US2583837A - Thermionic valve oscillator - Google Patents

Description

Jan. 29, 1952 B. M. HADFIELD r 2,583,837

THERMIONIC- VALVE OSCILLATOR Filed Sept. 24, 1945 2 SHEETS-SHEET 1 F W w a w 1 I 1 v o m l E ER 1 6! L v h v 0- INVENTDR T I BERTRAM MORTON HADFIELD ATTORNEY Jan. 29, 1952 LD 2,583,837

THERMIONIC VALVE OSCILLATOR Filed Sept. 24, 1945 2 SHEETS-SHEET 2 Harmonic INVENTOR BERTRAM MORTON- HADFIELD ATTORNEY Patented Jan. 29, 1952 land, assignor to Automatic Electric Laboratories Inc., Chicago, 111., a corporation of Delaware Application September 24, 1945, Serial No. 618,348 In Great Britain October 13, 1944 Claims. (01. 250'-36) The present invention relates to thermionic valve oscillation generators and is more particularly concerned with generators capable of delivering power to an output impedance.

One of the objects of the present invention is the provision of an oscillation generator employing a single thermionic valve both for the generation of oscillations and as a source of power of the oscillating frequency. A further object of the invention is the provision of an oscillation generator which will deliver a substantially constant output voltage of sinusoidal form over a wide range of values of the output load impedance According to one feature of the invention, a frequency selective transfer device is connected between the anode circuit and the grid-cathode circuit of a thermionic valve and the power output is taken from an impedance included in the cathode lead.

According to a further feature of the invention, a frequency selective transfer device is connected between the anode circuit and the grid/cathode circuit of a thermionic valve and means are provided for introducing a non-linear voltage regulating action in the anode circuit of the valve.

The valve employed may be a triode buttit is preferred to use avalve of the tetrode or pentode type, the non-linear characteristic of which may provide at least part of the non-linear voltageregulating action.

Alternatively or in addition the voltage-regulating action is provided by a non-linear network connected in the anode circuit and acting as an amplitude compressor or amplitude limiter. In such a case it is advantageous that the valve employed should be a pentode or a tetrode, since any change in anode circuit impedance due to the voltage regulating action will be substantially without effect on the relationship between the input voltage to the grid/cathode circuit and the current flowing in the cathode lead.

The invention will be better understood from the following description taken in conjunction with the accompanying drawings.

Fig. 1 illustrates the invention diagrammatically,

Figs. 2 and 3 show by way of example two alternative embodiments of the invention, and

Fig. 4 shows certain curves illustrating the results obtained with different values of the circuit parameters.

Referring first to Fig. 1, the valve VI combines the functions of an oscillator and output valve. The preferred type of valve shown is a pentode or tetrode and the normal direct current supplies are not shown; the currents i in the cathode, km in the anode, and (1-k)i in the screen electrode being the alternating component magnitudes, where It is the anode current-cathode current ratio the value of which is substantially constant when the valve is operated over its substantially linear ranges of voltage and current. The valve may be of the triode type, when there is no screen electrode and It becomes unity, but is not preferred since as mentioned above, it is desired that the relationship between the cathode current and the grid input voltage e2 shall be substantially unaffected by any changes in the anode impedance, for instance those due to the amplitu'de regulating action.

The output energy is taken from the cathode circuit and is dissipated in the load resistance L. The alternating anode voltage is developed by the passage of k1 through the net anode circuit impedance, Z, which is constituted by the shunt circuit comprising resistance R, amplitude regulating device A, and transfer device B. one form for A is shown within the block and comprises a resistance 53 in series with back-to-back rectifiers ME individually biased by voltages V. e The transfer device B has a voltage transfer ratio M at the desired frequency of oscillation determined primarily by the production of the necessary phase angle so that the output vbuage l is in phase with the grid input voltage e2.

Assuming for the moment that the dotted connection F is cut and that a sinusoidal input voltage of magnitude e2 produces an in-pliase sinusoidal output voltage el at the desired frequency, the necessary condition for self-oscillation when Fijs completed is that el shall be equal or greater than e2. Now el:7c.i.Z.M, and e2:i(L-|-r), where r is the working grid voltage/cathode current mutual resistance of the valve. More accurately, where a is the amplification factor and V0 the output voltage,

but for the purposes of the present invention the cathode follower factor ;r[ 1 will be neglected.

