US2362201A - Modulation - Google Patents

Description

NOV. 7, 1944. 1 1 HATHAWAY 2,362,201

MODULATION Filed Oct. 17, 1942 2 Sheets-'Sheet l lcjl- A TTO/P/VE Y Nov. .7, 1944. l.L l. HATHAWAY MODULATION Filed Oct. 17, 1942 2 Sheets-Sheet 2 m v Nh u SQ. fa

INVENTOR c/)ffffrr 4m/#WAX 4 770/?NE Y Patented Nov. 7, 1944 MODULATION Jarrett Lewis Hathaway, Manhasset, N. Y., assignor to Radio Corporation of America, 'a corporation of Delaware v application october 17,1942,seria1N0.462,3s1

13 Claims.

This application discloses a new and'improved modulation methodv and means for signalling-by timing modulated waves referred to herein as wave length modulated waves. My improvement involves the use of combined wave length and amplitude modulation in a system used for transmitting wave length modulating signals, and has as an` object the reduction of the noise level ina wave length modulation system. Considered from a different viewpoint, my method and means are for the purpose of improving in a wave length modulation system the signal to noise ratio.

Invdescribing my invention in detail reference willvbe made to the attached drawings wherein Figures 1 and 2 arecurves used in describing one source of noise or distortion in ka wave vlength modulation system and the manner in which the same is caused.

Figure 3 illustrates a Amodulation system arranged in accordance with .my invention wheretude modulation with wide and narrow band systems of frequency modulation, it has been ob- "served that with no modulation, noise levels are very low on both of the frequency modulation .systems compared with the amplitude modulation system. However, it was also noted that noise arose with modulation on the frequency modulation systems, whereas it stayed essentially constant on theamplitude modulation systern.

The result, as measured by a distortion meter,

is that for full level modulatiomless noise is present on the amplitudemodulation system Vthan on either the wide band or narrow band frequency modulation systems.

The noise level increase with deviation in timing modulation systems is, it is believed, due in part at least to the fact that in passing a wave ilengthmodulated wave through selective circuits,

the noise to signal ratio is increased when the wave is swung off carrier because of the rounding 01T of the characteristics of the circuits wherein the selectivity of the system resides. This can vbe seen by inspection of Figure 1, wherein curve A designates roughly the characteristic of the selectivity system used ina wave length modulated transmitter and in particular of such selective circuits used in the receiver of a Wave length modulation system. The selectivity characteristic at A results in an increase in the noise tosignal ratio N/S as the carrier. fc deviates from zero modulation in either direction, as shown by the curve B in Figure 2.

The noise rise observed in reception is also due in part to the improper cancellation of noise when the incoming wave is shifted in frequency from its mean frequency.. That is, the discriminator is normally tuned to the carrier frequency Where noise on each side cancels out, but when modulation occurs the noise is no longer cancelled.

The increasevin noise level or decrease in signal to noise Aratio puts an added burden on any limiter used in the system -atthe transmitter or receiver. At the receiver the burden on the limiter is increased considerably because the signal has fallen offvin strength and the limiter must be designed to operate on less signal and, as a consequence, the limiter at the receiver must be of better design and regulation Aor the gain of the receiver must be appreciably higher.

To cure this defect, I propose to modulate the amplitude of the wave length modulator output in accordance with the modulationpotential so that the output is increased in amplitude substantially .proportionally with the frequency deviation in either direction from zero modulation or carrier frequency. That is, in the presence of no modulation orv carrier frequency the output will be of a given lnominal value. As the modulation is applied, the output amplitude will be increased substantially proportionally as .the

frequency is shifted inf either directionA from the carrier'frequency.A

In my system no special or exact relation .betweenthe amount of amplitude modulation and the wave deviation in accordance with the modulation is required. It is essential however that the amplitude increase with deviation and this increase is substantially proportional to the frequency shift. In a sense them-the amplitude modulation applied to the wave length modulator 4in my system may be also represented by the curve B, Figure 2. Thus I obtain the desired iml provernent of the signall to noise ratio andthe reduction of noise during modulation and especially during high'level modulation. In my system the output at the carrier frequency or zero `modulation may be of lthe .desired nominal value.

