US2261800A - Automatic control system for carrier-wave signal beceivers - Google Patents

Description

N0 49 1.941 R. L. FREEMAN 2926159@ AUTOMATIC CONTROL SYSTEM FOR CARRIERWAVE SIGNAL RECEIVERS Filed Sept. 13, 1939 ATTORN EY tuned without losing the signal.

Patented Nov. 4, 194i iris sara AUTO'EFC CONTROL SYSTEM FOR, CAE- RiER-WAVE SHGNAL'RECEIVERS Robert L; man, Flushing, N. Y., assigner to y Hazeltine Corporation, a corporation oi Deia= WSE@ assuma@ september is, was, sensi no. erases i comme (ci. ases-ae) action should also be independent of signalI strength. The frequency interval over which the action is effective should be chosen as a compromise between two limitations; the control ac tion should be sufficient to correct for oscillator frequency instability caused by temperature and Y voltage changes and for mistuning caused by shortcomings in mechanical preset tuning devices; and in theother extreme, the. control action should be less than that which would he operative by the carrier signal on an adjacent channel. In a broadcast receiver this compromise suggests a choice 'of automatic frequency control action over a frequency range oi about 'l kilocycles on either side of the desired signalcarrier frequency. Thus, with conventional automatic frequency control, the sensitivity char acteristic or S-curve of the discriminator must f extend over a given frequency range of about 14 kilocycles. Attempts to obtain an S-curve of large slope sensitivity and consequently finer tuning correction generally are frustrated because any `increase in the slope of such characteristic S-curve tol a certain extent reduces the given frequency range to one smaller than the permissible ifi kilocycles.

As pointed out in the article entitled "Improvements in A. F. C. Circuits, pp. 20-23 of the November i936 issue of Electronics the frequency separation oi the S-curve peaks determines to some extent the amount of mistuning of a signal for which the automatic-frequency control system will lock in. Once this locking has occurred, the control-tube sensitivity determines how much the local oscillator may be mis- For a satisfactory locking-in, the peak separation on the S-curve of the discriminator should not be much greater than the receiver over-all band width at twice resonant voltage input as otherwise the' band width of the intermediate-frequency amplifiers preceding the discriminator network de termines the amount of mistuning of a signal for which the automatic frequency control system will loci: in. This limitation becomes severe and decreases the value of an automatic frequency control system whenever the deviation of the oscillator frequency from that which produces the correct intermediate frequency is larger than half the band width of the intermediate-frequency amplifier preceding the' discriminator network. Such condition is sometimes present in highly-selective broadcast receiversusing preset tuning mechanisms and is quite generally present in ultra-high-frequency receivers used for television reception or for frequency-modulated wave reception.

in the case of a television s uperheterodyne receiver, normal changes in temperature alone cause a frequency drift of the local oscillator of several hundred kilocycles. Because of certain advantages as to simplicity and economy, it is preferable to utilize the accompanyingl sound carrier to a television signal as the pilot signal to operate the automatic frequency control system. Thus. the discriminator network is placed in the intermediate-frequency amplifier channel which is responsive to the sound signal-carrier frequency. From the standpoint of reducing noise and interference itis desirable to make the band width of this sound amplifier channel no greater than necessary to accept the sound signal sidebands; in consideration of this, a band width of less than 50 kilocycles is desired. Suchl a choice is directly in conflict with the large band width required to give satisfactory automatic frequency control.

It is an object of the invention, therefore, to provide an improved automatic control system for a carrier-wave signal receiver which overcomes the above-mentioned disadvantages of such arrangements of the prior art.

It is a further object of the invention to provide an improved automatic frequency control system for superheterodyne receivers which has relatively high sensitivity and high tuning correction eiciency and which, at the same time, operates automatically from an amplier channel having a. relatively narrow frequency range without decreasing the permissible frequency tolerance for manual or automatic tuning or for oscillator frequency instability.

