US2181469A - Compensated diode detector - Google Patents

Description

28, 1939. A. w. BARBER COMPENSATED DIODE DETECTOR Filed April 9, 1937 AMP'LlFIER m m T .D 4 W D 4 a z .u 72 M16 m H 2 5 N2 FWIJ Aug? 1 M; p r .R F 0 L P G M A o C A AMPL'\FIER INVENTOR Patented Nov. 28, 1939 umrso STATES eATENT OFFICE 2,181,469 COMPENSATED DIODE DETECTOR Alfred W. Barber, Flushing, N. Y. Application April 9.1937, Serial o. 135,928

20 Claims.

My present invention relates to carrier wave receivers. More particularly it relates to a form of rectifier suitable for use in radio receivers and the like.

One object of my present invention is to provide circuits for generating automatic volume control and modulation frequency voltages in a radio or other carrier wave receiver. Another object is to eliminate the modulation frequency output coupling condenser and the effects of the automatic volume control filter on the action of the detector in such a receiver. A particular object is to eliminate the distortion which may be generated in such circuits employing diode detectors. 1

The most commonly used detector in radio receivers consists in a thermionic vacuum tube diode having a high resistance load. Thedemodulation voltages generated in this load, provided,

it is by-passed by a condenser having a reactance approximately equal to or' greater than the load resistance at modulation frequencies, consist in a direct current component proportional to the average amplitude of the impressed carrier volt-. age and a modulation frequency component proportional to the modulation of the carrier. Itis usual to filter the direct current component by means of a series resistor and a condenser to ground for application to the grid returns of amplifier tubes preceding the detector for purposes of automatic volume controh The modulation frequency component is selected by a series condenser and a high resistance to ground. At least a part of the modulation frequency drop across this latter resistor is applied to the grid of a modulation frequency amplifier tube. I have found that both the automatic volume control voltage filter and the modulation frequency coupling condenser cause distortions of the modulation frequency voltage generated by the system.

Both the automatic volume control filter condenser and the modulation frequency coupling condenser charge up to the average of one half of the input carrier wave modulation peak envelope. Whenever the modulation drops below this average these condensers feedback thru the diode load. resistor. 1 When the modulation envelope drops below the feed-back drop across the diode load resistor, therdiodegis cut-off and the modulation envelope and hence the modulation voltage iscut-off at this point. If the automatic volume control resistor and the modulation frequency coupling resistor in parallel are equal to a resistance R0 and the diode load resistance is R the modulation is cut-off at a envelope modulation percentage equal to 100Rn/Ro+R by this feed-back effect.

I have found that by separating the generation of automatic volume control andmodulation frequency voltages, by by-passing the automatic 5 volume control generatingload resistor by alarge condenser and by eliminating the modulation frequency coupling condenserthat these distortions are eliminated. In my Patent Number 2,129,726 entitled Rectifier circuits, granted September 13, 1938, and filed on January 30, 1936, I have shown the basic circuit by which I accomplish this elimination of distortion. I havefound, however, that certain improvements are possible .over this basic circuit.

I have found that an unby-passed resistor in series with the automatic volume control voltage generating load reduces, and may be made to eliminate to any desired deg ree, distortion due to the loading of the detector input circuit by the large capacity of this load circuit. In the basic system the automatic volume control voltage is subtracted from the voltage generated in the modulation frequency generatingdiode load producing a net modulation frequency voltage which is applied conductively to the grid of the first modulation frequency ament invention.

, Fig. '2 shows a modified form ofmy invention embodying automatic volume control fixation for tuning purposes.

