US1915483A - Radioreceiver - Google Patents

Description

Patented June 27, 1933 UNITED STATES PATENT OFFICE GEORGE L. BEERS, 0F COLLINGSWOOD, NEW JERSEY, ASSIGNOR T0 RADIQ CORPORATION OF AMERICA, A CORPORATION OF DELAWARE RADIORECEIVER Application filed January 15, 1932. Serial No. 586,783.

My invention relates to radio receivers and, more particularly, to receivers of the superheterodyne type.

During the operation of superheterodyne radio receivers, the phenomenon of image frequency response is, at times,.troublesome. That is to say, if the receiver is lacking in selectivity prior to the first detector, it will respond to signals from a transmitter, the frequency of which differs from that of the desired station by an. amount equal to twice the frequency to which the intermediate frequency amplifying stages are tuned, if the oscillator is tuned midway between the two frequencies. Signals from such a transmitter often cause serious interference. In other words, as disclosed in my Patent 1,703,079, when an interfering signal differs in frequency from the frequency of the local oscillator by an amount equal to one half the difference between the frequency of the said signal and the signal desired, the beat frequency produced is the same as that derived from the said desired signal and cannot be separated therefrom in later stages of the receiving system.

It is also possible that signals, at frequencies higher than the frequency of the desired signal, may be heterodyned by harmonics of the local oscillation frequency to produce an interfering beat-frequency. This disadvantage has been overcome in modern broadcast receivers, to some extent, though not entirely, by utilizing a relatively high intermediate frequency. Furthermore, two stations separated by the intermediate frequency may combine in the first detector toproduce the intermediate frequency and, thereby, cause interference with the reception of signals from a desired station.

In the past, as exemplified by my patent hereinabove mentioned, many attempts have been made to obviate the disadvantages referred to. For example, drain circuits tuned to the interfering signals have been utilized for their elimination and circuits designed to pass a narrow band of frequencies, and tuned to the desired signal, have likewise been used. Such arrangements, however, have required the simultaneous or sequential tuning of a plurality of circuits and, in uni-controlled receivers, it has been found diflicult to keep the various tuning devices properly aligned.

It is, accordingly, an object of my invention to provide a superheterodyne radio receiver that shall be devoid of the disadvantages hereinbefore enumerated.

Another object of my invention is to provide a radio receiver of the type described, that shall be simple in operation and substantially unaffected as to calibration irrespective of the constants of the antenna system with which it is employed.

The foregoing objects, and other objects ancillary thereto, I prefer to accomplish through the judicious use of band pass filters, for the purpose of so segregating both incoming radio signals and oscillations from a local source, previous to permitting them to interact to produce a beat frequency, that image frequency response, harmonic frequency response, and other undesired responses are prevented.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, to-

gether with additional objects and advantages thereof,- will best be understood from I the following description of a specific embodiment when read in connection with the accompanying drawing, wherein Figure 1 is a schematic view of a radioreceiving system embodying a preferred form of my invention, and

Fig. 2 is a schematic view of a radio-receiving system embodying a modified form of my invention.

According to my invention, referring to the drawing, I provide means whereby heterogeneous radio signals, received through an antenna system, are first separated into a plurality of minorbands. The separation may be accomplished through the use of a plurality. of band-pass filters 1, 3, and 5. If the receiver is intended to cover the present broadcast band, i. e. 550 to 1500 kilocycles, three filters are suflicient, the filters being so, designed as to, respectively, pass 500 to 850 in parallel toa common intermediate frequency amplifier 13 tuned to 400 kc, which is.