In accordance with the invention. there is provided an automatic control system for a carrierwave signal receiver comprising a network the output of which varies in response to variation of the frequency or other given characteristic of a signal input thereto over a given range of variation of the frequency or other characteristic,

i e teristic of the signal input to the network over a range which is wider than and which overlaps the given range. The system also includes means responsive to the resulting frequency or other characteristic of the signal input to the network for rendering the additional means inoperative when the resulting frequency or other' A characteristic has a value within the given range.

In a preferred embodiment of the invention as applied to a superheterodyne receiver, the intermediate-requency signal is applied to the frequency discriminator, and the additional means varies the frequency of the intermediate-irequency signal input to the discriminator so that it sweeps over a frequency range wider than, but which overlaps, the given frequency range ofthe discriminator network. The additional means eiectively applies to the oscillator control tube a relatively low-frequency sweeping or searching voltage whereby the receiver is tuned automatically to signals which initially are out of the given frequency-response range of the discriminator. Also, in accordance with a preferred 'embodiment of the invention, means are provided for rendering the additional means inoperative when the frequency of the signal input to the network is momentarily swept within said given frequency range, which means comprises a vacuum tube controlled by the output of the discriminator and controlling the application of the sweep voltage to the oscillator control tube.

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with' further objects thereof, will be best understood by reference to the specification, taken in connection with the accompanying drawing, in which Fig. 1 is a circuit diagram, partly schematic, ofy a complete superheterodyne receiver including an automatic frequency control system embodying the present invention; and Figs. 2

and 3 are circuit diagrams o different embodiments of automatic frequency control systems constructed in accordance with the invention.

Referring to Fig. `1 of the drawing, there is shown a circuit diagram, partly schematic, of a complete modulated-carrier signal receiver of the superheterodyne type embodying the invention. This receiver comprises, in cascade, an antenna-ground circuit it, ii, a radio-frequency amplier i2, a modulator i3 and associated oscillator 22, intermediate-frequency ampliiiers it and i5, a frequency-discriminator and detector network it, an audio-frequency amplier i'i of one or more stages, and a sound reproducer i8.

general, a conventional superheterodyne receiver r including automatic volume control and automatic frequency controlsystems, the operation of which, in general, is well understood by those skilled in the art. Any of a number of signals .1

intercepted by antenna-ground circuit it, it is selected and amplified in radio-frequency amplifier l2 and translated to oscillator-modulator 22, i3, wherein it is converted to. an intermediatefrequency signal. The intermediate-frequency signal is selectively amplied in intermediatefrequency amplifiers iii vand l5 and delivered to the frequency-discriminator and detector network l, wherein the audio-frequency signals and the automatic amplincation control and automatic frequency control biasing potentials are derived. The audio-frequency signals, in turn, are amplified by audio-frequency amplifier il and supplied to sound reproducer it for reproduction. The automatic amplication control bias derived from unit Wis effective to control the amplification of one or more of the units I2, til, it, and i5 to maintain the amplitude of the signal input to the unit 'i6 within a relatively narrow range for a wide range of received signal intensities, while the automatic frequency control bias derived from the unit i6 is effective to control the frequency-control tube 2| to maintain the frequency of the signal input to the network i@ at a substantially constant value.

The detector and Irequency-discriminator network it comprises a primary circuit including an inductance 2t tuned by a condenser 23.

and inductively coupled to an inductance 25 in a secondary circuit tuned by a vcondenser 26. Circuits 23, 2e and 25, 25 are tuned to the normal intermediate-carrier frequency of the receiver. Network it also includes diode rectiilers 21 and 23, to which are applied the sum and difference,

respectively, of the voltages'across the primary circuit 2.7i, 2t and across half of the secondary circuit 25, 28. To this end, a coupling condenser 3i is connected between the high alternatingpotential side of the circuit 23, 2d and a mid-tap on secondary inductance 25, while the mid-tap on inductance 25 is coupled by way of conductor le to a common terminal B of individual load resistors 29 and t@ of rectiflers 21 and 28, respectively. A by-pass condenser 32 is connected across load resistors 29 and 3D in series.

