US2194552A - Degenerative audio detector circuit - Google Patents

Description

March 26, 1940. s. HUNT DEGENERATIVE AUDIO DETECTOR CIRCUIT INVENTOR. E) 01/1? HUNT ATTORNEY.

Filed Oct. 8, 1938 NDPQNNMQ Patented Mar. 26, 1940 DEGENERA TIVE AUDIO DETECTOR CIRCUIT i Seymour Hunt, Jackson Heights, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application October s, 1938, Serial No. 233,906

4 Claims.

characteristic.

The so-called infinite impedance detector is.

well known in the'art, and is in reality a biased detector utilizing plate circuit rectification and employing an unbypassed load impedance in the cathode circuit. Such a detector network functions in the manner of adiode circuit; the, audio voltages developed across its cathode-load resistor are in the correct phase to causethe detector to be degenerative solely for audio voltage. v

Since all the audio voltage developed across the cathode load resistor is applied to the detector input the infinite impedance detector is degenerative for audio voltage.

It is I the degeneration, and the fact that the tubeis normally biased substantially to plate current cut-off, which linearizes the normal square law characteristic of the biased detector circuit.

load, hum at'60 cycles due to heatercathode leakage may result. Again itis diificult to obtain automatic volume control (AVG) voltage in a simple manner, since the direct currentgvoltage drop across the cathode load resistor places a negative potential on the diode plate, assuming that an auxiliary diode plate is employed adjacent the detector cathode. Furthermore, since the detector is 100% degenerativethe gain thereof is less than unity, and, hence, there is plification in the network.

The familiar plate rectification detector has a characteristic which is square law for small sig nals and becomes linear for large signals. Furthermore, the cathode thereof may be maintained at ground potential, and its grid return connected to a negative biasing potential source.

Such a detector is not degenerative for the carrier, and is productive of audio amplification. By utilizing inverse audio feedback between the plate and grid circuits of such a detector tube, the detection characteristic may be made substantially However, since the detector circuit is.

linear.

square law only for small signals it is not necessary to employ 100% feedback, and hence the audio amplification property of the biased detector tube is retained.

Accordingly it may be stated that it is one of the main objects of my present invention to provide" a detector network of the biased type which utilizes plate circuit rectification; there being employed substantially 25% inverse audio 5 feedback between the output and input circuits of. the detector network to render the detection characteristic substantially linear. Another important object of my invention is to improve generally the construction of biased detectors of the plate rectification type, by feeding backfrom the plate circuit to the grid circuit of the detector sufficient audio voltage in degenerative phase thereby to render the detection characteristic substantially linear over the entire signal amplitude range, and the detector further including a diode for deriving AVC bias from the carrier component in the detector output circuit.

Still other objects of my invention are to im- 1 prove generally the simplicity of biased detectors of the platerectification type, and more especially to provide detectors, with inverse audio feedback, which are not only reliable and efiicient in operation, but are also'economically manuiactured and assembled in radio receivers.

The novel features which I believe to be, characteristic of my invention are set forth in par ticularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the followingdescription taken in connection'with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into efiect.

I Referring to the receiving arrangement shown in thedrawing, let it be assumed that there is (shown therein the demodulator network, or seeflond detector, of a superheterodyne receiver. Of course, the invention is not limited to such a receiver, since any wave transmission circuit may be employed which utilizes a demodulator, Tube I can be of the diode-triode type; tubes of the 6J7, 6C5, 6P8, 6Q! or 6R7 types can be used for this purpose. The grid Z is connected to the high potential-end of. input coil 6, whereas condenser l, of 100 mmf., connects the low potential end of coil 6 to the grounded cathode 3. Any desired voltage source, such as the common voltage supply source of the system, may maintain grid ,2 5o

at a negative bias with respect to cathode 3. The letter C designates the negative. terminal of the bias source, the bias being applied through a 1 megohm resistor8. The coil 6 is fixedly resonated bycondenser 9 to the operating I. F., and the 55 detector input circuit lip-9 can be coupled to the output circuit of a prior 1. F. amplifier. The usual signal transmission tubes preceding the in? put circuit G9 have been omitted, but those skilled in the art will be aware that those comprise the usual signal collector feeding one or more radio frequency amplifiers, and the latter being followed by any type of converter stage whose I. F. output feeds one or more I. F. amplifiers.

The output plate 4, of the triode section 23 of tube 1, is connected to the subsequent audio amplifier network (not shown) through a path comprising radio frequency choke coils iii and l l in series with an audio coupling condenser E2, the latter having a value of approximately 0.1 mf. The audio amplifiers will feed a reproducer, such as a loudspeaker of any desired type. Shunt condensers i0 and H, each of mmf., provide with chokes l0li a carrier filter network to prevent I. F. carrier energy from being fed back to the detector input circuit and thus cause undesirable noises. The load resistor i3 has a value of about 250,000 ohms; it is connected in series between the positive terminal of the B supply (not shown) and the series carrier choke H. Audio .voltage is developed across load resistor 13, as well as direct current voltage by virtue of carrier rectification, and the audio voltage com ponent of the detected signals is fed back to thedetector input circuit in degenerative phase. This is accomplished by providing the path including condenser l4 and the adjustable tap 5. The feedback condenser has a magnitude of about 0.1 mf., and hence permits the passageof audio frequency currents. Adjustment of the tap l5 along the load resistor l3 varies the percentage of inverse feedback to the detector input circuit 6-9.

