US2446106A - Phase shift oscillator - Google Patents

Description

- July 27, 1948. D, s, ROBERTSON 2,446,106

PHASE SHIFT OSCILLATOR Filed June 29, 1944 gee ' l N V EN TOR. 04:40 ,5. P055? 7.90m

Evy-hagg- A TTORNEY Patented July 27, 1948 UNITED STATES PATENT, FFICE PHASE SHIFT OSCILLATOR David Stirling Robertson, Sydney, New South Wales, Australia, assignor to Amalgamated WirelessiAustralasia) Limited, Sydney, New South Wales, Australia, a company of New South Wales, Australia Application June29, 1944, Serial No. 542,797

In Australia November 5, 1943 1 I This invention relatesto phase shift oscillators andmoreparticularly to an improved method of stabilizing the output frequency and voltage from such oscillators. i

,A phase shift oscillatorconsists of an amplifier with a delay or phase shift network connected in the feedback ,path between itswoutput and input terminals. In order to produce self-sustained oscillations in the circuit, two conditions must be satisfied. Firstly, the voltage introduced from the output of the amplifier-to its input must be in phase with the input voltage; secondly, the overall amplification of the amplifier must be such as to compensate for attenuation caused by the network.

The first requirementis fulfilled by theuse of a non-resonant, multi-section, phase shifting network connected between the output and input of the amplifier valve or valves, each section of the phase shifting network comprising a series resistance and a shunt capacitor or vice versa. Since it is difiicult to obtain the desired phase shift of 180. degrees, in-two filter networks each composed of resistance and'capacity, three or more sections are usually employed.

.4 Claims. (01. 250- 3e) The second condtion is met by the use of an amplifying valve having an amplification factor equal to, or greater than, the attenuation of the phase shifting network.

In the past, considerable dificulty has been experienced in obtaining satisfactory operation from oscillators of the type referred to. For example, variations in gain of the amplifier are liable to occur over a period of time brought about mainly by changes of the plate resistance of the amplifying valve as a resultof fluctuations in the supply voltage. If the change in amplification is in a direction to cause the output to fall below the level necessary to compensate for the attenuation of the filter, oscillations will cease. Furthermore, as the plate resistance forms part of the phase shifting network, any variation of plate resistance will cause a change in the frequency of oscillation. This latter effect may be consider-j ably reduced by coupling the anode of the ampliher to the input of the phase shifting network through an impedance transformer, such as a cathode follower stage.

Also, the amplitude of oscillation is only limited by the curvature of the valve characteristic so that, if the gain of the amplifier is increased, see

of the valve characteristic, but on the dela bias applied tothe av cdrectifier, i i 1 By using both of the expedient-s outlined above, a fairly undistortedioutput can be achieved and the frequency stability of phase shift oscillators greatly improved. l q x It has been found, however, that the A, ;V, C. filter has a small effect onthe frequency of oscil; lation, andalso that theos'cillator isliable to stop and start at a frequency determined by the -time constant of the A. V. C. filter, (i. e.,'a low frequency, relaxation oscillation is liable to occur).

The object of the present invention is to provide a phase shift oscillator-of simple construction and improved frequency .Stabilityjn which all the desirable characteristics of ,prior art practice are retained and the attendant dis-advantages avoided. i The above stated objective is achieved, inaccordance with this invention, by providing animproved circuit arrangement which eliminates the customary A. VwC. filter and allows the A. V. C. bias to be fed into the inputof the phase shifting networkt-ogether with thefeedback voltage.

Broadly, an oscillator arrangement according to the subject invention comprises a thermionic valve having cathode, anode and .controlgrid electrodes, a feedback path connected between said anode and grid, ,animpedance transformer and. a non-resonant multi-section phase shifting network serially connected in said feedback path, means for rectifying a portion of the output .-energy from the oscillator to derive an, automatic biasing potential, and means for appyling said potential to the control grid of said-thermionic valve through the. phase-changing sections of said network, simultaneously: with application ;to saidvalve of feedback energy. h l i For a more complete understanding of the ,in-,- vention, and the manner iin. which it to be carriedout, attention is-nowdirected to the following description in connection with the ac: companying drawing which illustrates one;prac tical example of the invention. i i r Referring to the drawing, therein is illustrated an electron discharge device-provided with a feedback circuit which includes animpedance transformer and a three section non-resonant phase shifting network lmadeup of series conmeted-resistances and parallelconnected capacitties. The phase shiftingnetworkfunctionsto produce a voltage on the control electrode ofithe device which is substantially oppositeinphase to the voltage in its anode.

i In rexampleyunder consideration, the

cation is equal to or greater than the attenuation of the phase shifting network.

Operating potentials are supplied to the anode 6 of the valve VI from a source of D. 0. supply (not shown) through an anode load resistor 32 and lead 3I. Biasing potentials-for the screen grid 5 are taken from a potentialdivider compris ing the resistors 33, 34, 28 connected across the D. 0. supply source (not shown). The screen grid is held at cathode A. C. potential by means of the 5-;

' the condenser 24.

bypass condenser 35.

More specifically the anode 6 of the valve VI .is coupled to the control grid I3 of a cathode follower valve V2 through the D. C. blocking condenser I4. The valve V2 is shown as a triode having a cathode I5 and an anode I6, and a control grid I3. Although the valve V2 is shown as a triode, a screen grid pentode or other suitable type of valve may be employed without affecting the scope of the invention.

The anode I6 of the valve V2 is connected to the high potential terminal I8'of a source of D. 0. supply (not shown) through the lead I'I, whilst the cathode I5 is connected to the earth I9 and low potential terminal 20 of the D. C. supply source through the series connected resistors 2|, 22.

