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CLASS 331,	OSCILLATORS
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SECTION I - CLASS DEFINITION

This is the generic class for electrical oscillators.

SECTION II - LINES WITH OTHER CLASSES AND WITHIN THIS CLASS

ELECTRICAL OSCILLATORS OR WAVE GENERATORS OR PRODUCERS CLASSIFIED ELSEWHERE

See Classes in References to Other Classes, below.

OSCILLATOR COMBINED WITH OTHER APPARATUS OR SYSTEMS

See References to Other Classes, below.

SECTION III - REFERENCES TO OTHER CLASSES

SEE OR SEARCH CLASS:

SECTION IV - GLOSSARY

ACTIVE ELEMENT

A control device for exerting a control on a source of energy proportional to an applied control signal. A conventional triode, having cathode, control grid on anode, connected as a conventional amplifier, is an example of an active network, a control potential applied to the grid causing a flow of anode current, supplied by the anode biasing source, proportional to the magnitude of the control potential.

AMPLITUDE STABILIZATION

The correction for, prevention of, or compensation for an undesired change in amplitude of the generated waves of the oscillator from a desired value.

AUTOMATIC FREQUENCY STABILIZATION

The restoration of the generated frequency of the oscillator to a desired value by sensing the deviation in frequency, in direction and amount, from the desired value and instituting a corrective action proportional to sensed deviation to adjust the frequency determining element of the oscillator in such direction and amount so as to return the oscillator frequency to the desired value.

BEAM TUBE

An active element comprising a source of charged particles, means for concentrating the particles into a directed beam, means for exerting a control on the beam (e.g., beam accelerating electrode, control grid, deflecting means, slow wave structure, buncher type resonator, reflector electrode, etc.) and means for deriving output energy from the controlled beam.

BEAT FREQUENCY

The resulting difference (or sum) frequency wave, among other waves, produced when two waves of different frequencies are combined in a nonlinear device.

DISTRIBUTED PARAMETER RESONATOR

A resonator of the distributed network type, the capacitance, inductance and resistance of which cannot be isolated into separate lumped capacitors, inductors or resistors and wherein the time factor of propagation of wave energy in the network is appreciable.

ELECTRICAL NOISE OR RANDOM WAVE GENERATOR

A wave generator system wherein the frequency determining element consists of a material medium including electrically charged, chargeable or ionizable particles, the application of electrical energy to the medium by the driving means causing random translatory motion of the charged or ionizable particles resulting in the generation of an infinite number of waves of different frequencies which are fortuitously related, having no definite phase relationship, period, amplitude or shape.

ELECTROMECHANICAL RESONATOR

A resonator comprising an electrically driven material body wherein the mass and compliance parameters of the body determine the mechanical period of vibration of the body and wherein the driving electrical circuit for the body exhibits electrical resonance characteristics which are determined by the mechanical period of vibration of the body.

FREE RUNNING OSCILLATOR

An oscillator wherein the driving system continuously supplies the losses of the frequency determining means so as to produce sustained oscillations.

FREQUENCY ADJUSTING MEANS

Means for setting or controlling the generated frequency of the oscillator by varying a frequency determining element of the oscillator.

FREQUENCY DETERMINING ELEMENT

A passive network or device of the resonant or time constant type, which network or device forms the element of the oscillator which sets or determines the frequency or periodicity of the generated oscillations.

FREQUENCY STABILIZATION

The correction for, prevention of, or compensation for an undesired drift or change in the frequency of the generated waves of the oscillator from a desired value.

GASEOUS SPACE DISCHARGE DEVICE

A space discharge device having at least two electrodes in a gaseous or vapor medium, conduction between the electrodes taking place by ionization of the medium.

HARMONIC OR SINE WAVE OSCILLATOR

A free running oscillator for generating sinusoidal or nearly sinusoidal waves. They usually utilize a resonator of the lumped LC or the distributed parameter type as the frequency determining element.

HETERODYNE FREQUENCY

Beat frequency (which see).

KLYSTRON

A beam tube including at least two apertured cavity resonators, the beam of charged particles passing through the apertures of the resonators in succession, and a collector electrode being provided to intercept the beam after passing through the resonators. The first resonator causes bunching of the particles passing therethrough, the bunched particles then travel in a field-free region where further bunching occurs and then the bunched particles enter the second resonator giving up their energy to excite it into oscillations.

