US2118075A - Tone control circuit - Google Patents

Description

Ma'y 24, 11938. P. o. FARNHAM TONE CONTROL CIRCUIT Filed Oct. 14, 1931 ATTORNEY Patented May 24, 1938 UNITED STATES TONE CONTROL CIRCUIT Paul O. Farnham, Boonton, N. J., assignor to Radio Corporation of America, a. corporation of Delaware Application October 14, 1931, Serial No. 568,673

9 Claims.

My present invention relates to radio receivers, and more particularly to methods of, and means for, controlling the fidelity of reproduction of a radio receiver during tuning and gain control operation thereof.

In my copending application Serial No. 523,859 filed March 19, 1931, Patent No. 2,072,945 of March 9, 1937, there have been disclosed various methods of, and circuit arrangements for, ren- 10 dering a dynatron circuit suitable for use in radio receivers employing tuned radio frequency amplification. It was demonstrated in said application, from theoretical considerations, that a dynatron amplifier having a tuned plate load impedance will oscillate at any frequency for which the plate load is a pure resistance that is greater than the plate resistance. It was shown that this oscillating conditiony may be reached either by adjusting the control grid bias toward less negative values, or by increasing the plate load.

Hence, in accordance with the invention disclosed in said application, the tendency to oscillate as an electrode voltage was adjusted to increase amplification, was overcome by a simultaneous and automatic adjustment of another electrode voltage.

It was shown that one method of obtaining this action, when the amplification rate is controlled by varying the control grid bias, con-l sisted in introducing into the plate circuit a resistance of such value that the plate potential varies with grid bias to avoid conditions favorable for oscillation at any gain control adjustment of grid bias. In addition, the plate circuit of the dynatron amplifier was to include a corrective network, cooperating with the tuned output circuit, which maintained the load impedance, as viewed from the plate and cathode terminals, substantially independent of the tuning 40 of the output circuit. The result obtained by means of the invention of my aforesaid copending application showed that the gain, for different adjustments of the control grid bias in the dynatron circuit, was approximately inde- 45 pendent of frequency. It was also shown that two or more stages of tuned dynatron amplificationcould be readily cascaded, and freedom from oscillation secured, as Well as approximate independency of gain over frequency.

50 However, the frequency uniformity of gain in the tuned dynatron amplifier circuit was shown to be accompanied by an improvement in selectivity. This improvement in selectivity was considered as resulting from a decrease in the net 55 positive resistance of the tube and output as a (Cl.A Z50-20) whole, becoming greater as the load impedance approached the magnitude of the plate resistance. Therefore, When the stage gain is increased by using less negative control electrode bias voltage, and the ratio of load impedance to 5 plate resistance approaches unity, the selectivity increases. For the reception of distant stations, whose energy is collected at low intensity, this increase in selectivity with amplifier gain increase tends to reduce the high audio frequency 10 components of the noise output of the receiver. However, it will be readily appreciated that in the normal broadcast reception of local stations, the increase of selectivity with increased amplier gain results in the impairment of fidel- 15 ity of reproduction since the high audio frequency components are eliminated.

Accordingly, it may be stated that it is one of the main objects of my present invention to provide a radio receiver provided with a high frequency amplifier, a detector and a low frequency amplifier, the gain control characteristic of the high frequency amplifier being such that when the amplifier gain is increased, the selectivity increases, means being employed between the detector and low frequency amplifier output for compensating for the reduction of high audio frequency components, the last mentioned means being arranged for simultaneous operation with the gain control device of the high frequency 30 amplifier.

Another important object of the present invention is to provide in a radio receiver, comprising a tuned dynatron amplifier stage, a detector and an audio frequency amplifier wherein a tone control device is employed between the detector and the output of the audio amplifier for correction of increased selectivity when the gain control mechanism of the dynatron amplifier is adjusted for increased amplifier gain.

