US2286356A - Frequency modulation receiver - Google Patents

Description

` June 16, 1942. D. E. FOSTER FREQUENCY MoDuLATIoN RECEIVER Filed Feb. 18, 1941 Patented June in, i942 FREQUENCY MODULATEON RECEIVER Dudley E. Foster, South Grange, N. J., assigner to Radio Corporation of America, a corporation of Delaware Il Ciaims.

My present invention to frequency modulated carrier wave receivers, and more particularly to a frequency modulation receiver of improved and efficient design.

It is the present practice in the reception of frequency modulated carrier waves to utilize a center, or carrier, frequency located in the i2 to 50 megacycle range, and to utilize a maximum permissible frequency deviation of 'l5 kilocycles. As is well known to those skilled in the art, the modulation signals appear in the modulated carrier wave as carrier frequency deviations. The amplitude of the modulating signal at any modulation frequency corresponds to the extent of frequency deviation of the carrier, whereas the modulation frequency itself corresponds to the rate of carrier frequency deviation. Accordingly, it has been the practice to receive frequency modulated carrier waves with a superheterodyne type receiver in which the intermediate frequency networks were constructed to pass an overall band width of 200 kilocycles thereby securing transmission of all frequency components of the modulated wave to the frequency modulation detector.

However, it is found in actual practice that there exists a predetermined threshold value for the frequency modulated waves below which there is a relatively abrupt and marked increase in noise reproduction. It can be shown that the threshold value for the frequency modulated signals when the peak noise is approximately equal to the signal level at the detector input is a function of the intermediate frequency band width. This follows from the fact that the noise impulse amplitude is a direct function of the intermediate frequency amplifier band width. It necessarily follows, then, that the signal to noise ratio delivered to the detector input circuit can be lower than is usually considered desirable by employing a narrow intermediate frequency band width. However, when such restriction of the intermediate frequency band width is employed there inevitably results distortion by virtue of the fact that the frequency deviation of the carrier eX- tends beyond the narrowed band width, and, therefore, certain frequency components of the modulated carrier wave will not be reproduced.

Accordingly, it may be stated to be one of the main objects of my present invention to utilize in a frequency modulation receiver of the superheterodyne type, a first intermediate frequency network whose band width is co-eXtensive with the overall carrier deviation of the intermediate frequency waves, the first intermediate frequency network being followed by a network which functions to derive from the intermediate frequency waves other waves of a lower intermediate frequency and whose frequency deviation has been proportionately reduced, the second intermediate frequency waves then being transmitted to the frequency modulation detector through anetwork whose band width readily transmits without distortion the reduced intermediate frequency energy.

Another important object of this invention may be stated to reside in the provision of a method for receiving frequency modulated waves having a relatively wide carrier frequency deviation range while employing at the receiver a relatively low signal to noise ratio; the method essentially including the frequency division of the intermediate frequency energy derived from the frequency modulated signals, the frequency division step producing frequency modulated signals of a relatively low center frequency and a carrier frequency deviation which has been reduced from the transmitted wave frequency deviation by substantially the same amount which the center frequency of the intermediate frequency signals was reduced whereby the noise impulse amplitude at the frequency modulation detector input circuit is considerably reduced.

Another object of this invention is to provide a frequency modulation receiver of the superheterodyne type wherein there is located between the first detector and the second detector at least two intermediate frequency networks arranged in cascade, the first intermediate frequency network having an overall band width adapted to pass substantially all the modulation components of the frequency modulated signals, the second intermediate frequency network being `tuned to a center frequency which is half `the first intermediate frequency value, the second intermediate frequency network having an overall band width which is reduced from the first intermediate frequency network lband width by two, and there being employed between the two intermediate frequency networks a simplified frequency dividing network adapted to reduce the center frequency of the first intermediate frequency signals' to the center frequency of the second intermediate frequency network with a proportional reduction in the center frequency deviation.