Hence for self oscillation:

L+r2MJIz (1) There is thus an upper limit to the value of load resistance L, but, provided any lower value continues to satisfy (1), there is no lower limit other than that set by the incidence of the overload voltages and currents of the valve. The practicable lower limit to L may therefore be assumed to be that which will givethe maximum output energy for a given set of operating conditions for the valve, as in normal output stage design.

cuit is suitably designed so that the anode voltage excursions are within the normal limits, the design for power output in the cathode circuit can proceed on the assumption that the valve is a triode acting as a cathode follower. In these circumstances a minimum value for L in terms of r for maximum power output may be obtained, which will form a useful reference point in subsequent design; for it will be noted from (1) that the upper limit to L can be made higher at will merely by increasing the value of the voltage gain M.

As regards such a minimum value for L, it is unlikely that the load resistance will be permitted to fall below the known practical value of 2.Ra, where Rd is the assumed linear anode impedance. of the valve regarded as a triode; lower values will give a poorer conversion efiiciency from the direct current supply without any corresponding advantage in other respects. since 1' may be defined as Ra/u, Where c is the amplification factor of the valve regarded as a triode, then the likely minimum value for L/r is 2.Ra,' l/Ra, i. e. 2. A more strict analysis taking the power law characteristic of the valve into account can be shown to give a value of The lowest value of ,u likely to be used is that of the'power output type of valve, and is approximately 6. Hence on these grounds the likely minimum usable value for L/r 'will be, say, 10.

It will be understood that changes in the load impedance L will cause changes in the cathode current i and hence in the anode current .This will result in variations in the output volta e and the purpose of the amplitude regulating device A is toreduc'e these variations. Further by employing a non-linear network, smaller variations of'output voltage occur than if a linear network is used. To this extent the nonlinear network may be termed an amplitude compressor if it does not produce waveform, distortion or an amplitude limiter if it does. iIhe voltage 6! is fedinto the input circuit of the oscilation generator to maintain the valve in a state of oscillation, the voltage limiting device A being adapted to limit equally both halfwaves of the alternating voltage applied thereto. The limiting device will pass little or no current until the voltage applied exceeds the value of the bias voltage V at which time the impedance falls to a low value to allow the flow of a large amount of current. The maximum generated voltage, if the device is to operate perfectly, is dependent upon the polarizing voltage V of the rectifiers or. in the event that a gaseous discharge device is employed, upon the flashing voltage of the tube. The polarizing voltage for the rectifie'rs in Figure 1 is provided by a set of batteries and in Figures 2 and 3 by a tap on potentiometer R which is adjusted so that' the"potentiometer will produce the same In the case where VI consists of a pentode or tetrode, then, provided the anode cirinternal resistance regardless of which rectifier is conducting. If the device A operated perfectly, then above a certain value of Ici the output el would be constant and, provided Equation 1 were also always satisfied it is clear that the load voltage regulation would be due solel to the mutual grid resistance of VI; that is, V0/e2=L/(L+r), and the output voltage V0 would fall as L is decreased. It is therefore possible to improve this state of afiairs by a1- ranging that regulating device A is not perfect, but has a rising output 6! with rising input by causing it to have a regulation, or internal resistance effect, of equal and opposite sign to that produced by 1'. The internal resistance effect is produced by means of resistance S in Figure 1, SI or'the combination of s2, S3 and RI and R2 in Figure 2. In this manner,,whenever the oscillator output is such that A is regulating to maintain the voltage at the desired value the feedback voltage to the valve VI will vary with increased load to thereby maintain a constant output even while device A is operating and will eliminate to some extent the heretofore described difiiculties encountered wherever A is a perfect operating device.

