US2215284A - Frequency modulation signaling system - Google Patents

Description

Sept 17, 1940- E. H. ARMSTRONG 2,215,284

FREQUENCY MODULATION SIGNALING SYSTEM INVENTOR l [dw/f7 hf Army/*ong Sept 17, 1940- E. H, ARMSTRONG 2,215,234

FREQUENCY MODULATION SIGNALING SYSTEM Sept. 17, 1940.

E. H. ARMSTRONG FREQUENCY MODULATION SIGNALING "SYSTEM Filed Feb. 19, 1940 6 Sheets-Sheet 3 inve/ope 0f Moda/017270 [We/ope 0f Defecfof Carre/lf AT TORNE YS Sept. 17, 1940. E. H. ARMSTRONG FREQUENCY MODULATION SIGNALING SYSTEM Filed Feb. 19, 1940 6 Sheets-Sheet 6 Res/afer frequency/f7 C.

INVENTOR,

Patented Sept. 17, 1940 PATENT OFFICE FREQUENCY MoDULA'rIoN sIGNAmNG l y SYSTEM Edwin H. Armstrong, New York, N. Y. Aplplication February 19, 1940, Serial No.1319,569

nrc yJ, 7 1940 s claims. (or. 25o-6) This invention relates to improvements in frequency modulation transmission systems in radio signaling. It has for its object the improvement of the signal to noise ratio and the `improvement of the quality of transmission. It is particularly useful for very high fidelity transmission of music. The invention relates to the introduction of a distorting network at the transmitter and a restoring network at the receiver together with the 10 provision of certain detector characteristics in such manner as to produce a lgreat improvement in the quality of transmission.

Referring now to the figures which form a part of this specification, Fig. 1 illustrates the general 1:, arrangement of the transmitter and Fig. 2 the general arrangement of the receiver. Fig. 3 illustrates the arrangement of a predistorting network used at the transmitter and Fig. 4 a restoring or converse network. Fig. 5 shows the electrical characteristics of these networks. Figs. 6,

7, 8 and 9 are a series of characteristics for thev purpose of explaining certain operations occurring within the system. Fig. 10 illustrates the details of a modulating arrangement and Fig. 11

shows the characteristics of this arrangement. Figs. 12, 13, 14 and 15 illustrate some of the characteristic operations of the receiving system and Figs. 16 and 17 show a simplified form of the predistortion and restoring networks.

'Ihe difficulty which the invention proposes to overcome may be understood from the following explanation. It is well known that to produce realism in the reproduction of music that the total audible frequency range must be trans- 35 mitted and reproduced. This range is from perhaps 30 or 40 cycles per second to 15,000 or 16,000

cycles per second. It is also well known that present radio practice falls far short of this, barely half of this range being transmitted and,

40 for various reasons, less than half of this vrange being reproduced. It is also well known that in addition to failure to transmit the total frequency range that the present amplitude modulation methods introduce harmonics as a result of dis- 45 tortion in certain places in the transmitting and receiving systems. It is for these reasons that the present-day radio sets sound like a radio.

It is also well known that when an attempt is madelto transmit the higher frequencies of the 50 musical range in the ordinary manner, the widening of the band width of the receiver to accommodate the higher frequencies of the transmitted wave results in the admission of much additional noise. It is well known that the dis- 55 tribution of the energy among the various frequency components of the voice or of an orchestra is such that the greater part of it is concentrated in the lower frequency range so that REISSUED the higher frequencies are relatively much weaker Y than the lower ones. As a consequence of this when the transmitted wave is substantially fully modulatedby the lower frequencies,- the depth of 'modulation for the higher frequencies is relatively very small.

are the flrst to be lost in the noise. It is well known that this difficulty can be remedied to a considerable extent by the introduction of a distorting network at the transmitter which is4 so designed that it raises the higher frequencies to substantially the same level as the lower ones so that they may more effectively override the noise and by the introduction of a converse or restoring network at the receiver which recreates the proper relation between the various frequency components of -the transmission. It has been found that when this process is applied to a wide range frequency 'modulation system it produces a much greater gain than in an amplitude modulation system. It, however, greatly accentuates the distortion or creation of harmonics whichhas heretofore been referred to and produces a very unpleasant type of reproduction. It is the purpose of this invention to show how this difficulty may be avoided and the process applied to frequency modulation transmission with an increase in both the fidelity and noise level greater than can be obtained with the process as applied to the ordinary method of transmission.

