US2128997A - Automatic frequency control circuit - Google Patents

Automatic frequency control circuit Download PDF

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Publication number
US2128997A
US2128997A US136064A US13606437A US2128997A US 2128997 A US2128997 A US 2128997A US 136064 A US136064 A US 136064A US 13606437 A US13606437 A US 13606437A US 2128997 A US2128997 A US 2128997A
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United States
Prior art keywords
frequency
circuit
condenser
tube
reactance
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Expired - Lifetime
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US136064A
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English (en)
Inventor
Dudley E Foster
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RCA Corp
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RCA Corp
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Publication date
Application filed by RCA Corp filed Critical RCA Corp
Priority to US136064A priority Critical patent/US2128997A/en
Priority to CH202675D priority patent/CH202675A/de
Priority to GB11028/38A priority patent/GB513449A/en
Priority to DER102073D priority patent/DE685736C/de
Application granted granted Critical
Publication of US2128997A publication Critical patent/US2128997A/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03J—TUNING RESONANT CIRCUITS; SELECTING RESONANT CIRCUITS
    • H03J7/00—Automatic frequency control; Automatic scanning over a band of frequencies
    • H03J7/02—Automatic frequency control
    • H03J7/04—Automatic frequency control where the frequency control is accomplished by varying the electrical characteristics of a non-mechanically adjustable element or where the nature of the frequency controlling element is not significant
    • H03J7/042—Automatic frequency control where the frequency control is accomplished by varying the electrical characteristics of a non-mechanically adjustable element or where the nature of the frequency controlling element is not significant with reactance tube

