receiver to different stations.

It is a still further object of the present invention to provide a system of the character referred to wherein the first oscillator may be tuned variably through a frequency range effective to provide a relatively high iirst intermediate fre quency permitting an appreciable drift in the oscillatorl frequency without introducing distortion or detuning of a received signal. For this purpose, a relatively high first intermediate frequency is preferable, with a wider pass band characteristic than the second intermediate frequency amplier which operates at a relatively low frequency. The second oscillator tends to correct the elect of the frequency drift of the rst oscillator.

It is also a further object of the present invention to provide an impro-ved radio signal receiving system of the double heterodyne type for receiving signals in a plurality of differing input signal bands such, for example, as a standard broadcast band, a high frequency broadcast band, and a television signal also in a high frequency band, and in which the fundamental frequency and harmonics of the oscillators are prevented from introducing interference into the receiving circuits, with minimum electrical shielding.

A further object of the invention is the provision of a double heterodyne signal receiving system wherein a predetermined relation between the oscillator, intermediate and signal frequencies is effected whereby objectionable heterodyne beat frequency signals are largely avoided, and wherein the effect of frequency drift of the oscillators is substantially cancelled.

'I'he invention will be further understood from the following description when considered in ,plifiein connection with the accompanying drawings, and its scope -is pointed out in the appended claims.

In the drawings,

Figure 1 is a schematic circuit V'diagram of a double heterodyne radio sign-a1 receiving system embodying the invention,

Figure 2 is a diagram sho-wing the relation of the signal, oscillator, oscillator harmonics, and

intermediate frequencies in the frequency spectrum in which the system operates, and

Figure 3 is a schematic block diagram of a modication of the circuit of Fig. 1 adapted for signalling reception in a plurality of signal bands including television signals.

Referring to Fig. 1, input signals from a suitable source such 'as a collector or antenna 4 are applied through a tuned lter 6 to the input circuit 'l of a first converter provided by a balanced detector comprising tubes 8 and 9. The input system may include also a low impedance transmission line 5 coupling two parts of the filter 6 remote from each other. The converter may include a pair of triodes las shown, and the circuit arrangement is such that the noise-tosignal ratio and cross-modulation are reduced, while the oscillator voltage Vapplied to the input circuit from an oscillator I0 is prevented from passing through the output circuit Il of the first detector to the following portions of the receiving system to cause objectionable heterodyne beat signals at the second converter or detector, hereinafter referred to.

The oscillator may be of any suitable type variably tunable as by a variable capacitor I2 or 'a variable inductance i3 having a movable iron or other suitable core indicated at i4. Likewise, variable push-button tuning may be provided for the oscillator in any suitable and well-known manner, if desired. In the present example, the oscillator is of the Hartley type comprising an oscillator tube i5, the output circuit of which is indicated Iat I6, connected with the input circuit 'l of the first converter 8,-9, as shown.V

The output circuit Il of the rst converter is coupled to a rst intermediate frequency ampliiier comprising a tube I8 through a tuned input transformer i9, and this is followed by a second converter 28 coupled to a tuned output transformer 2i of the rst intermediate frequency am- The second converter includes an oscillator 22 having a fixed tuned oscillator circuit 23 and oscillator tube elements 24 included inthe present example, in the same envelope with the second converter 20.

Y The output of the second converter is coupled In one embodiment of the invention employingk a circuit in accordance with Fig. 1 for broadcast y reception, the input system 4, 5,6 is tuned to a pass band of substantially E30-161() kes. and the oscillator lll is arranged for variable tuning through a frequency range -of 5530-6510 kcs., thereby providing a first intermediate frequency of 5000 kcs., to which the transformers i9 and 2| are tuned. The second oscillator 22 is fixed tuned at 5260 kcs., thereby producing a second intermediate frequency of 260 kcs. to which the transformers 25 and 2 are tuned.

, The frequency stability of a high frequency oscillator is ordinarily less than that of an oscillator operating at a lower frequency. However, in accordance with the present invention, the first I. F. circuits pass a Wider than normal frequency band and the second oscillator will tend to drift in the same direction as the first oscillator and in the same direction as the resulting first I. F. frequency thereby tending to hold the second I. F. frequency constant. Thus, if the first oscillator frequency-tends to drift to a higher frequency, this raises the first I. F. frequency a corresponding amount but Will Anot cause detuning for. the reason that the first I. F. circuits are provided ywith a passbandof the order of 5 to l0 times that of the second I. F. amplifier. The normal tendency of the second oscillatoris to drift in the same direction as the first oscillator which is in the direction to hold the `second. intermediate frequency constantfor impressing on the narrow band second I. amplifier. .Inthe present example, the first IF. amplifier may have a pass band `of substantially plus and minus 15 kes.,

while .thesecond I. F. amplifier may have a pass band of plus or minus 3 kcs.

