US2445708A

US2445708A - Phase modulation

Info

Publication number: US2445708A
Application number: US557195A
Authority: US
Inventors: Murray G Crosby
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1944-10-04
Filing date: 1944-10-04
Publication date: 1948-07-20
Anticipated expiration: 1965-07-20

Description

July 20, 1948. M, G CR Y 2,445,708

PHASE MODULATI ON Filed Oct. 4, 1944 PHASE V MODUAAT/O/V F 3 1 TRANSM/Tffk T j FREQUENCY as I MOOULAT/ON 3 MON/7'01? S//v WAVE RECEIVER TEST sou/2c:

30 Mogu ,4 77 AMPLIIF/LER me/ e E SOU'PCE GAIN arc/u osco s 5 IND/CA T02 Fig 2 PHASE MODUL ,4 77 ON l I 25 C E/ VER Fig. 5.

a E FREQUENCY 3 MODULATION g TkANs/w/ Ift M00. 10 or H 1.

INVENTOR. Mae/war 6f Ceosar ATTORNEY "themodulated signal;

Patented July 20-, 1948 ICE" 2,445Q70s PHASEZMODULATIQN Murrax-Gr Crosby, Riverhead, Y.-, assignor to Radio a Corporation: of Y Delaware America, a corporation of Application OctoberA, 1944; Se'rial No; 557195 my invention is improved operation of phase modulators.

A more detailed object! ofijthe -channelim which the modulator is operanew m describing myinvention-in detail reference will be made to the attached drawings wherein by rectangle a phase 'modulation essential features-of'myinvention.

' Fig '2 unseat-me a similar mannera receiver mitter o-Fig;- 1.- Ffgiiiil-lustratesthebharact'er of the phase modulation transmitterof' unit I '0 f Fig.1 1.

'Trieinventioxris "applicable to, true phase modu 739. In this. phase modulator the modulation is modlfiedgni accordance ,with" its frequency and Usedifio modulate the, frequency oft an oscillation @Jcia lnrjthgidescription which .follOwsI have .used as an exampleah'phase modulatorpfthe corrected frequency modulation type; To those skilled-in the r f the signal-i-to noise ratio the; use ofi pte=emphasissandi (1e:

I emphasis, reference-is made to my article entitied; The service range of frequency; modular tion;'p ublished in the January 1940 issue oi RCA Review. I I

The improvement depends entirely onthe: nature of the modulation;

quency deviation width. An arrangement as shown in Fig. 1 was used.

In indicates aphase modulation transmitter such as. disclosed in the patents listed 20 1s radiating means tions of about a single frequency and ofafrebe observedn The-hand width of the phase modulated wave emitted by M is determined by the effective frequency deviation ofthat wave. For sinusoidal. modulation, the efiective frequency deviation. Ed, is. given by:

Where e is the phase deviation in radians and Fmis-ithe frequency of the modulating wave. '110 determine-the bandwidth for the test tone. the maxs quency. Hence imum value of Fm is substituted in (1). For instance, if Fm (max) were 15,000 cycles, and =5, Fld='l5,000 cycles. Hence the band width for the phase modulated wave resulting from use of the test tone is 150,000 cycles. As described in my article, Communication by phase modulation, Proc. 1. R. E., February, 1939, the signaltO-noise ratio gain for the system of this example is equal to the phase deviation in radians or 5.

The degree of frequency deviation applied to ID may be measured on monitor receiver 40 which gives an output proportional to frequency deviation. When the proper degree of modulation is applied, the indication of modulation input me-. ter 33 is noted.

When program modulation is applied to H), I have found that a considerably greater level may be applied across the terminals of the modulator and meter 33 before the proper frequency deviation is indicated at monitor receiver 40. In other words. the level fed to 33 may be emphasized to a degree higher than the normal tone modulation input. My experiments have shown the magnitude of this increase in level to be approximately equal to Fm (max) 1500 for an audio band of 15,000 cycles.

From the results of the experiments, for purposes of determining the effective frequency deviation of the phase modulated wave it may be seen that the effective frequency of the complex program modulating wave is 1500 cycles. Hence in applying the formula, Fd=Fm, the value of Fm is taken as 1500 cycles when program modulation is used. For the case of the example quoted above, a phase deviation of 5 radians would give an effective frequency deviation of 5 times 1500 or 7500 cycles. Thus to increase the phase deviation to produce the required 15,000 cycles frequency deviation of the example, the -phase deviation must be increased 75,000/7500 or times.

This increase in level applied to the phase modulation transmitter represents an additional signal-to-n-oise ratio gain over and above the factor equal to the phase deviation in radians. The total gain is thus X Fm (m n 1500 The additional gain is brought about by the fact that program modulation is so distributed in the audio spectrum that it produces a lowered eifective frequency deviation when applied to a phase modulation transmitter. This can be seen by a reference to Fig. 3 which shows a phase modulation transmitter of the corrected frequencymodulation type as described in my U. S, Patent #2985139. It will be noted that this phase modulator is a frequency modulator with an input network which raises the amplitude of the modulation frequencies in accordance with their frethe effective frequency deviation of a phase modulated wave is proportional to the modulating frequency. When a program wave is applied to the equalizing network, the energy in the wave is reduced since most of the energy is in the lower audio frequencies. This reduces the effective frequency deviation of the phase modulated program wave. As a result, additional modulation may be applied to the phase modulator input to bring band width of the wave up to the normal band width of the phase modulation receiver.

