US2162883A - Automatic frequency control system - Google Patents

Description

7'0 SIG/VAL June 20, 1939. D. E. FOSTER 2,162,883

AUTOMATIQ FREQUENCY CONTROL SYSTEM Filed March 31, 1957 iwirscrok SOURCE 0567114701? FREQUENC Y USC/UATOR 2 a FPFOUf/WY CONT R01 TUBE INVENTOR DUDLEYE. FOSTER ATTORNEY Patented June 20, 1939 UNITED STATES PATENT OFFICE 2,162,883 AUTOMATIC rasonnnoy CONTROL SYSTEM Delaware Application March 31,

. 6 Claims.

quency of an oscillator by heterodyning its oscillations with other oscillations of a different frequency to produce beats, the beats being combined with said other oscillations to produce control oscillations of a frequency equal to the oscillator frequency; the control oscillations being utilized to lock the oscillator to oscillation at its operating frequency.

Another object of the invention may be stated to reside in the provision of a superheterodyne receiver with a first detector of the electroncoupled type and a local oscillator of the negative resistance type, the I. F. output of the detector being combined with signals in a mixer to produce oscillations equal to the oscillator frequency; and the last mentioned oscillations being used to maintain the oscillator at its predetermined frequency. And still other objects of the invention are to improve generally the operation of local oscillators; and more especially to provide automatic frequency control circuits for superheterodyne receivers which are not only reliable and: efficient in operation, but readily embodied in receivers.

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

Referring now tothe accompanying drawing, there is shown in the latter only that portion of a superheterodyne receiver which is essential to a proper understanding of this invention. Specifically, there is shown the first detector tube l, the local oscillator tube 2, and a tube 3 whose circuits are arranged to cause the latter to act as an automatic frequency control arrangement 7.

for the receiver. The signal source for the first 1937, Serial No. 133,946

detector, or mixer, tube I may comprise the usual signal collector followed by one or more stages of tuned radio frequency amplification. The signals, symbolically designated as of a frequency Fnare impressed on the tunable input circuit 4 which includes the usual variable tuning condenser 5. The signal grid 6 and cathode 1 are connected to the input circuit 4; the cathode is connected to ground through the customary self bias network 8, and the low potential side'of input circuit 4 is at ground potential. The plate, or output electrode, 9 of tube I is connected to a source of proper positive potential B through the coil ii! of I. F. coupling transformer M. The coil It istuned to the operating I. F. by the shunt condenser H. The secondary circuit 12 of transformer M is tuned to the I. F. value, designated by the symbol F2. The I. F. output of the first detector may be amplified by one or more I. F. amplifiers, and then demodulated by a second detector. Of course, the audio voltage component of the demodulated I. F. energy can be utilized in any desired manner; further, the direct current voltage component may be employed for AVC action in any well known manner. Assuming that the signal frequency range is in the broadcast range of 500 to 1500 kc., the I. F. may have a value chosen from a range of '75 to 480 kc. Generally the local oscillations have a frequency higher than the signal frequencies at any setting of the tuning device, and the difference is equal to the I. F. Hence, the local oscillation frequency F3 is equal to F1+F2. The oscillator frequency F3 is impressed on the grid electrode E3 of tube 1; the oscillator grid being surrounded by a positive electrostatic shielding field produced by the screen grids I l, l5. The suppressor grid Iii, tied to the cathode, is disposed between plate 9 and screen grid I5. The tube i may be of the well-known 2A7 or 6L7 types; in either case the I. F. energy is produced by electron coupling between the signal and oscillator grids.

An auxiliary grid 21 is provided in the oscillator tube, and the grid is positioned between cathode 25 and screen IT. The electrode 2'! has alternating current voltage impressed thereon, the voltage having a frequency equal to F3. When such voltage is impressed on the electrode 21 the oscillator is pulled or locked into strong oscillation at the frequency F3. The locking oscillator comprises tube 3, which may be of the 2A7 or 6L7 type, provided with cathode 30, signal grid 3i, electrode 32 and plate 33. Positive screen grids are positioned on either side of the grid 32; a suppressor grid is disposed adjacent the plate 33. The cathode is maintained above ground potential by the self-biasing network 34. The plate 33 is connected to a point of positive potential through the resistor 35, the by-pass condenser 36 connecting the plate lead to ground. Signals are impressed on the grid 3| through the condenser til; I. F. energy is impressed on electrode 32 through lead 50.

The plate 33 of tube 3 is connected by condenser 6D to the electrode 27. The oscillatory output of converter 3 will include the sum frequency (F3) of F2 and F1. Hence, when the oscillatory output of tube 3 is impressed on electrode 21, the oscillator 2 automatically will be locked to the oscillation frequency F3. It Will, therefore, be seen that the oscillator of the receiver includes an auxiliary electrode which functions to maintain the chosen frequency F3 as soon as some I. F. energy is produced. Of course, the locking efficiency of tube 3 will depend upon the selectivity of the signal and I. F. circuits. If, for example, the transformer M is broadly tuned, then the converter 3 will begin to produce F3 as soon as a side band of the carrier is approached. However, the selectivity will seem to be increased because of the locking action.