The value of condenser 3i has a very marked eiect on the selectivity of the network i6 as to audio-frequency characteristics and also as to discriminator characteristics. When the capacitance of such condenser 3i approaches zero or is very small, the discriminator action lof the network l@ disappears', but the audio-frequency fidelity is as good as conventional intermediatefrequency diode detector stages. However, a compromise value for the capacitance of the condenser 3i can be found which gives Very good discriminator action as well as good audio-:Erequency delity. For an intermediate frequency of 450 kilocycles and with a value of capacitance of condenser3i ln the range of 4 to 10 auf, and with comparatively loose coupling of the'inductances 24 and 25, an automatic frequency control characteristic having a peak separation of 2 to 4 kilocycles is obtainable with a slope sensitivity of 30 to 40 volts per kilocycle (per peak volt applied to the preceding amplier tube grid) and with correspondingly high gain at the peaks. It will be appreciated that this is from 2 to 10 times the sensitivity ordinarily utilized in prior practice in which a peak separation of the order of 14 kilocycles is desired. However, if such a discriminator is used, it will not produce automatic frequency control action until the receiver is tuned almost to the frequency of the desired signal, that is to say, to within 4 to 5 kilocycles from resonance. Thus, it will be seen that network it has a unidirectional-voltage output which varies in response to variation of the frequency of the signal input to the network only over a given frequency range which is relatively small, preferably about 2 kllocycles. The attainment of such a narrow range oi response of the discriminator isfurther facilitated by inserting a resistor in series with lead I9 in con'el formance with the ideas set forth in the applicants copending application Serial No. 230,820, tiled September 20, 1938, which issued as Patent No. 2,184,072, December 19, 1939.

The oscillator control tube 2i and the'oscillator-modulator stage 22, i3 constitute means responsive to the output of the network it for .adjustingl the frequency of the signal input thereto to a frequency within a desired small range, or maintaining the frequency of the signal input thereto approximately at a predetermined value, within the given range of permissible frequency deviation when the frequency of the signal input is within the given frequency range. That is to say, the operation of the circuit described in detail up to this point, in general, is similar to a conventional automatic frequency control system except that an extremely sharp discriminator is used having a sensitivity from 2 to l0 times the sensitivity of the usual discriminator and a much smaller separation between peaks oi the discriminator characteristic s-curve.

However, according to this invention, there is provided additional means for periodically sweeping the frequency of the signal input to the network it over a frequency range which is wider than and which overlaps the normal or given frequency range of the discriminator and at a sweepv frequency lower than any frequency within such given frequency range, together with means for rendering the additional. means inoperative,` when the frequency of the signal input to the network it is within such given frequency. The additional means preferably comprises rneans for eftectively applying a low-frequency alternating potential to the control tube for varying periodically the oscillator frequency; specifically', this meanscomprises a periodic searching potential circuit tid inductively coupled by means of a transformer iid, SE to the input circuit E@ of the oscillator control tube 2i. The circuit 33 comprises the output circuit of a vacuum tube $3 the control electrode of which is applied a periodic voltage from a suitable source, such as conventional S0-cycle alternating current supply circuit te, over a conductor t. A filter network, comprising series resistors t, 50 and a shunt condenser 5i, preferably is coupled between the sup- I 3 ply circuit sa and the discriminar network ls to prevent the sweep voltage Afrom getting back into the automatic amplification control and audio-frequency system.

The application of a periodic voltage to the control electrode of the oscillator control tube 2i causes the oscillator frequency to be swept through a range which is wider thanbut includes the normal frequency range of the discriminator i6 and thus provides means' for augmenting the action of the sharp discrlminator it. When the receiver produces an intermediate-frequency signal of the proper frequency as it is tuned in the vicinity of the' desired broadcast signal and 'the oscillator '22 is swept through a relatively wide range as described above, the sharp discriminator takes eiect and locks in the signal in a conventional manner. as the discriminator action takes eiect, the diode rectifier 2&3 develops a neglative-bias voltage across its load resistor tu, re-

gardless oi' the side from which resonance is approached, which operates to* bias the control electrode of the vacuum tube tu to cui/on which,

in turn, renders inoperative the frequency sweep system of the oscillator 2i. The diode 2S and its associated circuit comprises 'means responsive to the resulting frequency of the signal input to the discrimlnator network for rendering the additional or frequency sweeping means inoperative when the resulting frequency has a value within the given range of the discriminator. Such action alone provides limited automatic frequency control action and, with a very sharp discriminator, the range of the automatic frequency control action depends only on the amplitude of the periodic sweep voltage and not at all on signal strength for signals above a certain threshold level.

