US2400948A - Noise squelch system - Google Patents

Description

May 28 1946- R. A. PETERSON 2,400,948

NOISE SQUELGH SYSTEM Filed March 19, 1945 ATTORNEY trol. ers, to utilize the grid current flowing in the Patented May 28, 1.946

UNITED ls'ra'rlezsl PATENT ortica .i

NOISE SQUELCH SYSTEM Robert A. Peterson, Haddoniield, N. J., assigner tol Radio Corporation of America., a corporation of Delaware Application March 19, 1942i, Serial No. 479,727 4 claims. (Cl. 25o- 20) My presentv invention relates to noise squelch systems for radio receivers, and more particularly to a noise-actuated control system for a receiver of angle modulated carrier wave energy.

In the past various schemes have Ibeen proposed forimpairing the reproduction of a radio receiver in response to a low signal-to-noise ratio at the receiver signal collector. In the case o f an angle modulated carrier wave receiver, such as a receiver of frequency modulatedcarrier energy, it has been proposed to use the discriminatorrectifier network output voltage for noise con.

It has, also, been proposed in such receivlimiter grid circuit. However, these various prior circuits have not been suiiiciently sensitive, 'reliable or eiicient to warrant widespread use thereof.

I It is one of the main objects of my present` l lto squelch, or mute, a modulation signal amplier stage. Y

Another object of my invention is to provide an improved muting circuit for an FM receiver, or a phase modulated (PM hereinafter for brevity) carrier wave receiver, which mutes the audio outputof the receiver when no signal is present at the receiver input terminals, and which permits the receiver to reproduce the modula` tion in eiilcient manner when a signal of proper strength is received.

Still another object of my invention is to provide a noise squelch circuit for an FM or PM receiver; the squelch circuit comprising a direct current voltage amplier which acts to .apply a cut-off bias to an audio ampler when the former is conductive, the noise voltage developed in the plate circuit of the limiter tubev being recti-J fled' to providey a.V bias control voltage for the direct current voltage amplifler, and the-rectied noise voltage being opposed by a negative voltage derived from :the limiter grid. 'circuit thereby to provide a residual Apositive voltage which is applied as said bias control voltagein the absence 4of a usable signal.

Still other objects of `this invention are to improve the sensitivityand eiliciency of noise squelch circuits vof receivers of the angle modulated type, and more especially to provide a receiver whose noise output is controlled by an economical circuit which can be readily manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; the invention itself, however,V 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 diagrammatically a system whereby my invention may -be carried into eiect.

Referring to the accompanying drawing, wherein there is shown a portion of a superheterodyne receiver of the VFM type essentialV to a proper understanding of my invention, it is pointed out that the usual networks prior to-the schematiy cally represented intermediate frequency (I. F.) amplifier I :are omitted. These networks are well known to those skilled in the art. Usually they comprise a signal collector device, such as a grounded antenna or a dipole, one or more tun'- able radio, frequency amplifiers, and a converter followed by one or more I. F. amplifiers. The receiver frequency may be any frequency that .is desirable for an angle modulated `carrier Wave receiver. vThe receiver frequency band may be in the 26-50 megacycle (mc.) range', or in the 'I0-100 mc. range., The operating I'. F. value may :'be in the mc. range, or it may be in the kilocycle (kc.) range. Wherethe latter is the case, double conversion can be used to reduce the car- 'rier frequency down to an I. F. value of, for example, 455 kc. Let it be assumed, then, that prior to amplifier I there has `been employed double conversion of the modulated carrier energy, and that the signal fed to stage I has an I. F. value'of 455 kc. The invention, however, is not limited to any specific frequency values. l

The signal energy may be FM or PM. The generic term angle modulated has been heretofore employed, and will be used in the'fol- .lowing descriptionand claims to indicate either PM or hybrids of the two. By way of specific example, itisv assumed that the received signal ,energy is FM waveenergy. It is well known to those skilled'lnthe artl of receiving FM waves that the noise output of the receiver rapidly increases when the signal-to-noise-ratio at the signal collector device falls below a predetermined value. ratio will decrease below the noise-free Value when tuning between stations, when moving an excessive distance from a desired FM transmitter, or when the receiver is moved into a. shielding area such as a bridge and the like.'` During such periods of noisy reception it is desirable to mute the receiver output. In the vously used in FM receivers have employed the reduction in-noise voltage at the demodulator output circuit in response to signal reception to operate the squelch circuit. However, such an arrangement has not proved to besufiiciently sensitive. It is, also, desirable to provide a muting circuit which will give improved operation in the presence of ignition noise interference.

The present noise muting arrangement satises all reasonable requirements in an FM receiver. Generally speaking, my invention is based. on the fact that the usual limiter tube circuit provides in its plate circuit noise voltage components in the absence of signal energy of a usable value. I utilize these noise voltage components to .provide cut-o bias for a signal transmission stage of the receiving system.