Still other objects of my invention are to improve generally the simplicity and efficiency of frequency modulation receivers of the superheterodyne type, and more especially to provide a frequency modulation superheterodyne receiver which is not only reliable in operation and has a lower than normal signal to noise ratio, but is economically manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 schematically shows a receiving system employing the invention,

Fig. 2 illustrates an illustrative circuit that can be used for the frequency divided network,

Fig. 3 shows the divider characteristic.

Referring now to the accompanying drawing, wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. l a frequency modulation superheterodyne receiver which generally comprises known networks, save for the networks embodied by virtue of the present invention. Thus, numeral l designates a signal collector which may be a dipole or any other type of signal collector adapted efficiently to collect frequency modulated carrier signals in the 42 to 50 megacycle band, it being understood that this is the presently assigned frequency modulation reception band. The collected signals are impressed upon a radio frequency amplifier network 2 which may usually embody one or more stages of tunable radio frequency amplification. The numeral 3 denotes the tunable circuit of the amplifier 2. The amplified radio frequency signals are impressed upon a flrst detector network l which is, also, fed with the locally produced oscillations from network 5. The numeral 6 designates the tunable tank circuit of the local oscillator, and the dotted line 'l denotes the unicontrol adjusting mechanism for simultaneously varying the tuning devices, such as the variable condensers, of the signal and tank circuits. These are instrumentalities well known in the superheterodyne reception art, and it is not believed necessary to explain them in any further detail.

There are produced in the output circuit of the first detector frequency modulated signals whose center frequency is of the intermediate frequency value, but whose overall center frequency deviation is the same as the overall center frequency deviation of the frequency modulated signals impressed on the input circuit of the network 2'. Assume for the purpose of this application, and merely to simplify the explanation, that the frequency deviation of the center frequency is 50 kilocycles, then the overall carrier frequency deviation will be 100 kilocycles. Let it, also, be assumed that the center frequency of the intermediate frequency signals produced in the output circuit of the rst detector is equal to megacycles. Accordingly, the intermediate frequency transformer 8 will have each of its primary and secondary circuits 9 and l0 respectively tuned to the center frequency of l0 megacycles, whereas the overall band width of the resonance curve of the tuned network 0i0 will be equal to 100 kilocycles. In this way there is transmitted through the network 9-l0 all the modulation components of the modulated carrier wave.

As stated previously, the peak noise is a function of the band width of network 9l0. The wider the band width the larger will be the amplitude of the noise transmitted to the frequency modulation detector. A mere narrowing of the intermediate frequency band width would, of course, introduce distortion since that would cut out the modulation components located at the extremities of the carrier frequency deviation. Accordingly, in my invention there is employed a frequency divider device ll which functions to reduce the carrier frequency valueV of the intermediate frequency signals to a substantially lower value. Simultaneously the carrier frequency deviation is decreased in proportion thereby permitting the use of an intermediate frequency network prior to the frequency modulation detector which is of a narrow intermediate frequency band width. This at once greatly decreases the peak noise at the second detector input Circuit, and yet does not introduce distortion, because, instead of cutting out any of the modulation components of the carrier, the frequency modulated wave has rather been compressed.

In frequency modulation the signal to noise ratio at the loudspeaker is proportional to the signal to noise ratio at the input terminals of the receiver, and to the ratio of the intermediate frequency amplifier band width to the audio frequency amplifier band width. For a constant audio frequency band width, if the intermediate frequency band width is decreased the output 'signal to noise ratio is decreased for a given input signal to noise ratio. However, there is a denite input signal-noise ratio belofw which frequency modulation exerts no improvement in noise. This improvement threshold occurs at the point where the peak noise voltage equals the carrier voltage at the detector. Below this threshold value the signal is submerged by the noise. Reducing the intermediate frequency band width reduces the peak noise voltage, and, hence, the threshold value. Thus, by reducing the intermediate frequency band width and center frequency deviation there is some reduction in the output signal to noise ratio for signals well above the threshold. However, near the threshold value reception may be had with a narrow intermediate frequency band width under noise conditions where reception with a wide intermediate frequency band width would be impossible.