summarizing the general features and operation of this system, an extremely sharp discriminatur network is used; then, in order that signais may be automatically tuned in, which are initially out of the normal frequency-response range of the receiver dscriminator circuit, an

l, auxiliary periodic voltage effectively is added to the control tube bias in addition to any unldirectional-bias potential developed by the discriminator. Thus, for (a-tune signals -that would otherwise be ineuective to develop a discriminator bias, the system pulls in to them because the control tube is sweeping the signal input to the network through a relatively wide frequency range. if the oil-tune signal is in this swept frequency range, discriminatori' voltages are 'developed .each time the oscillator. traverses the frequency that causes the correct intermediate frequency to be developed.

Referring to Fig. 2 of the drawing, there is shown an automatic frequency control system embodying a modiiied .formof the invention which, in general, is similar to that described above ,in connection with Fig. l, similar elements of the two circuits having identical reference numerals. In this system the diodes il', 29 of the discriminator i@ are reversed so that a positive bias is developed at the common terminal B oi' the resistors il@ and d@ of the networkfor biasing the control electrode of a gaseous vacuum-tube relay, such as a "Ihyratron, @il coupled to the circuit 3d. With no bias on the control electrode oi' the relay du, the tube'is nonconductive and a periodic sweep voltage appears at the transformer primary tit. When the control grid is made positive by the discnator bias developed by a signal locked in, the relay ill yactsas aconductor and vproduction of interchannel noises.

sweep voltage in the output circuit 83 of tube 3G.

'Such an arrangement is shown vin Fig. 3, in which the sweep voltage output of tube 36 is rectied by a diode rectifier il and the resulting bias voltage applied negatively over conductor 62 to the control-electrode of an audio-frequency stage tube to bias tle latter to cutod and thus suppress re- The diode il and its associated circuit thus comprises means responsive to the additional or frequency sweeping means for restricting the output of the receiver while the additional means is operative. When a signal is tuned in, this suppressor bias completely vanishes. In the circuit 'of Fig. 3, the transformer secondary 35 is coupled to the cathode of control tube 2i and the supply circuit i3d preferably is coupled in series with the cathode-biasing resistor it of vacuum tube tt. This arrangement eliminates the need of a filternetwork,such as that comprising resistors liti, E@ and condenser Ell of Fig. l, for preventing the sweep voltage from aecting the automatic amplification control and audio-frequency chel of the l receiver; otherwise, the systemsare substantially similar.

It will be understood that other types of frequency-discriminator networks may be utilized in place of the discriminator network illustrated and described.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modications as fall within the true spirit and scope of the invention.

What is claimed is:

l.. In a carrier-wave signal receiver, an automatic control system comprising, a network the output of which varies in response to variation of a given characteristic of a signal input thereto over agiven range of variation of said characteristic, means responsive to the output of said network for maintaining said given characteristic of said signal input to said network approximately at a predetermined value within said given range of variation when said characteristic of said input wave has any value within said given range, additional means for periodically sweeping the value of said characteristic. of said signal input over a range which is wider than and which overlaps said given range, and means responsive to the resulting characteristic of the signal input to said network for rendering said additional means inoperative when said resulting characteristic has a value within said given range.

2. An automatic frequency control comprising, a discriminator network the output of which varies in response to variation of the frequency of a signal input thereto over a given frequency range, means responsive to the outputv of said network for maintaining the frequency of said signal input to said network approximately at a predetermined frequency within said given frequency range when said frequency of said sigsystem 4 assises nal input falls within said given frequency rang, additional means for periodically varying the frequency of said signal input over a. frequency range which is wider than and which overlaps said given frequency range, and means responsive y to the resulting frequency of the signal input to said' network for rendering said additional means inoperative when said resulting frequency has a value within said given frequency range.