Referring now to the specific embodiments shown, by way of example, in Figs. 2 and 3, Fig. 2 shows a form 'of the invention in corporating a shunt connected tuned circuit as I the transfer device. The anode resistance R of the tetrode valve V2 corresponds to R in Fig. 1, but is shunted by a choke La. inorder to utilise fully the available direct current supply between the positive and negative supply leads. The transfer device is comprised by series resistance P and shunt transformer Tl whose primary is tuned to the desired frequency by condenser Cl, the phase of the voltage appliedto the grid of V2 being determined by the poling of the connections of the secondary winding, whilst the overall M value can be adjusted by the transformer ratio. The load resistance L is coupled to the cathode connection via transformer T2 whilst series resistance H (which may be comprised in part by the resistance of the cathode winding of T2) gives the normal grid bias voltage. The amplitude regulating device may be either the neon tube N and series resistance Si shown connected by dotted lines, or rectifiers MR2, MR3 polarised by voltages V derived from a supply potentiometer RI and R2.- In the former case the series resistance SI plus the differential resistance of N form the controllable regulation of the device adjusted to give a substantially constant output voltage for loads less than about one-third the maximum resistance sustaining oscillation, the striking voltage of the tube corresponding to the bias voltages V. In the second case series resistances S2, S3, are arranged in conjunction with the resistance Sc which comprises the bias voltage potentiometer R! and resistance R2 to give the same internal resistance. whether MR2 or MR3 be conducting, and of value .to give the above-mentioned constant output. Condenser C2 serves to couple the circuit to the anode of V2 whilst resistance Q enables the bias voltages to appear. individually on the respective rectifiers, and to be adjusted by the tapping so that the regulating effecj'd takes place over equal portions of the positive and negative anode alternating waveforms.

In the alternative arrangement shown in Figp of the devices A and B. The transfer device B now consists of three or more resistance/capacity networks which produce a phase angle of 130 at the desired oscillation frequency; four such networks are shown, RC, RICI, R2C2, R303, utilising the anode resistance R as part of the first network. Condenser C4 couples the transfer device to the grid of the pentode or tetrode valve V3, the necessary grid bias being obtained by cathode resistance 1'! and passed to the grid via resistance R4. The load L is coupled to the cathode via transformer T3. The amplitude regulating device A may again consist of a neon tube N and regulation resistances SI as for Fig. 2, or of an alternative form of the rectifier circuit by connecting the centre tap of the choke La to the positive supply lead whereby, rectifiers MR4, MR5 can be operated in a full wave circuit and biased by a common potential V derived from the supply potentiometer R5, R6, the resistive impedance looking into the tapping point constituting the regulation resistance.

It will, of course, be understood that the invention is not limited to the particular circuits shown in Figs. 2 and 3 and in particular other forms of amplitude regulating device may be used, such as, for instance, the known bridge connected lamp resistor circuits which do not introduce appreciable waveform distortion. The factors governing the design of circuits according to the invention will now be discussed with reference to Fig. 1 taking the typical waveformdistorting amplitude regulating device shown within the rectangle A.

Assuming the linear resistance R includes any 1- other linear resistance in the anode circuit (e. g. the input impedance of B at the oscillation frequency), then, when currents pass through S, the voltage waveform across R for a sinusoidal current 70. 2' will have a discontinuity at an angular displacement of a, and at 1ra, 'rr-I-(Z, 21ra and so on. Between the first two and last two angles, the waveform will consist of part of a sinusoid of reduced amplitude (due to the reduction of anode circuit impedance by the switchingin of the resistance S). The magnitude of the reduction of the maximum amplitude of the waveform from the previous sinusoidal value may therefore be denoted by the fraction 0, where R R-l-S I Fourier analysis of the resulting anode voltage waveform gives the following maximum amplitudes for the various harmonic components:

' 0 Fundamental R.lc.z'{1 c[1 a -ta) (2) 6 where .r represents the expression within the brackets. Also when the anode current attains the value ki sin a, the voltage on R is just equal to the rectifier bias voltage V i. -e. kiR sin a=V. Hence IciR=l- Sln (1 Further S1111 it Since a must clearly lie between 0 and 90 inclusive over the working range, then by as suming such values for a, and taking specific values for c and m in turn, the corresponding values of L/r may be obtained from (5). Using these in (7) gives the output voltage regulation curves in terms of L/r. The maximum L/r value will be obtained when the amplitude regulating device is just not functioning, i. e. when a is 90, which makes the bracketed terms in (5) unity independent of the value of 0. Since it has been shown that the minimum usable value of L/r is likely to be 10, then values of M .k.R/r less than 11 need not be considered; that is, m should be taken less than 0.091. As regards 0, its value must lie between 1 and 0 inclusive; a value of 1 inferring that resistance S is zero and that the amplitude regulation is as drastic as possible, whilst a value of 0 infers that S is infinite and that there is no amplitude regulation. Since the object is to use of value of S which will give a constant output voltage with changes in L/r, consideration of the equations shows that this is only possible when sin 2a tends towards 2a radians. Under these conditions so that if c is made equal to l-m, then V0 will be independent of L/r and will be given by If this value of c be taken, then the output voltage curves against L/r will rapidly tend to a constant value. Some typical curves are shown in Fig. 4 for L/r values between 1000 and 2 and for three values of m at 0.1, 0.01 and 0.001; the full line curves being for the correct value of c (equal to 1m), and the dashed curves being for the 0 value of 1. It is of course possible by taking arbitrary value of 0 less than 1-mto obtain a rising output voltage with chan es in L,

and vice-versa, but-generally speaking the equale ity will be used.

It will be observed from Fig. 4 that the full line curves show substantially constant output for values of L/r less than about one-third of the maximum value permissible for oscillation, even over the wide range of m values taken, so that provided the m value is fixed at a sufficiently low value the actual range over which constancy is obtainable (assuming that L/r: is the usable minimum) can be increased at will. For instance, when m is 0.001, there is a constant output range for L/r of from 300 to 10, or 30:1. With a given setup the value of m will of course vary with changes'in r, i. e. with life changes in the mutual resistance of the valve, but 0 may be restored by havingthe resistance S variable and testing for an output voltage change with load variations. The actual output voltage will then be different but may be independently restored to normal by adjusting the value of V. The net result of these adjustments will be that the maximum permissible L/r value for just sustaining oscillation will be different, but this is of little importance if the maximum value used lies at or below about one-third this value, i. e. on the constant output range. An alternative method for taking up changes in T which does not afiect any other feature, is to make r artificially largerby means of a series cathode resistance in the initial design and to vary this resistance according to the changes in r. The cathode bias resistance T1 in Figs. 2 and 3 can be used for this purpose, since, except at the value of L giving maximum watts output, the grid bias for a cathode follower is not a critical matter.

With the type of amplitude regulating device described above, the harmonic output of the oscillator depends largely on the efficacy with which the frequency selective circuit disposes of such harmonics as are generated. In the present oscillator the harmonic percentage will vary with the load L, and by taking the ratio of Equations 3 or 4 with 2 an estimate of the general harmonic with changes in a may be obtained. Taking the third harmonic as a typical example:

Third harmonic ratio:

20 sin 4a sin 2a a 4 2 2 MM 1r 2 For the case when sin 4a tends to 4a radians, i. e. over the constant output range before-mentioned, this expressionbecomes,

This agrees with the well-known fraction of 0.333 when-the waveform becomes rectilinear, i. e. for 0:1, but it is of interest to note that when 0 is not 1 as in the present case, then the ultimate harmonic ratio is not 0.333 but zero. Hence over the desired range of L/1- values the harmonic ratio will have a maximum valueand willtend to zero as L/r tends either to the maximum sustaining oscillation or to zero. Furthermore this will be the case for any order of harmonic, so that the harmonic curves shown dotted in Fig. 4 for the third may be taken as representative of the tendency, although the higher orders will, of course, have lower maxima. 1

Whilst these generated harmonic'ratiosmay appear large it should be noted that the derivation is quite general and applies equally well to all oscillators using the above form of amplitude regulating device, whether intentional or other: wise. For instance, if the oscillator circuit has fixed parameters (1. e. L fixed) the design must be based on a margin of say 6 db if oscillation is to be consistently ensured. This means for instance that the design figure for L as a' fixed value must be one half that which will just sustain oscillation, i. e. L/ 1* is one half its maximum value. Reference to Fig. 4 will show that the third harmonic then generated is about 27% which is not significantly difierent fromthe maximum of the circuit according to the invention of 33% from the point of view of designing the frequency selective circuit for adequate sup pression in the output. A resonant circuit of Q value equal to 50 (Fig. 2) will give at least 40 db suppression of the third harmonic, so that the maximum output third harmonic of the circuit according to the invention may attain a value of 0.33% as compared to 0.27% for a conventional fixed parameter circuit. Harmonic generation in the remaining portions of the circuit is reduced to a minimum with the present invention, even when the minimum load ratio L/r:10 is used and the watts output is the maximum pos sible for the valve working as a triode. For with this ratio the distortion due to the non-linear valve characteristics is reduced by the cathode follower action to the degree of 21 db feedback.