To explain the electrical action involved in the frequency division of the intermediate frequency signals impressed on network Il, let us consider a signal with a center frequency of megacycles and a carrier frequency deviation of plus and minus '75 kilocycles. That is, the center frequency varies between the limits 44.950 and 45.050 megacycles. As is well known, this center frequency deviation will produce a series of side band frequencies, the side bands depending upon the center frequency deviation and the modulating frequency. The relative magnitudes of the side bands is given by a Bessel series expansion. These side bands extend to at least the deviation plus the modulating frequency on each side of the center frequency. For the sake of simplicity of illustration consideration will be limited to the three frequencies 44.950, 45.00 and 45.050 megacycles. If the local oscillator has a value of megacycles there will result in the outputV of the converter stage the frequencies 9.950, -.00 and 10.050 megacycles; that is, 10 megacycles plus and minus '75 kilocycles. If, now, this signal is impressed on a frequency divider which divides by a factor of 2, 10 megacycles becomes 5 megacycles in the output, 9.950 megacycles becomes 4.975 and 10.050 megacycles becomes 5.025. That is, the frequencies involved become 5 megacycles plus or minus 25 kilocycles. Similarly, the other side band frequencies are divided by the same factor. Therefore, it is seen that frequency division divides not only the center frequency, but, also, the deviation frequency by the same factor.

What is desirable for the divider is a network which is capable of control by the intermediate frequency signals at input circuit I0, and which will produce at the output circuit i2 of the divider network second intermediate frequency signals whose center frequency is substantially smaller than the center frequency of the first intermediate frequency signals. For example, the second intermediate frequency signals may have a center frequency of 5 megacycles, and in that case the maximum carrier frequency deviation will be reduced proportionately so that it will be 2'5 kilocycles instead of the original 50 kilocycles. Hence, there may be utilized an intermediate frequency transformer E3 whose primary and secondary circuits l2 and tunedto 5 megacycles, while the overall band width is equal to 50 kilccycles. There is graphically shown above each of transformers 8 and I3 the action of the frequency divider network II.

The center frequency of the second I. F. energy may be further reduced in half to a value of 2.5 megacycles, with a concurrent reduction of the deviation to 12.5 kilocycles. Thus, numeral I5 denotes a second frequency divider which may be constructed in the same manner as divider I I. Above the transformer is shown the resonance curve of the coupled tuned circuits.

The third intermediate frequency signal energy may be transmitted through one or more intermediate frequency amplifiers, and the amplified signals may then be transmitted through a limiter network which functions to eliminate any amplitude variations introduced into the frequency modulated signals by virtue of the noise impulses or the action of the various resonant circuits. The limited frequency modulated signals are detected in any well known type of frequency modulation detector. At the frequency modulation detector input circuit the peak noise will be greatly reduced by virtue of the utilization of resonant circuits subsequent to the frequency division which are of relatively narrow band width. To enable those skilled in the art readily to construct the invention, there is shown in Fig. 2 a specific type of frequency divider network that can be employed for networks II and I5 of Fig, 1.

Referring to Fig. 2 there is shown a circuit which may be used for the frequency division function. The tube may be cf the pentode type. The input circuit i0 may be connected between the signal control grid and cathode. The resistor 3| is inserted between the low potential side of circuit I0 and the grounded cathode. Since in this application it is necessary that the output of the divider stage be proportional to the square root of the input over the operating range of frequencies, low capacitanceis required across resistor 3| so that tube I does v'not act as It are respectively -r a quasi-peak operated device. When the capacitance of transformer g-l to ground is high, as it may be in certain constructions, it is desirable to place resistor 3l in the high potential side of the circuit between transformer l0 and grid 30 rather than in the low potential side in order to reduce the capacitance across resistor 3l. The plate 32' is connected to a source of positive volttage through the coil of output circuit l2. Each of circuits l2 and I4 are resonate-d to the operating' intermediate frequency. The input-output characteristic of the divider network is shown in Fig. 3. As the voltage is increased in thev positive direction more current will be drawn by grid y30. This increases the voltage drop across resistor 3l and the operating bias on tube I. The tube output will then increase less rapidly than the input. By proper ch= ce of the resistance value of resistor 3l the desired output proportional to the square root of the input is secured.