Definitions

  • My present invention relates to automatic frequency control circuits for radio receivers of the superheterodyne type, and more particularly to a uniformly acting frequency control network for the local oscillator tank circuit of a superheterodyne receiver.
  • automatic frequency control circuits (AFC) for superheterodyne receivers generally comprise a discriminator unit for deriving a direct current voltage from the intermediate frequency (IF) energy when the latter shifts in frequency from the assigned IF value.
  • IF intermediate frequency
  • a frequency control tube network which is electrically associated with the local oscillator tank circuit in such a manner as to simulate across the tank circuit a reactance of a predetermined sign.
  • the voltage output of the discriminator is employed to regulate the magnitude 20 of the simulated reactance across the oscillator tank circuit, and the regulation is such that the oscillator frequency is shifted to a predetermined mean oscillator frequency at different settings 4 of the receiver tuning device.
  • Another important object of the invention may be stated to reside in the provision of a superflui heterodyne receiver of the type utilizing an AFC arrangement, wherein a frequency control tube is electrically associated with the local oscillator tank circuit of the receiver in such a manner as to simulate across the tank circuit a reactance of iff a predetermined sign, and a reactance of a different sign being employed in conjunction with the simulated reactance in such a manner that the percentage oscillator frequency .shift varies linversely with frequency whereby there is pro- 5O ⁇ 5 quizd a more constant absolute frequency shift over the tuning range of the receiver.
  • Sti-ll other objects of the invention are .generally to improve AFC arrangements for receivers of the superheterodyne type, and more especially 55 ⁇ to provide frequency control arrangements which are not only eiicient and reliable in operation, but are economically embodied in superheterodyne receivers.
  • a superheterodyne receiver which employs an AFC arrangement embodying the present invention.
  • the receiver may be of any conventional superheterodyne type; it will usually em- 2'0 body a signal collector l which feeds a tunable radio frequency amplifier 2.
  • 'Ihe tunable iirst detector 3 is supplied with amplified signal energy, and oscillations from local oscillator 4 are also impressed on the detector, or mixer, 3.
  • the IF energy output of the latter is amplified by an IF amplifier 5, and the output of the amplifier is demodulated by the usual second detector network.
  • the latter, and the following audio network, are omitted, because they are 80 well known to those skilled in the art. It is, also, pointed out that any type of automatic volume control arrangement may be used to maintain the signal intensity level at the demodulator input circuit substantially uniform. In this way 3'5 a substantially uniform signal intensity level is maintained at the input circuit of the discriminator.
  • 'Ihe signal circuits 2 and t have the rotors of the variable condensers thereof arranged for 40 mechanical uni-control; the rotor of the variable condenser t is mechanically coupled to the tun ing adjusting means, denoted by the dotted lines l, for the rotors of the signal circuit condensers.
  • the tank circuit of the local oscillator i includes 45 y the coil L1 shunted by the grounded variable condenser EE.
  • the fixed condenser 9 functions as a padder, and the latter acts to maintain the frequency of the oscillator tank circuit different from that of the signal circuits by a predeter- 5o mined constant amount throughout the adjustment range of the tuning device l.
  • the IF may have a value chosen from a range of from 75 to 480 kc.
  • an AFC arrangement may be of the type shown by S. W.,Seeley in his application Serial No. 45,413, filed Oct. 17, 1935.
  • the AFC generally comprises a discriminator I functioning to derive a direct current voltage (AFC bias) from the IF energy.
  • AFC bias a direct current voltage
  • the polarity and magnitude of the AFC bias is dependent on the sense and amount of frequency shift of the IF energy from the assigned frequency.
  • the AFC bias is applied through lead II to an electrode of the frequency control tube I2. The latter functions to produce a predetermined reactive effect across tank circuit Li-G.
  • the network can be of any desired type as long as it is capable of converting a frequency shift in IF energy into a direct current voltage change in polarity and magnitude.
  • the discriminator may comprise oppositely mistuned diode rectifiers, as shown by C. Travis in his application Serial No. 4,793, filed February 14, 1935.
  • the IF- tuned diode rectiers of the aforesaid Seeley application may be employed in the discriminator network if desired.
  • the plate I3 of control tube I2 is connected to the positive terminal (+B) of a direct current source through a path which includes the radio frequency choke coil 8.
  • the cathode il' of control tube I2 is grounded through the usual self-bias resistor-shunt capacity network I8.
  • the plate I3 is connected to the high potential side of oscillator tank circuit coil L1 through a condenser C3.
  • the platerside of condenser C3 is connected to ground through a series path whichV includes resistor Ri and the condenser C1.
  • the control grid S9 of tube I2 is connected to the junction of resistor R1 and condenser C1 through a condenser C2.
  • the control action of tube I2 is produced in the following manner.
  • the control circuit proper consists of tube I2, the resistors 20 and R1, and the condensers C1 and C2.
  • a certain alternating voltage, say E. exists between ground and the plate of control tube l2.
  • the same voltage exists across resistor Ri and condenser C1 in series. If the resistor R1 is a high resistance, the current through it is nearly in phase vwith voltage E.
  • the frequency control tube I2 connected as shown, electrically simulates in shunt across coil L1 an inductance with a small resistance and a condenser C3 in series therewith.
  • This shunt inductance capacity circuit acts to reduce the effective inductance of circuit Li-Ii; it increases the frequency of oscillation.
  • the AFC bias applied to grid I9 acts to vary the gain of the control tube, and 'the-magnitude of the simulated shunt inductance, so as to secure desired oscillator frequency correction in response to a frequency shift in IF energy from the assigned value.
  • the mean bias of grid I9 which bias is developed by network I8, is so chosen that there will be approximately equal frequency changes on both sides of the mean oscillator frequency at any setting of the receiver tuning means.
  • uniform oscillator frequency correction has not been secured as the variable condenser 6 is adjusted to change the operating frequency of the local oscillator from one en-d of the tuning range to the other end.
  • the magnitude of the condenser C3 is so chosen that this condenser resonates with the simulated shunt inductance to a frequency below the tuning range of the oscillator tank circuit.
  • Fig. 2 there is shown the equivalent network with respect to the oscillator tank circuit coil. It will be observed that the oscillator tank coil L1 has connected in shuntY therewith a seriesv path Vwhich includes the condenser C3 and a reactance designated as L.
  • the inductive reactance L is the equivalent inductance of tube I2.
  • the inductance L represents the simulated inductive reactance which is developed across the tank circuit due to the action of the frequency control tube I2.
  • the series path Cs-L is resonated to a frequency below the tuning range of the oscillator tank circuit in order to maintain substantial uniformity of frequency correction of the oscillator tank. circuit throughout the tuning range thereof.
  • the magnitude of .resistor R1 should be much larger than the reactance of condenser vC1.
  • the resonant frequency of Ca-L' decreaseswith decreasing Gm of tube I2; hence AFC bias change causes no difficulty.
  • the oscillator tank circuit Iii-6 is tunable through a range of frequencies of approximately 1,000 to 2,2 00 kc., and that the IF is 460 kc.; the signal circuits tuning from 540 to 1740 kc.
  • the shift would be 100 kc. at a signal frequency of 540 kc. and 220 kc. at a signal frequency of 1740 kc. 76
  • L stands for the inductance at a given frequency which would be equal to the combination of L and C3 in series. This inductance changes with frequency being smaller at the low frequencies (nearer LC3 resonance). It is this virtual inductance varying with frequency which produces the desired effect on shift with frequency, since a low inductance in shunt with another inductance has greater effect.
  • the percentage frequency shift of the oscillator tank circuit varies inversely with oscillator frequency, and, therefore, produces a more constant absolute frequency shift.
  • the condenser C3 is in series with the static capacity of the control tube l2 thereby decreasing the total capacity shunting coil L1. Again, the effective frequency variation of the tank circuit is greater, and hence the AFC system is more sensitive than with the condenser C3 large, or omitted.
  • a tube connected to said circuit to have the cathode to plate impedance of the tube simulate a reactance across the circuit, means for varying the gain of the tube to adjust the magnitude of said reactance, and a reactance of different sign from the first reactance in series with the impedance across said circuit, said two reactances being resonant to a frequency below said tuning range.
  • a tube connected to said circuit to have the cathode to plate impedance lof the tube simulate a reactance across the circuit, means for Varying the gain of the tube to adjust the magnitude of said reactance, a reactance of different sign from the first reactance in series with the impedance across said circuit, said two reactances being resonant to a frequency below said tuning range, said simulated reactance being inductive, and the second reactance being capacitative.
  • a tube connected to said circuit to have the cathode to plate impedance of the tube simulate a reactance across the circuit, means for varying the gain of the tube to adjust the magnitude of said reactance, and a reactance of different sign from the first reactance in series with the impedance across said circuit, said two reactances being resonant to a frequency below said tuning range, said gain varying means adjusting said magnitude when the circuit frequency departs from predetermined frequency values of said range.
  • a tube having connections thereto to have the cathode to plate impedance of the tube produce an inductance effect across the coil, a condenser in series with said impedance across the coil, said condenser resonating said inductance to a frequency below said desired frequency.
  • a resonant circuit of the type including a coil and a condenser in shunt therewith, said circuit being tunable through a desired frequency range, an electron discharge tube including at least a cathode, control grid and an anode, means for impressing the output current of said tube on said resonant circuit, a circuit element connected to said resonant circuit whereby the voltage across said circuit element is substantially in quadrature with the voltage across said resonant circuit, means for applying said quadrature voltage to said control grid whereby the effective reactance of the coil of said resonant circuit is decreased by virtue of a simulated inductive reactance produced across said coil, and a condenser effectively connected in series with said simulated inductance across said coil, said condenser and simulated inductance being resonant to a frequency below the tuning range of said resonant circuit.
  • said automatic frequency control circuit being of the type which includes a frequency control tube having input and output electrodes electrically coupled with the local oscillator tank circuit of the receiver to produce a simulated inductive reactance across the tank circuit, a condenser connected between the output electrode of said control tube and the high alternating potential side of said tank circuit whereby the condenser is effectively in series with said simulated reactance across the tank circuit, and said condenser and simulated reactance being resonant to a frequency below the lowest frequency of said tank circuit.
  • a superheterodyne receiver of the type including a local oscillator network provided with a tank circuit tunable over a relatively wide frequency range, an intermediate frequency network, an electron discharge tube having input and output connections to the tank circuit such that the cathode to plate impedance of the tube acts as an inductance across the tank circuit, and a discriminator, responsive to shifts in the intermediate frequency energy from an assigned frequency, for controlling the gain of said tube in a sense to cause the inductance to correct the frequency of the tank circuit and maintain said assigned frequency value; the improvement which comprises a condenser in series with said cathode to plate impedance across said tank circuit, and said condenser resonating the said inductance to a frequency below said frequency range whereby said correction is substantially uniform over said range.