The oscillator may be used as a source of alternating energy of constant or variable fre" quency, and has particular application to the provision of signal tones for transmission over communication circuits to effect the operation of selective receiving devices, since it may be made to feed many such circuits at a time without significant change in transmitted signal level, and is of compact and economic form for moun ing as common equipment in telephone exchanges and the like.

I claim:

1. An oscillation generator comprising a therm: ionic valve having a cathode, a cathode lead connecting the cathode to ground, an input circuit, and an anode circuit, a frequency selective transfer device comprising a series resistance, a transformer and a condenser coupling said anode and input circuits, one of the windings of said transformer tuned to a desired frequency. by means of said condenser, an elementin said cathode to ground lead, a load impedance'inductively coupled to said element, whereby a constant voltage is supplied to said load impedance over a wide range of values of the impedance.

2. An oscillation generator as claimed in claim 1 in which said thermionic valve is of the type which has non-linear characteristics, and having an amplitude regulating device comprising a non-linear network for introducing a nonlin': ear regulating action in said anode circuit, to

- thereby act as an amplitude limiter or an ampli-;

tude compressor.

3. An oscillation generator comprising a thermionic tube having an anode, a grid (and a cath ode circuit, an element in said cathode circuit, an

output circuit coupled to the cathode circuit across said element, positive and negative supply leads, a non-linear regulating device compris: ing a full wave rectifier arrangement anda p0: tentiometer connected across said supply lines, said rectifier arrangement connected between points on said potentiometer and said positive lead, and a transfer device comprising a series resistance, a shunt transformer, and a condenser connected between said grid, anode and regulating device, one of the windings of said transformer tuned to a desired frequency by means of said condenser, in which the internal resistance of said non-linear device is eifective to provide a substantially constant output in said output circuit for values of said load impedance less than approximately one-third the maximum value of impedance which sustains oscillation of the generator.

4. An oscillation generator comprising a thermionic tube having a grid cathode circuit and a cathode anode circuit, a non-linear voltage regulating circuit including an amplitude regulating device and a voltage transfer device connected in said cathode anode circuit coupling said first two circuits, said transfer device comprising a series resistance, a shunt transformer and a condenser, one of the windings of said transformer being tuned to a desired frequency by means of said condenser connected in parallel with said one winding, a lead common to and connected in both of said first two circuits, an element included in said lead, and an output voltage derived from said element to maintain the load voltage constant in spite of load circuit variations.

5. An oscillation generator comprising a thermionic valve having a cathode lead, an impedance connected in said cathode lead, an input circuit for said valve which is arranged to work with a signal circuit of high impedance, an anode circuit for said valve, a series resistance in said anode circuit, an amplitude regulating device connected in shunt of said series anode resistance comprising a series resistance and a neon bulb, a frequency selective transfer device and said'amplitude regulating device arranged to couple said anode circuit to said input circuit, and an output circuit for said valve connected across said cathode impedance which is arranged to work with circuits of low impedance, whereby the oscillator generator is eifective to couple circuits of high and low impedance.

BERTRAM MORTON HADFIELD.

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

UNITED STATES PATENTS Number Name Date 2,066,333 Caruthers Jan. 5, 1937 2,163,403 Meacham June 20, 1939 2,173,427 Scott Sept. 19, 1939 2,353,493 Parkin July 11, 1944 2,382,198 Bollinger Aug. 14, 1945 FOREIGN PATENTS Number Country Date 425,308 Great Britain Mar, '7, 1935 426,396 Great Britain Mar. 28, 1935 OTHER REFERENCES Proc. IREvolume 26, Number 10, October 1938, pages 1278-1294.

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