US2347398A - Modulation system - Google Patents

Description

April 25, 1944 M. G. CROSBY MODULATIN SYSTEM Filed May l, 1942 2 Sheets-Sheet l n mf. T

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ATTORNEY Patente'liApr. 25, 1944 2,341.3 monum'nos srs'rsu Murray G. Crosby, Riverhead, N. Y.. assignor to Badin Corporation of America, a corporation of Delaware Application May 1, 1942, Serial No. 441,323 9 Claims. (Cl. 179-1715) In this application a new and improved method and means for producing amplitude modulation. phase modulation, or carrier-eliminated amplitude modulation is disclosed.

In my improved method and means a carriereliminated amplitude modulated wave *is produced by frequency modulating a wave which is then passed through a circuit having zero output at the carrier frequency. In this circuit the carrier is reduced substantially to zero and the side bands passed. This carrier-eliminated amplitude modulated wave may be used as such or, if preferred, carrier may be added thereto with proper phase to -produce a resultant wave modulated in amplitude or in phase. As is well known, the side bands and carrier in a phase modulated wave are relatively displaced and phased 90 with respect to the same relationship in an amplitude modulated wave.

The principle used in this invention is similar to that employed in my Wave modulator disclosed in my U. S. Patent No. 2,248,745 and in my Frequency modulation receiver disclosed in Crosby application #434,509, filed March 13, 1942.

, In describing my invention in detail reference will be made to the attached drawings wherein Fig. 1 illustrates by block diagram the essential features of a system arranged in accordance with my invention, While Fig. 2 shows somewhat schematically the essential features of the circuit connections of an improved modulator system arranged in accordance with my invention.

Referring to Fig. l the modulation input which may represent voice signals, telesraphy signals, television signals, fascimile signals, etc. `appears at jack I and is applied from the jack to a frequency modulation system in unit I4 to produce a frequency modulated carrier. The unit I4 is assumed to include any approved frequency modulation system including a source of wave energy of carrier frequency and means for modulating the wave length ofthe said carrier wave energy. 'I'he frequency modulated wave is fed to unit Il which includes a mixing tube wherein it is heterodyned with oscillations of substantially constant sponding amplitude modulation so that the output of 24 is wave energy In the form of side bands modulated in amplitude and frequency. This combination of amplitude and frequency modulation ls fed to unit which is also fed the original modulated carrier from unit I4. In unit Il the amplitude and frequency modulated output of 24 is heterodyned back to the carrier frequencyy by beating against the frequency modulated Wave from I4 so that a constant frequency output is obtained in lines 34 which has the frequency of the carrier source 20.

In the heterodyning process carried out in converter 30 the frequency modulations on the wave cancel out, while the carrier is tuned or ltered out with progressively lesser alternation as the' modulation are selted and made available for frequency, from a stable wave carrier source 2li, i

and in which the difference frequency, which is an intermediate frequency, is modulated as to wave length in a manner corresponding to the modulations applied in unit I4. This difference frequency, or intermediate frequency, is passed from converter i8 through lines I 9 to a series tuned circuit in unit 24 which gives zero or substantially zero output at the carrier frequency (mean intermediate frequency). 'I'he characteristic of this series tuned circuit is substantially as shown in unit 24 of Fig. 1.

As is well known the means in V24, by dropping out the carrier, converts the wave length modulations on the wave energy passed into correutilization at lines I4. The modulation on this constant frequency wave is carrier-eliminated amplitude modulation which consists of the amplitude modulation side bands only.

This output may be used directly from lines 34, if such modulation is desired, but if phase or amplitude modulation is desired, the side bands may be combined in unit 40 with the carrier from unit 20 with the proper phase relations as adjusted in unit 44. Where phase modulation output is desired from 40, the new carrier supplied from 20 through phase adjuster 40 is out of phase substantially with respect to the phase position it would have for the case of amplitude modulation. The combining tubes in stage 40 are biased and adjusted for linear, class A amplifier operation.

Insetting forth the operation of the system. reference will be made to Fig. 1 wherein is shown by block diagram the essential components of the system. The carrier source 20 feeds converter I8 which is also fed by frequency modulated oscillator I4 to produce a heterodyne output in lines I9. This output may be the sum or difference of the currents from 20 and I4. This heterodyne output is fed through a circuit in 24 which has an amplitude characteristic in accordance with the characteristic drawn in that block. Such a characteristic gives zero output at carrier frequency and rises linearly on either side. Thus as the` frequency is modulated either side of the carrier frequency the output rises linearly. This superimposes carrier-eliminated amplitude modulation upon the frequency modulated wave which is passed through the circuit.' 'I'his frequency modulated wave is fed to a converter in ll where it is heterodyncd with the original frequency modulation wave from I4 so that a constantfrequency beat note of the difference or sum frequencies is obtained for the heterodyned output. The carrier-eliminated amplitude modulation remains and the carrier frequency has the frequency of carrier source 20. The output of converter 30 may be utilized directly if a carriereliminated amplitude modulation is desired.

