US2278030A

US2278030A - Radio receiving apparatus

Info

Publication number: US2278030A
Application number: US346271A
Authority: US
Inventors: Rennie I Weber
Original assignee: Zenith Radio Corp
Current assignee: Zenith Electronics LLC
Priority date: 1940-07-19
Filing date: 1940-07-19
Publication date: 1942-03-31
Anticipated expiration: 1959-03-31

Description

March 31, 1942. R WEBER 2,278,030

RADIO RECEIVING APPARATUS Filed July 19, 1940 IN VENTOR.

V H .Ezwu'e f Weber Patented 31, 1942 RADIO RECEIVING APPARATUS Rennie I. Weber, Franklin Park, 111., assignor to Zenith Radio Corporation, Chicago, 111., a corporation of Illinois Application July 19, 1940, Serial No. 346,271

. 2 Claims.

This invention relates to radio receiving apparatus, and more particularly concerns the provision of an untuned radio frequency stage therefor comprising two or more parts, each of which parts comprises a circuit path resonant at a particular frequency, the resonant frequencies of the various circuit paths being at difi'erent points along a given frequency range or scale which is greater than and embraces the frequency tuning-flange of the apparatus.

She object of the invention is to eliminate some of the mechanically operated parts heretofore required in radio apparatus for tuning, without reducing the efilciency, selectivity, or sensitivity of such apparatus.

Another object is to obtain a relatively corrective gain at all frequencies throughout the frequency tuning range of the radio apparatus, to thereby accomplish equal antenna sensitivity over the entire tuning range thereof.

Still another object is to reduce the intermediate frequency response in the lower section of the tuning range of radio receiving apparatus.

A further object is to reduce the image interference throughout the tuning range of radio receiving apparatus, and particularly in the high frequency section thereof, as it is in the upper region of the tuning range where detrimentally high image interference is most often encountered.

A still further object is to obtain strong signal attenuation at frequencies outside the tuning range of the apparatus.

Referring now to the accompanying drawing:

' Figure l is a circuit diagram of radio receiving apparatus embodying my invention, and

' Figure 2 is a slightly modified detail view of that portion of the circuit diagram of Figure 1 which embodies my invention and which is defined therein by dotted lines.

In radio receiving apparatus of the superheterodyne type, the signal is usually received through a tuned antenna stage, which is a circuit or stage that is selectively tunable to reso-' nance at the frequency of the carrier wave. of the desired signal, and in some superheterodyne circuit arrangements, the signal thus received is passed directly into the modulator or converter stage, while in others, the signal impulses are passed through a tuned radio frequency ampliflcation stage into the modulator or converter stage. The system of passing the received signal impulses directly into the modulation stage has been found unsatisfactory under many conditions. There is a strong tendency, for example, for the intermediate frequency response signals to interfere with the reception of the desired signal. Another drawback has been image signal interference in the upper section of the frequency tuning range of the apparatus. In order to overcome these difliculties, a stage of tuned radio frequency amplification has been employed between the first or antenna stage and the modulator or converter stage, but this system, while quite satisfactory in many respects, requires that the antenna stage and the radio frequency stage be tuned synchronously and that both be tuned in synchronous relation with the oscillator circuit.

In the construction of compact receivers, the usual fixed arrangement of the rotors of the three variable condensers (which are commonly used in the last mentioned system for tuning the antenna and radio frequency stages and the oscillator circuit) upon a common shaft to effect the necessary synchronization during tuning creates a serious design limitation, and where automatic or push-button tuning is provided as an auxiliary condenser-adJusting tuning means, the bulkiness of a three-gang condenser becomes an even more serious consideration. With my invention of the untuned radio frequency stage, one of the above-mentioned tuning condensers is no longer needed, and a greater design freedom results therefrom.

With automatic or push-button tuning of the variabl inductance type, it is most desirable that the tuning be limited to the antenna stage and the oscillator circuit. For example, if automatic tuning of this type is provided to selectively cover five transmitting stations, it is customary to use five separate tuning units for tuning the antenna stage and five for tuning the oscillator circuit, and, if a radio frequency stage is to be tuned, five more tuning units are required. The elimination of the five last-mentioned tuning units, through the use of my untuned radio frequency stage, adds another advantage in designing.