followed bya second detector 7 and the usual "audio frequency amplifier; I I I In order that each minor band offrequencies may be suppliedjwith a band of locally generated oscillation frequencies properly chosen to produce the 400 kc,intermediate frequency from any frequency Within the said minor band, I provide an oscillator 15'which is tunable from 900'kc to 2000 kc. Theoutput. circuit .of the oscillator is connected in parallel to the input circuits of a plurality of band-pass filters 17 19, and 21, the said filters being, respectively, adapted to pass 900 to 1250 kc, 1250 kc, to-1600 kc, and 1600-to 2000 kc. The, first mentioned filter is connected tothe receiving channel adaptedto pass 500 to 850 kc, the second mentioned filter to the channel adapted to carry 850 to 1200 kc, while the last filter is connected to the channel carryinglQOO to 1550 kc. r a t It will, accordingly, be obvious that, as the oscillatoristuned overits frequency range, the several bandpass filters, to which its output circuit is connected will, consecutively, permit the localoscillations to be impressed upon 'the three input channels. If the desired signal lies between 500 to 850 kc, it appears in the first input channel'and can only be heterodyned with local oscillations over the range between 900 and 1250 kc. Such beingthe case, the'intermediate frequency of 400' kc, cannot'be produced through interaction withanyincoming' signal which lies in the bands passed by either of the othertwo input 7 filters; Accordingly, only the desired signal is heterodynedand is rendered audible. It is also evident that, since each minor band is restricted to 350 kc, and since the intermediate frequency is 4:00" kc, signals separated by 400 kc, cannot be impressed 011 any one of the first-detectors to produce the intermediate frequency. It will also be noted from a careful inspection of the drawing that no two incoming frequencies, the sum or difference of which could produce a beat with some oscillation frequency, are permitted to simultaneously pass through any single input channel whereon an oscillationfrequency, which would heterodyne the-said sum or difference, is impressed. 5 3

Should the desired incoming-signal lie within either'of the bands 'aassed by the remaining two in'put filters, thes'ystem func- 'tions in the same manner with the result that image and harmonic frequency response is prevented and the entireibroadcast range may be covered through:manipulation'of a single tuning control associated with the local oscillator.

My invention is also applicable to radio systems of the type wherein all incoming signals are amplified at radio frequency in aperiodic amplifiers before impression upon frequency-selective circuits, for subsequent demodulation and amplification at audio frequency. Y

Since, as is well known in the art, it is substantially impossible to secure straight line amplification throu the use of thermionic tubes, a certain amount of rectification takes place in the aperiodic amplifiers. Hence, if the sum of or difference between two incoming signals is a frequency that is capable of being heterodyned in later stages of the receiver, should it be of the superhe'terodyn'e type, .to produce the same beat frequency'as that produced by a desired signal, or is a frequency equivalent to that of a desired signal, interference results. For example, assume a beat frequency of 400 kc,

produced through the interaction of a desired signal, say at 550 kc, with a localoscillation frequency of 950 kc. Under such circumstances,if an incoming signal at, say, 500'kc, and another signal at 1050 kc, are simultaneously being impressed on the aperiodic amplifier, a-ditlerence beat frequency of 550 kc, appears in the output circuit thereof. The said beat frequency, beingthe same as that of the desired incoming signal, produces, with the local oscillations at 950 kc, a beat frequency of 400 kc, thus causing objectionable interference with the said desired signals. Also, in the event that the .receivers are of the tuned radio frequency type, the beat frequency produced in. the aperiodic amplifier, being the same as that of the desired incoming signal, also causes interference;

It is, accordingly, Within the scope of 'my invention to'provide means whereby incoming signals may be separated into a plurality of minor bands and to further provide additlonal means whereby only the frequencies in the'said bands are separately amplified previous to being impressed upon later stages.

Specifically, referring to Fig. 2, I find it expedient to. connect an energy collecting device, such as an. antenna 23, in parallel to the input circuits of a plurality of bandpass filters 25 and 27 so designed, respectively, as to i pass minor bands of 550 to 1,000 kc, and

1,000 to 1,500 kc. The output circuit of each band-pass filter is connected to a corresponding aperiodic amplifier 29 and the output current from each amplifier is fed into an individual band-pass filter 31 so designed as to pass the same band of frequencies as is impressed upon the input. c1rcuit of the a1nplifier. For example, referring to Figure 2 the input band-pass filter 25, which passes the band of 550 to 1,000 kc, is provided with an output band-pass filter also passing 550 to 1,000 kc, while the amplifier connected to the input band-pass filter 27 passing 1,000 to 1,500 kc is provided with an output band pass filter designed to cover the same range.