4 voltage impulse is applied through condenser I4 to the grid I3 of valve V2. The valve V2, as already stated, functions in known manner as an impedance transformer. The voltage developed between the cathode and ground is almost equal to the grid-to-ground voltage and is of the same phase. So far as frequency of oscillation and attenuation in the feed-back path is concerned, this valve may be neglected. The use of an impedance transformer of this type, however, re-

' duces to anegligible value the undesirable effects, of the valve VI, on the operating characteristic of the circuit.

Biasing potentials, derived from the potential I drop across the resistor 2|, are applied to the grid I3 of valve V2through the grid resistor 23.

The valve V2 thus connected functions in known manner to couple a high impedance input into a low impedance load.

The output of the valve V2 is developed across the serially connected cathode resistors 2 I, 22 and is applied to the input of the filter through the coupling condenser 24 and the lead 25. The phase changing network comprises the series connected resistors I, 8, 9 and the parallel connected condensers I0, II, I2 arranged in a three-section inverted L network.

The output from the cathode resistors 2 I, 22 is also applied through the condenser 24 to the anode 26 of a diode rectifier. Inthe present example the diode rectifier is shown as comprising a pair of anodes 26, 26a included in an envelope also common to the above mentioned electrodes of the valve VI, and cooperating with a'portion of 'the cathode 3 thereof. 'Such valves are well known and provide a convenient method of obtaining the desired functions with a minimum of component parts. It is not desired to limit the invention to the precise arrangement shown, as equal results may be obtained by the use of a separate rectifier.

The anode 26 of the diode rectifier is connected to cathode 3 of the valve VI through a D. C. path comprising resistors 21, 28 the junction point of said resistors being also connected to earth I9.

Resistor 28 and condenser 29 are employed'to provide a negative biasing potential which is applied to the grid 4 of the valve VI through resistor 21, leads 30, 25, and resistors I, 8, 9.

As the biasing path for the control grid 4 is connected to the diode anode 26, the biasing potentials developed across the resistor 28 also serve as a delay bias for the dioderectifier.

When operating potentials are applied to the circuit thus described, a voltage impulse is developed across resistor 32 of the valve VI. This The voltage impulse appearing across the cathode load resistor of the valve V2 is applied to the input of thephase shifting network through This voltage impulse, when analyzed, is composed of a number of alternating current voltage trains, each train having a different frequency. It is well known that components of different frequencies travel through networks of-the type illustrated at difieren't velocities so that their relative arrival times at the end of the network differs from their relative starting times. The velocity of any one train through the filter is dependent upon the values of resistances and capacities chosen in constructing the network. The values of resistance and capacity are chosen so that a train of A. C. voltage, having a predetermined frequency, is so delayed in the network that the voltage appearing across the output and applied to the grid 4 is degrees out of phase with the voltage of the same train when it enters the network through the coupling condenser 24.

I This causes the valve VI to sustain oscillations at the frequency whose network input and output voltages are 180 degrees out of phase.

Output potentials, from the valve 2, which are applied to the input of the phase changing filter network are also fed through the conductive connection 30 to the rectifier anode 26. The D. C. component of the rectified energy developed across the rectifier load resistor 21 is applied to the control grid 4 through the conductive path including the connection 30 and the network resistors I, 8, 9.

It will thus be seen that in a circuit arrangement in accordance with this invention, the customary A. V. C. filter is dispensed with and the A. V. C. and feedback potentials are fed by a common path through the phase'changing filter network to the control grid 4 of the valve VI. The elimination of the customary A. V. C. filter removes from the circuit the undesirable effects of the time constants of such filters and greatly improves the stability of the oscillator.

I claim: I

1. A phase shift oscillator in combination with a system for stabilizing the amplitude and frequency of the oscillations generated, the components of said combination being cons'titutedas a circuit arrangement having an oscillation generating discharge tube in which the electron emission from a cathode traverses a grid-controlled space path to a main anode and an uncontrolled space path to at least one rectifier anode, a capacitively shunted resistor connected between the cathode and ground, a resistive connection between ground and a grid which controls said controlled space path, said connection including a resistance'element in series witha phase shifting network, a connection from said rectifier anode, to the junction between said resistance element and said network, and a feedback circuit coupled between the main anode and said junction.

2. The combination according to claim 1 wherein said feedback circuit includes electronic means for amplifying the feedback potential-s.

3. The combination according to claim 1 wherein said resistive element in the connection between ground and a grid constitutes means for so biasing said rectifier anode that rectifier action takes place only when the output energy exceeds a predetermined value.

4. In apparatus for generating oscillatory energy of substantially constant frequency and amplitude, a first electron discharge device having input and output electrodes including an anode, cathode and control electrode and also having an additional electrode cooperating with said cathode to form a rectifier, a second electron discharge device having electrodes including an anode, a cathode and a control electrode, connections for applying electro-positive potentials to the anodes of said tubes and biasing potentials to the control electrode of said last tube, a coupling between the anode of said first named tube 6 and the control electrode of said second named tube, an impedance in the cathode return of said second named tube, a phase shifting network coupling the cathode of said second named tube to the control electrode of said first named tube, a rectifier circuit including an impedance connecting said additional electrode to the cathode of said first tube, and a connection between the input end of said phase shifting network and said rectifier circuit to control the bias on. the control electrode of said first named device.

DAVID STIRLING ROBERTSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Edson Mar. 7, 1944 OTHER REFERENCES Number

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