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  • Channel Selection Circuits, Automatic Tuning Circuits (AREA)
  • Inductance-Capacitance Distribution Constants And Capacitance-Resistance Oscillators (AREA)
US136064A 1937-04-10 1937-04-10 Automatic frequency control circuit Expired - Lifetime US2128997A (en)

Priority Applications (4)

Application Number Priority Date Filing Date Title
US136064A US2128997A (en) 1937-04-10 1937-04-10 Automatic frequency control circuit
CH202675D CH202675A (de) 1937-04-10 1938-04-08 Zwischenfrequenzüberlagerungsempfangsschaltung mit selbsttätiger Abstimmungskorrektur.
GB11028/38A GB513449A (en) 1937-04-10 1938-04-11 Improvements in or relating to frequency control systems in high frequency electric circuits
DER102073D DE685736C (de) 1937-04-10 1938-04-12 Empfaenger mit selbsttaetiger Scharfabstimmung

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US136064A US2128997A (en) 1937-04-10 1937-04-10 Automatic frequency control circuit

Publications (1)

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US2128997A true US2128997A (en) 1938-09-06

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US136064A Expired - Lifetime US2128997A (en) 1937-04-10 1937-04-10 Automatic frequency control circuit

Country Status (4)

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US (1) US2128997A (de)
CH (1) CH202675A (de)
DE (1) DE685736C (de)
GB (1) GB513449A (de)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
BE455642A (de) * 1943-05-06

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Publication number Publication date
GB513449A (en) 1939-10-12
CH202675A (de) 1939-01-31
DE685736C (de) 1939-12-27

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