Referring to the specific circuits of the drawing, the numeral 2 designates the limiter tube. While the tube is shown as of the pentode type. it will be understood that any other suitable type of tube may be employed. The limiter tube has its input grid 2' connected to ground through a path which comprises the secondary winding 3 of the I. F. transformer 4 and the resistorRz. Each of the primary and secondary circuits of I.- F. transformer 4 is tuned to the operating I. F. value. The condenser C1 shunts the resistor R2. The network Ra-Ci functions as the usual signalresponsive grid biasing network to provide grid circuit limiting action for input signals whose amplitude exceeds a predetermined amplitude. The network Rz-Ci has a relatively small time constant value. The cathode of tube 2 is grounded, while the screen grid thereof is connected to a point of positive direct current potential through resistor R1. The latter is grounded for I. F. currents by condenser Ca.

The plate of the limiter tube is connected to the +B terminal of the direct current source by means of .the primary winding 6 of the' transformerfeeding the discriminator. The limiter operation is well known to those skilled in the art. Its ideal input vs. outpu characteristic is depicted above tube 2. The function of the limiter is substantially to eliminate amplitude variations from the FM signal energy applied to the discriminator circuit. l

The FM detector network itself may be of any well known form. -The speciilc detector illustrated is of a well known and simple type. Essentially it comprises a pair of'diodes which are y connected in opposed relation,. and whose tuned inpuI-,Ucircuits are equally and oppositely mistuned in frequency with respect to the mean frequency of the applied signal-energy. The detector tube 'I is shown as "of the 6H6 type by way of example.

One diode section 8 has connected between its electrodes the series arrangement of the input circuit S and the output load resistor l0. 'I'he second diode section 8' has connected between its electrodes -the input circuit 9' and the output load resistor I0'. The cathode of diode section 8' is grounded, as isl the case with the lower end of load resistor I0. The resistors I0'and III' are in series, and each of them is bypassed by appropriate condensers for I. F. currents.

The input circuits 9 and 9' are each reactively coupled to the primary circuit 6, but there is very` little coupling between-the -two input circuits 9 and 9. The reference characters F1 and F2 denote the respective off-tune frequencies of these two circuits. The mean frequency of the applied signal energyis indicatedA by the symbol Fc, and in the present case the value of Fc is the operating I. F. value. Above the FM detector there is depicted the ideal frequency deviation Vs. rectified output characteristic of the detector. This-is the well known inclined characteristic curve of the FM detector. The .peaks of the detector characteristic may be spaced, if desired, by a distance greater than the maximum frequency deviation of the FM signal energy. Itl

is not believed necessary to describe the functioning of the detector. circuit, sinceit is very well known to those skilled in the'art. It is sufficient to indicate that there will be taken off from the upper end of resistor I0 the modulation signal, in this case audio frequency, which was originally applied to the carrier at the FM transmitter.

Where the signal energy to be received is a PM carrier wave, itvmay still be necessary to utilize an amplitude limiter prior to detection. The PM detection network will be of any well known type. It is sufficient for the purposes of this application to point out that the essential difference between a PM carrier wave and an FM carrier wave resides in the fact that in the former case the extent of frequency deviation is greater for the higher audio frequencies.

The modulation signal utilization network is indicated as comprising a tube Il, as for example a triode. whose cathode I2 is connected to ground through a biasing resistor I3 in series receives its bias from the voltage .across resistor The biasing of tube II is controlled by the direct current voltage amplier I1 which is shown, by way of specific illustration, as a triode. The plate I8 of tube I1 is connected to the grid of tube I'I through a resistor I9, the lower end of resistor I9 being bypassed to ground for pulsating current components bycondenser 20. The plate I8 is connected through resistor 2| to the upper end of resistor I3", and the cathode of tube I1 is grounded. A positive voltage exists across I3", and this is used as plate voltage for tube I1. The control grid 22 of tube I 1 is returned back to ground through a path which the opposite ends thereof bypassed to ground denser 23'. 'I'he resistor 24 acts as the output load of a voltage doubling rectifier which comprises the GHS type tube 30.`

The'diode section 3| of tube 30 has its anode connected to the upper end of resistorv R1 by the coupling condenser; 32, while the cathode of the diode section 3l' is connected to the junction of resistors 24 and 23. The diodeI section 33 of tube 30 has its anode connected to the left end of'resistor 24, while its cathode is connected to the output terminal of coupling condenser 32. The noise voltage vcomponents developed across the resistor R1 are transmitted through the coupling condenser 32 to the diode sections 3| and 33 of tube 30. By virtue of the voltage doubling connections of the diode sections 3| and 33 with respect to output resistor 24, there will be developed across the resistor 24 rectified noise voltage whose amplitude is substantially double the amplitude of the noise voltage components at R1.