From trigonometry it is known that the square root of an angle may be expressed as half the angle plus a constant. 1t be shown that for an input wave coswt and a square root characteristic, the output will be essentially:

Therefore, if a circuit is set up which operates so that the output is proportional to the square root ofthe input, the output will be at half the input frequency (plus a D. C. component). Hence, there is secured by means of the circuit of Fig. 2 a stage whose characteristic is as desired. The resistor 3G produces a partial limiting action, as Fig. 3 shows. Accordingly, by using a following divider network at i5 which is constructed in the manner shown in Figs. 2 and 3, there will be secured a further division of the I. F. value, a corresponding reduction in the band width and further partial limiting. Of course, if additional limiting is desired, then any well known form of limiter may be employed subsequent to transformer 20.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. The method which includes beating frequency modulated waves with local oscillations to produce beat waves whose center frequency deviation range is the same as the original signal deviation range, deriving from the beat energy a signal energy output which is the square root of the beat energy whereby the center frequency is reduced in half with respect to the center frequency of said beat energy and whose center frequency deviation range is proportionately reduced, then further reducing by the same factor the center frequency of the desired signal energy, and partially limiting amplitude variation during each of said center frequency reduction steps.

2. In a superheterodyne receiver of the type adapted to receive frequency modulated carrier waves and including a converter network for converting the frequency modulated carrier waves to intermediate frequency signals whose frequency deviation range is substantially the same as the frequency deviation of said frequency 1/2- cos modulated carrier waves, a frequency divider network for deriving from said intermediate frequency energy a second intermediate frequency energy whose center frequency is substantially reduced with respect to the center frequency of said first intermediate frequency energy and Whose frequency deviation range is proportionately reduced, and a network for transmitting said second intermediate frequency energy, the improvement which comprises said frequency divider network including a tube having an input circuit upon which is impressed said first intermediate frequency energy, said tube having an output circuit for developing said second intermediate frequency energy, and said tube being constructed to have a characteristic such that its output is proportional to the square root of its input.

3. In a superheterodyne receiver of the type adapted to receive frequency modulated carrier waves and including a converter network for converting the frequency modulated carrier waves to intermediate frequency signals whose frequency deviation range is substantially the same as the frequency deviation of said frequency modulated carrier waves and a frequency divider network for deriving from said intermediate frequency energy a second intermediate frequency energy whose center frequency is substantially reduced in half relative to the center frequency of said first intermediate frequency energy and whose frequency deviation range is proportionately reduced; the improvement which consists of said divider network including an electron discharge tube having input and output electrodes, a tuned circuit in series with a resistor coupling said input electrodes, means applying said intermediate frequency signals to said tuned circuit, and said resistor having a magnitude such that the divider output is proportional to the square root of the divider input.

4. In a superheterodyne receiver of the type adapted to receive frequency modulated carrier Waves and including a converter network for converting the frequency modulated carrier waves f to intermediate frequency signals whose frequency deviation range is substantially the same as the frequency deviation of said frequency modulated carrier waves, a frequency divider network for deriving from said intermediate frequency energy a second intermediate frequency energy Whose center frequency is substantially reduced with respect to the center frequency of said first intermediate frequency energy and whose frequency deviation range is proportionately reduced, and a network for transmitting said second intermediate frequency energy, the improvement which comprises said frequency divider network including a tube having an input circuit upon which is impressed said first intermediate frequency energy, said tube having an output circuit for developing said second intermediate frequency energy, and said tube input circuit including a resistor in series therewith so chosen that the divider has a characteristic such that its output is proportional to the square root of its input.

DUDLEY E. FOSTER.

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