If amplitude modulation with the carrier present is desired, the carrier from 20 is recombined by means of the carrier and side-band combiner 40 which is fed by the carrier and side bands. Phase adjuster 44 adjusts the phase of the carrier with respect to the side bands so that the proper relation for amplitude modulation is obtained.

If phase modulation is desired, phase adjuster 44 is set to introduce a 90 shift with respect to the setting for amplitude modulation. This follows from the well-known fact that phase modulation may be considered as amplitude modulation with the phase of the carrier shifted 90.

In Fig. 2 I have shown circuit connections of the means shown by block diagram in Fig. 1. Modulation of the desired type is applied at Jack and impressed by the transformer on an electrode of the reactance tube |0|. Condenser C and resistance R. form the phase shifter for the reactance tube |0|. The internal impedance between the anode |00 and cathode of the reactance tube is shunted across the tuned tank circuit |01 or a part thereof. The tuned tank circuit |01 is the frequency determining circuit of an oscillation generator tube |02 which is frequency modulated by the reactance tube |03 under the influence of the control potentials at |00 in well-known manner.

This reactance tube frequency modulated is of the type disclosed in many of my prior applications. See, for example, U. S. Patent No.. 2,279,659, dated April 14, 1942. The operation of this reactance tube modulator, therefore, is well-known in this art and a detailed description thereof at this point isbelieved unnecessary.

This frequency modulated wave is fed by lines ||0 and H2 to grids IIS and H8 of the two converter tubes and |24. A carrier wave generator tube |36 with crystal |40 generates a wave of nearly fixed frequency and this wave is fed by blocking condenser |3| to the grid'l44 of converter tube |24 to produce a frequency modulated intermediate frequency in the output of converter |24. This intermediate frequency output is fed from the anode |50 of tube |24 through the rejection circuit consisting of tuned circuit |52 and |54 which has the characteristics shown in unit 24 in Fig. l. To obtain this characteristic condenser |52 and lnductance |54 are series tuned to the mean frequency of the frequency modulated intermediate frequency (sum or difference) output from |24. The resulting carrierelimi'nated amplitude modulated, frequency modulated wave is then fed by lines |55 to the grid |58 of tube |20 and therein heterodyned with the original frequency modulated wave fed from the tank circuit |01 to the grid ||6 of tube |20 to produce a (difference or sum frequency) constant frequency amplitude modulated wave output in tube |20. This output which appears in parallel tuned circuit |60 may be utilized directly if carrier-eliminated amplitude modulation is desired. In this case the output is supplied by blocking condenser |2| to additional ampliers and radiating means or translating means of any type.

If phase or amplitude modulation is desired, the side bands are combined with a new carrier in tube |10. The new carrier may be derived from the constant frequency source including tube |38. The side bands are fed by lead |1| to the grid |14 of tube |10. 'I'he carrier is fed from tube |36 to the grid |18 of this tube |10 through a phase shifter consisting of a push-pull secondary winding, in the tuned circuit |11, coupled to the oscillator tank circuit |15 and to a variable resistor |18 and condenser |19. 'I'he phase adjustment is made by adJ'usting variable resistance |18 to a point such that the carrier is in phase with the resultant of .the side bands when amplitude modulation with a carrier output is desired, or is 90 out of phase with respect to the resultant of the side bands when phase modulation output is desired. The phase modulation or amplitude modulation output is taken from the tuned output circuit through blocking condenser IBI.

The operation of the oscillator |05 and the mixer stages including tubes |20 and |24 is well known in the art and it is believed detailed description thereof and of the various biasing circuits, plate supply leads, chokes, etc., is unnecessary. It is noted, however, that tube |10 is adjusted and biased to operate as a linear class A amplifier.

I claim:

l. In a signalling system, a source of wave energy the wave length of which is controlled in accordance with control potentials and apparatus for deriving therefrom side band energy correspondingly modulated in amplitude only including a source of oscillations of substantially constant frequency, a mixer stage coupled to said first named source and to said second named source, a filter circuit coupled to said mixer stage, said filter having a characteristic such that its output for wave energy of a mean frequency` equal to the beat of the frequencies of said second and ilrst mentioned sources is substantially zero and increases for frequencies which depart in either direction from said mean frequency, and a second mixer stage coupled to said filter and to said rst named source.

2. In a signalling system, a source of wave energy the wave length of which is controlled in accordance with control potentials and apparatus for deriving therefrom energy correspondingly modulated in phase or in amplitude including a source of oscillations of substantially constant frequency, a mixer stage coupled to said first named source and said second named source, a filter circuit coupled to said mixer stage, said lter circuit having a characteristic such that its output for wave energy of a mean frequency equal to the beat of the frequencies of said second and rst mentioned sources is substantially zero and increases for frequencies which depart in either direction from said mean frequency, a second mixer stage coupled to said first source and to said filter circuit, and a combining stage coupled to said second mixer stage and to said second source.

3.` A system as recited in claim 2 wherein a phase adjuster is included in said coupling between said combining stage and said second source.