LC RESONATOR

A resonant circuit comprising separate inductance and capacitance elements, i.e., lumped inductor and capacitor elements.

MAGNETICALLY CONTROLLED SPACE DISCHARGE DEVICE

An active element comprising means for producing a space discharge of charged particles and having further means for subjecting the space discharge to the direct control of a magnetic field and an electric field.

MAGNETRON

A magnetically controlled space discharge device comprising a linear cathode, an anode, usually cylindrical, coaxial therewith, the magnetic field being parallel to longitudinal axis of the cathode, while the electric field is transverse thereto.

MOLECULAR OR PARTICLE RESONANT OSCILLATOR

An oscillator wherein the frequency determining element consists of a material medium comprising particles, molecules or atoms, the application of electrical energy by the driving means to the medium setting the particles, molecules or atoms into a state of vibration or oscillation, the vibration or oscillation being that of the particle, molecule or atom itself and not the vibration or oscillation caused by the translational motion of the particle, molecule or atom as a whole.

MOLECULAR RESONATOR

A resonator comprising a material medium and wherein the vibration or oscillation of the molecules of the medium determines the resonant frequency of the resonator. The vibration or oscillation is of the molecule itself and not that due to the translational motion of the molecule as a whole. See, also, above, the definition of a molecular or particle resonant oscillator.

NEGATIVE RESISTANCE OR NEGATIVE TRANSCONDUCTANCE DEVICE

An active element of the two terminal type having a volt-ampere characteristic with negative slope over the range of voltages or currents wherein it is operative, that is, an increase in voltage results in a decrease in current, or vice versa.

OSCILLATOR

A system for initiating and maintaining oscillations whose frequency or period is fixed or determined by the physical parameters of the system. The fundamental elements required by an oscillator system are: (1) a frequency or period determining element, such as a resonator or timing means, (2) a driving system for the frequency or period determining element, and (3) means for deriving a useful output from the oscillator system. This class is restricted to oscillators for generating electrical oscillations or waves and specifically excludes alternating current generators of the mechanically driven dynamo-electric machine type.

RC OR RL FREQUENCY DETERMINING NETWORK

A network of the nonresonant type comprising either resistive and capacitive or resistive and inductive components. The network, by way of example, may be employed: (1) as a frequency determining phase shift network in a sine wave oscillator of the phase shift type, (2) as a frequency determining bridge network in sine wave bridge oscillators, such as the Wien bridge type of the double-T type or (3) as a time constant network in a relaxation oscillator to determine the period of the generated relaxation oscillations.

REFLEX KLYSTRON

A klystron utilizing only a single apertured cavity resonator through which the beam of charged particles passes in one direction, a repeller electrode being provided to repel or redirect the beam after passage through the resonator back through the resonator in the other direction and in proper phase to reinforce the oscillations set up in the resonator.

RELAXATION OSCILLATOR

A free running oscillator for generating decidedly non sinusoidal waves. They usually utilize a time constant network of the RC or RL type as the frequency determining element.

RESONATOR OR RESONANT CIRCUIT

A frequency determining means comprised of substantially pure reactances of opposite signs (i.e., mass and compliance in a mechanical resonator or inductive and capacitive reactance in an electrical resonator) wherein the phenomenon of resonance (i.e., when the positive and negative reactances are equal) is relied upon to determine the frequency of the generated waves.

RETARDING FIELD TUBE

A tube having at least three electrodes, i.e., a source of electrons (cathode), control electrode (grid) and anode or plate electrode, the control electrode being biased positively with respect to the other electrodes. The electrode bias potentials are so chosen that the electrons attracted from the cathode by the positive grid pass through the grid and are slowed down by the repelling effect of the less positive (or negative) anode field and are returned back to or through the grid. This phenomenon is repeated again and again so that a cloud of electrons are caused to sweep back and forth through the grip, giving up energy to the grid at a frequency which is a function of the transit time of the cloud of electrons. The Barkhausen Kurz, Gill-Morrell and the reflex klystron are examples of oscillators utilizing a retarding field tube.

