US2395737A - Angle modulation noise squelching system - Google Patents

Description

Feb. 26, 1946., c. w. -l-lANsELl.

ANGLE MODULATION NOISE SQUELCHING SYSTEM .indi/70 INVENTOR.

A Trae/VE Patented Feb. 26, 1946 ANGLE MODULATION NOISE SQUELCHING SYSTEM Clarence W. Hansell, Port Jefferson, N. Y., assign- Vr to Radio Corporation of America, a corporation of Delaware Application August 1l, 1943, Serial No. 498,185

(Cl. Z50-20) 9 Claims.

My present invention relates generally to a noise squelching system for angle modulation receivers, and more particularly to an automatic squelching circuit arrangement for a receiver of angle modulated carrier waves.

One of the main objects of -my present invention is to prevent noise of large volume in the modulation output from a phase or frequency modulation receiver when there is no modulated carrier current being transmitted from the receiver input terminals to the limiter.

The generic expression angle modulated carrier wave used in the description and claims is intended to include frequency modulated and phase modulated carrier waves or hybrids thereof. The term hybrid signifies modulation which possesses characteristics common to both frequency and phase modulation.

Another important object of this invention is to provide a simple and effective method for 0btaining control over a receiver of angle modulated carrier waves so as to eliminate the rush of noise which is usually associated with a decrease of the received modulated waves below a usable intensity level; the method comprising the introduction into the limiter input of just enough locally-produced carrier current to suppress the noise.

Another important object of my invention is to provide in combination with a conventional frequency modulation (FM hereinafter for simplicity) receiver, an auxiliary oscillator whose frequency is spaced from the limiting frequency of the received signal band by a superaudible frequency value; the oscillator energy being iniected into the receiver at a point prior to the limiter input terminals and at such a magnitude that inter-station noise is suppressed, and the effect upon the output of the receiver of the auX- iliary oscillator energy itself being suppressed, just like noise, in response to the amplitude of said signal attaining a predetermined usable intensity level.

A still more specific object of my invention is to provide an automatic squelch system for an FM receiver of the superheterodyne type, wherein there is provided an oscillator operating at a frequency to one side of the intermediate frequency (I. F.) of the receiver, the oscillator being coupled to the input of the limiter and having its energy level about equal to or somewhat greater than the peak values of noise depending upon the percentage of normal current amplitude at which limiting begins, and the oscillator frequency preferably being removed in excess of 10,000 cycles from the extreme edge of the pass bandof the I. F. network thereby to prevent audible beats between the oscillator current and the signal carrier current.

Yet another object of my invention is to kprovide a novel method of operating an FMreceiver and rejecting all signals or noise having amplitudes below the intensity level of an auxiliary carrier wave; the auxiliary Wave being injected into the limiter input thereby to provide a noisesuppressing improvement threshold despite the absence of usable desired signal amplitude.

Still other objects' of my invention are `to improve generally the efficiency and reliability of FM receivers, and more especially to provide a noise-free FM receiver capable of being economically and simply manufactured and vassembled.

The novel features which I believe to be characteristic of my invention are set forthwith particularity in the appended claims; Vthe invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated schematically a circuit organization whereby my invention may be carried into effect.

In the drawing:

Fig. 1 schematically shows an FM receiver embodying the invention,

Fig. 2 depicts qualitatively the amplitude relations between the desired signal band, noise and auxiliary oscillator current at the limiter input. l

Referring now tothe accompanying drawing, the FM receiving system shown in Fig. 1 has its conventional networks schematically represented. Those skilled in the art of radio communication are fully aware of the manner of constructing and operating an FM receiver adapted to operate on the superheterodyne principle. While my invention is not restricted to use in a radio receiver nor to any specific range of frequencies, by way of specific example let it be assumed that the receiver shown in Fig. 1 is adapted to be operat d in the present FM broadcast band of 4.2 to 50 megacycles (ma). In this band each station is permitted an overall maximum frequency swing of kilocycles (kc). The operating I. F. value may be chosen from a range of 4 to 20 mc. While Ian I. F. value of 8.33 mc. is generally found to be a satisfactory compromise for all factors, I have utilized a specific I. F. value of mc. by way of simplified illustration.