3. An automatic frequency control system comprising, a discriminator network the output of which varies in response to variation of the frequency of a signal input thereto over a given frequency range, means responsive to the output of said network for adjusting the frequency of said signal input to said network to a frequency within a desired small frequency range within said given frequency range when said frequency of said signal input is within said given frequency range, additional means for periodically varying the frequency of said signal input so th'. t it sweeps at a frequency lower than any frequency within said given frequency range and over a frequency range which is wider than and which overlaps said given frequency range, and means responsive to the resulting frequency rof the signal input to said network for rendering said additional means inoperative when said resulting frequency has a value within said given frequency range.

4. An automatic frequency control system comprising, means including a discriminator network the output of which depends upon the frequency of a signal input thereto over a given frequency, control means responsive to said output for maintaining the frequency of .said signal input approximately Aat a predetermined frequency range within said given frequency range when the frequency of said input. wave falls within said given frequency range, additional means comprising a searching potential circuit for varying periodically'the frequency of said signal input at a frequency lower than any frequency within said given frequency range and over a frequency range which is wider than and which overlaps said given frequency range, and means responsive to the output of said discriminator for rendering said additional means ineffective when the resulting frequency of said signal input falls within said given frequency range.

5. In a carrier-wave signal receiver of the superheterodyne type, an automatic frequency control system comprising, a discriminator network, an oscillator control tube, means including said discriminator network for developing a frequency-control bias and applying it to said tube, additional means for effectively applying a low-frequencyalternating potential to said control`tube for varying periodically the oscillator frequency, whereby automatically to tune the receiver to signals which initially are out of the frequency-response range of the discriminator, and means responsive to the resulting frequency of the signal input to said network for removing said low-frequency voltage from said control tube when said resulting frequency has any value within the frequency-response range of said discriminator.

6. In a carrier-wave signal receiver of the superheterodyne type, an automatic frequency control system comprising, an exceedingly sharp discriminator network the output of which varies in response to variation of the frequency of an intermediate-carrier signal input thereto over a given frequency range, an oscillator control tube, means responsive to said output for controlling said ccyitrol tube to adjust the frequency of said intermediate-carrier signal input to a frequency within a desired small frequency range when said frequency of said intermediate-carrier signal input has any value within said given frequency range, additional means for applying a low-frequency alternating potential to said oscillator control tube for varying periodically the oscillator frequency so that it sweeps at a quency range, additional means for varying the frequency lower than the intermediate-carrier frequency of said receiver and overa frequency range which overlaps and is wider than said given frequency range, and means responsive to the output of said discriminator network for rendering said additional means inoperative when the resulting frequency of said intermediate-carrier signal input falls within said given frequency range. I

7. In a modulated-carrier wave signal receiver, an automatic frequency control system comprising, a discrlminator network the output of frequency of said signal input over a frequency range'which is wider than and which overlaps said given frequency range, means for rendering said additional means inoperative when the resulting frequency of the signal input to said network fallsY within said given frequency range, and means responsive to said additional means for restricting the output Aof the receiver while saidadditional means is operative, whereby to suppress intercarrier noises when the receiver is not tuned to a carrier-wave signal.

ROBERT L. FREEMAN.

DISCLAIMER A 2,261,800.Ro1bert L. Freeman. Flushing, N. Y. A'ronA'nc CoNTnoL SYSTEM Fon CARRIER-WAVE SIGNAL Rncmvmns. Patent dated November-1, 1941. Dis-v claimr filed September v2, 1943, by the inventor; assignee, Hazeltine -.(J'orporatwn, assent1ng. I Hereby enters this disclaimer' to claims 4, 5, and 6 of said patent.

[Oficial Gaztte Septembr 28, 1943.]

DISCLAIMER 2,261,800.-Robert L. Freeman, Flushing, N. Y. AUTOMATIC CONTROL SYSTEM FOR CARRIER-WAVE SIGNAL RECEIVERS. Patent; dated November v4, 1941. Disclaimer filed June 22, 1944, by the inventor; the assignee, Hazeltne Corporation, assenting. Hereby enters this disclaimer to claims 1, 2, and 3 of said patent.

v [Oficial Gazette August 1, 1.944.]

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