US2269126A

US2269126A - Frequency modulation monitoring

Info

Publication number: US2269126A
Application number: US373552A
Authority: US
Inventors: Roger J Pieracci
Original assignee: Individual
Current assignee: Individual
Priority date: 1941-01-08
Filing date: 1941-01-08
Publication date: 1942-01-06
Anticipated expiration: 1959-01-06

Description

3 Sheets-Shet 1 Filed Jan; a, .1941

SSQ Q `Jaa.; n. 6, 1942. R. J. PIERACCI FREQUENGY MODULTION MONITORING Filed Jan. 8, 1941 5 she'elts-svheet 2 INVENTOR ATTORNEYS Jan. 6, 1942. R. J. PIERAccl FREQUENCY MODULATION MONITORING Filed Jan. s, 1941 `s lsheets-sneu s Y( Rage/J Pemcc/ BY ATTO/wf: Ys

Patented Jan. 6, 1942 UNITED STATES PATENT OFFICE 2,269,126 l FREQUENCY MoDULATloN MONITORING Roger J. Pieracci, Cedar Rapids, Iowa Application January 8, 1941, Serial No. 373,552

8 Claims.

This invention relates to a method of monitoring frequency modulation transmissions in radio signaling. It has for its object the provision of means whereby the instantaneous frequency deviation of the transmitted wave may be visually observed so that a continuous watch on the performance of the transmitter may be malntained. Other objects of the invention will appear hereinafter.

Referring now to the gures which form a part of this specification, Fig. I and Fig. II show graphically the components of afrequency modulated wave in terms of the amplitude and the distribution of the side bands in the spectrum for various conditions of modulation. Fig. III shows schematically the arrangement of the elments.

of the invention for directly observing in an oscilloscope the components of the frequency modulated wave as illustrated in Figures I and II.

The theory of operation of the monitoring system will be understood from the following explanation.

It is well known that it can be shown mathematically that a frequency modulated wave consists of a carrier and an infinite number of side frequencies. The expression for a frequency modulated wave is e=E1 sin (wt-i-mp sin but).

where w=21r times the carrier frequency Fc. l

p=21r times the modulating frequency Fm.

`mp=ratio of the deviation in frequency to the modulating frequency.

E1=amplitude of the unmodulated carrier wave.

the component frequencies for a few specific' cases. It will be observed that in each case the frequency modulated wave consists of the carrier and aseries of side frequencies each spaced from the carrier and from each other by an amount equal to the modulating frequency. The

. by the reactance tube modulator 6.

For the modulating frequencies withl diagrams of Figures I and II are self explanatory. The arrangement of Figure III shows the means by which the components illustrated in Figures I and l1 may be viewed directly on an oscilloscope screen.

' Referring now to Figure III, I represents the input to the monitoring system from the transmission which is to be observed. 2 represents a converter or first detector of a superheterodyne receiving system. 3 represents an I. F. amplifier of 2 mgc. and 4 an amplitude modulation detector. The output of the detector is connected across the vertical plates 8-9 of an oscilloscope Il having a screen I2, as shown. The converter 2, in addition to the signaling currentwhich is to be observed and which for this example is taken as a 40 mgc.i75 k.c. deviation frequency modulated current is likewise supplied with current from a 38 megacycle oscillator shown at 5. This oscillator is frequency modulated thru k.c.

V The reactance tube modulator is controlled by a 30 cycle linear sweep voltage from a source 1, as shown, which voltage is also applied to the horizontal deflection plates Ill- II of the oscilloscope 0 by the leads shown.

'Ihe operation of the system will be understood from the following description with reference to.

Figure III.

'I'he FM signal (E1) to be monitored is applied from source I to the input circuit of the converter 2. The pass band of this circuit is sufficiently broad to accommodate without attenuation, a band equal to or wider than that occupied by the carrier and all of its side bands. A locally generated FM signal (Ez) is injected into conand below the center frequency +100 k.c. or more by the application of a saw 'tooth wave form voltage to the grid of the reactance tube modulator 6, generated by the linear sweep voltage source 1. This excursion above andy below the center frequency takes place at a rate approximatelyA 30 cycles per second.

Assume the incoming FM signal (E1) is unmodulated and consists of only a carrier or center frequency. As the local oscillator voltage (Ez) passes through its center frequency, the

mixture of E1 and E2 produces a voltage at the intermediateA frequency, since it will be recalled that the difference of the center frequencies of E1 and E2 is equal to the intermediate frequency. The output of the intermediate frequency amplier tube will be a pulse of Voltage whose` maximum value will occur at the instant the frequency difference between E1 and E2 is exactly equal to the intermediate frequency. The voltage will drop s-harply about this point, when the frequency difference between E1 and E2 is slightly 'greater or less than the intermediate frequency. The sharpness of the pulse is dependent on the selectivity of the intermediate frequency transformer (which may bea few k. c. wide) and the rate at which the local oscillator frequency is swept about its center frequency. It can be seen from the foregoing explanation, that if a curve of IF output voltage vs. oscillator frequency (E2) were plotted, that a pulse similar to a s-teep resonance curve would result in the region of i5 k. c. of the center frequency, while the region from i5 k. c. to i100 k. c. would show no output.

The sharp pulse of radio frequency voltage c from the IF amplifier 3 is applied to the second detector 4. This is a half wave rectifier which eliminates the negative portion of the cycle, and delivers a unidirectional pulse whose shape is the envelope of the individual RF cycles received from the IF amplifier. This sharp inverted V pulse is applied to the vertical plates 8 9 of the oscilloscope 0 and is directly proportional to the amplitude of the incoming FM carrier Voltage (E1) It would appear as a very sharp resonance curve on the screen l2 of the oscilloscope.