In accordance with my invention then the test tone of 15,000 cycles per second from 22 is appre-emphasis,

modulation material amplitude program wave. The crest factor is plied as modulation to the transmitter in through switch T, and the tone amplitude brought up for full modulation of the carrier, as indicated on the oscilloscope 2. The reading on the meter V for full modulation as denoted on the target of the oscilloscope is reversed and the program material from source 28 is'applie'd to the terminals of the modulator HI and across meter 33. usual FM broadcast systems and operation known heretofore, the is made such that the peak volt meter V is the same taken when tone from the source 22 was applied. If meter V is theroot-mean square type of meter, consideration has to be given to the crest factor of the from 6 to 10 decibels so in the usual FM systems not, using program input is made 6 to 10 than tone input as read on an R.- This adjustment gives equal peak voltages of the tone and program input.

In accordance with my invention, however, the modulation material applied has its amplitude boosted, by say a variable gain amplifier in 30, so that the peak voltage at the modulator input and across meter V not only equals the reading taken as described above when tone was applied, but exceeds the said meter 33 reading by a factor the reading on as the reading described above,

decibels lower M.-S. meter.

Fm (max) In a particular example investigated Fm was 15,000 cycles per. second, giving an improvement factor of 10 times. This resulted in considerable improvement in signal-to-noise ratio in the system.

The modulation meter at 40 and 42 may be of any approved type, and will not be described in detail herein. Receiver 40 consists of a conventional frequency modulation receiver which gives a known output for a given frequency deviation applied at the input. This known output is read on oscilloscope 42, which has its deflection calibrated in frequency deviation. The receiver may be calibrated by the use of a frequency modulated generator. The frequency modulated generator may be calibrated by the method described in my U. S. Patent #2,293,022, issued August 11, I942.

The phase modulation receiver of Fig. 2 may be of any approved type, but is preferably of the type disclosed in my U. S. Patent #2229540, dated January 28, 1941, or Patent #2230331, dated February 4, 1941, or in Patent #2330232, dated February 4, 1941.

In actual operation there is room for considerablevariation in the factor. The complex program wave is exceedingly variable and judgment is required to determine its peak value. As an example, we have the disagreement as to the crest factor of program modulation--some broadcast engineers use 6 decibels and others use as high as 10. Hence a quantity which depends'upon the peak voltage of the program wave, as 'Fm (max) /1500 does, can not be precisely designated. Someone elses judgment might choose the values to make the quantity Fm (max) 1000 or Fm (max) /2000.

Iclaim:

1. In combination, an angular modulator including a circuit wherein wave energy of carrier wave frequency flows and a source of modulating currents representin a program arranged to be coupled with said circuit to modulate'the-anance with the currents, a source of oscillations,

of substantially single frequency, arranged to be coupled to said circuit to modulate the angular velocity of the wave energy in accordance with the oscillations, means for indicating the amplitude of the oscillations required to accomplish about 100% modulation of the carrier energy by the oscillations, and means for increasing the amplitude of the modulation currents beyond the amplitude of the oscillations when the modulation currents are used to modulate the wave energy.

2. The method of operating a modulation sys" tem having means for modulating the angular velocity of wave energy by program currents which includes these steps, generating oscillations of a substantially single frequency within the program current frequency spectrum, modulating the angular velocity of said wave energy by said oscillations and increasing the degree of modulation to about 100%, then measuring the amplitude of the oscillations, increasing the amplitude of the program currents materially above the amplitude of the oscillations, and modulating the timing of the wave energy by the modulation currents of increased value.

3. The method of operating a phase modulation system having means for modulating the phase of wave energy by program currents which includes these steps, generating oscillations of a substantially single frequency Within the program current frequency spectrum, modulating the phase of said wave energy by said oscillations and increasing the degree of phase modulation of the wave energy by the oscillations to about 100%, producing an indication of the amplitude of the oscillations required to produce said phase modulation, producing an indication of the amplitude of the program currents, increasing the amplitude thereof until said last mentioned indication is materially above the said indication of the amplitude of the oscillations, and modulating the phase of the wave energy by the amplifled program currents.

4. The method of operating a phase modulation system having means for modulating the phase of wave energy by program currents which includes these steps, generating oscillations of a substantially single frequency within the program current frequency spectrum, modulating the phase of said wave energy by said oscillations to about 100% modulation, producing an indication of the amplitude of the oscillations required to produce said about 100% modulation, producing an indication of the program current amplitude and increasing the amplitude of the program currents until said last named indication exceeds said first named indication by a factor of the order of Fm (max) 1500 Where Fm (max) is the program maximum current frequency, and modulating the Wave energy by the modulation currents of increased Value.

5. In combination, a phase modulator including a circuit wherein Wave energy of carrier wave frequency flows and a source of modulating currents representing a program arranged to be coupled with said circuit to modulate the phase of the wave energy in accordance with the currents, a source of oscillations of substantially single frequency, arranged to be coupled to said circuit to modulate the angular velocity of the wave energy in accordance with the oscillations, means for indicating the amplitude of the 0scillations required to accomplish modulation of the carrier energy by the oscillations, and means for increasing the amplitude of the program modulation currents by a factor of the order of Fm (max) 1500 beyond the amplitude of the oscillations when the program modulation currents are used to modulate the wave energy where Fm (max) is the maximum frequency of the program current.

6. The method of improving the signal-to noise ratio of a phase modulation system which transmits a complex modulating wave having a maximum frequency which includes, modulating the angular velocity of a carrier wave by a sinusoidal wave of a frequency about equal to said maximum frequency of said complex modulating wave, modulating the angular velocity of said carrier wave by the complex modulating wave, and controlling the degree of said last angular velocity modulation so that the effective peak frequency deviation of the carrier is made equal to the effective peak frequency deviation of said carrier when the same is being angularly velocity modulated by said sinusoidal wave.

MURRAY G. CROSBY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Crosby Feb. 10, 1942 Number