US2277261A - System for transmission and reception of frequency modulated signals - Google Patents

Description

March 24, 1942 J. E. SMITH ETAL SYSTEM FOR TRANSMISSION AND RECEPTION OF FREQUENCY MODULATED SIGNALS Original Filed April 27, 1939 i uw 'vu' u."

Patented Mar. 24, 1942 SYSTEM FOR TRANSMISSION AND RE- CEPTION F FREQUENCY MODULATED SIGNALS James Ernest smith, Brooklyn, N. Y., James N.

Whi

taker, Weehawken, N. J., and Gilbert R..

Clark, Brooklyn, N. Y., assignors to Radio Corporation of America, a corporation of Delaware Original application April 27, 1939, Serial No.

270,332. Divided and this 1949, Serial No. 335,204

13 Claims. (Cl. P18-5) This invention is a division of our application led April 27, 1939, Serial No. 270,332, andrelates to the variation of carrier waves by change in frequency instead of amplitude. It will be described in connection with photo radio transmission, although it is capable of various other uses.

As is now well known, changes in the condition and height of the ionosphere produce fading effects in radio transmission. These changes may at times affect all frequencies within a wide range and at other times affect only a particular frequency or frequencies within a quite narrow range. The rst is usually referred to as general fading and the second as "selective fading." I'hese two eifects may be produced at different times or they may occur simultaneously.

To overcome the effects of fading ithas been proposed to modulate the carrier wave by changing the frequency rather than the amplitude, since fading produces amplitude variations of the signal. Frequency modulation would thus enable one to discriminate between the signal modulation and that produced by fading. By using frequency modulation at the transmitter 'and rigidly limiting the voltage or current at the receiving station, the amplitude modulation produced by fading can be eliminated. For a disclosure of this general method of reducing fading, reference is made to the patent of Wright and Smith, No. 1,964,375, June 26, 1934.

Also, in producing frequency variation it has been found advantageous to vary the frequency of a sub-carrier and then amplitude modulate the radio frequency carrier by this frequencymodulated sub-carrier. While the improvements produce an improvement in the transmission of pictures as compared to systems using amplitude modulation, selective fading effects are still noticeable. Most objectionable is the selective fading of the radio frequency carrier. Whenever this occurs the received signal appears to be overmodulated by the side bands, thus producing spurious components inv the detected output of the side-band separation-frequency and harmonics thereof. The main spurious component is the second harmonic of the sub-carrier caused by this beating together of the two side bands. Other even harmonics may be produced, but to a less extent.

application-May 15.

a radio carrier modulated by a sub-carrier frequency and its harmonic, the latter two frequencies having been modulated by the signal.

y Another object is to receive both the modulated sub-carrier frequency and the double sub-carrier frequency and select the latter at all times.

Another object of the invention is to provide a novel arrangement for frequency-modulating a sub-carrier frequency.

Other objects will appear in the following description, reference being had to the drawing, in which:

Fig. 1 is a part diagrammatic and part block illustration of a transmitting system embodying our invention. Y

Fig. 2 is a block diagram of the receiving circuit.

Fig. 4 gives voltage-frequency graphs of the frequency detector.

Referring to Fig. 1, the picture scanner may be of any desired kind as indicated at I. It has a tone output by control of a source 2 of audio oscillations. The tone output at 3 varies the frequency of a sub-carrier frequency oscillator, which in turn modulates a radio frequency carrier. 'Ihe sub-carrier frequency oscillator is of the beat or heterodyne type. The tone output 3 of the scanner feeds into double rectifier 4 and the rectified output is applied to tube 5 as a negative bias that varies with the scanner output.

Two high frequency oscillators 6 and 1 are adjusted to oscillate at frequencies differing by a low frequency beat, for example, 800 cycles. Oscillator 6 maintains a constant frequency, but

, oscillator 'I has its frequency varied by the vac- It is one object of our invention to overcome A the effects of selective fading by receiving and utilizing the second harmonic of the sub-carrier at all times.

Another object of the "invention is to transmit 56 uum tube stage 5. Tube 5 has its plate-cathode circuit in shunt with the high resistance 8 and condenser 8' and its plate connected through the condenser 9 to the tank circuit consisting of coil I0 and condenser I0. The anode circuit of tube '5 is thus in parallel with the grid portion of the coil I0 of the tank circuit..