Those skilled in the art are well acquainted with the phenomenon which occurs during tuning of the conventional radio broadcast receiver which employs tuned radio vfrequency ampliflcation. It has been observed `that as the selecting device is adjusted for selecting the low radio frequencies of the broadcast range that increased selectivity is secured. In other words, the selectivity characteristic of the receiver becomes greater as the receiver is tuned from the `high frequency end of the range to the low`frequency end. This phenomenon will usually occur in high frequency amplifiers employing a tuning condenser device. That-is to say, it is immaterial to the change of selectivity with frequency whether the amplifier is a dynatron circuit, as disclosed in my aforementioned copending application, or any conventional tuned radio frequency amplifier. It will therefore be seen that here', again, it is desirable to provide a means for insuring fidelity of reproduction at the low frequency end of the tuning range.

Hence, it may be stated that it is another important object of the present invention to provide a radio receiver including a frequency selecting device adapted to tune the receiver throughout a desired frequency range, and additional means, simultaneously operative with the selecting means, for correcting the high audio frequency reduction when the selecting device is adjusted to the low frequency end of said range.

Another object of the present invention is to provide in a radio receiver at least one stage of tuned radio frequency amplification including a station selecting means and a gain control device,

.and additional means between the detector and the output of the audio frequency amplifier for permitting the high audio frequency components to be amplified to a greater extent than the low audio frequency components when the tuning means of the receiver is adjusted to a station disposed near the low frequency end of the broadcast range, and/or in the case of a dynatron radio frequency amplifier, when the gain control device is adjusted for increasing the gain of the radio frequency amplifier.

Still other objects of the invention are to improve generally the efficiency of radio receivers employing tuned dynatron amplifier circuits, and particularly to provide a receiver of this type which is not only stable and reliable in operation, but capable of faithful reproduction throughout the tuning range of the receiver and throughout the gain control range of the receiver.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically one circuit arrangement whereby my invention may be carried into effect.

Referring to -the accompanying drawing, wherein there is shown a radio receiver, it will be noted that the receiver comprises a source of signal energy I coupled, as at M1, to an inductance coil L1 disposed in the input circuit of the radioI frequency amplifier tube 2. TheY radio frequency amplification circuit, disposed between the source l and the detector 3, comprises a. tunable dynatron amplifier circuit which has been disclosed in my aforementioned copending application. Since the latter discloses the theoretical and practical features of this dynatron circuit, it is not believed necessary in this case to disclose more than the essential features of the circuit.

It will therefore be understood that the dynatron amplifier circuit in this application represents merely one satisfactory embodiment of the invention for securing operation which conforms to the requirements, already stated. The electron discharge tube 2 is of the tetrode, or screen grid, type having an input circuit which may be a resonant circuit L1, C1 connected between the control electrode G1 and an equi-potential cathode K which is energized for electron emission by a filament H. The source o f current supply for impressing voltages on theI several tube electrodes is indicated diagrammatically as a source B, but it is obvious that any suitable source, such as the customary, power supply devices, may be employed. As indicated, a negative bias Ec is applied to the control grid by means of an adjustable tap 4 connected to the cathode K through lead 5. The screen grid G: is connected to a point on the current source B through a lead 6, in which the voltage is greater than the supply source voltage which determines thevoltage of the anode P.

The tuned output circuit L2, C2 is not connected directly across the anode P and cathode K, but is coupled to the anode through a condenser Cm. The direct current potential from supply source B is impressed upon anode P through a shunt circuit which includes a choke coil L, and resistors R1 and R2. Condensers C are provided to by-pass the anode resistance R2, the screen grid, and cathode leads for carrier frequencies.

The inductance L, resistor R1 and associated condenser C form one path from anode to cathode with a second path through the coupling con-y denser Cm and parallel tuned circuit L2, C2. The resistor R1 is necessary to prevent oscillation at a frequency determined substantially by the choke L in shunt with capacity Cm and the inherent anode-cathode capacity of the tube. It also serves in holding down the value of the load impedance at the lower end of the frequency band. The combined resistors R1 and Rz function to increase the anode Voltage,` for a constant anode supply voltage, as the control grid bias Ec is reduced to increase the stage gain.

