US2031441A - Radio signaling apparatus - Google Patents

Description

J. c. WARNER RADIO SIGNALING APPARATUS Filed March 8, 1929 Fig.1.

Fig.5.

NEGATNE GRH) BIAS Feb. 18; 1936.

Inventor; John CM/or net, SLAM LN His Atborneg.

Patented Feb. 18, 1936 PATENT OFFICE RADIO SIGNALING APPARATUS John C. Warner, Schenectady, N. Y., assignor to General Electric Com New York pany, a corporation of Application March 8, 1929, Serial No. 345,498

14 Claims.

My invention relates to radio signaling apparatus and more particularly to a detector circuit for such apparatus in which an electric discharge device, having a control grid, anode and cathode, is used as a detector, with the control grid biased for anode circuit rectification to provide what is commonly known as a bias detector.

A detector and circuit of the type to which the invention is particularly adapted, is shown and described in Patent No. 1,770,838 of Wendell L. Carlson, in which is provided a bias detector which produces substantially a linear ratio between the rectified response and the percentage modulation of the carrier wave, the arrangement being characterized by the fact that radio signals may be received without harmonic distortion and without distortion due to audio frequency regeneration and irregularities from power supply circuits.

A detector of this character may be of the hotcathode type having three or more electrodes for example, and is operated with input or impressed voltage and power output of such magnitude that a single stage of low frequency or audio amplication following it is sufiicient for the normal operation of the usual loudspeaker or other similar output device. This particular type of detector is now variously known as a high-swing detector or power detector.

In the operation of a detector of the above character, the application of a radio frequency voltage or carrier wave thereto causes an increase in the average anode current, depending upon the amplitude of the impressed voltage. While relatively high and unusual negative grid bias and anode voltages are used with this type of detector, the limit in the amount of signal voltage which can be applied is reached when the peak of the modulated radio frequency voltage reaches zero grid voltage. Beyond this point distortion occurs due to conduction current flowing in the grid circuit in a manner which is well known. Therefore, the lower the percentage of modulation the less desirable is the output characteristic, that is, the audio output which can be obtained before this limit is reached. This is also true to a greater extent in other and less efficient forms of bias detectors.

The principal object of this invention is therefore to provide an improved circuit arrangement in connection with a bias detector, whereby the limitations in input and output voltages are automatically raised in proportion to the amplitude of the impressed carrier voltage, and whereby the output characteristic for a given anode voltage is materially improved. Stated in another way, the object of the invention is to provide in connection with a biase detector, means for automatically controlling the bias voltage in response to changes in the amplitude of the impressed radio frequency or carrier wave voltage whereby, for a given anode voltage, an increase in the output may be obtained.

My invention will, however, be better understood from the following description when considered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Fig. 1 is a wiring diagram of a power detector embodying my invention; and Figs. 2 and 3 are curve diagrams illustrating the operation of the embodiment of my invention shown in Fig. 1.

Referring to Fig. 1, 5 is an electric discharge device of the 3-element hot cathode type, the cathode 6 of which is heated by a suitable element 1 supplied with heating current through a center tapped transformer 8, being thereby adapted for alternating current operation, although the invention is not limited to the use of alternating current devices but may be of any suitable type having a control grid, an anode and a cathode. A control grid 9 is connected with an input circuit ID of any suitable type and which in the present example includes the secondary ll of a tuned radio frequency transformer l2 and a variable tuning capacitance I 3 connected in shunt with the secondary II.

The grid return from the input circuit is taken to the cathode 6 through a bias resistor M which is shunted by a fixed capacitance I 5 having a relatively low impedance to audio frequencies or other modulation frequencies.

Between the resistor l4 and the input circuit is inserted in the present example a source of fixed bias potential l6, having a polarity as indicated. The cathode 6 and its heater element 1 are connected together by lead I! and the center tap in the secondary of transformer 8.

The anode I8 is connected through an anode or audio frequency output circuit I 9 with the primary 20 of an audio frequency output transformer 2|, from the secondary 22 of which is taken the output from device 5. Anode voltage is supplied by suitable means such as a battery 23, connected in the anode circuit between the low potential end of the primary 2|] and lead 24 which is connected to the common cathode return for both the grid circuit and the anode circuit through resistor l4 and capacitance IS. A

bypass capacitance 25 is connected as shown to provide a radio frequency bypass path from the anode I8 to the cathode return lead 24. Resistor I4 is thus connected in the anode circuit and the rectified anode current flows through it. The total bias is therefore the sum of the fixed bias I6 and the voltage drop due to the anode current flowing through the resistor I4. The magnitude of the fixed bias voltage provided by source I6 is dependent upon certain characteristics of the device used as the detector and in certain cases may be reduced to zero, the entire bias then being obtained from the resistor I4.