SEMICONDUCTOR ACTIVE ELEMENT

A solid state active element comprised of a solid material having a conductivity intermediate that of a good insulator and a good conductor.

SHOCK EXCITED RESONATOR OSCILLATOR

An oscillator of the nonself-sustaining type wherein the driving system applies an electrical impulse to the frequency determining element (i.e., resonator), which element is then permitted to oscillate freely at its natural frequency.

SOLID STATE ACTIVE ELEMENT

A two-terminal or fourterminal active element of electrically conductive, semi-conductive, ferromagnetic or ferroelectric material in the solid state. Examples are: The Hall effect plate, semi-conductor (transistor), magnetic type and dielectric type amplifiers or negative resistance devices.

SPACE DISCHARGE DEVICE

A device comprising at least two spaced electrodes and wherein conduction by charged particles, e.g., electrons, or ions, takes place between the electrodes.

STABILIZATION

The maintenance of a desired condition or state of the oscillator which condition or state may be subject to change.

TRANSISTOR

A semi-conductive active element having at least three electrodes so arranged that the application of electrical energy to one electrode controls the flow of current between two other electrodes.

TRANSIT TIME OSCILLATOR

An oscillator system wherein the time of flight or transit angle of charged particles between electrodes of a space discharge device is an appreciable part of the cycle of the generated oscillations, the energy derived from the moving particles being continuously supplied to the frequency determining network of the oscillator in proper phase to sustain oscillations. Transit time effects are utilized in magnetron, beam tube and retarding field type oscillators.

TUBE

An active element of the space discharge device type. See: active element; space discharge device.

SUBCLASSES

1	AUTOMATIC FREQUENCY STABILIZATION USING A PHASE OR FREQUENCY SENSING MEANS
	This subclass is indented under the class definition.  Subject matter wherein the oscillator has means to adjust its generated frequency and is also provided with a control circuit or loop for controlling the oscillator frequency adjusting means in response to deviation of the generated frequency of the oscillator from a desired frequency or range of frequencies in such direction and amount as to restore the oscillator frequency to the desired frequency or range of frequencies. The control circuit or loop includes: (1) discriminator means for sensing the deviation of the generated frequency of the oscillator in direction and amount and for producing control energy proportional to such deviation, and (2) means responsive to the control energy and coupled to frequency adjusting means of the oscillator to vary the frequency adjusting means in such direction and amount as to restore the generated frequency of the oscillator to the desired frequency or range of frequencies. (1) Note. The automatic frequency stabilizing systems defined above are to be distinguished from the frequency stabilizing systems classified lower in the schedule (see the search notes below) by the fact that the A.F.S. systems rely on sensing a drift in frequency of the generated oscillations and asserting a corrective control to adjust a frequency determining element of the oscillator in the proper sense to bring it back on frequency, whereas the frequency stabilizing systems classified elsewhere do not sense a drift of frequency and retune the oscillator as a function of such drift, but rely on adding means to the oscillator system to correct or compensate, or to prevent, changes in oscillator circuit parameters that tend to cause an undesired change in oscillator frequency. Such means may, for example, compensate for the affects of changes in humidity, temperature, current load impedance, tube impedance, electrode bias potential, etc. SEE OR SEARCH THIS CLASS, SUBCLASS: 41, for beat frequency oscillators with frequency stabilizing means. See also, (1) Note, above. 88, for magnetron type oscillators with frequency stabilizing means. See, also, (1) Note, above. 175+, for oscillators in general with frequency stabilizing means. See, also (1) Note, above. SEE OR SEARCH CLASS: 322,  Electricity: Single Generator Systems,   subclass 32 for dynamoelectric generator systems with frequency responsive devices or networks for automatically maintaining the frequency of the generated wave constant. 329,  Demodulators,   subclasses 315+ for frequency demodulators and subclasses 345+ for phase demodulators. 332,  Modulators,   subclasses 123+ for average condition control in frequency modulator distortion prevention and subclasses 155+ and 159+ for average condition control in an amplitude modulator. 334,  Tuners,   subclass 13 for a tuner having a saturable core element combined with means to automatically center the frequency of the circuit; subclass 16 for a tuner having a reactance tuning means combined with automatic frequency centering means, and subclasses 26+ for tuners having an electromagnetic operator combined with automatic frequency centering means. 348,  Television,   subclasses 536+ for synchronizing systems which may utilize oscillators of the automatic frequency stabilized type and subclass 735 for television tuners which may utilize oscillators of the automatic frequency stabilized type. 375,  Pulse or Digital Communications,   subclasses 354+ for synchronizing systems which may use oscillators of the automatic frequency stabilized type. 455,  Telecommunications,   subclasses 91+ for radio transmitters using oscillators of the automatic frequency stabilized type; and subclasses 130+ for receivers using oscillators of the automatic frequency stabilized type.