The oscillator 6 has atank circuit Illa and lll'a and is identical with oscillator 1 except for the frequency variation element shunted across the lower half of the coil I0. VThe output of oscillator 6 is adjustably connected to control grid Il of detector tube I2, which is surrounded by screen grid I3, The voltage on this screen is preferably lower than that on the plate of the tube. The output of oscillator 'I is adjustably connected to another control grid I 4 of tube I2.

Fig. 3.is a diagrammatic illustration of the band pass frequency detector of Fig. 2.

The output of detector tube l2 is passed through a low pass lter I5 to filter out all frequencies except the varying beat frequency produced by\the constantv frequency of oscillator 6 and the varying frequency of oscillator 1. Part of the low pass filter output, which consists of the frequency modulated sub-carrier frequency, is then amplified at 20 and passed through a band pass filter 2| into the mixing circuit 22. The filter 2l is constructed to pass the frequency band of the varying sub-carrier frequency but not any sub-harmonics that may be present. The other part of the output of filter I5 passes through the frequency doubler 23 to produce a modulated second harmonic of the sub-carrier frequency. The doubled frequency is then passed into mixing circuit 22 through band pass filter 24, made to pass the frequency band of the modulated second harmonic. The combined output in 22 then passes into the modulator 25 to modulate a high frequency carrier produced by generator 26. Transmitter 21 radiates the modulated high frequency carrier by means of antenna 28. The modulator 25, generator 26 and transmitter 21 may be of any type desired, as no special requirements are imposed thereon by our improvement.

In Fig. 2 the receiving antenna 29 is connected to an appropriate receiving apparatus 30, which may consist of radio frequency amplifiers, a detector and audio frequency amplifiers and any other desired devices for converting the signals into audio frequencies of the desired intensity. The output of the amplifier 30 passes through a' band pass filter 3l and a voltage limiter 32 into a frequency detector 33 and rectifier 34 to recorder 35. The recording may be carried out by the photographic process, visual process, or by any other method, as our invention is independent of the particular type of the recorder.

The band pass filter 3l is made to exclude the frequency band of the fundamental of the subcarrier frequency and to pass the band of the second harmonic thereof, for reasons that will appear later in the explanation of the method of operation of our improvement.

The voltage limiter 32 limits both halves of the alternating waves and may be of any suitable type, a number of which are well-known in the art, but we prefer the one described in the parent application above referred to.

The frequency detector 33 of Fig. 2 is shown in detail in Fig. 3. The input lines 36 introduce the signal into a low pass filter through transformer 31. It is essentially a low pass filter consisting of series inductances 38, 39 and shunt capacities 40, 4I and 42, arranged in the usual way. These inductances and capacities are of such number and value that the characteristic curve A shown in Fig. 4 will have a substantially straight portion a, b between the frequencies f1 and lz. The modulation frequencies f1 and fz may have various values, but in practice we have found it satisfactory to use a sub-carrier frequency varying between 800 and 1,000 cycles per second for the fundamental, which, when doubled by the frequency doubler, would have aV sited, and then the characteristic will be like curve B in Fig. 4.

The operation will now be described:

The tone output of scanner I is rectified by double rectifier 4 and the rectified potential varies the plate-cathode impedance of tube 5 and thus varies the tuning effect of condenser 9 in oscillator 1. By varying the eective impedance (anode-cathode impedance) in series with condenser 9, the impedance of the shunt around the lower half of coil l0 is varied and this varies the frequency of the tank circuit, which is the frequency producing element in the oscillator.

Oscillator 6 is set to oscillate at some suitable frequency, for example, 75 kilocycles, and oscillator 1 will oscillate under control of the scanner at a frequency that'varies above or below the frequency of oscillator 6 a suflicient amount to produce a beat, the latter varying from, say, 800 to 1,000 cycles. This beat or sub-carrier frequency is produced in the output of detector tube I2 by connecting the output of oscillators 6 and 1 to control grids II and I4, respectively. Low pass filter I5 permits the varying sub-carrier frequency to pass and the higher frequencies are filtered out.

A part of the fundamental band (800 to 1,000 cycles) of th'e sub-carrier frequency, passes through amplifier 20 and band pass filter 2| into mixing circuit 22 and a part has the frequency doubled to 1,600 to 2,000 cycles before it passes into the mixing circuit 23 through band pass fliter 24. The combined fundamental and second harmonic frequencies thus modulate the high frequency carrier from 26 and the Wave transmitted from the antenna has side bands produced by the fundamental and side bands produced by the second harmonic, both of which contain the frequency modulation produced by the output of scanner I.