There are two sources of interference which most often affect the lower tuning rangefrequencies. Signals coming from stations transmitting at or near the intermediate frequencyof the receiving apparatus is one source, and feedback of the intermediate frequency signal of the final intermediate frequency amplifier of the apparatus itself is another. As to the firstmentioned source, although the radio receiving apparatus is not tuned at or near the aforesaid intermediate frequency, the apparatus with only an antenna stage in the radio frequency section. or portion thereof will not, except under very favorable conditions, prevent undesired signals coming from the aforesaid stations transmitting at or near said intermediate frequency from interfering with the desired signal for which the apparatus is receptively tuned. As to the second-mentioned source, feedback of the intermediate frequency signal will occur whenever there is an opportunity for a pickup of the intermediate frequency signal of the apparatus by the atenna stage. The problem of preventing the interference by such feedback of the intermediate frequency signal is most serious in the construction of compact-radio receivers, which include a loop antenna built into or supported within the casing or cabinet thereof. The attenuation, or lack of gain, of my untuned radio frequency stage at frequencies below the tuning range of the apparatus blocks the passage of these interfering signals to the modulator. or

' converter stage and thereby prevents interfersignal to which the apparatus is tuned.

ence with the desired signal.

Another type of interference is encountered when the receiving apparatus is tuned to a frequency in the upper section of its frequency tuning range. This interference is created by an image signal which passes through the antenna stage to the converter and conflicts with the It is well known that, if the receiving apparatus is tuned to 1200 kilocycles and the intermediate frequency thereof is 455 kilocycles,. the oscillator frequency is 1655 kilocycles. If there should be a signal which is 455 kilocycles higher than the oscillator frequency, that is to say 2110 kiloeycles, the' 2110 kilocycles signal will tend to pass through the apparatus along with the 1200 kilocyeles signal, thereby creating serious interference. The herein described attenuation, or lack of gain, of my untuned radio frequency stage at frequencies above the frequency tuning range of the apparatus effectively prevents the passing of the conflicting image signal into the converter or modulator stage.

It is desirable in radio receiving apparatus that, the gain or sensitivity throughout the tuning range thereof shallbe constant. That is to say, the gain at any one point in the frequency tuning range should equal the gain at any other point therein. My untuned radio frequency stage has this equal "gain characteristic within itself and is sufliciently flexible to effect compensations for unevenness in gain that may exist in other stages. If, for example, the gain or sens'tivity of the apparatus is low at a frequency of 700 kilocycles, the arrangement of my untuned radio frequency stage with compensatory gain at the same frequency of 700 kilocycles maintains the equal sensitivity throughout the tuning range of the apparatus. Likewise, if a high gain obtains at a certain point in the frequency range in the other sections of the apparatus, it may be reduced by lowering the gain at that point in my untuned radio frequency stage.

Figure 1 shows the essential circuits of a superheterodyne radio receiver. The power supply circuit isnot shown. It will not be neces-- sary to describe all the parts which are shown in Figure l, snce standard electrical symbols have been applied. Reference numeral I indicates the variable condenser of the antenna or first radio frequency stage, and reference numeral 2 the mediate signals.

variable condenser for the oscillator circuit. My untuned radio frequency stage is defined by the dotted lines. The parts thereof shown in Figure 1 are resistance units 3' and l, inductance coils 5 and 6, and condensers I, 8 and 9.

In Figure 2, my untuned'radio frequency stage is shown separated from the radio receiving apparatus. Also indicated, in dotted lines by standard electrical symbols, are a number of so-called stray capacities. Because the location and value of these stray capacities may be controlled, we may consider them for the purposes of this explanation as actual condensers placed as indicated in Figure 2. The arrangements and the values of the units shown are for receiving apparatus with a frequency tuning range between 550 and 1600 kilocycles, but they should in no way be considered as limiting this invention,

since not only may a frequency tuning range or band be 'used in different positions within the complete range of radio frequencies, but the outer limits of such frequency tuning range may be further separated or moved more closely together.