Fig. 3 shows'still another modification of my invention. 1 I

In Fig, 1 I have shown one form of my improved detector circuit interposed between an amplifier C and an output device D. The amp1i ;fier C may be connected to an antenna A and ground B for the reception of radio signals or it may be connected to a transmission lineor other signal source circuit. The amplifier C may be a tuned radio frequency amplifier with one or more stages controlled by the automatic volume control voltage developed by the detector or it may be taken to represent the portion of a superheterodyne receiver up to the second detector in which case the automatic volume control voltage may be applied to one or more intermediate frequency amplifier stages, one or more radio frequency amplifier stages, the first detector or any suitable combination of the three. The output of amplifier C appears across a coil l which is inductively coupled to the detector input coil 2 which may be tuned to resonance by condenser 3. The output coil I will ordinarily be in the plate circuit of the final stage of the amplifier C and may or may not be tuned to resonance. Other means than the magnetic coupling may be used between the amplifier C and the detector as for instance capacity coupling and the detector input tuned circuit 2 and 3 may be replaced by other impedances such as a radio frequency choke or a resistance although I prefer the tuned circuit as shown.

The detector consists of two diodes and their associated circuits. One diode consisting of plate 4 and cathode 5 generates a steady voltage suitable for automatic volume control purposes by virtue of the high time constant load circuit consisting of resistor 6 and condenser I in parallel. The resistor 8 is connected in series between the diode cathode 5 and the high time constant load circuit 6 and l. This resistor 8 should be large, of the order of 100,600 ohms, and its function is to prevent heavy loading of the input circuit on modulation peaks by the diode and the high time constant load. In operation condenser "l charges up to approximately the average of the positive modulation envelope which is the same as one half the peak to peak value of the unmodulated radio or intermediate frequency input voltage. When the input voltage is modulated the modulation peaks rise above and fall belowthis condenser voltage. While the modulation envelope is above the voltage across condenser l the condenser charges thru the diode and resistance 8. Without resistor 8 this charging current is high and the input coil 2 is effectively shunted with a distortion producing load. Since the input voltage is also applied to the second diode which produces modulation frequency voltages it is important to preserve the input wave form undistorted. I have found that by limiting the charging current of condenser '1 by means of the series resistance 8 that this distortion of theinput voltage may be reduced by any desired amount. If resistor 8 is made equal to the tuned impedance of the input circuit 2 and 3 or greater I have found that the input wave distortion is reduced to a negligible value.

If the cathode end of condenser 1 is grounded at G as shown, the other end of condenser I is negative with respect to ground and this nega-' tive voltage may be used for automatic volume control in the amplifier Cby suitable connections well known in the art. A series filter may beused in the automatic volume control circuit consisting of a series resistor 9 and shunt condenser ill to ground G for purposes of decoupling between the detector and the amplifier. If resistor 9 taken in conjunction with condenser! has a time constant of the order of 0.1 or more condenser l cannot feed-back any distorting voltage during inward modulation swings since it will take appreciable time for the current from condenser Ill to change the voltage across condenser '1 compared to the duration of the modulation peaks if the modulation is of audio frequencies. Evidently for other'types of modulation the time constant may be made such that feed-back is effectively prevented during modulation peaks. Thus the voltage across condenser '1 remains essentially constant at all times. When the modulation peaks are above the voltage across condenser I resistor 8 limits the charging current flowing into condenser l and when the modulation envelope is below the voltage drop across condenser 1 no current flows thru the diode. Thus the only distorting efiect of the automatic volume control voltage generating load and diode is an alternation between no load for inward modulation swings and a load equal to resistor 8 for outward modulation swings. If resistor 8 is made large enough so that when it is shunted across the input circuit essentially no change takes place in the input voltage, the first diode and its load cannot distort the input voltage wave under any conditions of modulation.