As stated heretofore, the bias applied to grid 2 is such as to operate the triode section of tube 1 as a conventional biased detector which employs plate rectification. Such a detector possesses infinite impedance since it draws no grid current. It possesses a square law detection characteristic for small amplitude signals, and the characteristic becomes linear as the signal amplitude increases. The detector cathode .is .at ground potential, and the detector itself is not degenerative for the carrier frequency currents. As is well known, this type of detector provides audio amplification. audio feedback through path iii-l4 the detection characteristic is made substantially linear over the entire signal amplitude range, and in addition audio amplification is secured. This is accomplished by using substantially less than 100% inverse feedback. Actual experimentalresults in the broadcast band of 550-1500 k. c. with a superheterodyne receiver demonstrated that approximately 25% negative audio feedback through path i5-l4 is sufficient to secure linear detection. Adjustment of the tap l5 along registor 13 shows that as the magnitude of feedback is increased, the quality of the audio reproduction, as measured by the ear, gets better until about 25% feedback is attained. Increasing the feedback beyond this value produces little, or no, change in the quality of the audio reproduction. It will now be clear that since the detector is only 25% degenerative, considerable audio gain is secured.

Another'advantage of this circuit is the fact that the auxiliary diode anode 5 cooperates with the grounded cathode 3 to provide an AVG rectisecond detector.

By utilizing the inverse her. The I. F. carrier energy appearing in the plate circuit of the detector section exists therein in amplified form, and is impressed upon the diode anode 5 through an I. F. by-pass condenser it which has a magnitude of approximately 100 inmf. The AVC load resistor I1 is connected between the anode 5 and ground, and the resistor may have a value of 0.5 megohm. As is well known to those skilled in the art, the AVG bias developed across resistor I1 is transmitted through lead I8, which includes the audio filter network IE), to the signal grids of one or more of the signal transmission tubes preceding the The AVC circuit functions to vary the gain of each of the controlled tubes in a sense such as to maintain the carrier amplitude at the detector input circuit 09 substantially uniform over a wide range of signal carrier amplitude variation at the signal collector of the receiver.

Experimentation with the detector network disclosed herein further reveals thatthe detection characteristic desired is secured even without the use of a negative direct current potential biasing source. With the bias grid return left open, and utilizing solely the inverse feedback path, it was observed that the audio output increased a noticeable amount, and approximately 30%, without affecting the fidelity of reproduction. With such an arrangement the degenerative feedback is substantially closer to 100%. It may be stated that the feedback voltage apparently biases the detector tube automatically.

WhileI have indicated and described a system for carrying my invention into effect, it Will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a detector network, a tube including at least a cathode, control grid and plate, said cathode being at ground potential, a signal input circuit connected between the grid and cathode,

. tion of said characteristic and to secure substantial audio gain over said linear portion for strong signals.

2. In a detector network, a tube including at least a cathode, control grid and plate, said cathode being at ground potential, a signal input circuit connected between the grid and cathode, a resistive load connected to the plate, means for biasing the grid to render the tube operative as a plate rectification detector whose charac- I teristic is square law for weak signals and linear stantial audio gain over said linear portion for strong signals, an auxiliary anode disposed in said tube adjacent said cathode to provide a diode, a second load resistor connected between the cathode and diode anode, and means for impressing the carrier energy component in the detector plate circuit upon said diode anode.

3. In a detector network, a tube including at least a cathode, a control grid and plate, said cathode being at ground potential, a signal input circuit connected between the grid and cathode, a resistive load connected to the plate, means for biasing the gridto render the tube operative as a plate rectification detector whose characteristic is square law for weak signals and linear for strongsignals, and means for feeding back in degenerative phase to the grid substantially 25% of the audio voltage developed across said load resistor thereby to linearize the square law portion of said characteristic and to secure substantial audio gain over said linear portion for strong signals, said feedback means comprising a path which includes a, condenser and a tap slidable along said load resistor.

4. In combination with at least one signal transmission tube, a detector network of the plate rectification type and including a tube having a cathode at ground potential and a signalinput circuit, said detector havinga detection characteristic which is square law for weak signals and linear for strong signals, a load impedance in the output circuit of the detector for developing audio voltage, and means for feeding to said detector input circuit in a degenerative phase a relatively small portion of said audio voltage thereby to linearize the square law portion of the characteristic for weak signals and to secure substantial audio gain over said linear portion,

and means electrically associated with the grounded cathode and responsive to the amplified carrier component in the detector output circuit for varying the gain of said signal transmission tube. SEYMOUR. HUNT.

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