Referring now to Fig. 1, which illustrates the `general arrangement of the transmitter, I represents the microphone, 2 the usual preamplifier, 3 the predistorter, an amplifier of the speech frequencies, 5 the modulator of a frequency modulation transmission system, 6 a series of frequency multipliers exciting a finalpower amplier stage 1 which in turn' feeds the radiating system 8. The frequency modulation transmitter components 5, 8 and 1 maylbe of the type described in my U. S. Patent #1,941,068. l

Referring now to Fig. 2, I0 represents the receiving antenna, Il an amplifier for thereceived current, and I2, I3 and I4 the usual converter, oscillator and intermediate frequency amplifier of a superheterodyne receiver. I5 represents a limiter for removing amplitude modulation, I6 a filter, and I1 an amplifier for the filter output. The networks I8, 20, 22 and I9, 2|, 23 are for the purpose of converting frequency changes into amplitude changes and the circuit is similar to that described in my U. S. Patent #1,941,069. 24,

Hence the higher ' frequencies 23 and 23, 21 represent detectors arranged across each half of the circuit, and 3l represents a bal-- anced output transformer connecting into the restorer 3l the audio ampliiier 32 and the speaker or other translating device 33.

The circuits of `the predistorter and restorer are illustrated respectively in Fig. 3 and 4. 'I'he transmission characteristics of these two networks are illustrated by curves A and B, respectively, in Fig. 5. As shown by curve A, the frequencies around 15,000 cycles are raised by the predistorter to some ten times the amplitude of the lowest frequencies, and in the restorer as shown by curve B these high frequencies are cut down an exactly similar amount so as to give an overall uniform transmission characteristic for transmitter and receiver. Fig. 6 shows the envelope of modulation as it appears in an amplitude modulation system at the transmitter under normal circumstances, and Fig. 7 shows the same characteristic as modiiied when predistortion is included in the transmission. Fig. 8 illustrates the envelope of the current in the detecting system, and Fig. 9 shows the current voltage curve of the ordinary detector. Fig. 10 shows the detailed circuit arrangement of the modulator 5 of Fig. 1, and Fig. 11 certain oi' its characteristics. Fig. 12 shows the characteristics of the networks I8, 20, 22 and I9, 2|, 23 of Fig. 2 under one condition of adjustment, and Fig. 13 shows the characteristics of two networks having a somewhat different design. Fig. 14 illustrates the comparative noise voltage distribution with respect to frequency in the detector systems of an amplitude and frequency modulation receiver, respectively, and Fig. 15 shows the relative improvements which are obtained when the restorer is applied to the two systems.

It is now in order to consider problem for which this invention furnishes a solution. Referring now vto Fig. 6, there is illustrated the envelope of the modulation of an amplitude modulated transmitter. The curve shows the variation of the low frequency modulating voltage, the rather complex wave form indicating a deep low frequency modulation with a shallow modulation of higher frequency tones. It will be understood, of course, that this curve represents likewise the peak value of the radio frequency currents radiated. It is .essential in amthe particular plitude modulation in order to produce the best signal to noise ratio to operate the transmitter as nearly as possible at full modulation. To accomplish this, the gain of the modulation ampliiier is set so that the depth of the modulation approaches the zero line as closely as possible'without actually touching it. Points A AI in Fig. 6 illustrate this condition.