2	Plural oscillators controlled
	This subclass is indented under subclass 1.  Subject matter wherein the generated frequency of two or more oscillators is automatically stabilized. The oscillators may have A.F.S. loops individual to each oscillator or a single A.F.S. loop may control two or more oscillators, or any combination of A.F.S. loops may be employed, provided the generated frequency of two or more oscillators is automatically stabilized. SEE OR SEARCH THIS CLASS, SUBCLASS: 46+, for plural oscillator systems in general.

3	Molecular resonance stabilization
	This subclass is indented under subclass 1.  Subject matter wherein the frequency discriminator or sensing means is of the molecular or atomic resonance type, comprising a medium, which may be solid, liquid or gaseous, the frequency selective properties of which are due to the vibration or oscillation of the individual molecule or atom itself and are not due to the translational motion of the molecule or atom as a whole. SEE OR SEARCH THIS CLASS, SUBCLASS: 9, for A.F.S. systems utilizing a distributed parameter type discriminator. 25+, for A.F.S. systems utilizing a signal or phase comparing type discriminator having at least two inputs for the signals (of the controlled oscillator and reference source) to be compared. 32+, for A.F.S. systems having a single input type discriminator. 78, for electrical noise or random wave generators wherein the translatory motions of charged or excited particles or molecules are utilized to generate oscillations. 94.1, for oscillators of the molecular or particle resonant type (e.g., maser) wherein a molecular or particle resonant solid, liquid or gas comprises the active element of the oscillator. SEE OR SEARCH CLASS: 250,  Radiant Energy,   subclass 251 for devices for producing and propagating a unidirectional stream of neutral molecules or atoms through a vacuum, usually at thermal velocity and including means to excite the molecules or atoms at a resonant frequency.

4	Search sweep of oscillator
	This subclass is indented under subclass 1.  Subject matter wherein means are provided, in addition to the A.F.S. loop, for varying the oscillator frequency through a range of frequencies. The systems classified herein are chiefly directed to means for restoring A.F.S. control wherein such control is lost due to the fact that the oscillator frequency for some reason, such as during "warm-up" or some other disturbance, lies outside the "capture" range of the frequency discriminator, the sweep range of frequencies includes the "capture" range of the discriminator and serves to bring the oscillator frequency within the "capture" range and thereby render the A.F.S. effective. SEE OR SEARCH THIS CLASS, SUBCLASS: 23+, for oscillators wherein low frequency modulation of the generated oscillations is utilized for A.F.S., signals representative of the frequency modulated oscillations being compared with signals of the modulating source. 106, for oscillator systems of the periodic amplitude and frequency varying type. 178, for oscillators in general provided with cyclic frequency sweeping means.

5	Magnetron oscillator
	This subclass is indented under subclass 1.  Subject matter wherein the oscillator provided with A.F.S. is of the magnetic and electric field controlled space discharge device type. SEE OR SEARCH THIS CLASS, SUBCLASS: 86+, for magnetron type oscillators in general, particularly subclass 88, for such oscillators provided with frequency stabilization means of the nonautomatic type.

6	Klystron oscillator
	This subclass is indented under subclass 1.  Subject matter wherein the oscillator provided with A.F.S. is of the beam tube type utilizing hollow resonator electron bunching or electron bunching and catching means for determining the generated frequency of the oscillator. Such oscillators are generally designated as klystron oscillators. SEE OR SEARCH THIS CLASS, SUBCLASS: 83, for multi-cavity type klystron oscillators. 84, for reflex type klystron oscillators.