The radio frequency carrier will be received by antenna 29 and receiving apparatus 30 of Fig. 2. It will be amplified and detected and the resultant signal amplified at audio frequency in the usual way. 'The output of the receiver, when there has been no fading of radio frequency carrier, will contain both the varying sub-carrier frequency and the varying doubled frequency thereof. The former is blocked by the band pass filter 3|, but the latter passes readily through. Due to the effect of the upper atmosphere, the signal will have spurious amplitude modulation in most cases. When the signal passes through limiter 32, a sufficient portion of the positive and negative tops of the alternating wave will be cut off to eliminate this spurious amplitude modulation and the signal will pass into the frequency detector 33 free from this addition, but with all the frequency modulation produced by the scanner I.

On the straight portion a-b of the curve of filter 33, the output varies in amplitude directly with the frequency. Hence filter 33 is a frequency detector that converts the frequency modulation of the sub-carrier second harmonic into amplitude modulation whichis rectified at 34 and recorded at 35.

It will thus be seen that, normally, that is, with no selective fading, the transmitted second harmonic of the sub-carrier frequency is received, limited, converted and recorded and the transmitted fundamental of the sub-carrier frequency is not utilized.

When the radio frequency carrier selectively fades, the transmitted harmonic of the subcarrier frequency will be doubled by the beating of the side bands and it will not pass through the band pass filter 3|. The fundamental of the sub-carrier frequency will likewise be doubled by a similar beating action. Hence it will be passed by the filter 3l and will reach the recorder in a way that will be understood from the previous description.

Briefly, with our invention, the receiving station utilizes the second harmonic produced at the transmitting station when there is no selective fading and utilizes the second harmonic formed by the beating of the side bands when there is selective fading.

The conversion of the frequency modulation into amplitude modulation can obviously be accomplished by means other than low or high pass filters, as other devices are known in the art.

The frequency modulation arrangement in our invention may, of course, be used to transmit only the modulation produced by the sub-carrier fundamental for reception by the receiving apparatus of our parent application, by appropriate disconnection of the frequency doubler 23 and its associated apparatus.

Various modifications of our invention may be made without departing from the spirit of the invention.

Having described our invention, what we claim 1s:

1. In a signaling system, means for producing a sub-carrier frequency, means for varying said sub-carrier frequency in accordance with a signal, means for producing a second harmonic of said varying sub-carrier frequency, means for producing a radio frequency carrier, means for modulating the radio frequency carrier by both said sub-carrier frequency and said second harmonic, means for transmitting the modulated radio frequency carrier, and means at the receiverfor utilizing the second harmonic only of the sub-carrier frequency and rejecting the fundamental thereof.

2. In a signaling system, means for producing a sub-carrier frequency, means for varying the said sub-carrier frequency in accordance with a signal, means for producing a second harmonic of said varying sub-carrier frequency, means for producing a radio frequency carrier, means for modulating the radio frequency carrier by both said sub-carrier frequency and said second harmonic, means for transmitting the modulated radio frequency carrier, a receiver having means for detecting the second harmonic only of the sub-carrier frequency and means for limiting the amplitude of the detected frequency.

3. In a signaling system, means for producing a sub-carrier frequency, means for varying said .sub-carrier frequency in accordance with a signal, means for producing a second harmonic of said varying sub-carrier frequency, means for producing a radio frequency carrier, means for modulating thel radio frequency carrier by both said sub-carrier frequency and said second harmonic, means for transmitting the radio frequency carrier, a receiver having means to detect the fundamental and the second harmonic of the sub-carrier frequency and means for selecting the second harmonic frequency only.

4. In a signaling system, means for producing a sub-carrier frequency, means for varying the said sub-carrier frequency in accordance with a signal, means for producing a second harmonic of said varying sub-carrier frequency, means for producing a radio frequency carrier, means for 5. In a signaling ystem, means for producing a sub-carrier frequency, means for varying said sub-carrier frequency in accordance with a signal, means\for producing a second harmonic of said varying sub-carrier frequency, means for producing a radio frequency carrier, means for modulating the radio frequency carrier by both said sub-carrier frequency and said second harmonic, means for transmitting the modulated radio frequency carrier, a receiver having means to detect the fundamental and the second harmonic of the subcarrier frequency, means for selecting the second harmonic frequency and means for limiting the amplitude of the selected frequency.