The FM wave energy is collected by an antenna, and the latter may be of the dipole type Y if desired. The collected FM wave energy may 150 kc. The converter l acts to reduce the centei` frequency Fe of the amplified FM waves to the lower I. F. value of 5 mc. without changing the frequency deviation or swing. The I. F. energy may be amplified in one or more amplier stages, as indicated schematically at 2. Above the I. F. amplifier rectangle 2 I have represented ideal pass band characteristic of the I. F. amplifiers. It will be noted that there is passed a signal energy band having a width of 150 kc. The mean or center frequency Fc is equal to 5 mc.

The usual amplitude mddulation limiter 3 is employed in order greatly to reduce any amplitude modulation effects which may have developed on the FM Waves in their passage to the input terminals of the limiter. l The limiter may be of any conventional and well known type. Its input-output characteristic is ideally shown above the rectangle 3. Generally speaking, the limiter has a flat output energy level above a predetermined signal input intensity level. Both grid circuit and plate circuit limiting action may be employed in the limiter tube.

The limited FM wave energy with a mean frequency of Fc at the operating I. F. value is applied to the FM detector circuit 4. Any conventional and well known FM detector circuit having the idealized characteristic shown above rectangle 4 may be employed. The detector characteristic relates' frequency of signal input to detector output voltage. As is well known, it is common practice to have a pass band width in excess of the 150 kc, swing at thel detector input circuit. 'I'he discriminator section of the usual FM detector stage acts to translate the FM wave energy into corresponding amplitude modulated wave energy. The latter is rectified in the rectiiier section to provide the modulation signals originally used .to modulate the carrier at the transmitter station. The FM `ment threshold) the audio frequency output of the subsequent audio amplifier is essentially noise voltage. The latter, when reproduced by the reproducer, is very disturbing to the listener. This noisy reception will usually occur during tuning between stations, and causes the familiar interstation rush of noise in an FM receiver. It will, also, occur as a result of deep fading, since in wave energy supplied to the detector 4 is a frequency-variable wave. That is,

this case the signal to noise ratio becomes very low.

The production of noise during periods when the signal to noise ratio is below the improvement threshold may be explained by a consideration of the following phenomena: A limiter is not capable of removing amplitude modulation having a depth of modulation as great as This is true since in the condition of 100% modulation the amplitude of the carrier wave is zero for certain intervals. Hence, to provide complete limiting action the limiter must supply a signal gain which raises the signal level from zero to a nite value. This is, of course, impossible. Consequently, when the condition of 100% modulation occurs at the limiter input, the output energy of the limiter is amplitude modulated regardless of the degree of limiting obtainable by the limiter. The condition of 100% amplitude modulation in the FM Wave energy applied to the limiter input terminals occurs at the improvement threshold. At this threshold point there is a substantial equality of the peak voltage of the noise and that of the incoming carrier. Reference is made to the paper by M. G. Crosby entitled Frequency modulation noise characteristics in the April, 1937, issue of the Proceedings of the Institute of Radio Engineers for a more complete analysis of the improvement threshold characteristics.

At the aforesaid point of equality there is a rcomplete cancellation of the signal by the noise.

'I'his produces intervals of zero signal. In general, my invention contemplates the injection or introduction of an auxiliary carrier into the limiter input terminals at those instants when there exists a condition of equality, or less than equality, of the peak voltage of the signal with respect to the peak voltage of the noise. carrier energy is provided by an auxiliary oscillator 5 whose energy is of a predetermined frequency and of constant predetermined amplitude. The auxiliary oscillations are fed into the receiving system at any desired point between the converter output terminals and the limiter input terminals. Preferably, the auxiliary oscillations are introduced into the receiving system at the input terminals of the limiter network. If the I. F. circuits are not too sharply selective the auxiliary oscillator or squelching current may be introduced at any point ahead of the point of limiting on the weakest signals. A

As stated previously, the general purpose of the present invention is to prevent noise of large volume in the output from the FM receiver whenthere is no carrier current being transmitted from the input terminals of the receiver to the limiter input terminals. My basic method is to introduced just enough locally-produced carrier current, of a frequency other than the desired carrier frequency, to suppress the noise. The locally-produced carrier current is beyond audible beat with a correctly tuned received carrier current, and preferably beyond audible beat with current rof any frequency which is not substantially reduced in amplitude by frequency selectively in the I. F. amplier. The locally-produced carrier current is itself suppressed, just like noise, when a strong enough desired carrier current is received. More specifically,l at the input terminals of the limiter there is injected an auxiliary radio frequency current whose frequency is removed a considerable frequency distance from the desired signal frequency, the auxiliary current having an amplitude about equal to, or somewhat greater than, the peak values of the noise. Accordingly.