The operation of a detector and circuit of this general character is described in the above-mentioned copending application of Wendell L. Carlson. In this arrangement the anode voltage and bias voltage are relatively high and the radio frequency signal voltage supplied through transformer I2 and input circuit I0 is also relatively high, permitting a power amplifier or even an output device such as a loud speaker to be operated directly from the secondary 22 of output transformer 2 I. However, in that application the entire bias voltage is obtained from a source like that at I6, which in value would equal the sum of the voltages supplied by source I6 and resistor I4 in the present example under conditions of no signal voltage.

The negative bias voltage is such that the detector is adjusted for substantially maximum rectification and operates on what is known as the lower bend of the plate-current, grid-voltage curve, the bias voltage being such that with a common form of B-element electric discharge device of the type shown, an anode current at nearly the cut-off value flows in the anode circuit, with no radio frequency or carrier voltage impressed upon the grid circuit.

The limit of the power output of this type of detector is determined by the bias voltage applied to the grid. Increasing the bias voltage will allow an increased input radio frequency voltage and consequently a higher output audio frequency voltage, but the distortion of the signal will be greater, especially at high percent modulations, and with weak signal input the detector will be operating beyond its anode current cut-off point. The relation between the output for two values of bias and anode voltages is shown in Fig. 3, the output voltages obtainable at the secondary 22 of transformer 2| in relation to the input voltage delivered through transformer I2 to input circuit I0 being taken to illustrate this relation.

Referring to Fig. 3, curve A shows the output voltage obtainable at difierent impressed radio frequency or carrier voltages with a certain fixed negative bias voltage supplied by source I6, and anode voltage supplied by source 23 sufficient to operate device 5 as a bias power detector. Curve B shows the increased output voltage obtainable from the same device by increasing both the anode voltage at source 23 and the fixed negative bias voltage at source I6. In curve A for example, the negative bias voltage may be taken as 25 and the anode voltage as I80, while in curve B the negative bias voltage is increased to 35 and the anode voltage to 250. These values do not represent limitations upon the use of such detectors but are values found to be suitable for a common form of electric discharge device known commercially as radiotron UY-227 and which is represented at 5 in Fig. 1.

Because of the limitation in the working voltage which may be applied to certain forms of commercially available electric discharge devices suitable for use as bias detectors, and because of the problem of voltage supply in radio apparatus such as receivers, in which this type of detector is used, it is more desirable to use the lower or normal working values of bias and anode voltage for such devices. In curve C I have shown the output voltages obtainable in accordance with my invention, while using the same device 5 as in curves A and B and the same normal and more desirable values of negative bias and anode voltage as in curve A. This improved result is obtained by the use of the resistor I4 of a suitable resistance value in the cathode return lead as shown, which permits the bias voltage to be controlled by the impressed carrier wave or applied radio frequency voltage. For the above mentioned radiotron UY-227, a resistor of 6000 ohms may be used at I4, although the voltages and resistance values used in any case depend upon the characteristics of the device used as the detector, the permissible maximum distortion and the percentage of modulation of the signal to be detected.

With this arrangement the negative bias voltage applied to the detector is made to increase automatically with increased signal or carrier voltage supplied through the input circuit I0, and permits a given output at the modulation or audio frequency from a given electric discharge device at a lower anode voltage than has hitherto been possible. This may be more clearly understood with reference to Fig. 2 in which the effect of resistor I4 is shown with various values of impressed carrier or radio frequency voltage.

Referring to Fig. 2, the relation between the average anode current and negative grid bias are shown by five curves. These curves indicate the static and signal characteristics of a typical electric discharge device when used as a power detector with my improved circuit. Curve Eg=0 is the usual static grid-voltage anode-current curve. The other curves are taken with various A. 0. signal or carrier wave voltages as indicated applied to the grid in addition to the D. C. bias.

In this figure, Ec is the bias voltage-supplied from source I6 and is fixed in value. The additional bias supplied by resistor I4 is represented by the distance between point A and vertical dash line EC. 'Ilhe distance of dash line E0 from origin D is then the total bias applied to the grid of the detector with no signal voltage.

Both sources of bias voltage are so adjusted that the total is that, which in accordance with the circuit arrangement shown in the copending application of Carlson, above mentioned, will give the desired detector operation. In the present example this voltage is such that with the desired normal voltage applied to the anode from source 23, the anode current is reduced under static nosignal conditions to nearly its cut-off value, such as a value of /2 milliampere, for example.