6. In a. signaling system, means for producing a sub-carrier frequency, means for varying the said sub-carrier frequency in accordance with a signal, means for producing a second harmonic of said Varying sub-carrier frequency, means for producing a radio frequency carrier, means for modulating the radio frequency carrier by both said sub-carrier frequency and said second harmonic, means for transmitting the modulated radio frequency carrier, a receiver having means to detect the fundamental and the second harmonic of the sub-carrier frequency, means for selecting the second harmonic frequency, means for limiting the amplitude of the selected frequency, and meansfor converting the frequency e modulation of the limited frequency into amplitude modulation.

7. A transmitting system comprising a source of modulating potential, a source of constant al-l ternating potential', means for modulating said alternating potential by said modulating potential, means for rectifying the modulated alternating potential, means for producing a carrier frequency, means for producing a sub-carrier frequency, means for varying the sub-carrier frequency by the output of said rectifying means, and means for modulating the carrier frequency by the varying sub-carrier frequency.

8. A transmitting system comprising a source of modulating potential, a source of constant low frequency alternating potential, means for modulating said low frequency alternating potential by said modulating potential, means for rectifying the modulated alternating potential, means for producing a. carrier frequency, vmeans for producing a sub-carrier frequency, means for varying the sub-carrier frequency by the output of said rectifying means, and means for modulating the carrier frequency by-the varying subcarrier frequency. A

9. A transmitting system comprising a source of modulating potential, a. source of constant low frequency alternating potential, means for modulating said low frequency alternating potential by said modulating potential, means for rectifying the modulated alternating potential,

means for producing a carrier frequency, means and means for amplitude modulating the carrierl frequency by the varying sub-carrier frequency.

10. A transmitting system comprising a source of modulating potential, a source of constant low frequency alternating potential, means for modulating said low frequency alternating potential by said modulating potential, means for rectifying the modulated alternating potential, means for producing a carrier frequency, means for producing a sub-carrier frequency, means for varying the sub-carrier frequency by the output of said rectifying means, means for amplitude modulating the carrier frequency by the varying sub-carrier frequency, and means for transmitting a radio wave having the modulated carrier frequency.

1l. A transmitting system comprising means for producing a varying output potential, a generator of low frequency alternating potential, means for varying the amplitude of said alternating potential in accordance with said varying output potential, means for rectifying the modulated alternating potential, means for generating a high frequency carrier potential, means for generating a constant intermediate frequency potential, means for generating a second intermediate frequency potential, means for varying the frequency of the second-mentioned intermediate frequency potential with the amplitude of the potential of the output of said rectifying means, means for beating together the outputs of the two intermediate frequency generators, and means for modulating the high frequency carrier potential by the beat frequency potential of said two intermediate frequency generators.

12. A transmitting system comprising a picture scanner producing a varying output potential, a generator of low frequency alternating potential, means for varying the amplitude of said alternating potential in accordance with the varying output potential of said scanner, means for rectifying the modulated alternating potential, means for generating a high frequency carrier potential, means for generating a constant intermediate frequency potential, means for varying the frequency of the'second-mentioned intermediate frequency potential with the amplitude of the potential of the output of said rectifying means, means for beating together the outputs of the two intermediate frequency generators, and means for modulating the high frequency carrier potential by the beat frequency potential of said two intermediate frequency generators.

13. A transmitting system comprising a picture scanner producing a varying output potential, a generator of low frequency alternating potential, means for varying the amplitude of said alternating potential in accordance with the varying output potential of said scanner, means for rectifying the modulated alternating potential, means for generating a high frequency carrier potential, means for generating a constant intermediate frequency potential, means for generating a second intermediate frequency potential, means for varying the frequency of the second-mentioned intermediate frequency potential with the amplitude of the potential of the output of said rectifying means, means for beating together the outputs of the two intermediate frequency generators, and means for amplitude modulating the high frequency carrier potential by the beat frequency potential of said two intermediate frequency generators.

JAMES ERNEST SNIITH. JAMES N. WHITAKER. GILBERT R. CLARK.

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