The second diode comprising plate H and cathode l 2 is connected in series with the modulation frequency load circuit consisting of resistor i3 and condenser M in parallel. Plate H is connected to the high end of the input coil 2 and the load circuit is connected between cathode l2 and a variable tap I8 on resistor 6. Condenser M should have a capacity C equal to l/ 6.28 1?, where is the highest desired modulation frequency and R is the resistance of resistor 13. With this connection modulation frequency voltages and a direct current component appear across resistor I3. If a variable tap I is taken from resistor 13 and applied to the grid iii of a modulation frequency amplifier tube ll, no coupling condenser is necessary between the diode and the amplifier tube grid. Taps i8 and 55 are mechanically coupled by arm it in such a manner that as tap I5 is moved toward cathode I12 tap I8 is moved toward the coil end of resistor E and as tap I5 is moved in the opposite direction toward the low end of resistor l3-tap i8 is moved toward the grounded end of, resistor 5. If the taper of resistors 6 and. I3 is such that the same percentage of the total resistance of resistor t is placed between tap l8 and ground as the percentage of resistor l3 placed between tap it: and the low or return end of resistor I3, the direct current drops between ground G and grid IE will essentially cancel each other leaving a net modulation frequency voltage between ground G and grid l6. Thus an essentially bias free modulation frequency voltage is applied to grid I6 and no blocking condenser is needed. This circuit therefore completelyeliminates distortions due to the: automatic volume control filter condenser and modulation frequency voltage blocking or coupling condenser experienced in conventional circuits.

Modulation frequency amplifier 41 may have a cathode 2i! with a resistor 2! by-passed by condenser 22 connected from cathode 28 to ground G or other well knownmethods of bias may be used. The use of the automatic bias by means of the cathode series resistor has the advantage that any unbalance in direct current biasapplied to the control grid is accommodated to some degree by a change in plate current and hence also a compensating change in cathode bias. Tube I! may have a plate 23 feeding an output device D which may be additional amplifier stages, a loud speaker or other utilization means.

While not limited to these values I have found the following constants satisfactory to the operation of my invention in a radio sound broad- Resistor 8 equal to 250,000 ohms Resistor 6 equal to l megohm Condenser '7 equal to 0.1 microfarad Resistor 9 equal to 1 megohm v Condenser 10 equal to 0.1 microfarad Resistor 13 equal to 0.5 megohm Condenser '14 equal to 50 micro-microfarads Resistor 21 equalto 3000 ohms Condenser 22 equal to 25 microfarads The circuit of Fig. 2 functions in a manner similar to that of Fig. 1 in so far as the components bearing similar numbers are concerned. The return of the modulation frequency load resistor I3 andcondenser I4 is made to thecoil end of the automatic volume control circuit so that when the output to grid- 6 is reduced there is no reduction in the bias applied from the automatic volume control load circuit. The direct current bias on grid 6 thus increases as the output is reduced.

This increase in bias, however, will be partially oifset by the decrease in the drop across cathode resistor 2i due to the reduced plate current .which the increase in grid bias causes, A high time constant load consisting of condenser 26 shunted ;by resistor 25 may be placed in series with the componentgenerated in the modulation frequency load so that the net bias on grid 6 is at all times essentially zero. Switch 24 is provided to open the discharge path of condenser E thruresistor 6 thus preventing the automatic volume control circuit from discharging but permitting it to build up thru the diode and resistors 8 and 2'! as described in the above mentioned patent" for purposes of tuning to a signal. Resistor 21 functions much like resistor 8 in preventing loading of the input circuit. Placing condenser M on the tap on resistor l3 decreases the detection efficiency of the modulation diode on lowered Volume control settings and aids in the changing. of the drop across condenser 26 to match theout'put' setting as described above.

The circuit of Fig. 3 functions similarly to that of Fig. l in so far as components bearing similar numbers are concerned. Condenser 1, however, discharges thru resistors I3 and 2B in series so that if they are equal the cathode end of resistor 28 has 'efifectively one half the direct current component generated in resistor 13 cancelled. Similarly if resistor 28 is larger or smaller than resistor l3 moreor less than half the direct current component cancellationtakesplace. Thus the drop across'resistor 28 consists of the modulation frequency component across resistor i3 plus the direct current component across resistor i3 due to its rectification currents minus a portion of the direct current dropacross condenser '1. Volume may be controlled by varying the tap 29 on resistor 28 and applying the drop from tap 29 I to ground G to grid 6 of tube ll.

While I have shown and described only a few systems whereby my invention may be carried out, other forms are possible within the spirit and scope of the invention as set forth in theappended claims.