4. In apparatus of the class described, a timing modulated oscillation generator and means for varyingthe timing of the oscillations generated in accordance with signals, a source of oscillations of substantially nxed frequency, an electron discharge mixing tube having input electrodes and an output electrode, a coupling between an inputv electrode of said mixing tube and said timing modulated oscillation generator, a coupling between said source of oscillations of substantially xed frequency and an input electrode of said mixing tube whereby timing modulated oscillations and oscillations of substantially xed frequency are impressed on said input electrodes and heterodyned in said tube, a circuit series tuned to a beat note obtained by mixing the timing modulated oscillations and the oscillations of substantially fixed frequency coupled with the output electrode of said mixing tube, an electron discharge device having input electrodes and an output electrode, a coupling between an input electrode of said device and said timing modulated oscillation generator, a coupling between an input electrode of said discharge device and said series circuit, and a circuit coupled with the output electrodes of said device for selecting therefrom amplitude modulated energy wherefrom the carrier has been substantially suppressed.

5. In apparatus of the class described, a timing modulated oscillation generator and means for varying the timing of the oscillations generated in accordance with signals, a source of oscillations of substantially x'ed frequency, -an electron discharge mixing tube having a plurality of control electrodes and an output electrode, a coupling between one control electrode of said mixing tube and said timing modulated oscillation generator, a coupling between said source of oscillations of substantially xed frequency and another control electrode of said mixing tube whereby timing modulated oscillations and oscillations of substantially fixed frequency are impressed on said control electrodes and heterodyned in said tube, a circuit seriestuned to a beat note obtained by mixing the timing modulated oscillations and the oscillations of substantially xed frequency coupled with the output electrode of said mixing tube, an electron discharge device having control grid electrodes and an output electrode, a coupling between a control electrode of said device and said timing modulated oscillation generator, a coupling between another control electrode of said discharge device and said series circuit, and a circuit coupled with the output electrode of said device for selecting therefrom amplitude modulated energy wherefrom the carrier has been substantially suppressed.

6. In apparatus of the class described. a timing modulated oscillation generator and means for' varying the timing of the oscillations generated in accordance with signals, a source of oscillations of substantially constant frequency, an electron discharge mixing tube having input electrodes and an output electrode, a coupling between an input electrode of said mixing tube and said timing modulated oscillation generator, a coupling between said source of oscillations of substantially constant frequency and an input electrode of said mixing tube whereby timing modulated oscillations and oscillations of substantially constant frequency are impressed on said input electrodes and heterodyned in salu tube, a circuit series tuned to a. beat note obtained by mixing the timing modulated oscillations and the oscillations of substantially constant frequency coupled with the output electrode of said mixing tube, an electron discharge device having input electrodes and an output electrode, a coupling between an input electrode of said device'and said timing modulated oscillation generator, a coupling between an input electrode of said device and said series circuit, whereby timing modulated oscillations from said generator and wave energy from the output of said mixing tube are impressed on the input electrodes of said device and beat frequency energyappears on said output electrode of said device, an additional combining tube having input electrodes and output electrodes, a coupling between said source of oscillation of substantially constant frequency and the input electrodes of said additional combining tube, a coupling between the input electrodes of said additional combining tube and the output electrode of said device, and connections for deriving from the output electrodes of said additional combining tube signal modulated energy.

7. Apparatus as recited in claim 6 wherein a phase adjusting network is included in the coupling between the source of oscillations of substantially constant frequency and the input electrodes of the additional combining tube.

8. In apparatus of the class described, a timing modulated oscillation generator and means for varying the timing of the oscillations generated in accordance with signals, a source of oscillations of substantially fixed frequency,van electron discharge mixing tube having a plurality of control electrodes and an output electrode, a coupling between a control electrode of said mixing tube and said timing modulated oscillation generator, a coupling between said source of oscillations of substantially constant frequency and a control electrode of said mixing tube whereby timing modulated oscillations and oscillations of substantially fixed frequency are impressed on said control electrodes and heterodyned in said tube, a circuit series tuned to a beat note obtained by mixing the timing modulated oscillations and the oscillations of substantially fixed frequency coupled with the output electrode of .said mixing tube, an electron discharge device having control grid electrodes and an output electrode, a coupling between a. control electrode oi said device and said timing modulated oscillation generator, a coupling between a control electrode of said discharge device and said series circuit, whereby timing modulated oscillations from said generator and wave energy from the output of said mixing tube are impressed on the control electrodes of said device and beat frequency energy appears on said output electrode of said device, an additional combining tube having two control electrodes and output electrodes, a coupling between said oscillation generator of substantially fixed frequency and a control electrode of said additional combining tube, a coupling between a control electrode of said additional combining tube and the output electrode of said device, and connections for deriving from the output electrodes of said additional combining tube signal modulated energy.

9. Apparatus as recited in claim 8 wherein a phase adjusting network is included in the coupling between the source of oscillations of substantially constant frequency and the input electrodes of the additional combining tube.

MURRAY o. CROSBY.

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