In actual practice, of course, it is very diiii'- cult to prevent overmodulation even with the most careful supervision, and although the larger or low frequency modulations may be kept from overshooting the zero line the higher frequency ripples will at times inevitably ovexshoot it. This means distortion and the production of harmonics, particularly of the frequencies composing the ripples on the main modulation,.and the value of these may reach relatively high levels withrespect to the levels of the fundamental ripple frequencies. As a consequence of this. a disagreable harsh tinge will be introduced in the sound reproduction. Now when predistortion is inserted in the modulating system so that the high frequency ripples are increased in amplitude relatively to the low frequency ones, this form of and distortion is greatly aggravated. The' general effect is as illustrated in Fig. '1, where points B and B1 show the accentuation of the over modulation, and consequently of the distortion. The curves have been drawn in a way to illustrate cxtreme cases of distortion. It is, however, not necessary to actually overshoot the zero line to produce it. Distortion is likewise encountered when the depth of the modulation is such that nearly complete modulation is produced because the voltage-current curves of vacuum tube ampliiiers o r oscillators are not strictly linear but bend at their lower end, so that the envelope oi the radio frequency current is n ot strictly proportional to the wave form of the modulating voltage. This type of distortion is almost always present, unless relatively low percentages of modulation are used.

At 'the receiving end various .forms of distortion occur. v A certain amount is due to lack of linear amplication but for the moment this may be disregarded in View of a more disturbing form. Fig. 8 shows the envelope of the radio frequency current in the rectier or detector circuit. It will be observed from observation of the points C and C1 that two forms of distortion may occur. One is that due to the lack of linearity of the detector characteristic as shown in Fig. 9, and the .other is actual destruction of the form of the fine ripples by cross modulation with the noise components., The last form may be very bad indeed. In the application of the predistortion method to amplitude modulation it becomes necessary, therefore, to reduce the percentage modulation at the transmitter to prevent distortion. This results in a rise in the noise level so that the net theoretical gain of the system is substantially reduced.

Examining now the sources of distortion at the transmitting end of afrequency modulation system, particularly the phase shifting frequency multiplying type described in my U. S. Patent #1,941,068, it will be found' that distortion due to lack of linearity in tube characteristics can be eliminated. Figs. 10 and l1 show how the distortion due to lack of linearity in the characteristics of the tubes used for modulating may be avoided. Fig. 10 shows a general form of phase shifter which may be used. represents the source of exciting current which is applied to the grid of the carrier amplifier 36 and to the tuned circuit 38 through a phase adjusting network 31, so arranged that the E. M. F.s applied to the grids of 39 and 40 are 90 out of phase from the E. M. F. applied to the grid of 36. The circuit 33 excites the grids of the balanced modulator 39, '40 differentially so that there is no output when no modulation is applied. The screen voltage is modulated by means f the transformer 4l, the voltage being applied differentially so as to unbalance the modulator. Referring now to Fig. 1l, which shows the relation between the screen voltage and the radio frequency voltage output, assume the normal voltage applied to the screens by the battery D to be O A change in the voltage of the screens will/according to the invention described in thq: #1,941,068 patent, add a voltage NS or NT at 90 phase displacement to the output voltage MN of the tube 36. The maximum required is about of the voltage MN to give the shift. Therefore, by properly designing the relative amplications of the tubes 36 and 39, 40it is possible to produce unbalanced voltages of such maximum usable phase distortion due to lack of tube linearity is, en-

countered either without or with predistortion since there is nol aero anywhere near the part of the curve which is utilized. Hence, in the modulating system of the frequency modulation transmitter described it possible to eliminate compietely thaty type of distortion described in the amplitude modulation case.

In the receiving system it is customary to make use of a. balanced detector system as shown in Fig. The reactance characteristics of the two detector branches are shown, respectively, in Fig. 12 arranged to produce 100% modulation over the range Fi-Fz. It will be observed that one branch is arranged to be non-reactive at F1 and the other to be non-reactive at F2, the extremes of the frequency swing. In this way the changes in frequency are converted completely into changes in amplitude.