The auxiliary when the desired signal carrier disappearsifrom the :input to the limiter the limiterstill will tbe limiting on the current from the auxiliaryradio frequency current source. By this means it is kpossible to secure an automatic noise squelching action.

It should be noted that, to secure complete squelching action, the amplitude of input-energy to the limiter from the auxiliary source minus the peak values of combined noise currents, should be great enough to produce a substantial amplitude limiting in the limiter so that little amplitude modulation of the auxiliary source current will appear in the output of the limiter. In practice, of course, the auxiliary current will simply be adjusted in amplitude until a desired degree of noise squelching is obtained. It is necessary in order that beats between the auxiliary current source and the desired signal carrier may produce no audio output 'from the receiver, that the auxiliary current be removed in frequency by more than, say, at least 10,() cycles from the Aedge of the pass band of the I. F. circuit, depending upon the cut-off characteristics of the audio system of the receiver, taking into account the frequency response characteristics of human ears.

In Fig. l it is shown that to obtain the auxiliary radio frequency current for accomplishing the automatic squelching action there is employed an oscillator tube operating at a frequency located to one side of the limiting edge of the pass band of the I. F. network. A practical advantage of the present invention resides in the fact that for this type of noise squelching there is involved an addition of no more tubes than would be required for other squelch systems used in FM receivers.

In Fig. 2 I have attempted to picture in a purely qualitative and ideal manner the relations which exist between the I. F. signal band, the noise components and the auxiliary oscillator current. The numeral 6 designates the idealized passfband characteristic of the I. F. amplifier network, and it is assumed that the amplitude level of the I. F. signal'band is of a certain predetermined value. The dotted line 'l denotes the peak noise level and also the level at Which limiting takes place in the limiter, which is considerably'below the normal amplitude of the received modulated carrier Wave. The auxiliary oscillator current is represented by an arrow 8 indicated as being approximately twice the noise level line l, and is shown as spaced approximately 10 kc. from the right-hand edge of the I. F. pass band 6.

So long as the amplitude of the I. F. signal within band 6 is sufficiently above the noise level I there will be no noise reproduced at the receiver reproducer, except that produced due to phase modulation beats between the noise currents and the signal current, which is small in magnitude compared With noise output when there is no carrier current present with the noise currents. However, when the amplitude of the I. F. band falls to the improvement threshold point, noise will be reproduced for the reasons given above. The improvement threshold is reached when the amplitude of carrier current in the pass band 6 is approximately equal to, or a little greater than, the noise level 1, it being understood that peak values are being considered and that the average of noise currents is substantially less than the peak values. However, when the signal carrier current falls Amuch below the improvement threshold, or is absent, the auxiliary oscillator current 8 will provide an artificial or simulated carrier which will bring the signal to noise ratio at the limiterinput terminals iabove `the improvement 'threshold point and thus automatically prevent Athe reproduction of noise.

'Should thereceived FM wave diminish in am- '5 plitude substantially to zero there will still be no reproduction of noise, because the auxiliaryoscillator current will simulate the carrier and so far asv the limiter is concerned there will be offered to its input terminals a signal to noise ratio in l0 excess of the improvement threshold value. By

l5 is so set that only the noise is rejected thereceiver is quiet between stations during tuning, and produces ,an Aaudi'ofrequency output When a desired signal is present. The very weak signals having an amplitude .close to that of vthe noise are distorted somewhat by the presence ofthe auxiliary oscillations, and are made somewhat more noisy than when the squelch oscillator is cut out of the circuit. However, this loss isinconsequential since such weak signals would be rated unserviceable anyway, and should be received, vif

desired, with the squelch circuit shut off.