The variable tuning condensers C1 and C2 are shown arranged for uni-control by a conventional arrangement 1, while the grid bias control 4 is shown arranged for manipulation by a convention device 8. The detector3,while conventionally represented, is to be understood as comprising any well known type of detector circuit, either of the grid leak or anode detection type, and has its input connected across the. resonant circuit L2, C2.

The output of the detector 3 is shown coupled by an audio frequency transformer Mz to an audio frequency amplifier 9, the output of which amplifier may be coupled by an audio frequency transformer M3 to any desired and well known type of utilization means, such as head phones, loud'l speaker and the like. It will, of course, be understood that the amplifier 9 may comprise one or morestages of amplification.

It has' been pointed out heretofore that the tuned dynatron amplifier circuit shown herein, While possessing a frequency uniformity of gain, simultaneously becomes more selective as the gain control tap 4 is adjusted to decrease the negative bias on the control electrode G1. This improvement in selectivity may be considered as resulting from a decrease in the net positive resistance of the tube and output circuit as a whole, and becomes greater as the load impedance approaches the magnitude of the anode resistance. While for reception of weak signal energy transmitted from distant points this increase in selcctivity tends to reduce the audiofrequencycomponents of the noise output of the receiver and thus improves distant reception of weak stations, it is also true that with increased selectivity the transmission eiiiciency of the high audio frequency components is seriously impaired.

Accordingly, there is provided in the output of the audio amplifier 9 an arrangement for compensating fo-r such reduction in transmission efficiency of the high audio frequencies. The compensating means comprises an aperiodic reactive path including a resistor I0 in series with a fixed condenser II, connected in shunt with the primary winding of the transformer M3. An adjustable tap I2 is arranged for varying the fiow of high audio frequency currents through the path I0, II. That is to say, when the tap I2 is adjusted to one end of the resistor I0 and adjacent the high potential side of the condenser II there will be a maximum diversion of high audio frequency currents through .the condenser II. Conversely, when the tap I2 is adjusted towards the opposite end of the resistor I0 there will be a minimum diversion of high audio frequency currents through the path I0, II. 'I'he gain control device 8 is arranged for unicontrol, through a conventional linkage I3, with the resistortap I2. 'I'he arrangement is such that when the grid bias tap 4 is moved towards the positive side of the source B, that is to increase the gain of the dynatron amplifier, the resistor tap I2 will be adjusted towards the upper end of the resistor I0, that is to divert less of the higher audio frequency currents through the path I0, II. Of course, when the tap 4 is adjusted to diminish'the gain of the dynatron amplifier then resistor tap I2 will be adjusted to divert more of the higher audio frequency currents.

In the same manner there is provided an aperiodic reactive path I0', I I', in shunt with the primary winding of the transformer M2. 'Ihe adjustable tap I2 is arranged for uni-control with the tuning condensers C1 and C2 through the linkage 1. It will be readily realized that when the tuning condensers are adjusted to the low frequency end of the desired frequency range, that the tap I2' will be adjusted to the upper terminal of the resistor I0', that is to divert less of the higher audio frequency currents. In this way, for `increasing selectivity, and also for increased amplifier gain, there will be less diversion of higher audio frequency currents through the aperiodic reactive, or tone control, path. It should be pointed out thatthe transformers M2 and M3 should be chosen so as to impart a rising frequency amplification characteristic to the part of the receiver between the detector 3 and the utilization means. Thus, the higher 'audio frequencies will inherently be amplified to a greater extent than the lower audio frequencies. Hence, the taps I2 and I2' will function to decrease the transmission of the greatly amplified higher audio frequencies when the selectivity of the dynatron amplifier is at a minimum, as when the tuning condensers are adjusted to the high frequency end of the range, and the gain control is adjusted to increase the amplifier gain.

While there has been shown only a single stage of amplification disposed between the source I and the detector 3, it is to be clearly understood that two, or more, stages of dynatron amplification may be readily cascaded, and the present invention employed therewith. In my aforementioned copending application such a cascaded tuned dynatron amplifier has been disclosed, and I wish it to be clearlyI understood that the present tone control arrangements can be employed in conjunction with the tuning devices and gain control device shown in said application.