Referring now to Fig. 2 along with Fig. 1 the relative positions ofthe Various frequencies employed are shown in a frequency spectrum in which the systemoperates, the scale being drawn to cover the spectrum between 100 kcs. and 90 mcs. It will be noted that the first, second and third .harmonic frequencies 35,35 and 3l, of the variable flrstoscillator frequency 38 do not confiict to produce objectionable I. F'. or A. F. beat interference signals at either intermediate frequency, indicated at i9 and'lill, such as 260 kes. and 5G00 kcs. or at audio frequencies. 'Furthermore, the higher order harmonics than indicated are relatively weakand do not causeV interference even though their beat frequencies may fall Within one of the pass bands.

The second I. F. frequency is chosen preferably to lie below the signal receiving band, as shown. The first and second oscillator frequencies are above the first-I. frequency. Thus, with the system arranged in this' manner, a minimum degree of shielding of the electrical circuits is necessary.

Furthermore, it will be noted that the harmonies of both oscillators are above the signal receiving bandsof both the R. F. and first andv Among the advantages derived from the system as shown is that no variable input radio frequency selectivity is employed. The circuit largely avoids interference due to image signal frequencies, sum or difference signal frequencies, and signal frequencies at the intermediate frequencies. Furn thermore, the first oscillator is required to cover only a 1 to 1.2 frequency range to receive the 540 to 160() kcs. broadcast band.

The circuit shown in Fig. 1 may be employed for relatively high frequency signal reception as shown-for example, in Fig. 3, to which attention is now directed.

Referring to Fig. 3, the double heterodyne receiving `system of Fig. 1 is shown in its application for the .reception of signals in a plurality of differing frequency bands, for example, the reception of sound 'signals on the 5530-1600 kcs.

band and the 41-44 mc. band, and for the reception of a television channel, both sound and picture signal, at 44-50 mc.

The input circuit comprises three band pass filters 5f), 5l, and'52 for the R. F. signals in the three receiving bands `each of which maybe of the type shown in Fig. 1 and precede the first converter 5-3, to which they Yare selectively connected by suitable selector switching means 54, the input circuit from the signal source l being similarly controlled .by selector switch means 55, so that signals from thesource may be selected in a predetermined band and applied to the first converter 53.

Three first oscillators indicated at. 55, 5l and 58' are likewise provided for the 4first converter and connected therewith throughselector switch means 59 conjointly operable with switch means 5ft and 55 as indicated'by the dotted connections fill. f

It should be understood that the three input band pass filters may, if desired, includeV amplification and variable tuning and, likewise, the first oscillators, preferably .55 and 5i, and variably tunable, as indicated. The third oscillator may be kadjusted to a predetermined fixed frequency such as 58 mcs., for the reception off-the single television channel.

In the present example, the first intermediate frequency amplifier indicated at 5I is tuned to a slightly vhigher frequency than in the preceding example .and may have a mean pass band frequency of 8.25 mcs., as indicated, 4the pass band being plus or minus v25 kcs. for the purpose of permitting oscillator drift, as hereinbefore de-k scribed.

For this intermediate frequency, the variably tunable first oscillator 56 for the broadcast band may tune through a range of 8.78 to 9.85 mcs., while, for the `@L44 mc. receiving range, the oscillator 51 is tunable, as indicated, between 19.25 an`dr52.25 mcs. Y

The second converter is indicatedat E52 and may include amplification, and is coupledtoa second oscillator 63 providing afixed' output frequency/'of 8.51 mcs. to derive from the first I. F. signal the' same 260 kcs. second I. F. signal for the second I. F. amplifierfd as in the circuit of Figpl. The band pass characteristic of thesecond I. F. amplier or filter '54 is kpreferably `adjustable' 'with change in wave band as indicated by the selector switch means 65 which is coupled'with"selector switch means '59, 55 and 54, as indicated bythe dotted ccnnectionf, for adjusting the selectivityv .or pass band characteristic Yas indicatedin a range of6 kcs. to 50 kcs. for the three receiving bands shown. It will thusbe seen that for receiv.Y

ing signals in the higher frequency ranges, the selectivity or band pass characteristic ofthe second I. F. amplifier is changed to more nearly equal the width of the pass band of the first I. F. amplifier for the reason that the wider band is desirable for high fidelity reception in the ultra high frequency broadcast band.

From the first converter 53, the second amplifier channel is provided for the video signal and comprises a band pass lter or amplifier 'ID which may be an intermediate frequency amplifier of which the amplifier portion is indicated at 'Il followed by a secondhand pass filter l2 in the output circuit indicated at i3 for the video signal, cor responding to the output circuit 14 for the audio frequency detector and usual amplifying circuits of the usual sound channel. The video channel is controlled by selective switching means 15 conjointly controllable with the switching means previously referred to, to provide a connection with the first converter when the band change switching means is moved to receive signals in the television channel.

