US2062956A - Image suppression system - Google Patents

Description

Dec. 1, 1936. R. B. ALBRIGHT 2,062,956

IMAGE SUPPRESSION SYSTEM Filed Nov. 5, 1955 I 5;; xvi; 6

Patented Dec. 1, 1936 UNITED STATES PATENT OFFICE IMAGE SUPPRESSION SYSTEM Application November 5, 1935, Serial No. 48,407

5 Claims.

This invention relates to novel signal transfer means for radio receivers and like systems, wherein it is desired to transfer signals of certain frequency and to reject signals of certain other frequencies. More particularly, the invention relates to signal transfer means of the stated character for transferring desired signals from a signal source, such as an antenna, to an amplifier stage while suppressing or attenuating undesired signals emanating from the said source.

One object of the invention, therefore, is to provide novel signal transfer means adapted to transfer desired signals and to reject undesired signals. 7

Another object of the invention is to provide signal transfer means which, in addition to rejecting undesired signals, is adapted to transfer desired signals with substantial gain.

The invention is particularly adapted for use in a superheterodyne radio receiver to couple the antenna to a tuned radio frequency stage and, when thus employed, the invention is adapted to suppress or reject the image signals commonly encountered in such a system. For the purpose of disclosure, therefore, the invention will be described with particular reference to this adaptation but it will be understood that the invention is not thus limited in scope.

In the accompanying drawing, the single figure is a diagrammatic illustration of a preferred form of the invention as applied to a superheterodyne radio receiver.

As is now well known, in a superheterodyne receiver, the desired signal is modulated by a signal from a local oscillator producing signals having frequencies equal to the sum and difference of the desired signal frequency and the oscillator frequency. The difference frequency signal, having the intermediate frequency, is selected and implified. The oscillator frequency is thus the desired signal frequency plus the intermediate frequency. Likewise undesired signals known as image signals, whose frequency is equal to the oscillator frequency plus the intermediate frequency, or twice the oscillator frequency minus the intermediate frequency, will, when modulated with the oscillator signal, produce undesired signals of intermediate frequency. For example, if the desired signal frequency band is from 150 to 300 k. c., i. e., the long wave band, and the intermediate frequency is 450 k. c., the image signal frequency band will extend from 750 to 1200 k. c.

The present invention, when applied to a superheterodyne radio receiver, is adapted to suppress or reject undesired or image signals, such as those above mentioned. For the purpose of the present specification and appended claims, an image signal may be defined as a signal other than the one it is desired to receive which will heterodyne with the local oscillator signal or a harmonic thereof to produce a signal, at least one component of which is of the intermediate frequency.

Referring now to the single figure of the drawing, the antenna A may be connected through an inductance L1 to one extremity of the primary winding P of a tuned transformer T, the other extremity of the primary Winding being connected to ground. The secondary winding S of the transformer may be shunted by a variable condenser C which serves to tune the secondary to the signal which it is desired to receive, as well known in the art. One extremity of the secondary winding may be connected to the control element of a space discharge device V of a radio frequency transfer stage, while the other extremity of the secondary winding may be connected to any suitable source of biasing potential such as is commonly employed in the art. The transfer stage may be part of any suitable means for'utilizing the desired signals and in the specific adaptation of the invention to a superheterodyne receiver, this transfer stage will form a part of the conventional superheterodyne radio receiver. The primary of the transformer T may be untuned and may be self-resonant at some frequency below the range of frequencies which it is desired to receive, as well known in the art.

In accordance with the present invention, there is provided a capacitive element C1 shunted about the primary of transformer T. It has been found that the provision of this element in cooperative relation with the other elements of the system results in effective attenuation of image signals and has other desirable features, as set forth hereinafter. Preferably, there is also provided a trap circuit L2C2 connecting the antenna to ground, which circuit is tuned to the intermediate frequency and serves to reject signals of that frequency, as more fully explained later.

The invention may be clearly understood by a consideration of the design and functioning of the elements of the disclosed system in a particular case. Assuming, for example, that it is desired to receive signals within the frequency range of 150 to 300 k. c. as above mentioned, this being the standard long wave band, and assuming further that these signals are to be received with a superheterodyne radio receiver having an intermediate frequency of 450 k. c., then it is apparent that image signals having the frequencies above mentioned will be encountered and the system should be designed or adapted accordingly. The inductance L1 may have such distributed capacity that it is self-resonant at a frequency of about 750 k. 0. Likewise, the primary winding of the transformer T, due to its distributed capacity, may be self-resonant at a frequency of about 85 k. 0. These inherent capacities are indicated on the drawing by brokenline representations, as is also the inherent capacity between the windings of the transformer T. It will be seen that the transformer constitutes a signal transfer device having a certain amount of inherent capacitive transfer impedance as well as inductive transfer impedance, and the directions of winding of the primary and secondary may be designed so that transferred signals due to the one add with signals transferred by the other. In the preferred form of the invention, it is also preferable that the inductance L1, the capacitance C1, and the primary of the transformer, when considered as a unit, have an over-all resonant frequency just above the long-wave frequency band above mentioned but below the intermediate frequency. For example, this resonant frequency may be 335 k. c.

Considering now the reception of desired signals within the long-Wave frequency range of 150 to 300 k. c., the trap circuit L2C2 will appear to such signals as a capacitive reactance having a very high impedance. The self-resonant inductance L1 will appear to such signals as an inductance, while the self-resonant primary winding of the transformer will appear as a capacitive reactance. Therefore, the primary of the transformer and the shunt capacitance C1 may be considered together since they will appear to the desired signals as a capacitive reactance. These characteristics of the elements in question will be obvious from the various frequencies involved.

