US2200038A - Automatic frequency control circuit - Google Patents

Description

May 7,'1940. G. MouNTJoY Erm. 2.200,038

' AUTOMATIC FREQUENCY-CONTROL CIRCUIT Filed March 19, 193e ATTORNEY.

Patented May 7, 1940 AUTOMATIC FREQUENCY CONTROL CIROUTT y signor to Radio Corporation of poration rif-Delaware America, a cor# Applicatiemarch. 19, 1938,seri1 No. 196,870'. claims. (01.-,2505-12m Our present invention. relates to automatic frequency control (AFCf'circuitsof vthe type used at present in superheterodyne receivers, and more particularly to an yimproved and simple AFC cir- 8 .fruit which dispenses with-the usual frequencyv contr.oltube.\-`

'I'11ere:.-has.lpreviously beenfrdisclosed an AFC system-employingwJ reactive impedance, -as an inductanceein the cathode circuitlof a frequency loochanger tube;rthe AFC; bias being'applied directly to-lthe signal input felectrode of'thentube: yIn such a systemwvariation Vof ithefinput electrode 'poten-4 tial results in asubstantial change; inithe frequencyI of the oscillator tuna-bleftank--.circuit;

Whene-utilizing. such. an -AFG system in va receiver tunable over the broadcast band of -500-to 1550 kc;, it is essential to provide va-wide yrange of -frequency shift. at the successive -settingsfof-the oscillator tank .condenser.- For exampley-frequency: lshifts zoaof the. order of l0-25'fkc.-are desirablafso-tl'iat'l frequency'correctionfof the tank-circuitfs readily accomplishedn l l Accordingly,V it maybe stated 1to-be one ofthe mainobjects of our-invention to provide an'AFC the .variablecondenser `Il andcoil I2. The electrode .lDf-is Yconnected'to feedback coil i3, .thev latter beingzreactively coupled to tank .coil l2.

e-system employing a combined local oscillator-first detectornetworka discriminator being used tor provide .an I. f lfrequency.eresporisive'frpotential which is applied to thasignalgrid ofthev vsaid netwcrktubeutilizing an=impedancefwhich pref csents essentially a capacity effectto the oscillator'r tankr circuit and thereby p-roducesrelatively :Wide frequencyshifts -of-:the llatterfas the signal grid potentialV is varied. l

4 Anotherk important .Object offrthe inventiorris oscillatorcircuitf. a resonant circuit ytuned to 1a ffrequency belowthe :oscillator circuit frequency; I and. means being1 utilized to f adjust the gai-n fof the .changen tubethereby-to vary the effective frequency of:theioscillator-lcircuit.

Stillotherobjects offouroinvention are -f to im 45.provegenerally the :simplicity and efficiencyy of AFC circuits for receivers, and more-especiallyto f providean AEC circuitwhich. is reliabl-erand effecs tive .in operation-,^and is economically--rnanuface tured andlwassembled.

.t The novel features .which .we believe .to be char-.

acteristic o-f our...invention are.set .forth in pare... ticularity in the ,appended claims; the invention self,..,however, asto bothits organization.and....

methodofoperatonwill best be understood by T energy is fed to a second detector, and the latter litfreferencewtottlie following .description .taken in connection with 'the drawing in wl'iiclfi` fwehaver--A I indicated diagrammatically a circuit organization-"2 whereby ourinvention may be carried into effect.

Referrng now to thefdrawingfthere is shown'.` the? converter network cfa superheterodyne re.- ceiver remaining circuits being schematically.' represented. The-converter tube AI may be a pentagrid tube; say of the BASG typeV The input and-output circutslth'ereof are. purely conven-v tional :those skilled in the art will readily appre ciatetheconstruction'of the network. The signal input'..circuit 2 includes a` variable tuning condenser 3; .the input circuit is connected between the .signal grid'li andtground. The cathode 5 is connected to ground through a coil 6; the con denser .1 being utilized to 'resonate the `coil to a frequency. below. the oscillator tank frequency f range. A selff-.biasing resistor-condenser network 8 may-.abe `included in seriesbetweenground and coil 6 therebyfto establish grid l at a normal negav 20a-f tivebiaswithrespect to cathode 5.

