US2315308A - Method and means for transmitting frequency modulated signals - Google Patents

Description

March 30, 1943. E H, ARMSTRQNG 2,315,308

METHOD AND MEANS FORTRANSMITTING FREQUENCY MODULATED sIGNALs Filed Aug. 2. 1940 A T TUR/v5 X5 Patented Mar. 30, 1943 METHOD AND MEANS Fon TRANSMITTING FREQUENCY MoDULArEn srGNALs Edwin H. Armstrong, New York, N. Y.

Application August 2, 1940, Serial No. 349,870

(ci. 25o- 17) 4 Claims.

This invention has for its object the elimina tion of disturbances resulting from frequency changes in radio transmitting systems, particularly of the type described in my U. S. Patent #1,941,068 for producing wide band frequency modulated signals. In this type of transmitter the frequency modulated signal is produced by changing the phase of a current derived from a relatively low frequency oscillator whose frequency is usually crystal controlled.' This initial phase shift is then multiplied a great many times until the desired frequency change is produced at the frequency which is to be radiated. On ac'- count of the great number of frequency multiplications employed, small irregularities in the constancy of the frequency of the oscillator give rise to various difficulties, and it is the purpose of this invention to eliminate them.

Referringnow to the drawing which forms a part of this specification, the single ligure illustrates the general arrangement of the apparatus which will be understood from the following description. An arrangement now in common use for producing a frequency modulated current of 100 kc. deviation at approximately 40 megacycles consists of a crystal controlled oscillator having a frequency of 200 kc., a`phase shifting device.

therefor, and a seriesl of frequency multipliers. The frequency multiplication is usually accomplished in two steps, the first step being up to some convenient multiple of the 200 kc. from which it is heterodyned down to some relatively lowerfrequency that is a sub-multiple of the frequency to be radiated, and this in tum is multiplied upto the nal value. It has been common practice to multiply up to a frequency of 12.8 megacycles, which is then heterodyned down to some frequency in thevicnity of 900 kc. This frequency is then in turn multiplied up to the vicinity of 40 megacycles. Ina specific case for producing a 42 megacycle wave a heterodyning current of 11.925 megacycles would be combined to produce a difference frequency of 875 kc. which would then be multiplied 48 times to produce a 42,000 kilocycle current. The total frequency multiplication amounts toA 3072 times.

It has been found that the frequency of the 'initial crystal oscillator is not a constant quantity but varies minutely due to irregularities of filament emissions and to various other effects and while these changes are extremely minute, after a multiplication of several thousand fold they become perceptible. In addition, the drift of frequency due to temperature changes of either crystal may cause a resultant change in frequency of the transmitted wave of an undesired amount. This is particularly true with respect to the higher frequencies. The present invention has for its object the reduction of all these effects to such'v a very small percentage of their i original amount that they become of negligible importance.

'Ihe invention proposes to accomplish this by deriving the current for the second oscillating source from the initial oscillator so that any changes in frequency of the first oscillator, whether slow or fast, will be substantially cancelled out in the conversion of frequency down to the submultiple of the frequency to be radiated.

An embodiment of the invention is illustrated in the single ligure which forms a part of this specification. Referring now to the figure, I represents the oscillator of some lowfrequency, 2 the usual carrier amplifier and 3 the usual balanced modulator employed in producing the initial phase shift. 4, l, 6, l, 8, i represent the usual series of doublers forraising the oscillator frequency which in the present case is taken for example as 200 kc. pliers I0, I I, I2, I3 I4 composed of two triplers and three doublers raise the frequency to 14,400 kilocycles. I5 is a converter for combining the 12.8 megacycle modulated'current with the 14.4 megacycle unmodulated current to, give a resultant frequency modulated current of 1600 kilocycles. I6, I1, I8, I 9 and 20 represent a series of doublersand triplers to raise the frequency to 115.2 megacycles. 2| represents the usual power amplifier and 22 the antenna. 23 represents the usual correction amplifier.

The theory of operation will be clear from an examination of the figure. Any change in frequency of .the oscillator I, either at a slow or rapid rate, will be multiplied by the multipliers in the two-paths by a nearly equal amount so that they will either increase or decrease in frequency simultaneously, leaving the beat or difference frequency substantially unchanged. In this process, the slight frequency changes due to irregularities of the emission in the' electron. stream in the oscillator tube are cancelled out as well as the larger changes due to thermal drift.

It will be obvious that by choosing a particular oscillator frequency and selecting the proper combination of harmonics in the two paths that any desired resultant frequency having any desired frequency deviation may be obtained.

I have described what I believe to be the best embodiments of my invention. I do not wish, however, to be conned to the embodiment shown,

A second series of multi-- to the amplitude of the modulating current and inversely proportional to frequency of said current, said device being free" from inherent disturbances of greater magnitude than that of the said disturbances inherent in said oscillator, said path also including means for multiplying the frequency of the modulated currents, a second path connecting said converting device to said initial oscillator and including means for multiplying the frequency of the currents generated by said initial oscillator, means connected to the output of the frequency converting device for multiplying the frequency of the beat currents formed therein, the total multiplication effected by the several multiplying means being of the order of several thousand fold, and means for radiating the modulated currents of the multiplied frequency. v l

2. The method of reducing disturbances which arise due to the extremely minute inherent variations in the frequency of the initial oscillator of a wide swing frequency modulation system, which consists in dividing the current from the oscillator into a plurality of paths, changing the phase of the current derived from said oscillator in one of said paths without introducing into said path disturbances of greater magnitude than that of the disturbances inherent in the said oscillator, multiplying in said path the frequency of the modulated currents to produce a higher second frequency, multiplying in said other path the frequency of the current derived from the initial oscillator to produce a third frequency which differs from the second frequency by an amount which is a sub-multiple of the frequency to be transmitted, beating the second'frequency and third frequency together to produce a fourth frequency, multiplying the fourth frequency to the frequency to be transmitted, the total multiplication effected by the several multiplying steps being of the order of several thousand fold, and transmitting the currents of the resulting frequency.

3. A transmitting device as set forth in claim 1 in which the frequency difference between the currents impressed on the converting device by the first and second paths is equal to a submultiple of the frequency of the currents radiated.

4. A transmitting device as set forth in claim 1 in which the means for multiplying the frequency of the currents in'the second path consists solely of a plurality of frequency multipliers connected together in series.

EDWIN H. ARMSTRONG.

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