7	Plural controls
	This subclass is indented under subclass 6.  Subject matter wherein the oscillator is provided with two or more oscillator frequency controls as part of the A.F.S. system. For example, the A.F.S. system for a reflex klystron oscillator may (1) control a motor to tune the cavity for coarse tuning and at the same time (2) supply a bias voltage to the reflector electrode for fine tuning of the oscillator. SEE OR SEARCH THIS CLASS, SUBCLASS: 10+, for single oscillators provided with two or more A.F.S. controls.

8	Transistorized controls
	This subclass is indented under subclass 1.  Subject matter wherein the A.F.S. circuit of the oscillator includes semiconductor means, such as transistors. SEE OR SEARCH THIS CLASS, SUBCLASS: 107+, for oscillators wherein the active element is of the solid state type and indented subclasses 108+ wherein the active element is of the transistor type.

9	Oscillator with distributed parameter-type discriminator
	This subclass is indented under subclass 1.  Subject matter wherein the A.F.S. control circuit of the oscillator is provided with a frequency sensing means of the distributed parameter type (e.g., cavity resonator). SEE OR SEARCH THIS CLASS, SUBCLASS: 3, for A.F.S. systems utilizing a molecular resonance type discriminator. 25+, for A.F.S. systems wherein the discriminator is of the signal or phase comparing type having at least two inputs for the signals (of the controlled oscillator and reference source) to be compared. 32+, for A.F.S. systems having a single input type discriminator.

10	Plural A.F.S. for a single oscillator
	This subclass is indented under subclass 1.  Subject matter wherein oscillator is provided with two or more A.F.S. means, each having a different effect in tuning the oscillator. For example, one A.F.S. means may provide a coarse tuning control and another A.F.S. means may effect a fine tuning control of the oscillator. SEE OR SEARCH THIS CLASS, SUBCLASS: 2, for plural oscillators involving plural A.F.S. control loops. 7, for klystron oscillators provided with two or more oscillator frequency controls as part of the A.F.S. systems.

11	Plural comparators or discriminators
	This subclass is indented under subclass 10.  Subject matter wherein the oscillator system is provided with two or more comparators or discriminators, each of which may be associated with a separate A.F.S. control. By way of examples (1) one discriminator may have a broad band frequency response characteristic and another discriminator may have a narrow band frequency response characteristic or (2) one discriminator may be of the frequency sensitive type, while another discriminator may be of the phase sensitive type.

12	With phase-shifted inputs
	This subclass is indented under subclass 11.  Subject matter wherein at least two comparators are so arranged that the outputs of the controlled oscillator and a reference oscillator are directly compared in one comparator and the output of the controlled oscillator is phase-shifted and then compared with the reference oscillator in a second comparator, the outputs of the comparators then being combined in a control circuit to effect tuning of the controlled oscillator.

13	Motor control of oscillator
	This subclass is indented under subclass 10.  Subject matter wherein at least one of the A.F.S. controls includes an electric motor for electromechanically adjusting a tuning reactance of the oscillator. Usually the motor control is utilized for coarse tuning while another A.F.S. control of the electronic type (e.g., including a reactance tube) is utilized for fine tuning of the oscillator. SEE OR SEARCH THIS CLASS, SUBCLASS: 24, for oscillators wherein low frequency modulation of the generated oscillations is utilized for A.F.S. and wherein the signal comparator is of the electric motor type. 29, for oscillators wherein an electromechanical signal comparator is utilized in the A.F.S. loop. 35, for A.F.S. oscillators with electromechanically controlled oscillator frequency adjusting means.

14	With intermittent comparison controls
	This subclass is indented under subclass 1.  Subject matter wherein means are provided to open and close the A.F.S. loop of the oscillator periodically. By way of example, a periodically actuated switch may connect the input of the comparator in the A.F.S. loop of an oscillator alternately to the oscillator output and the reference oscillator output.