When a signal is applied to the detector the average rectified current in the anode circuit increases as will be seen from the curves in Fig. 2, and the voltage drop in resistor I4 increases proportionally, thereby increasing the total bias and allowing a greater grid voltage swing without the grid becoming positive and the increased output or efficiency as indicated in curve C of Fig. 3.

This action is entirely different from that of an amplifier under the control of an impressed signal voltage. An amplifier is operated at such a point along its characteristic curve that the average value of its anode current does not change with normal variations in signal voltage. Hence there is no rectification or detection and its bias voltage does not change. The i bias voltage is fixed and hence determines merely the limits of the applied voltage which may be amplified without distortion.

The bias detector however is operated for substantially maximum rectification with its anode current reduced to nearly its cut-off value, by suitable bias, and any application of signal voltage eifects an increase in the average anode current. With the present arrangement the additional bias provided by the bias resistor extends the operating range of the grid in a negative direction, thereby permitting a greater range of detector action. The value of this arrangement is greatest with low percentage modulation. For example, at 15% modulation of the incoming signal, the improved circuit which I have provided will allow increased output voltage of the order of 50% to 100% depending on the value of the automatic bias resistance.

When operating the detector with a fixed bias, as in the case Ec represents a fixed bias only, the average anode current will rise with increases in carrier voltage as shown by the vertical Ec line. With my improved circuit however, the fixed bias is represented by distance E'c and the average rectified current in the anode circuit and bias both increase with increased signal or carrier wave voltage along the line AB, the average rectified current and total bias for each value of signal voltage being indicated at points M, N, O, P, along the line AB.

The efiect of removing the fixed bias and applying only a bias as provided by a fixed resistor, is indicated by dash line CD. As mentioned hereinbefore, the exact proportioning of fixed and variable bias and the best value of bias resistance is dependent upon a. number of factors including the characteristics of the electric discharge device, the permissiblemaximum distortion and the minimum percentage of modulation at which full output is to be obtained. For a given value of carrier or impressed voltage resulting in a certain total bias, the performance of the detector is the same as if a fixed bias equal to the instantaneous. total bias were used.

The audio frequency by-pass capacitor I across the bias resistor M is desirable in order to maintain the audio frequency impedance of the anode circuit substantially the same as if the bias resistor were not used.

From the foregoing description it will be seen that in connection with an electric discharge device and means for operating it as a power detector, or high swing detector, there has been provided additional means responsive to changes in the amplitude of an impressed signal voltage or carrier wave or the value of rectified anode current resulting therefrom for automatically increasing the power output or voltage obtainable from the device used as the detector. This additional means operates to apply a bias voltage to the grid of the detector, which voltage increases with the amplitude of the impressed carrier or signal voltage or the magnitude of the average rectified anode current.

The above described arrangement has the ad-' device having a control grid, an anode and a cathode, and means providing an anode voltage and a normal static negative bias voltage for said grid, said voltages being of such values that said device operates as a bias detector for substantially maximum rectification, of means connected in series with said grid and said firstnamed bias'means for applying an additional negative bias voltage to said grid in a direction to add to the normally applied bias voltage in response and proportional to the amplitude of an impressed carrier wave, to increase the detector efficiency of said device in response to an increase in the amplitude of said wave.

2. The combination of an electric discharge device provided with a grid for controlling the current transmitted between its cathode and anode, means providing an anode potential and means for subjecting said grid to a bias potential which tends to reduce said current substantially to zero, said potentials being of such values that said device operates as a bias detector for substantially maximum rectification, means for causing said current to flow, and means for applying to said grid a bias potential controlled by said current.

3. The combination of an electric discharge detector device provided with a grid for controlling the current transmitted between its cathode and anode, means providing an anode potential and means for subjecting said grid to a. bias potential which tends to reduce said current substantially to zero, said potentials being of such values that said detector provides substantially maximum rectification, means for causing said current to flow, and means for applying to said grid a. bias potential produced by said current, said bias potential varying in accordance with said current.

4. The combination of an electric discharge detector device provided with a grid for controlling the current transmitted between its cathode and anode, means for impressing a modulated carrier wave on said grid, means providing an anode voltage, means for subjecting said grid to a normal negative bias voltage which tends to reduce said current substantially to zero, said anode and grid voltages being of such values that said device is adjusted for substantially maximum rectification, and means connected in series with said first-named grid bias means for applying to said grid an additional negative bias voltage in a direction to add to the normally applied bias voltage, said additional bias voltage increasing in accordance with an increase in the amplitude of said wave, whereby the maximum undistorted output of said device is increased in response to an increase in the amplitude of said impressed carrier wave.