Assume that the FM signal to be monitored (E1) is now modulated at 175 k. c. more or less, with a sinusoidal audio frequency. From FM sideband theory, individual discreet side bands will be produced above and below the carrier at frequency intervals equal to the audio frequency, and whose amplitudes Vary in accordance with the equation of voltage for the side bands. As the local FM oscillator voltage (E2) makes its frequency excursion above and below its center frequency, voltages at the intermediate frequency are produced each time the frequency difference of the side bands and local oscillator is equal to the intermediate frequency. Proper phasing of the two'voltages is obtained by a slight adjustment of the saw tooth sweep frequency. The production of a sharp unidirectional pulse at the output terminals of the detector 4 for each side band component occurs in the same mannery as described above for the carrier of the unmodulated FM wave. The amplitude of each of these pulses is directly proportional to the amplitude of the corresponding side band component.

summarizing the above description, the pulses are produced by a mixture of two FM waves whose center frequencies differ by the intermediate frequency. When the two FM waves are Ain properphase, .the carrier and side band comvFM signal being monitored. The several 'components are properly spaced on the screen I2 of the oscilloscope, by applying to the horizontal deflection plates I0-|I, the same 30 cycle linear sweep voltage applied to the local FM oscillator.

When the electron beam is at the center of the tube ll, the linear sweep voltage is zero as is the control voltage at the grid of the reactance tube modulator, hence the local FM oscillator 5 is at its center frequency, and the peak value of the carrier occurs at this point on the screen. As the electron beam moves to the right or left of center, the frequency of the local FM oscillator follows, and the vertical beam deflections produced by the pulses representing the spectrum, appear on the screen l2 in exact relation of frequency spacing with respect to each other.

I have described what I believe to be the best embodiments of my invention. I do not wish, however, to be confined to the embodiments shown; but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

1. The method of visually indicating the several frequency components of a frequency modulated wave which comprises the steps of generating a second wave having a mid-frequency differing from that of the frequency modulated wave by a given intermediate frequency, automatically varying the frequency of the second wave above and below its mid-frequency in a substantially linear manner, beating the second wave of varying frequency with the frequency modulated Wave to produce a series of beat frequencies spaced apart in accordance with the frequency components of the frequency modulated wave and energizing a visual indicating device by energy derived from the said series of beat frequencies.

2. The method of visually indicating the several frequency components of a frequency modulated wave which comprises the steps of generating a second wave having a mid-frequency differing from that of the frequency modulated wave by a given intermediate frequency, automatically varying the frequency of the second wave above and below its mid-frequency in a substantially linear manner by an amount at least equal to the frequency deviation of the frequency modulated wave, beating the second wave of varying frequency with the frequency modulated wave to produce a Series of beat frequencies spaced apart in accordance with the frequency -components of the frequency modulated wave, and controlling the ray of an oscilloscope by energy derived from the said series of beat frequencies.

3. The method of visually indicating the several frequency components of a given frequency modulated current which comprises the steps of generating a 'second current having a mid-frequency differing from that of the given current by a desired intermediate frequency, automatically varying the frequency of the second current above and below its mid-frequency, beating the second current of varying frequency with the frequency modulated current to produce a third current having frequency components spaced apart in accordance with the frequency components of the frequency modulated wave, detecting the individual frequency components of the third current to derive the modulation component thereof and energizing a visual indicating device by means of the detected modulation component.

4. Means for visually indicating the several frequency components of a given frequency modulated current comprising, in combination, means for generating a second radio frequency current,

Vdicatng device comprises control means for automatically varying the frecurrentl with the given frequency modulated current to derive a third current of predetermined intermediate frequency, a detecting device con-v nected to said last named means and having an output circuit'arranged to pass a current corresponding to the modulation envelope of the intermediate frequency current and a visual indicating device connected to the output circuit of said detecting device. l

5. A lvisual indicating means as et forth in claim 4 in which the control means for varying the frequency of the second generator arranged to generate Ya wave `of sawtoothed shape.

6. A visual indicating means as set forth in claim ,4 in which the visual indicating device comprises an oscilloscope having a pair of opposed defiectng plates connected to the output circuit of the detecting device.

7. A visual indicating means as set forth in claim 4 in which the control means for varying the frequency of the second current comprises a generator arranged to generate a current of low frequency which increases to a maximum value substantially linearly and in which the visual inan oscilloscope having a first pair of deecting plates connected tothe output circuit of the detectingdevice land a' second pair of deflecting platesdisposed substantially normal to those of the rst pair, and 'connections between vthe plates of the second pair'4 and said low frequency current generator.

' 8. Means for visually indicating the several frequency components of a given frequency modulated current comprising, in combination, an oscillator for generating a second radio frequency current, a reactance tube modulator having terminals connected to said oscillator, a low frequency source of linear sweep voltage connected l to thegrid of said reactance tube and. operative current comprises a to cause the reactance tube to vary the frequency i of the said second current generated by said oscillator over a range of frequencies at least twice as wide as the frequency deviation of the given frequency modulated current, means for combining the secondl current with the given frequency modulated current to derive a third current of predetermined intermediate frequency, a detecting devicel means and operative .to detect the individual frequency components of the thirdy current as they are applied successively to the detecting device, an oscilloscope having a first pair of deflecting plates connected to the output terminals of the detecting device and a second pair of deflecting plates disposed substantially normal to those of the rst pair and connections between the plates of the second pair and the terminals ofA said source of sweep voltage.

ROGER J. PIERACCI.

connected to said last named