15	Amplitude compensation
	This subclass is indented under subclass 1.  Subject matter wherein the A.F.S. loop circuit of the oscillator is provided with means to limit the amplitude, or to maintain the amplitude constant, or to correct or otherwise compensate for undesired changes in amplitude of a control signal in the loop circuit, or to disable the A.F.S. loop if the amplitude of the control signal (e.g., reference oscillator signal) is below a predetermined level (e.g., the level wherein noise signals may adversely affect the oscillator A.F.S. and thereby cause the generated frequency to be shifted outside its proper range). SEE OR SEARCH THIS CLASS, SUBCLASS: 40, for beat frequency oscillators provided with amplitude adjustment or control means. 109, for transistor oscillators provided with amplitude stabilization or control means. 182+, for oscillators in general with amplitude control or stabilization.

16	Tuning compensation
	This subclass is indented under subclass 1.  Subject matter wherein the A.F.S. controlled oscillator includes means for setting the frequency of the generated oscillations to any selected frequency within a predetermined frequency range, means being provided to interrupt or otherwise deactivate the A.F.S. control loop of the controlled oscillator while changing the generated frequency from one value to another so as to prevent the A.F.S. loop from tending to hold the oscillator frequency to its previously set value, that is, the A.F.S. tends to drag. SEE OR SEARCH CLASS: 455,  Telecommunications,   subclasses 255+ for similar A.F.S. systems utilized in radio receivers.

17	Particular error voltage control (e.g., intergrating network)
	This subclass is indented under subclass 1.  Subject matter wherein the A.F.S. loop of the oscillator is provided with means to modify or control the error voltage so as to compensate or correct for undesired changes in the error voltage brought about by some deficiency in the oscillator system, such as a drift in reference frequency, hunting due to instability around the A.F.S. loop, undesired shift in phase of side band frequencies, and so forth.

18	With reference oscillator or source
	This subclass is indented under subclass 1.  Subject matter wherein the A.F.S. circuit of the controlled oscillator includes a source of constant frequency, which source serves as a frequency standard, means being provided for comparing the frequency of the oscillations generated by the controlled oscillator with the standard frequency, the frequency comparison means producing an electrical control or error signal which is a measure of the deviation, in direction and amount, of the controlled oscillator frequency from the standard frequency, the control or error signal being applied to the frequency adjustment means of the controlled oscillator in such a manner as to reduce the frequency difference between the generated oscillations of the controlled oscillator and the oscillations of the constant frequency source to a minimum. SEE OR SEARCH CLASS: 329,  Demodulators,   subclasses 323+ for a frequency demodulator using an oscillator and subclass 346 for a phase demodulator using an oscillator.

19	Spectrum reference source
	This subclass is indented under subclass 18.  Subject matter wherein the reference source is a spectrum generator producing a plurality of waves of different frequencies, which waves are simultaneously fed to the comparator or discriminator of the A.F.S. circuit of the controlled oscillator means being provided wherein the generated waves of the controlled oscillator may be selectively locked-in to the desired frequency of the spectrum. (1) Note. This subclass does not include those systems wherein a filter or harmonic selecting network is provided for selecting a single frequency from the range of frequencies of the spectrum source, the selected frequency being fed to the comparator. Such systems are elsewhere in subclasses 1+, depending on the type of oscillator or type of A.F.S. control.

20	T.V. sync type
	This subclass is indented under subclass 18.  Subject matter wherein the oscillator system is of the type peculiarly adapted for use in television systems (e.g., vertical or horizontal sweep generators), the reference source comprising synchronizing pulses (usually of short duration) which pulses are compared with signals representative of the generated waves of the controlled oscillator in a phase comparison means, the resulting control or error signal from the comparison means being utilized to adjust the frequency of the controlled oscillator to restore it to the desired relationship with reference frequency. SEE OR SEARCH THIS CLASS, SUBCLASS: 143+, for relaxation oscillators in general, especially indented subclass 145, for multivibrators with synchronizing, triggering or pulsing circuit, indented subclass 149, for blocking oscillators with synchronizing, triggering or pulsing circuit and indented subclass 153, for relaxation oscillators in general with synchronizing, triggering or pulsing circuit. SEE OR SEARCH CLASS: 315,  Electric Lamp and Discharge Devices: Systems,   subclasses 1+ for cathode-ray tube circuits, especially indented subclasses 378, 379+ and 391+ for cathode ray sweep circuits which may utilize sweep generators with A.F.S. control combined with significant cathode ray control means or cathode-ray tube structure, e.g., deflecting plates, deflecting coils, etc. 348,  Television,   subclasses 536+ for automatic frequency stabilizing systems as an element of a more comprehensive system, e.g., controlled oscillator combined with sync separator. 375,  Pulse or Digital Communications,   subclasses 354+ for synchronizers in general.