The number of resonant circuit paths which constitute my untuned radio frequency stage are two or more. In a given network of inductance units, resistance units, and capacitance units, the number of resonant circuit paths which may be traced are only a few less than the total number of separate units in the network. For this reason, it is quite possible to find more than the tour which will now be described. However, in fullyv describing the invention by this example, it is preferred to consider my untuned radio frequency stage, as arranged in this form of the invention, as consistingof four resonant circuit denser I, as shown in Figures 1 and 2. This path is usually designed for resonance at a frequency below the frequency tuning range of the apparatus. The frequency chosen for resonance is usually that of the intermediate frequency, which in this example of the invention is 455 kilocycles. Asthe impedance of this series-connected circuit path is low, the voltage is also low, and thus the sensitivity at the resonant frequency is low. This low sensitivity, or low gain, at frequencies below the tuning range of the apparatus effectively blocks any interfering inter- The second path'to be considered consists of the inductance coil 5 and the stray capacitance indicated by reference numeral II, the condenser I and the stray capacitance II. The inductance coil 5 is in parallel connection with the capacitances l0 and II and in series with condenser I.

The resonance of this path is usually placed within the lower half of the tuning range of the apparatus. Inthis example, the frequency is 600 kilocycles and the tuning range of the apparatus is between 540 and 1600 k-ilocycles. The impedance of this circuit path being relatively high, a high voltage results and thereby high gain.

Thethird path to be considered consists of inductance coil 0, condensers I and Q, and the stray capacitance It. In this circuit, inductance coil 6 is in series connection with two parallel branches, one branch consisting of condenser I and the other "branch consisting of condenser 0 in series with the stray capacitance l2. The impedance of this circuit path is low, but in this case the gain is high, since the output is connected across the capacitive elements 8, 9 and i2, and is thus taken at a point of high voltage. The resonant frequencyis chosen to be within the upper half of the frequency tuning range of the apparatus. kilocycles.

The fourth resonant circuit path to be considered in this explanation of my untuned radio frequency stage consists of inductance coil and In this particular example, it is 1400 the distributed capacitance l3. The resonant fre I quency is chosen to be above the tuning range of the apparatus. At frequencies above the tuning range of the apparatus, the impedance is high and this impedance, being in series with the output, reducesthe gain. The resonant frequency of this circuit, as shown inthis particular arrangement of my untuned radio frequency stage, is 2100 kilocycles.

As may be seen from the description of the various resonant circuits, there is strong signal attenuation at frequencies below and above the tuning range, and relatively equal amplification throughout the tuning range frequencies. There are two convenient means of controlling the gain throughout the tuning range. The first is the resistance unit 3. Changes in the resistance in this unit affect the gain throughout the frequency tuning range, and particularly in the lower half thereof. Variations in the resistance of unit 3 varies the overall impedance of the path and will vary the voltage developed in the second de-' scribed circuit path. The second control means is the condenser 8. Variation of the capacitance of this condenser will vary the developed voltage, and in this manner serve as a convenient means for controlling the gain in the upper half of the frequency tuning range.

I claim:

1. In radio receiving apparatus, an untuned radio frequency transmission stage comprising a radio frequency amplification tube and a plurality of output circuit paths connected therewith and resonant at different frequencies, two of said circuit paths having relatively low gain characteristics and two of said circuit paths having relatively high gain characteristics, one of said relatively low gain circuit paths extending between said tube and a radio frequency ground connection and another of said relatively low gain circuit paths being located effectively inthe output of said stage, and both of said relatively high gain circuit paths extending-between said tube and a radio frequency ground connection, the over-all frequency transmission range of said stage and the tuning range of said apparatus lying between and determined by the frequencies at which the aforesaid relatively low gain circuit paths are resonant, and the equalization of the signal amplification throughout the tuning range of said apparatus being effected through the employment of the aforesaid relatively high gain circuit paths at frequencies within the frequency transmission range of said stage.

2. In radio receiving apparatus, an untuned radio frequency transmission stage comprising a radio frequency amplification tube and a plurality of output circuit paths connected therewith and resonant at different frequencies, two of said circuit paths having relatively low gain characteristics and two of said circuit paths having relatively high gain characteristics, one of said relatively low gain circuit paths extending between said tube and a radio frequency ground connection and another of said relatively low gain circuit paths being located effectively in the out put of said stage, and both of said relatively high gain circuit paths extending between said tube and a radio frequency ground connection, the over-all frequency transmission range of said stage and the tuning range of said apparatus lying between and determined by the positions occupied by the resonant frequencies of the aforesaid relatively low gain circuit paths in the radio frequency spectrum, and the level of the amplification characteristics of said stage and the equalization of thesignal amplification of said apparatus being effected by the positioning of the resonant frequencies of the aforesaid relatively high gain circuit paths in. the radio frequency spectrum between the resonant frequencies of the aforesaid relatively low gain circuit paths.

RENNIE I. .WEBER.