What I claim is:

1. In a carrier Wave receiver, the combination of two diodes, a commoninput circuit, a loadcircuit connected in series with said input circuit and with each of said diodes and a substantially capacity free resistance circuit connected between one of said diodes, its associated load circuit, and

- said input circuit.

and said input circuit, a second load circuit hav-- ing an alternating current impedance low compared to the alternating current impedance of thetfirstsaid load circuit and a substantially capacity free resistance circuit connected in series connected between the other of said diodes and said input circuit.

3. In a carrier wave receiver, the combination of two diode rectifiers, an input circuit, a load circuit connected in series with each of said diodes and said input circuit, and additional means comprising an impedance circuit having a value of the order of the impedance of said input circuit at carrier frequencies connected in series with one of said loadcircuits.

4. In a carrier wave receiver, the combination of two diode rectifiers, an inputcircuit, a load circuit comprisinga resistance and condenser connected in parallel connected in series with each offsaid diodes and said input circuit, wherein one circuit comprising a condenser and resistor in parallel connected in series with each of said diodes and said input circuitwherein one of said combination of two diode rectifiers, an input circuit, a load condensers is large compared to the other of saidv condensers and additional means comprising a substantially capacity free resistor circuit having a value of the order of the magnitudeof the tuned impedance of said input circuitconnected in series with the larger of said loadconf densers.

:6; In a carrier wave receiver, the combination of two diodes, a diode input circuit, a loadcircuit comprising a resistance and condenser condiodes and said input circuit, wherein one of said condensers is large compared to the other of said condensers, an amplifier tube comprising atleast 55 connection between one of said load circuits and ground and a conductive connection between the a plate, a control grid and a cathode, a conductive other of said load circuits and said grid anda resistance connected in series with said ground ed load circuit and its associated diode.

a conductive connection between the ungrounded end of said grounded load circuit and at least one volume control point in said preceding amplifier.

8. A circuit as: set forth in claim 6 and, in-

nected in, parallel connected between each of said to 7 A circuit as set forth in claim 6 and including an amplifier preceding said input circuit and eluding a carrier frequency amplifier preceding said 'inputscircuit and a series resistance connected between the ungrounded end of said with each of said diodesand said input circuit, wherein one of said condensers is large compared to the other of said condensers, a resistor connected between said higher capacity diode circuit and its associated load, a conductive ground con-- nection between the point common to said last resistor and said larger condenser, an amplifier tube including at least a plate, a control grid and ,a cathode, a variable tap point on each of said load resistors, a conductive connection between said grid and the tap on the resistor of the lower capacity load circuit, and a conductive connection between the end of said lower capacity load circuit remote from its associated diode and the other of said resistor taps.

10. A system as set forth in claim 9 wherein said taps are mechanically coupled.

11. In a carrier wave receiver, the combination of a carrier current amplifier, an amplifier output impedance, a second impedance coupled electrically to the first said impedance, two diode rectifiers each comprising at least a cathode and a plate, a conductive connection between each of said diode plates and one end of said second impedance, a resistor connected between the cathode of the first of said diodes and ground, a first variably tapped resistor connected between ground and the other end of said second impedance, a second variably tapped resistor connected between the cathodeof the second of said diodes and the tap point of said first tapped resistor, a condenser connected across said second tapped resistor, a condenser large compared to the last said condenser connected across the first said tapped resistor, a conductive connection between one end of said larger condenser and at least one point in said carrier current amplifier, an amplifier tube including at least a plate, a control grid and a cathode, a condenser bypassed resistor connected between said cathode of said amplifier tube and ground and a conductive connection between said grid and the tap point of said second tapped resistor.

12. In a carrier wave receiver, the combination of a carrier frequency amplifier, a double diode rectifier, a diode input circuit and a modulation frequency amplifier wherein the first section of said diode feeds a modulation frequency load circuit serially connected with said input circuit and said diode, means for conductively connecting said load circuit to said modulation frequency amplifier and wherein the second section of said diode feeds a second load circuit having a low impedance to modulation frequencies serially connected with said input circuit and said second diode section, automatic volume control connections between said second load circuit and said carrier amplifier and including a high resistance in series with said second diode section and said input circuit.