21	Lock to power line
	This subclass is indented under subclass 20.  Subject matter wherein means are provided to stabilize or lock the controlled oscillator to the low frequency (e.g., 50, 60 C.P.S.) power current derived from a commercial alternating current supply line.

22	Plural significant heterodyne stages
	This subclass is indented under subclass 18.  Subject matter wherein two or more frequency beating or heterodyning means, or stages, are effectively connected in the A.F.S. loop of the controlled oscillator. SEE OR SEARCH THIS CLASS, SUBCLASS: 31, for oscillators with plural heterodyne stages in the A.F.S. loop, but wherein the discriminator is of the single input type, i.e., no comparison of the oscillator frequency with a reference frequency or source is made at the discriminator. 38+, for beat frequency oscillator systems including plural beating or heterodyning means.

23	Sensing modulation (e.g., frequency modulation controlled oscillator
	This subclass is indented under subclass 18.  Subject matter wherein means are provided to frequency modulate the generated oscillations of the controlled oscillator by a low frequency reference source, signals representative of the modulated signal being compared with signals from the low frequency reference source in a signal comparator, the error signal from the output of the comparator being utilized to restore the generated frequency of the oscillator to the desired relationship with the reference frequency. SEE OR SEARCH THIS CLASS, SUBCLASS: 4, for A.F.S. oscillators wherein the frequency of the oscillator is swept over a range of frequencies so as to bring the generated frequency within the "capture" range of the discriminator.

24	With motor comparator
	This subclass is indented under subclass 23.  Subject matter wherein the signal comparator comprises a polyphase motor (usually of the two phase or split-phase type) having at least two phase windings, the signal representative of the low frequency modulated oscillations of the controlled oscillator being applied to one phase winding and the signal representative of the low frequency source being applied to another phase winding of the motor, the rotor of the motor is mechanically coupled to the frequency adjusting means of the oscillator. SEE OR SEARCH THIS CLASS, SUBCLASS: 13, for a single oscillator with plural A.F.S. means wherein at least one of the A.F.S. means includes an electric motor for adjusting an oscillator frequency control means. 29, for A.F.S. oscillators with electromechanical signal comparator means. 35, for A.F.S. oscillators with electromechanical controlled oscillator frequency adjusting means.

25	Signal or phase comparator
	This subclass is indented under subclass 18.  Subject matter wherein a particular or significant signal or phase comparing network is utilized in the A.F.S. loop of the controlled oscillator. SEE OR SEARCH THIS CLASS, SUBCLASS: 11, for single oscillator with plural A.F.S. means and including plural comparators. 32, for A.F.S. systems wherein the discriminator is of the single input type and wherein no signal comparison is made. SEE OR SEARCH CLASS: 324,  Electricity: Measuring and Testing,   subclasses 76.52+ for systems for measuring the frequency of a cyclic current or voltage by phase comparison with a standard cyclic current or voltage, and subclasses 76.77+ for systems for measuring electricity utilizing phase comparison means (e.g., phase comparison between cyclic pulse voltage and sinusoidal current). 327,  Miscellaneous Active Electrical Nonlinear Devices, Circuits, and Systems,   subclasses 2+ for miscellaneous phase comparison circuits and subclasses 39+ for miscellaneous frequency comparing circuits. 329,  Demodulators,   subclasses 315+ for frequency demodulators and subclasses 345+ for phase demodulators.

26	Plural diode type
	This subclass is indented under subclass 25.  Subject matter wherein the signal or phase comparator includes two or more unilaterally conducting devices (e.g., diodes) as elements thereof. SEE OR SEARCH THIS CLASS, SUBCLASS: 33, for oscillator systems utilizing a single input, nonsignal comparing, discriminator of the plural diode type in the A.F.S. control loop thereof.