The output circuits of the band-pass filters 31 are connected, in parallel, to a dis tribution network 33.

For example, if, in the operation of the system just described, when it is desired to receive signals at 1350 kc, two incoming signals at 600 and 750 kc, respectively, are also impressed upon the band-pass filter 25, which passes 550 to 1,000 kc a spurious frequency of 1350 kc, might appear in the output circuit of the amplifier. This beat frequency, interacting with local oscillations at 950 kc, would produce an intermediate frequency in a superheterodyne receiver of 100 kc, which is the same frequency that would be produced through the interaction of the desired incoming 1350 kc, and the said local oscillations. If the receiver is of the tuned radio frequency type, tuned to'1350 kc, the same interference results.

However, by providing the output filter 31 for the amplifier 29, the beat frequency ofv 1350 kc, is prevented from reaching the later stage of the receiver and interference, thereby is obviated.

Additional frequency combinations which, through simultaneous rectification in the amplifiers, could produce interference with a desired signal, will be apparent to those skilled in the art, without further explanation, it being sufficient to say, at this point, that the provision of the output filters effectively removes such spurious interfering beat frequencies to greatly improve the selectivity of the system.

It should also be apparent, from a consideration of the foregoing, that the filters described in connection with Fig. 1 of the drawing, may each be divided into a plurality of sections separated by aperiodlc amphfiers. In such manner, the losses in the filters may be compensated to any desired degree.

Although I have selected a specific receiver arrangement for purposes of illustration, my invention is not restricted thereto, but is only limited by the prior art and the scope of the appended claims.

I claim as my invention:

1. The method of reducing interference with the reception of radio signals lying in a given major band, which comprises separating incoming signals into a plurality of minor bands, causing the si nals in one of said bands to interact with locally generated oscillations at a frequency falling within a restricted band, whereby a plurality of beat frequencies are derived, and thereafter isolating a pre-determined one of said beat-fresignal manifest.

quencies for the purpose of making a desired 2. The method defined in claim 1 wherein each minor band is SO lGSliIlCtGCl that no two simultaneously incoming signals lying there-' indiifer from each other by the isolated beat-frequency.

The method defined in claim 1 wherein each minor band is so restricted that neither the sum nor difference between any two sig' nals lying therein is capable of producing the isolated beat-frequency through interaction with any frequency lying in the restricted band of locally generated oscillations.

the difference between the upper and lower frequency limits of each minor band is less than the isolated beat frequency.

5. A radio receiving system including a plurality of band pass filters adapted to be connected in parallel to a source of signal energy, an oscillator, a plurality of band pass filters connected in parallel to the output circuit of the oscillator, the last named filters 7. The invention defined in claim 5 where in the first named filters each cover a specified minor frequency range and further characterized in that the kilocycle width of the band passed by each of the second named filters is the equivalent of the width of the band passed by the filter with which it is associated, the upper limit of the band passed by each one of the said second named filters differing from the upper limit of the band passed by its associated filter by a predetermined beat-frequency.

8. In a radio receiving system, means for separating incoming signals into a plurality of minor bands, a local source of oscillations and means for supplying to each band from said source oscillations covering a frequency band having the same width as the band to which they are applied, the frequency range of the supplied oscillations being so shifted in the frequency spectrum as to enable the derivation of a predetermined beat frequency from any signal in any minor band. i

9. The invention defined in claim 8 wherein the signal separating means is constituted by a plurality of band-pass filters, the difference between the upper and lower limits of the band passed by each filter being less than the predetermined beat frequency.

10. The invention defined in claim 8 where-- 4. The method defined in claim 1 wherein" in the signaLseparating means is constituted by a plurality of band-pass filters, each filter in so proportioned that no two frequencies simu taneously passed thereby are capable of so combining with the oscillations supplied thereto as to produce the predetermined beatfrequency. I

In testimony whereof, I have hereunto subscribed my name this 29th day of December GEORGE L. BEERS.

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