As stated above, my system for reducing noise on frequency modulation reception during modulation involves a combination of wave length and amplitude modulation. In Figure 3 I have shown an embodiment of my system comprising a wave length modulator at I including a source of oscillations and a means connected with said source of oscillations and with a source of modulating potentials l2 to accomplish in I0 wave length modulation of the carrier. For example, I may use in I0 a timing modulated transmitter of the Crosby type, such as, for example, illustrated in Crosby U. S. Patent No. 2,279,659, dated April 14, 1942. The transmitter may include frequency multipliers and amplifiers and has an output stage comprising tubes I5 and I8. The input electrodes of the tubes I6 and I8 are supplied With wave length modulated wave energy and the output electrodes thereof are coupled by rier in I0 is deviated in accordance with the modulating potentials. At the same time the modulation is applied by transformer 30 to the grids of 26 and 28, the combined impedance thereof, which shunts resistance 48, is lowered and the direct current potential applied to the anodes of tubes I6 and I8 increases to thereby increase the intensity of the output as the carrier deviates in either direction from zero modulation. The tubes 26 and 28 may be biased close to, or even at, or beyond cut 01T. Since the tubes 26 and 28 are differentially excited by the modutuned circuit to'a utilization means, such as,

an antenna or to lines connected with a utilization means directly or by way of additional amplifiers and frequency multipliers.

The output electrodes of tubes I6 and I8 are supplied direct current by a line 24 connected to the cathodes 21 and 29 of a pair of rectiriers 26 and 28, the anodes of which are tied together and connected to a source of direct current not shown having a negative terminal connected to the cathodes of tubes I6 and I8. The direct current potential circuit for tubes I6 and I8 is from the positive terminal of the source to the anodes of tubes 26 and 28 through the tubes to the cathodes 21 and 29, thence by line 24 to thek anodes of tubes I6 and I8, to the cathodes of tubes I6 and I8 and thence to the negative terminal. of the direct current source. kThe resistance 48 is placed in parallel with the internal impedances of tubes 26 and 28 and acts as an additionalpath for the direct current to reach the anodes of tubes I6 and I8.

'Ihe control grids of tubes 26 and 28 are coupled by transformer 30, or any other suitable phase inversion system known to the art, to the source of modulating potentials I2. Bias for these tubes is supplied by way of resistor 34 from any source, such as, for example, source 36. The tubes 26 and 28 are biased nearly to cut on by way of resistors 34, and the source 36 may be bypassed for potentials of the order of modulating frequency by condenser 4I).

In my system, as stated above, modulation is applied to the wave length modulator I0 in source I2 and the carrier therein is modulated in wave length in a conventional manner. Simultaneously, amplitude modulation is supplied to the transformer 30 and the bias of the tubes 28 and 28 is adjusted so that on peaks, either positive or negative, lthe carrier is increased. The transmitted wave is picked up by a receiver such as shown in Figure 4. This amplitude modulation is then smoothed in the receiver by fast action automatic gain or volume control or by conventional limiters. Thus, the

signal on peak modulation is stronger relative tothe interfering noise than it would be with conventional frequency modulation only.

In the modification of Figure 3, the carrier may be normally adjusted to be low in amplitude. The bias on the grids of tubes 26 and 28 is adjusted to approximately cut off. The bias may be above or below cut off. When the bias is greater than cut oiT the entire anode supply current to tubes I G and I8 is passed thru resistor 48 in the absence of modulation since the impedances of tubes 26 and 28 are innite. As modulation is applied the wave length of the carlating potentials, the grid of one tube or the other swings positive on both the positive and negative cycles of the modulating potentials.

In a. modification shown in Figure 3a, the transformer 30 supplies its output to a tube 5I of the duo-diode-triode type. The modulation potentials are rectified in the diode section and appear across resistor 50 to be impressed on the control grid 52 so that both halves of the modulation cycle are amplified and appear in the transformer 54, the secondary winding of which is in the direct-current-supply circuit for tubes I6 and I8 at points marked Y. The transformer 3l) of Figure 3a has its primary connected to the modulation potential source at points marked X to thereby replace the amplitude modulator of Figure 3. This system, as in Figure 3, increases the carrier amplitude on both -I- and peaks of modulation as the carrier is deviated from zero modulation. The secondary terminals marked Y are connected in the high voltage supply line to tubes I6 and I8 with such polarity that this high voltage is increased during both the positiveand the negative excursions of modulation, which implies a reduction of high voltage at times when the modulating voltage crosses the zero axis. The action here, which is an increase of the potential in the anodes of tubes I6 and I8 as the carrier is swung from no modulation position in either direction, is apparent from the preceding description, it is believed.

.The amplitude modulation may be applied in the transmitter at any suitable point, Whether in the output stage or one of the earlier stages. It may be applied in the anode supply as shown or in the screen grid circuits or in the grid circuits if desired.