As stated above, when the received modulated waves increase to or attain an amplitude such as substantially to exceed the noise level and the magnitude of the auxiliary oscillator current,

then automatically the latter current will be `suppressed as if it were noise. Of course, the oscillator 5 may have its frequency located at a point spaced from either ofthe limiting `edges of the I. F. pass band. vIts intensity level is preferably predetermined or pre-set for the thermal agitation noise level of the receiver, or for the particular noise level encountered kin the area where the receiver is being operated, if this noise level is higher than the receiver noise level. If desired, the amplitude of squelch current maybe made adjustable by the listener. 'Ihe numeral 9 indicates an adjusting device which is preferably included in the auxiliary oscillator network so 4" that the intensity of the auxiliary oscinations may be controlled in order to lcope with the various noise levels encountered in practice. Should it be found, as might be the case in some types of receivers, that the gain of the tunable high frequency amplifier circuit and converter circuit Yis greater at the high frequency end of the tuning range than at the low frequency end of the tuning range, then it will be desirable concurrently to adjust the amplitude of the auxiliary oscillations to compensate for such change of gain.

For example, the tuning device of the receiver could in that case be mechanically coupled to the oscillator amplitude adjusting means 9 so that the oscillator amplitude level would be set to squelch properly at both the low and high frequency ends of the tuning device. In this way the variation in gain of the receiver over the tuning range would be compensated for, since if this were not done the oscillator 5 might very well squelch properly at the low frequency end of the tuning range and would not squelch sufficiently at the high frequency end. Any other wellknown method for compensating for variation in gain over the tuning range may be utilized. For

instance, a selective equalizer circuit could be inserted in the converter and prior circuits, or additional limiting could be inserted ahead of the point of connection of the squelch oscillator 5.

While I have indicated and described a system for carrying my invention into effect, it will be lapparent to one skilled in the art that my invention is -by no means limited to the particular organization shown and described, but that many -modications may be made without departing from the scope of my invention. I l What I claim is: l

l. A method for obtaining control over a receiver of angle modulated carrier waves of the type' including a limiter so as to eliminate the rush of noise accompanying a decrease of the -received modulated waves below a usable intensity level, which comprises introducing into the limiter input locally-produced carrier current of a fined frequency substantially different from that received from the modulated waves and of sufficient magnitude to suppress the noise.

f2. In a frequency modulation receiver of the type including a limiter network; the noise squelching improvement which comprises an os- -cillator adapted to produce oscillatory energy having a frequency spaced from the limiting frequency cf the received signal band by a superaudible frequency value, and means for injecting the oscillatory energy into the receiver at a point prior to the limiter at a magnitude such that lauxiliary oscillator operating 1at a, frequency spaced by a superaudible frequency value from the operating intermediate frequency of the receiver, means applying the output of said auxiliary oscillator to the limiter input circuits, and said auxiliary oscillator having its energy level about equal to the peak values of noise components in the received signal energy.

4. A method of operating a frequency modulation receiver of the type provided with a limiter network, which comprises locally producing auxiliary waves having a frequency of the order of the desired signalrcarrier but different from the latter, and injecting the auxiliary waves into the limiter network input at a magnitude such as to provide aneifective signal to noise ratio above the improvement threshold at the limiterjnput terminals.

5. A phase and frequency modulation receiver having means for preventing a large increase in output due to noise, when there is no substantial signal current in the receiver circuits, comprising means to introduce a currei'it having an'amplitude lying between the average amplitude of noise currents and the amplitude of least useable signal currents, and said introduced current having a, frequency removed from the band of signaling currents by an amount equal to or greater than the highest important modulation frequency.

6. In a receiver of angle modulated carrier waves of the type provided with a limiter; an improvement to eliminate the rush of noise accompanying a decrease of the received modulated waves below a usable intensity level, which comprises means for introducing into the limiter input of the 'receiver locally-produced carrier current of substantially constant frequency and of suicient magnitude to suppress the noise.

7. In a frequency modulation receiver of the type including a limiter network; the noise squelching method which comprises producing oscillations having a frequency spaced from an extreme frequency of the received signal band by a superaudible frequency value, injecting the oscillations into the receiver at a point prior to the limiter at a magnitude such that inter-station noise is suppressed and deriving from the limiter energy of the frequencies of said signal band.

V8. In a frequency modulation receiver of the superhetercdyne type provided with an intermediate frequency network feeding into a, limiter network; the improvement which comprises an auxiliary oscillator operating at a Yfrequency spaced by a superaudible frequency value from the operating intermediate frequency of the receiver, means applying the output of said auxiliary oscillator to the limiter, and said auxiliary terminals above the improvement threshold and deriving from the limiter output terminals energy of the frequency of said received signals.

CLARENCE W. HANSELL.

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