The electrodes 9 and it function as the oscil-.'r

' latorlgridV and. anode respectively. yThe tunable tank; circuitfor the oscillator section comprises electrode l0 may have'a positive .potential applied 30ethereto'from-any desired source; the oscillator. grid.Y electrode A9 is connected lto ground by the leak resisten-I5, .andthe blocking condenser I6 is :connected betweenthe high potential side of. 355.120 providelafrequency changer .tube-having signal and oscillatorlfcircuits yassociated-tl'1er-.ewith; and. "r thererbeingincluded in thespace current'path` of: f the ...tube,-,.anrl. .in operative s association with theI r condenser Handzthe grid 9. The rotors of condensers and Il may be arranged for mechanical uni-control adjustment; the dotted line Il desigf nates such tuning instrumentality.

Thesignals applied to input circuit Zmay'be derived fromV an antenna, after being amplified by one orrmore tunable radio frequency amplifiers.

'I'liesignal range'may be ther broadcast band of 500 to 155.0 kc.; or it may be any of the ranges used in .a multi-rangezreceiver. The intermediate frequency (LF.) maybechosen froma range of. Yto 450 kc.; say, forr example, the I. F. is 400 kc.

. The network Zil'is, then,'resonated to the 400 kc.

value; ,the network being connected inthe plate circuitof tube` l. l The signallgrid 4, as is well known, is electrostatically shielded from the oscill lator electrodesv by the `positive screen electrodes. The I. F.- amplifier 22 has its'input circuit 2| coupledto thel. F. circutZ; the amplified I. F.'

output of amplier 22 a direct current voltageA whose polarity and magnitude are dependent upon the sense and amount of the frequency departure of the carrier, at the output of amplifier 22, from the predetermined I. F. value. The AFC bias, that is the discriminator output voltage, isv

zero when the carrier frequency of the I. F. signal energy is equal to the I. F. By connecting the discriminator output to the signal grid 4, as` by lead 40, there is provided a device for varying the gain of tube I in response to frequency variations of the I. F, energy carrier. The lead 40 includes the filter 4I for suppressing the pulsating components in the AFC bias; the blocking condenser 42 is connected between lead d0 and the grounded side of condenser 3.' The tank circuit H l2 is tunable over a frequency range of approximately 900 to 1950 kc.;

lit constantly differs from the signal circuit frequency by the I. F. value. If the tank frequency departs from that frequency value whichproduces the desired I. F. value, then the I. F. energy carrier frequency will shift and produce AFC bias.

:This occurs whether the tank frequency departure is 'caused by the inter-station tuning process, orv by oscillator frequency drift from a desired setting. The AFC bias is applied to signal grid li thereby to adjust the frequency of tank circuit 356i I IZ in a sense to maintain the I. F. value of 400 kc. Since'the network 0 7 is resonant to a frequency below the lowest frequency of the tank circuit frequency range, the impedance of the network is essentially capacitative insofar as the tank circuit H IE is concerned.

As the potential of grid f3' is made increasingly negative, the frequency of tank circuit Il l2 is found to increase. Conversely, when the'grid i is rendered less negative, with respect to the ynormal grid bias value, the tank circuit frequency decreases. This means that the capacitative effect of network 6 1 is varied, with signal grid bias adjustment, in a manner such as to supplernentthe condenser il, and in a direction to maintain the predetermined I. F. value. The

bias network 8 may be used to establish the nora mal grid bias value of grid l; it is recommended that the normal bias value be sufciently high to prevent the grid i being swung positiveupon the AFC bias assuming a relatively high positive potential. f

To explain the actual operation of the AFC circuit, let it be assumed that a signal carrier of 1000 kc. is desired to be received. Assuming, furythenthat the tuner il is being adjusted to adjust now be seen that when the AFC lcias goes negative, the frequency of network Il l2 increases towards 1400 kc. The constants of network 0 1, and the bias range of grid 4, are chosen to shift the frequency of circuit lI-IZ sufficiently to secure the desired I. F. value.