The rectied noise voltage is applied to the grid 22 of tube in a positive polarity sense. The network 23-23' functions as a filter to suppress any pulsating voltage components which a point on R2A such that negative voltage developed across Rz will be su-fliciently less than thepositive voltage developed across resistor 24|y to provide a residual positive bias for grid 22, This residual positive bias will exist, of course,

through 4condenser 32. However, when the may .exist in the output voltage across resistor y duces the positive voltage developed across resistor 24 to the proper value. l

When the signal-to-noise ratio at the receiver Vcollector device rises to a usable value, the noise voltage components at R1 rapidly decrease. This means that the voltage developed across resistor 24 rapidly decreases. However, the voltage across R2 does not decrease. It either will remain constant, or it will actually increase. This means that the grid 2'2 of the tube |1 will be biased sufficiently negative to cut off its space current iiow,

and the cut-off bias across resistor 2| will be removed from the grid of audio frequency amplier If an additional limiter tube-is employed prior to thelimiter tube 2, the voltage drop across Ra will be invariable if the no-signal noise originating in the iirst circuits of the receiver produce enough voltage to saturate the first limiter,

"tube'H completely muted in the absence of received signal energy.' However, for received signals of 0.1 microvolt, or greater, it is,desirable to have the cut-off bias removed from the grid of tube l. Of course, these constants are purely illustrative, and the set designer can adjust the vconstants to suit the particular requirements of transmission.

` 'only if suiilcient noise voltage is transmitted residualpositive voltage applied to grid 22 has a y sufficient and predetermined magnitude tube I'l will be conductive, and will provide space cursuiicient voltage to cut oi the space current now in tube |'l by virtue oi' the fact that the voltage across resistor 2| is applied in a negative polarity sense to the grid of tube Il. The condenser 20 acts to delay the change in bias across resistor 2|.

In the no-signal state it is desirable to adjustthe slider 26 on resistor R2 so that the voltage ap- While Ihave indicated and described a system for carrying my invention into eiiect, it will be apparent Vto one skilled in the art that my invention is by no means limited to the particular ortype including an amplitude limiter stage having plied to grid 22 will be slightly positive thereby causing the audio amplifier tube Il to be cut oif. This means that there will be no noise reproduction at the receiver reproducen. It is pointed out that the noise voltage components produced in the plate circuit .of the limiter tube may go up to several kilocycles in frequency, depending on the bandwidth' of the I. F. amplifier. In other words, the limiter tube acts in the `the limiter output electrodes, a load resistor connected between the cathode of said one diode and anode of the second diode for providing a. rectiiied voltage whose magnitude is proportional to y noise voltage appearing in the limiter output cirmanner oi' a detector for the radio, frequency 1 noise components so far as its plate circuit is cuit in response toa substantial reduction in the signal-to-noise ratio at the receiverl input terminals, a resistor-condenser network in circuit potential of said network to saidload resistor concerned, and, hence, develops across its plate vThe resistor 25 acts to isolate network Ra-Ci from condenser 21. It is this recti'cation which provides the'opposing negative voltage that refor combining the voltage output of the rectiiier and said control voltage to provide a residual voltage which is positive, an electron discharge device responsive to said positive voltage for producing a space current, and means responsive -to the last-named space current for rendering said 4input and output electrodes, a ireeuency modulation detector and a modulation signal amplifier;

tlhe improvement comprising a rectifier-doubler coupled to the limiter output electrodes for providing a rectined voltage whose magnitude is prof portional to noise voltage appearing in the lim- 4iter output circuit, in response to a substantial reduction in the signal-to-noise ratio at the receiver input tenninals, a resistor-condenser network for deriving from the input limiter elec-V trodes a control voltage Whose magnitude depends upon the amplitude of received signal energy, means combining the voltage output of the rectier-doubler and said control voltage to provide a residual voltage which is positive, an electron discharge device responsive to said positive voltage for producing a space current, and means responsive to the last-named space current for rendering said modulation signal amplier inopera` tive.

3. In a frequency modulation receiver of the type including an amplitude limiter tube having input and output electrodes, a frequency modulation detector and a modulation signal amplier tube; the improvementl comprising a rectier con- ,sisting of opposeddiodes having a common cou-V pling to the limiter output electrodes, an output means responsive to said positive voltage for producing a space current flow in said device, and resistive means, connected'between the amplier input electrodes and responsive to the last-named space current, for rendering said modulation signal amplifier tube inoperative.

4. In a frequency modulation receiver of the type including asignal transmission tube having input and output electrodes, a frequency modulation detector coupled to the output 'electrodes and a modulation signal amplifier coupled to the detector; the improvement comprising a pair of opposed diodes, a load resistor common to the space current paths of the opposed diodes, a condenser connecting unlike electrodes of said diodes to said output electrodes for providing a rectiied voltage across said load resistor whose magnitude is proportional to noise voltage, a resistor-condenser network connected to said input electrodes for producinga control Vvoltage whose magnitude depends upon the amplitude of received signal energy, means comprising a connection from the negative potential end of said load resistor to a slider adapted to be adjusted along the resistor of said resistor-condenser network for combining the voltage output of the rectier and said control voltage to provide a residual voltage which is positive, an amplifier tube having an input connection responsive to said positive voltage for producing space current ow, means responsive to the last-named space current for rendering said modulation sigmeans combining in polarity opposition the voltage output of the rectifier and said control voltage to provide a residual direct current voltage which is positive, an electron discharge device,

nal amplifier inoperative, said .last means comprising a resistor connected between the modu- `als lation ampliiier input electrodes and adapted to be traversed .by said space current.

ROBERT A. PETERSON.

Images