US1789371A

US1789371A - Signaling by frequency modulation

Info

Publication number: US1789371A
Application number: US205071A
Authority: US
Inventors: Harold O Peterson
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1927-07-12
Filing date: 1927-07-12
Publication date: 1931-01-20
Anticipated expiration: 1948-01-20
Also published as: GB293803A

Description

Jan. 20, 1931. H. 0. PETERSON SIGNALING BY FREQUENCY MODUL ATION Filed July 12; 1927 UPfRA TING RANGE INVENTOR mow o. mensou 2Z RNEY was 20, 193

UNITED STATES PATENT ornea mm 0. PETERSON, OF NIVERHEAD, NEW YORK, ASSIGNOB TO RADIO CORPORATION OF AMERICA, A CORPORATION 01 DELAWARE SIGNALING BY FREQUEN MODULATION 2 r Application filed .I'uly 1B, 1927. Serial No. 205,071

This invention relates to frequency (modulation and includes a method and means for transmitting and receiving a signal wave over a high frequency carrier wave by 5 slightly changing the frequency and'not the amplitude of the carrier wave in accordance wit the signal wave. 1

The increasing trafiic of all kmds employing various portions. of the available.

requency spectrum for the transmission of intelligence makes it desirable that the necessary wave band re uired for any one channel be greatly re uced. When using amplitude modulation the wave band required for speech telephony extends .over

twice the width of the useful range of audible frequencies, owing to the resulting side 25 object of my invention.

I accomplish the aforesaid objects by slightly changing the frequency of the carrier wave at'a frequency dependent on the frequency of the si al wave and to an exnal wave, but without appreciably changing the am litude of the carrier wave at any time. y this frequency modulation, instead of amplitude modulation, I inherently introduce a frequenc wobble which helgs eliminate fading, an avoid the use-of si e band frequencies, in the ordinary sense, with their attendent disadvantages.

To change frequency modulation to amplitude modulation it has been suggested that the receiver be detuned. Forresponse to'small fre uency variation I first heterodyne the received energy to obtainfrequency modulated energy of intermediate frequency, and then analyze the intermediate freguency modulated energy by means of a etunedlcircuit to obtain the. signal wave. 4 The requisite maximum wave band may be made as small as desired, since the relative frequency change, for the purpose-of analy tent dependent on t e amplitude of the Sig-- sis, is dependent u on the magnitude of the frequency selecte .as the intermediate frequency. By this means a ran' 'e of only five hundred cycles per second, lnsteadof twenty thousand cycles or more, min the case of amplitude modulation, suflices for successful operation, even on very short wave lengths.

The invention is described more in detail in the following specification fwhich is accompanied by a drawing in which,

Figure 1 indicates a\transmitter for frequency modulation;

Figure 2 indicates an alternative form of variable reactance for obtaining frequency change in response to a signal wave;

F1gure3 represents a heterodyne receiver capable of analyzing and translating frequency modulated signals; and

Figure 4 is explanatory of the method of reception.

Referring to Figure 1 there is a. microphone circuit or other source of a signal wave 2, whichmay be coupled to a suitable amplifier 4, and then conve ed over a wire "line 6 for use in obtaining requency modu-.

lation' in a transmitter. A high frequency carrier wave is generated in a vacuum tube osclllator 8 having input and output inductances 10 and 12, and a coupling andtuning condenser 14. The anode circuit is .energlzed from a source of direct current 16, which is by-passed for high frequency oscillations by a condenser 18.

In parallel with the tuning oondenserl-l there is a variable condenser 20, one plate of which is vibrated by means of an armature 22 which acts under the influence of an. electro-magnet 24 which is energized by the energy ofthe signal wave. It is clear that the magnitude of the frequency variation is determined by the amplitude of movement of the armature 22, and that the'efi'ective frequency variation for a given amplitude of movementmay be made very small, being governed by the relative magnitude of. the two parallel connected condensersm and 20,

and that for a fixed magnitude ofthe vari able condenser 20 the mean frequency of the carrier wave is adjustable by means of the tuning condenser 14:. v

The frequency modulated output of the oscillator is coupled to a power amplifier 26, the output from which may be conveyed over high frequency lines, or radiated from a suit able antenna 28.