It has been found that when this type of conversion system is used with a transmission which employs a predistorted output that bad distortion is encountered. This may be prevented as in amplitude modulation by a reduction of the percentage of modulation at the transmitter, with the resultant loss in the signal to noise ratio. However, I nd that the transmitter adjustment may be left unchanged, the distortion removed, and the signal to noise ratio left unimpaired provided a characteristic as illustrated in Fig. 13 is employed. By designing the characteristics of the two branches so that the non-reactive points come at F3 and F4 which are outside the range of the frequency swing F1 F2 which is being used, complete freedom from distortion and cross modulation with noise components is secured. This is accomplished Without any loss in the signal to noise ratio provided the noise is not greater than 50% of the signal. It is particularly valuable to be able to realize the full gain of predistortion because it is much more effective on the high frequency components in frequency modulation signaling than it is on amplitude modulation. The reason for this will appear from an examination vof the noise characteristics of Fig. 14 which shows the relative voltage distribution of the noise components in frequency modulation and amplitude modulation reception. It will be observed that the noise distribution, which in the present case for purposesl of illustration is assumed to be that due solely to thermal agitation and shot effect is uniformly distributed over the audible range while that in the frequency modulation system increases linearly from zero at zero frequency to the same value as the amplitude modulated receiver at the upper end of the band. Hence a restorer having such a characteristic as that of Fig. 4 will produce a very muchl greater reduction in the noise in the frequency modulation system than in the amplitude modulation one. The relative noise voltage-frequency characteristics which a. restorer having the characteristics of Fig. 4 willgive when applied to both systems is shown by the curves of Fig. 15. It will be observed that in the range of good audibility a large improvement=in the Ireduction of the noise voltage is obtained. As the noise produced is proportional to the energy involved, or the square of these values, the importance of the improvement is apparent. While only one form of noise h as so far been considered,` namely, tube and circuit noise, the general result is the same for all sorts ofr disturbances such as ignition noise, commutatin'g machinery noise, X-ray 'machines and the like. In practice on all these types of noise it has been found that very subt stantialv aural improvement is realized.

It will be observed that the networks shown in Fig. 3 and 4 are somewhat complicated. While this is of no importance at a. transmitting sta.- tion, it does become of importance when a large number of receivers are to be constructed as the element of cost is quite important. Figs. 16 and l? show farms of networks which produce substantially the same characteristics as those shown in Figs. 3 and 4. In the predistorter while'a relatively large range of values may be chosen from, I find the following set to be pracf tical and inexpensive.

The capacity 44 has a value of .001 'nui'."rofared the resistance l5 is '15,000 ohms. The input resistance 46 should be 300 ohms and the output resistance 41 should be 30007ohms.

In the restorer shown in Fig. l, 48 represents the detector output impedance and may be of the order of 100,000 ohms. 49 is a resistance of 50,- 000 ohms and 50 a capacity of .001 microfarad.

The volume control potentiometer 5I should have sists in amplifying'the high frequencies of the band to a substantially greater degree than the low frequencies thereof, varying the frequency of plitude are minimized, and amplifying the lowy frequency currents of said band to a substantially greaterdegree than the high frequency currents thereof.

2. In a system for transmitting and reproducing a band of signaling currents, in combination, means arranged to amplify the high frequency currents of the band to a substantially greater degree than the low frequency currents thereof, means for generating a carrier wave of substantially constant frequency, means Vfor causing the amplified currents to vary over a wide range the frequency of the carrier wave, means for transmitting such wave, means for receiving the wave and amplifying the received currents, means coupled to said last-named means for converting the frequency variations into a. band of currents of variable amplitude and having 'an admittance band width substantially greater than the width of the said wide' range of carrier frequencies, a detectingdevicecoupled to said converting means 10 mized.

3. The combination as set forth in claim 2 ,in which the means for converting the frequency variations into currens of variable ampliiudr` comprises two parallelpatilsyone of said paths including a. circuit, having a resonant frequency lower than the lowest frequency oi the transmitted band of frequencies and the other of said paths including a circuit havingv a resonant, fxe- Y quency higher than the highest,

frequency of the transmitted ban.

EDWIN H. ARMSTRONG.

EPI

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