5. The combination with an electric discharge device having an anode, a cathode and a control grid, and means for operating it as a bias detector under conditions for substantially maximum rectification, of means responsive to changes in the amplitude of an impressed carrier wave for controlling, the output characteristic of said device whereby said device provides increased output with increased amplitude of animpressed carrier wave, said means including a resistor arranged to transmit the rectified anode current from said detector, a control grid circuit for the device connected with said resistor to receive bias voltage therefrom, and means providing a low impedance path to modulation frequencies in shunt with said resistor.

6. The method of detecting modulated waves by means of an electric discharge device havin a control element, which comprises applying to the control element a negative voltage of such value as to reduce the average anode current of said device substantially to zero whereby the device is adjusted for a condition of a substantially maximum rectification, and automatically increasing the negative voltage in a ratio proportional to the amplitude of said waves, thereby increasing the output eificiency of said device.

7. The combination with an electric discharge device having an anode, a cathode and a control electrode, of means arranged to apply normal working values of bias and anode potentials to said device, the relative values of said bias and anode potentials being such that said device is operative as a power detector of modulated high frequency waves having substantially maximum rectification characteristics, and means for increasing the output efliciency of said device in response to increases in the amplitude of applied modulated carrier waves without increasing the values of said potentials, said last named means including a relatively high bias resistance inserted in circuit between the anode and the cathode whereby it provides a variable drop in potential which increases in response to increases in the amplitude of an impressed carrier wave, and a circuit connection between the control electrode and said resistance for applying said drop in potential to the control electrode in a direction to add to the normally applied bias potential.

3. The combination with an electric discharge device having a control grid, an anode and a cathode, means to impress a carrier wave on said device, and means providing an anode voltage and a normal static negative bias voltage for said grid, said voltages being of such values that said device operates as a bias detector for substantially maximum rectification, of a resistor connected in series with said first-named bias means for applying an additional negative bias voltage to said grid responsive to the current in said anode and in a direction to add to the normally applied bias voltage, to increase the detector eificiency of said device in response to an increase in the amplitude of said wave.

9. In combination, an electron discharge detector having an anode, a cathode and a grid, means for impressing signal modulated high frequency oscillations on said grid, an output circuit connected between the anode and cathode, means to impress a constant negative bias potential on said grid to reduce the anode current substantially to the cut-off value whereby the intensity of the detected output signal currents varies in a substantially linear relation with the intensity of the impressed high frequency oscillations, and means for supplying to said grid an additional negative bias potential varying responsively to the intensity of the supplied high frequency oscillations in such a way that said linear relation between the intensity of said output signal currents and the impressed oscillations is maintained while the grid swing of said discharge device is increased.

10. In combination an electron discharge device having an anode, a cathode and a grid, means for impressing signal modulated high frequency oscillations on said grid, an output circuit connected between the anode and cathode, means to supply operating potential of a certain value to said anode and a constant negative bias potential to said grid, said operating and bias potentials having such values that said device operates as a detector producing output signal currents varying substantially linearly with the impressed high frequency oscillations over a certain range, and means to increase substantially said range while maintaining said anode potential at said certain value and said bias potential at said constant value, said means comprising means for supplying an additional bias voltage to the grid having a value varying responsively to current in said anode thereby to vary the grid swing of the discharge device and in such a way as correspondingly to increase the signal output of said detector.

11. The method of operating a vacuum tube detector which includes the step of adjusting the polarizing potentials to effect linear detection for a limited range of input voltages, amplifying received signals to bring the detector input voltages to a value not less than those falling within said limited range, and extending the range of input voltages for which linear detection obtains by applying to one element of the tube a bias voltage derived from the rectified current in a tube circuit other than the control grid circuit.

12. In a detector stage, the combination with a vacuum tube having input and output circuits, of means for extending the range of detector input voltages for which said tube functions as a linear rectifier, said means comprising a resistance common to said circuits.

13. A radio receiving system comprising a detector of the thermionic type containing a grid, means for automatically altering the average potential of the grid negatively as the carrier voltage increases, said means comprising an element in series with the grid and having high impedance to steady currents and low impedance to carrier and modulation frequencies, means for amplifying the received signal voltages, and means for impressing the amplified voltages upon the grid of the detector.

14. The combination in a modulated carrier Wave transmission system of a detector of the thermionic type containing a control grid, means for automatically increasing the average potential of the grid negatively as the signal voltage increases, said means comprising a resistance in series with the grid, and a condenser shunting said resistance and designed to oil'er low impedance to carrier and modulation frequencies.

JOHN C. WARNER.

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