I claim as my invention:

1. In a double heterodyne signal receiving system, the combination of band pass signal receiving circuits responsive to signals in a predetermined frequency band, a first converter coupled thereto, a first oscillator for said converter tunable in a predetermined frequency range, a second oscillator tuned to a frequency outside of and relatively close to said first oscillator frequency range, and signal amplifying and converting means including a second converter and a first intermediate frequency amplifier coupled between said converters having a predetermined pass band in a range below the frequencies of said oscillators, said means providing a second intermediate frequency signal of substantially constant frequency in response to drift in the frequencies of said oscillators.

2. In a double heterodyne signal receiving system, the combination of a first converter of the balanced type, a first oscillator coupled thereto and tunable in a predetermined frequenlcy range above said band, a second oscillator tuned above said band to a frequency outside of and relatively close to said first oscillator frequency range, signal amplifying and converting means including a first intermediate frequency amplifier coupled between said converters having a predetermined pass band in a range below the frequencies of said oscillators and above the signal receiving band, providing a second intermediate frequency signal of substantially lconstant frequency in response to drift in the frequencies of said oscillators, and a second intermediate frequency amplifier for said last-named signal tuned for response thereto in a relatively narrow pass band.

3. In a double heterodyne signal receiving system, the combination of a full wave first converter, a variably tunable first oscillator, a first intermediate frequency amplifier, a second con verter and a second intermediate frequency amplifier following said first .converter in the order named, a second oscillator coupled to said second converter, means for tuning said first and second oscillators to provide higher oscillation frequencies than the first intermediate frequency and harmonics above the first and second intermediate frequencies, thereby to minimize beat frequency interference, and said tuning means providing oscillator frequencies so spaced in the frequency spectrum that the harmonics of the second oscillator fall between the harmonics of the first oscillator, thereby further minimizing beat frequency interference insaid system.

4. Ina .double heterodyne signal receiving system,.the combination of a first os'cillatorzvariably tunable through a frequency range pro-- viding a relatively high first intermediate frequency signal, a first intermediate frequency amplifier for said signal having a relatively Wide band pass characteristic, a second oscillator, a converter coupled to said second oscillator and to' said first intermediate frequency amplifier providing a relatively low second intermediate frequency signal, said second oscillator being tuned to a fixed frequency adjacent to the operating frequency range of the first oscillator whereby, as the oscillator frequencies drift in operation, the second intermediate frequency is held substantially constant, a second intermediate frequency amplifier .connected with said converter and having a relatively narrow pass band characteristic, and means for determining the oscillator frequencies such that the lower harmonies thereof fall outside the pass bands of said first and second intermediate frequency amplifiers and. the lower harmonics of the second oscillator fall between the lower harmonics of the first oscillator, thereby to provide minimum beat frequency interference with minimum shielding.

5. A multiple channel double heterodyne signal receiving system comprising in combination, a first converter, a first band pass signal channel, a second band pass signal channel, a plurality of band pass lters preceding said first converter, means for selectively applying signals to said converter througheach of said band pass channels, means providing a plurality of differing oscillator signals for said first converter, means for selectively applying said signals to v said first converter to establish a predetermined intermediate frequency for said first signal channel and for said second signal channel se-l. lectively, a second oscillator for said first signal .channel and a second converter coupled thereto and operative at a predetermined frequency to provide a relatively low second intermediate frequency, and means coupled to said second converter providing an adjustable band pass filter for said second intermediate frequency, the operating frequencies of said oscillator means being higher than the first intermediate frequency and such that the lower harmonics thereof fall outside the pass bands of said rst and second signal channels, and the harmo-nics of the second oscillator fall between the lower harmonics of the first-named oscillators.

6. In a multiple channel double heterodyne signal receiving system, the combination of a plurality of selectable band pass input filters, and a plurality o-f selectable tuned oscillators, said oscillators having operating frequencies above the pass bands of the respective band pass filters, a first converter selectively connected with said oscillators and band pass filters to provide relatively high intermediate frequency signals, band pass lter means for said signals, and means for converting said intermediate first signal to a relatively low intermediate frequency below the lbands of said band pass input filters.

'7. In a double heterodyne signal receiving system, the combination of means providing a band pass signal input circuit, a band pass first intermediate frequency amplifier, a band pass second intermediate frequency amplier, a variably for controlling the frequency of both oscillators rtunable rst oscillator, Athe rst oscillator ybeing such that the harmonics thereof are outside of tunable through a frequency range providing a the frequencyranges established fer the signal relatively high rst intermediate frequency and and intermediate frequencies, Whereb-yinterfer- 'a second oscillator tuned to provide a relatively A5 ence beat frequencies are minimized. v

low second intermediate frequency, and means WENDELL L. CARLSON.

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