What I claim is:

1. In a system wherein the instantaneous frequency of wave energy is swung in proportion to the intensity of signaling current,vthe method of increasing the signal to noise ratio which includes the following steps, increasing the amplitude of the wave energy as th'e frequency thereof deviates from carrier frequency in one direction, increasing the amplitude of the wave energy as the frequency thereof deviates from carrier frequency in the other direction and maintaining the signalling channel width substantially constant during said operation.

2. In a system wherein the instantaneous frequency of wave energy is deviated through a range considerably greater than 90 in accordance with signals, the method of operation which' includes the following steps, increasing the amplitude of the deviated carrer as the same deviates from carrier condition in one direction an amount which grows as the deviation from the carrier condition increases, increasing the amplitude of the deviated carrier as the same deviates from the carrier condition in the other direction an amount which grows as the deviation from the carrier condition increases and maintaining the signalling channel Width substantially constant during said process.

3. The method of demodulating a wave of the nature recited in claim 1 which comprises, limiting off said amplitude changes and demodulating the instantaneous frequency changes.

4. The method of demodulating wave energy of the nature recited in claim 2 which comprises,

limiting off said amplitude changes and demodulating the instantaneous frequency changes.

5. The method of compensating noise and distortion in a wave length modulation system wherein the carrier is deviated about a mean value in accordance with modulating potentials and wherein selectivity of the circuits and discriminator circuit characteristics introduce noise and distortion which increases with deviation of the carrier from the no modulation position which includes the following steps in the `transmitter rectifying the modulation potentials and increasing the amplitude of the wave length modulated wave inaccordance with the rectified potentials in both the positive and negative polarities thereof.

6. The method of signalling which includes the following steps, generating carrier Wave energy the frequency of which is modulated in accordance with signals and increasing the amplitude of said frequency modulated wave substantially in proportion to changes of th'e amplitude of the signals in both directions from a base value to increase the signal to noise ratio during transmission. l

7. A signaling method including with the steps recited in claim 1 the following steps, transmitting said amplitude varied instantaneous frequency modulated Wave, receiving the transmitted Wave, limiting the amplitude of the received wave to remove said amplitude variations an demodulating the limited wave. i

8. In a system wherein the instantaneous frequnecy of wave energy is swung through more than 90 in proportion to the intensity of signalling current, a modulator for increasing the amplitude of the Wave energy in both directions of swing to reduce losses due to circuit selectivity in the transmitter or receiver.

9. In a signalling system, a Wave length modulated Wave generator and transmitter, a wave lengthlmodulated wave receiver including a pickup element and a demodulator, band pass circuits in said system for selecting the generated wave energy and passing the same along, `and means for compensating for distortion introduced in said wave energy by said band pass circuits comprising a modulator for increasing the ampliiication of the wave length modulated Wave energy when deviations thereof approach the limits of said band pass circuits.

10. A method as recited in claim 6 including these' additional steps, picking up said wave energy, limiting off said amplitude variations and demodulating th'e limited wave energy.

11. In a 'signalling system wherein wave energy of carrier Wave frequency has its instantaneous phase shifted in accordance with signals through a range considerably greater than 90, transmitting means associated with said generator and signal receiving and amplifying means, a selective circuit in at least one of said means, said selective circuit being arranged to pass therein the shifted wave energy and means for reducing the effects of the characteristics of said selective circuit on said shifted Wave energy comprising an amplitude modulator through which said Wave energy is passed, and connections in said amplitude modulator for increasing the amplitude of the shifted wave energy substantially in proportion to its instantaneous phase excursions above or below carrier frequency while maintaining the signal channel Width substantially constant.

12. In a wave length modulation system, a wave length modulated transmitter including a source of modulating potentials and carrier wave length modulating means coupled thereto with an output stage therefor comprising an electron discharge amplified tube having output electrodes coupled in an output circuit and connected with' a source of direct current, and means for improving the signal to noise ratio of the radiated energy including a full Wave rectifier having input electrodes coupled to said source of .modulating potentials and having output electrodes, and connections including the impedance between the output electrodes of said rectiiiers in the connections between the 'output electrodes of said amplifier tube and said source of direct current.

13. Inv a wave length modulation system, a wave length modulated transmitter including a source of modulating potentials and carrier wave length modulating means coupled thereto with an output stage therefor comprising an electron discharge amplifier tube having output electrodes coupled in an output circuit, a source of direct current, a full Wave rectifier having input electrodes coupled to said source of modulating potentials and having output electrodes, an ampli-

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