Tube 2 has its circuit arranged to produce local oscillations and is operated in a negative transconductance manner, and may be of the type such as 57 or 606. The second grid i7 is connected to a source of positive potential S through resistor 29; the plate l8 is connected to a source 'of positive potential S2 Which is less positive than that of S. The third grid i9 is at a negative potential relative to cathode 25 by reason of the voltage drop across the self bias resistor 24- which is by passed for radio frequencies by condenser 62; the D. C. path for this grid is through coil 20 and resistor 23 to ground. The first grid 27 is operated at a negative potential relative to the cathode; it plays no part in the oscillating action of the tube, but its negative potential influences the strength of oscillation. Resistor 6| in this grid circuit may, therefore, be connected to any desired point on resistor 24 to adjust oscillation amplitude. In a tube with this arrangement of grids and operating potentials the phase of grid H and grid l9 is the same; that is, negative transconductance exists between grid l1 and grid [9 so that the circuit is self-oscillatory by reason of condenser 28 connecting grids If and i9. The frequency of oscillation is approximately determined by coil 23 and condenser 2i, but may be varied over a relatively narrow range by a radio frequency voltage applied to grid 21. Thus, in the absence of voltage on grid 21, the oscillation frequency will be that of circuit 21l2l22, but when a voltage differing by a few kilocycles from that of circuit 23--2l-22 is applied to grid 2?, the oscillations will take place at the frequency injected into grid 2?. The oscillation amplitude is adjusted so that tube l is not saturated, i. e., any increase in oscillation amplitude produced by tube 2 will increase the output of frequency F2 with a given amplitude of signal input F1. Therefore, tube 2 will be caused to oscillate most strongly at a frequency where maximum amplitude of frequency F2 is produced, i. e., at the correct intermediate frequency.

In operation, then, the tuner 26 is adjusted to a signal carrier. As soon as some signal energy is converted to I. F. energy, the latter istransmitted through lead 50 to grid 32. Signals being already impressed on grid 3!, the output of con verter 3 will include F3. When oscillations of the latter frequency are impressed on grid 21, the oscillator 2 is pulled into F3 oscillation. The action is cumulative, and it requires but a little signal energy to produce the locking effect. Should the local oscillator tend to drift in fre quency for one reason or another, the tendency will be compensated for by the controlling effect of converter 3.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. A method of reception which includes collecting signal modulated energy of a desired carrier frequency, generating local oscillations of a different frequency, combining the signal energy with said oscillations to produce energy of a different frequency, combining said signal energy with difference frequency energy to produce other oscillations of said different frequency, and combining the said local oscillations with said other oscillations at the same frequency in such electrical relation as to maintain synchronism therebetween.

2. In a superheterodyne receiver of the type ineluding a first detector and a local oscillator, the

method which includes collecting signal modulated energy of a fixed frequency, generating at said oscillator oscillations of a different frequency, combining signal energy and oscillator energy at said detector to produce beat frequency energy, combining said signal energy with beat frequency energy to produce other oscillations of said different frequency, and impressing said other oscillations upon said oscillator in such electrical relation to said first oscillations as to maintain frequency synchronism therebetween.

3. In combination, a first detector, a local oscillator of the negative transconductance type, means impressing oscillations from the latter on the detector, said detector having an intermediate frequency energy output circuit, a converter network having a signal input circuit and an intermediate frequency input circuit, means impressing signals on the signal input circuit, means impressing energy from said intermediate frequency output circuit on said intermediate frequency input circuit, said converter having an output circuit coupled to said oscillator to impress thereon the SllIIll of the signal and intermediate frequencies whereby said oscillator is caused to have maximum oscillatory amplitude at said sum frequency.

4. In combination, a first detector, a local oscillator, means impressing oscillations from the latter on the detector, said detector having an intermediate frequency energy output circuit, a converter network having a signal input circuit and an intermediate frequency input circuit, means impressing signals on the signal input circuit, means impressing energy from said intermediate frequency output circuit on said intermediate frequency input circuit, said converter having an output circuit coupled to said oscillator to impress thereon the sum of the signal and intermediate frequencies, said local oscillator comprising a tube having reactively coupled input and output electrodes, and an auxiliary electrode in said tube, said sum frequency being impressed on the auxiliary electrode.

5. In combination, a first detector, a local oscillator tube, means impressing oscillations from the latter on the detector, said detector having an intermediate frequency energy output circuit, a converter network having a signal input circuit and an intermediate frequency input circuit, means impressing signals on the signal input circuit, means impressing energy from said intermediate frequency output circuit on said intermediate frequency input circuit, said oscillator tube having an auxiliary electrode, said converter having an output circuit coupled to said oscillator auxiliary electrode to impress thereon the sum of the signal and intermediate frequencies.

6. In combination, a'first detector, a local oscillator tube, means impressing oscillations from the latter on the detector, said detector having an intermediate frequency energy output circuit, a converter network having a signal input circuit and an intermediate frequency input circuit, means impressing signals on the signal input circuit, means impressing energy from said inter-- mediate frequency output circuit on said intermediate frequency input circuit, said oscillator tube having an auxiliary electrode, said converter having an output circuit coupled to said oscillator auxiliary electrode to impress thereon the sum of the signal and intermediate frequencies, said oscillator having a tank circuit tuned to said sum frequency.

DUDLEY E. FOSTER.

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