13. In a carrier current receiver, the combination of a carrier current amplifier, two diode rectifiers, a coupling impedance connected between said rectifiers and said amplifier, independent load circuits connected between each of said diodes and said coupling impedance, wherein one of said load circuits comprises a high impedance to modulation frequencies in series with a low impedance to modulation frequencies, and the other of said low circuits comprises a resistor in series with a condenser and resistor in parallel, and means for opening the circuit of the last said resistor. 1

common to said' resistor 14. In a carrier wave receiver, the combination of a carrier frequency amplifier, two diode rectifiers, a modulation frequency amplifier tube including at least a plate, a control grid and a cathode, means for coupling said carrier frequency amplifier to said rectifiers including a rectifier input impedance, wherein said diodes each comprise at least a plate and a cathode, conductive connections between each of said diode plates and one end of said input impedance, a circuit connected between the cathode of the first of said diodes and the other end of said input impedance comprising a high resistance in series with a condenser and resistance in series connected in parallel with a resistance and switch in series, a conductive ground connection to the connection between said high resistance and said condenser, a resistance connected between the cathode end of said input impedance and at least one point in said carrier frequency amplifier, a condenser connected between at least one of the last said points and ground, a circuit connected between the cathode of the second of said diodes and the cathode end of said input impedance comprising a potentiometer in series with a condenser and resistor in parallel, a conductive connection between the tapof said potentiometer and said control grid, a condenser connected between said control grid and the second diode series condenser, a resistor and condenser in parallel connected between the cathode of said modulation frequency amplifier and ground and a connection between the plate of said modulation frequency amplifier and an output device.

15. In a carrier frequency receiver, the combination of a carrier frequency amplifier, two diode rectifiers, a modulation frequency amplifier tube including at least a plate, a control grid and a cathode, means forcoupling said carrier frequency amplifier to said rectifiers including a rectifier input impedance, wherein said diodes each comprise at least a plate and a cathode, conductive connections. between each of said diode plates and one end of said'input impedance, a circuit connected between the cathode of the first of said diodes and the other end of said input impedance comprising a resistor and condenser in series, a conductive connection between the point and condenser and ground, a circuit connected between the cathode end of said input impedance and at least one point in said carrier frequency amplifier comprising a series resistor, a condenser connected between at least one of said points said carrier frequency amplifier and ground, a circuit connected between the cathode. of the second of said diodes and the cathode end of said input of a carrier frequency amplifier, two rectifiers, an impedance coupling said amplifier with said iectL fiers, independent condensers connected in series with each of said rectifiers and said impedance, wherein one of said condensers is much larger than the other, means for limiting the carrier frequency charging current of the larger of said condensers, and utilization means conductively connected to one end of the smaller of said condensers. Y I

17. A carrier wave rectifying system comprising, two rectifiers, a rectifier input circuit, meam for charging independent condensers connected in series with said input circuit and each of said rectifiers, wherein one of said condensers is large compared to the other of said condensers and means for limiting the instantaneous charging current of the larger of said condensers.

18. A system as set forth in claim 17 wherein said current limiting means comprises a substantially capacity free resistor having a value greater than the magnitude of the impedance of said input circuit to unrectified currents. 1

19. In a carrier wave receiver, the combination of a diode rectifier, a source of carrier signals to be rectified, a resistor and a condenserall con nected in a series circuit, wherein at least a portion of said series circuit is completed by said resistor substantially free of shunting capacity.

20. In a carrier wave receiver, the combination of, a diode rectifier, asource of carrier frequency signals to be rectified, a carrier frequency impedance and a condenser all connected in a series circuit, wherein at least a portion of said series circuit is completed by said impedance substantially free of any carrier frequency low impedance shunting path.

ALFRED W. BARBER.

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