Shifts of 24 kc. were obtained at the low frequency end of the broadcast band, 'and 12.3 kc. at the high frequency end. This was accomplishedv when 6 1 was tuned to 440 kc. Network 6 7 is tuned below the oscillator band, `and preferably below the desired signal band.- gIt eserves to produce a quadrature'current-in the cathode electrical stream. When the bias of grid 4 is changed this quadrature current is changed, and the effect of tube i in shunting circuit Il l2 is varied. For example, when grid 4 is sent negative by the discrirninator bias, the quadrature current of cathode 5 is changed, and theshunt across circuit ll-IZ produced by this quadrature current is changed.. The frequency Iof Il l2 is increased by this process.

By way of illustration the following circuit constants are given, it being understood that these values are in no way limiting:

Coil 6: 3 lnillihenries Condenser 1=100 mmf. Condenser l4=450 mmf. Condenser l6=100 Inni-f.

Resistor I5: 50 megohms While we have indicated and described a system for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular I output circuit associated with an output electrode quency, said capacitative impedance comprising a network tuned to a frequency below any frequency to which thel tank circuit may be rese onated but above the said intermediate frequency. 2. In a frequency changer circuit provided with' a tube having an oscillator section and an assoiof: 1

ciated oscillator tank circuit, a signal input elec-r trode in said tube. an intermediatev frequency output circuit associated with an output electrode of the tube, a signal input circuitcoupled to the signal input electrode, a capacitativeirnpedance, disposed in the space current path con` trolled by the signal electrode, operatively associated with the tank circuit, and means for varying the potential of said input` electrodeY thereby to cause said impedance to adjust the tank circuit frequency, said capacitative imped-y ance consisting of a resonant network tuned to a frequency below the oscillator tank circuit frequency range. v

3. In combination with a tube having a cathode and plate, a signal grid, oscillator grid and oscillator anode electrode disposed between the cathode and plate, a tunable signal input circuit connected between the signal gridvand cathode,

. a tunable oscillator circuit operatively associated diate circuit to produce a. variable direct currentr voltage, ymeans applying'the variable voltage to the signal grid, and a network in the cathode circuit resonant to a frequency below the oscillator frequency range.

4. In combination with a tube having a cathode and plate, a signal grid, oscillator grid and oscillator anode electrode disposed between the cathode and plate, .a tunable signal input circuit connected between the signal grid and cathode, a tunable oscillator circuit operatively associated with the oscillator grid and anode an intermediate frequency output circuit coupled to the plate, a discriminator coupled to said intermediate circuit to produce a variable direct current voltage, means applying the variable voltage to the signal grid, and a network in the `cathode circuit which is resonantto a frequency which is less than the lowest frequency of the oscillator frequency range but is higher than the intermediate frequency.

5. In a superheterodyne receiver of the type employing a combined local oscillator-first detector network provided withv a tube having at least a cathode, an output electrode, a signal grid, anoscillator grid and an oscillator anode,

an oscillator tank vcircuit reactivelycoupling said oscillator grid and anode to provide an oscillator section, said tank circuit including means for` adjusting its frequency over a relatively wideA range of frequency values, a signal input circuit coupled to said signal grid and cathode and in`.y

cluding means for tuning it over a signal frequency range which differs from said oscillator range by a predeetrmined. frequency value, an output circuit coupled to said output electrode tuned to said predetermined frequency, a discriminator, responsive to frequency departures of signal energy in said output circuit from said predetermined frequency, for developing a direct vcurrent voltage Whose magnitude is dependent on the amount of departure, means for applying said voltagelto said signal grid, and a resonant network, tuned to a frequency below theoscillator frequency range, connected inthe space current path of said tube to simulate a capacitative reactance in such relation to said tank circuit that the tank circuit frequency is supplementally adjusted by said discriminator voltage.

GARRARD MOUNTJOY. CHARLES W. FINNIGAN.

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