In the foregoing modification the frequency variation was obtained by varying a capacitance, but a variable inductance may equally well be used, in accordance with the modification shown in Figure 2. There the output a frequency of the oscillator is determined by a resonant circuit comprising a fixed inductance 42, a tuning condenser 44, and an inductance 46 which is varied by movement of a copper plate 48 located in its flux field. This plate is reciprocated about, its pivot 49 by means of an armature 50 which responds to variations in the magnetic field of an electromagnet 52, to which the signal wave is applied.

In case telegraphy signals are to be transmitted an alterating current of audible frequency may be keyed, as is indicated by the key 54 controlling the alternator 56. The alternator frequency may be super-audible, in'

- quency output is fed to an intermediate amplifier 66, which may include several stages of tuned amplification in cascade to accentuate the resonance curve in order to obtain a steep slope on its side. This receiver apparently resembles an ordinary super-heterodyne receiver, but differs in thatit must include some means to analyze the frequency modulated energy of intermediate frequency- This is most simply done by tuning the in-' termediate frequency amplifier 66 to a frequency which lies outside of the operating range of the intermediate frequency.

Thus, referring to Figure 4, the intermediate frequency amplifier is tuned not to the the mean carrier frequency f but rather to a frequency f,, which is so different from the mean frequency that all of the operating range of intermediate frequency lies to one side of the resonance frequency f,. This resultsin an amplitude change in response to the frequency variation, which is rectified in a suitable detector tube 68, the output from which is translated by translating means 70.

It is to be understood that the input circuit of the detector tube 68 may itself be made the analyzing circuit ifla less steep resonance curve is required. It is also to be understood that where the intermediate frequency amplifier is used as the analyzer the tuning need not be done by varying the tuning of the am-- atively detuned frequency f I consider the use of 'a'prelimifnary' hetero-- dynev desirable in all cases, and essential if slight frequency variation is employed, in-

asmuch as it increases the frequency variation relative to the frequency 0 the carrier on which it is impressed,thereby making analysis more practicable. Merely steepenin the resonance curve will not solve the pro lem, for besides the difiiculty of constructing exceedingly efficient circuits commercially, it 7 must be remembered that such circuits will tend to average out rather than follow the rapid frequency variations impressed on them.

Having thus described my invention 1 claim:

1. The method of receiving signals sent by frequency modulation which includes the step of heterodym'ng the received energy with energy of constant frequency to obtain frequency modulated energy of intermediate frequency in order to increase the degree of frequency modulation of the intermediate fr uency energy relative to the degree of mo ulation of the incoming signal energy whereby analysis anddetection of the signal is facilitated.

2. The method of receiving signals sent by frequency modulation which includes heterodyning the received energy to obtain frequency modulated energy of intermediate frequency of augmented modulation relative to the signals sent by frequency modulation, analyzing and. rectifying the intermediate frequency energy of augmented modulation, and translating the rectified energy.

3. The method of receiving signals sent by frequency modulation which includes heterodyning the received wave to obtain an inter mediate frequency modulated wave of relatively augmented modulation, amplifying the intermediate frequency wave in a resonant circuit tuned to a frequencywhich lies outside of the operating or modulation ran of the intermediate frequenc rectifying t e amplified energy, and trans ating the rectified energy.

4. A receiver for frequency modulated signals comprising means to heterodyne the received energy to obtain energy of intermediate frequenc of relatively augmented frequency mod ation, an analyzing circuit tunedto a frequency outside ofthe operating or modulation range of intermediate frequencg, and signal translating means.

5; receiver for frequency modulated siginc 1,7aa,ar1

nals comprisinga source of local oscillations of constant frequency and a combining device to heterodyne a received frequency modulated wave to a wave of intermediate frefl y of r latively augmented frequency modulation, a resonant intermediate frequency amplifier tuned to a frequency outside of the operating or modulation range of intermediate frequency, means to rectify the m amplified energy, and means to translate the rectified enemy OLD O; PETERSON;