Furthermore, while two separate tone control paths are shown; it will vbe apparent to those skilled in the art that a single path could be employed, which path would include two resistors, each resistor being associated with the tuning control or the gain control. Furthermore, it is within the scope of the present invention to permit the linkages I and I3 to be so arranged that the gain control may be disconnected from the tap I2 and the tuning control I disconnected from the tap I 2' with the result that the increased selectivity inherent in the dynatron amplifier circuit when the amplifier gain is at a maximum can be taken advantage of for distant reception.

While I have indicated and described one arrangement 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 setn forth in the appended claims.

What I claim is:

l. In the operation of a radio receiver including a dynatron amplifier having a resonant output circuit adapted to be tuned over a band of frequencies, a detector and an audio frequency amplifier, the method which consists in amplifying collected signal energy, detecting the energy, amplifying the detected energy, reducing the variation of the ratio of anode load impedance to the internal anode-cathode impedance as the output circuit of the dynatron is tuned over a band Iof frequencies and simultaneously controlling the transmission efficiency of the audio amplifier.

2. In a method, as defined in claim 1, the step of maintaining the anode load impedance substantially constant throughout the tuning range of the dynatron amplifier.

3. The method, as defined in claim 1, which includes the step of automatically varying the internal anode-cathode impedance as a function of the anode load impedance as the dynatron amplifier is tuned over its frequency range.

4. In combination, in a radio receiver, a dynatron amplifier including tuning means, a detector, an audio amplifier, means for controlling the gain of the dynatron amplifier, additional means in the audio amplifier output circuit for varying the transmission efficiency of the audio amplifier for higher audio frequencies when the said gain control is varied and further means in the detector output circuit for varying the amplification of the higher audio frequencies simultaneously with adjustment of said tuning means.

5. In combination, in a radio receiver, a dynatron amplifier including tuning means, a detector, an audio amplifier, means for controlling the gain of the dynatron amplifier, and additional means in the audio amplifier output mechanically coupled with the gain control means for varying the transmission eiciency of the audio amplifier for higher audio frequencies when the gain control means is adjusted to increase the gain of the dynatron whereby the apparent selectivity of the dynatron amplifier is increased, said additional means comprising an adjustable aperiodic reactive path.

6. In combination, in a radio receiver, a dynatron amplifier, including tuning means, a detector, an audio amplifier, means for controlling the gain of the dynatron amplier, and additional means in the audio amplifier output circuit for varying the transmission efficiency of the audio amplifier for higher audio frequencies, said additional means including an adjustable impedance larranged for uni-control adjustment with the means for controlling the gain of the dynatron amplifier.

7. In combination, in a radio receiver, a dynatron amplifier including tuning means, a detector, an audio amplifier, means for controlling the gain of the dynatron amplifier, and additional means in the audio amplifier output circuit for varying the transmission efficiency of the audio amplifier for higher audio frequencies, said additional means being so arranged that the higher audio frequencies are efficiently transmitted when the gain control device of the dynatron amplifier is adjusted for increasing amplifier gain.

8. In combination, a tuned radio frequency amplifier, detector, and audio frequency transmission circuit, means for adjusting the gain of n to adjustments of said gain adjusting means, for automatically increasing the transmission of the higher audio frequencies in said transmission circuit as the selectivity of said radio frequency ampliiier is increased by adjusting its gain to higher levels, said gain adjusting means and said responsive means being constructed and arranged for simultaneous adjustment.

9. In a radio receiving system a high frequency amplifying portion including means for tuning it to a desired frequency, a low frequency amplifying portion, and separate manually controllable means vconnected with said high and low frequency amplifying portions for simultaneously decreasing the gain in the high frequency amplilfying portion and the high-frequency response of the low-frequency amplifying portion at`the said desired frequency.

PAUL 0. FARNHAM.

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