It will be seen that the voltage across the inductance L1 will be approximately 180 out of phase with the voltage across the capacitance C1 and, therefore, the voltage appearing across the primary of the transformer or signal transfer device will be somewhat greater than the input voltage supplied by the intenna. In other words, the voltage applied to the primary of the transformer will be equal to the sum of the input voltage and the voltage across the inductance L1. Thus, the desired signals are transferred with a certain amount of gain. Furthermore, since the elements in question, considered as a unit, are resonant at a frequency of about 335 k. c., the gain will be greater at the higher end of the longwave frequency band due to the upward slope of the response characteristic in the vicinity of the resonance peak.

At the desired signal frequencies in question, the transfer impedance between the primary and secondary of the transformer will be largely inductive and will comprise substantially the mutual inductance between the two windings. Furthermore, the impedance of the primary winding will be small compared with the impedance of condenser C1. Thus, it will be seen that substantially all of the signal current flowing through inductance L1 will flow through the primary winding of the transformer and the signal transfer to the secondary winding will be large.

Considering now the reception of signals having the intermediate frequency or having frequencies in the neighborhood of the intermediate frequency, the tuned trap circuit L202 will, of course, present very low impedance to such signals but the impedance of the remainder of the circuit to such signals will be relatively high, as above mentioned. Thus, the antenna is substantially short circuited by the intermediate frequency trap L202 and there will be substantially no signal transfer through the device to the receiver.

Considering now the reception of signals in the image signal frequency range, that is, the frequency range from 750 to 1200 k. c., which signals it is desired to attenuate as much as possible, it will be seen that the trap circuit L2C2 will appear to these signals as an inductance having very high impedance. The inductance L1, which is self-resonant at a frequency in the lowest part of the image signal frequency band, will appear to signals at that frequency as a very high impedance and will appear to signals of higher frequency as a capacitive reactance having high impedance. It has been found further that the signal transfer device T presents a transfer impedance which is largely capacitive at such frequencies and due to the capacitive coupling between the'primary and secondary windings.

- It will be seen then that image signals from the antenna will pass through the high capacitive impedance presented by the element L1 and will then be transferred to ground through the condenser G1 which is shunted by the primary winding and by the circuit comprising the capacitive signal transfer impedance and the tuning condenser of the secondary circuit. Obviously, the smaller the reactance of the condenser C1, i. e., the larger the capacitance, the greater will be the attenuation of the image signals. It is further desirable that the transfer impedance to image signals be as large as possible to increase further the image signal attenuation. For this reason it is necessary to use a separate condenser C1 rather than design the primary to have a larger distributed capacity. It is not practical to wind a transformer having sufiicient distributed capacity to replace C1 and even if it were, the device would not function in the same manner as with the external condenser due to the presence of secondary resonance frequency peaks which are to be found in the impedance characteristic above the fundamental resonant frequency of a coil having distributed capacity.

In order to obtain as great attenuation of the image signals as possible, the condenser C1 should be made as large as possible but yet sufficiently small so that the unit comprising the inductance L1, the condenser C1 and the primary winding is resonant at some frequency below the image frequency range and above the frequency range of the signals which it is desired to receive.

From the above description, it will be seen that the use of the invention aids materially in suppressing or rejecting the undesired signals while transferring the desired signals with sufficient gain. Although the invention has been illustrated in preferred form and with respect to a particular adaptation thereof, it will be understood that it is not thus limited but is capable of modification within the scope of the appended claims.

I claim:

1. In a superheterodyne radio receiver, an antenna, a signal transfer device comprising a primary and a tuned secondary inductively coupled thereto, and having at least some capacitative transfer impedance therebetween, an impedance element connected between said antenna and said transfer device, said impedance being inductive for signals of the desired frequency and capacitive for signals of at least some image signal frequencies, and means for reducing image signals comprising a separate capacitative impedance element in shunt relation with said primary.

2. In a superheterodyne radio receiver, an antenna, a signal transfer device comprising a primary and a tuned secondary inductively coupled thereto, and having at least some capacitative transfer impedance therebetween, an inductance element serially connected between said antenna and said transfer device, said inductance element having sufficient capacitive reactance associated therewith so that said element will appear as a capacitive reactance for signals of at least some image signal frequencies, and means for reducing image signals comprising a separate capacitative impedance element in shunt relation with said primary.

3. In a superheterodyne radio receiver, an antenna, a signal transfer device comprising a primary and a tuned secondary inductively coupled thereto, and having at least some capacitative transfer impedance therebetween, an inductance element serially connected between said antenna and said transfer device, said inductance element being self-resonant at a frequency at the lower portion of the image frequency band of said receiver, and means for reducing image signals comprising a separate capacitative impedance element in shunt relation with said primary.

4. In a superheterodyne radio receiver, an antenna, a signal transfer device comprising a primary and a tuned secondary inductively coupled thereto, and having at least some capacitative transfer impedance therebetween, an impedance element connected between said antenna and said transfer device, said impedance being inductive for signals of the desired frequency and capacitive for signals of at least some image signal frequencies, a trap circuit tuned to the intermediate frequency connecting said antenna to ground, and means for reducing image signals comprising a separate capacitative impedance element in shunt relation with said primary.

5. In a superheterodyne radio receiver, an antenna, a signal transfer device comprising a primary and a tuned secondary inductively coupled thereto, and having at least some capacitative transfer impedance therebetween, an impedance element serially connected between said antenna and said transfer device, said impedance being inductive for signals of the desired frequency and capacitive for signals of at least some image signal frequencies, and means for reducing image signals comprising a separate capacitative impedance element in shunt relation With said primary, said device and said elements having an over-all resonant frequency above the frequency range of desired signals but below the intermediate frequency.

ROBERT B. ALBRIGHT.

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