US2299481A - Neutralizing system - Google Patents

Description

Oct. 1942. 5, JQHANSSQN 2,299,481

NEUTRALIZIN G SYSTEM Filed march 26, 1940 lllplll llllll y s. a. JOHA/VSSON q. H-Mdt A TTORNEV Patented on. 20, 1942.

. UNITED STATES PATENT OFFICE I NEUTRALIZING SYSTEM Sven G. Johansson, Cranford, N. J., assignor to western Electric Com York, N. Y., a corpora ion of New York Application March 26, 194i), Serial No. 325,939

50laims.

This invention relates to tuned vacuum tube circuits and more particularly to a neutralizing means therefor.

In developing tuned intermedite frequency ampliiier circuits for a precision detector operating at an intermediate frequency of about 1900 kilocycles it was found that the tuning shifted as the gain was changed resulting in double peaks in the transmission characteristic. This detunin was of such a magnitude that it could not be tolerated in the class of equipment for which it was intended and was found to be caused by a change of coupling capacitance.

It is the object of this invention to eliminate the aforesaid detuning effect byproviding a means for nautralizing the efiect of the change in coupling capacitance caused by the changes in gain and in the more general case to neutralize the efiect of the coupling capacitance in such circuits wherever double peaks appear.

The foregoing object is attained by providing in combination a vacuum tube coupling two circuits tuned to the same frequency, an input and output circuit for said tube including a portion common to both circuits and an inductance inserted in the common portion of such magnitude as to substantially neutralize all feedback at the said y, Incorporated, New

become so low as to injure the tube. By-pass condenser C shunts the alternating current components of the plate current around resistors R, R1 and has connected in series therewith the neutralizing inductance L.

Screen grid SG is connected to a suitable source of voltage +SG, positive with respect to cathode K. The output circuit 0 is connected in series with the plate circuit and connects plate P to a suitable source of plate voltage +13, also positive with respect to cathode K. The negative pole B of the plate supply source is connected to cathode K through bias resistor R as shown. It will thus be clear that bias resistors R, R1, by-pass condenser C and inductance L form a network common to both input and output circuits.

vA modification of the cathode circuit is shown in Fig. 1A wherein resistor R shunts only its bypass condenser C while neutralizing inductance L is in series with the combination. This circuit is of value where resistor R is relatively small but is not limited to that case as it may be used with any value of resistor R.

tuned frequency, whereby detuning is eliminated.

For a more thorough understanding of the invention reference maybe made to the drawing wherein:

Fig. 1 discloses the invention applied to a tube circuit utilizing a self-bias resistor;

Fig. 1A is a modification of part of 1;

Fig. 2 shows how the invention may be applied to a circuit utilizing a separate bias source; and

Figs. 3 and 4 are analytical modifications of Fig.

Referring now to Fig. 1, input circuit I and output circuit 0 are tuned to the same frequency as in the intermediate frequency stages of a hetero'dyne amplifier and are coupled together by a vacuum tube T which, for convenience, has been shown as a screen grid tetrode but may be any other type of tube whether capableof variable gain adjustment ornot. Input circuit I is connected to control grid G and to cathode K as shown, the conmotion to the cathode being through self-bias resistors R, R1. Resistor R is shown variable as" a convenient means of varying the gain of the tube in a-well-known manner, while fixed resistor R1 provides a minimum limit so as to prevent annulling the effect of inductance L. Fixed resister-R1 also assures that adjustments of resister R will not permit the grid bias voltage to Fig. 2 is essentially identical to Fig. 1 except that here a separate bias source C is connected to the input circuit I to provide suitable bias to grid G. In order to provide gain control, this bias voltage C is made variable by any suitable means, not shown. Blocking condenser C is provided to by-pa'ss the alternating current input. Neutralizing inductance L is inserted in the oath-. ode' circuit as shown.

It is a well-known fact that for circuits such as 'hereinabove described double resonant. peaks appear whenever the common coupling impedance between two substantially identical tuned circuits of relatively high Q becomes greater than a value approximating the circuit resistance. Applicant has discovered that the detuningeflect experi enced in variable gain intermediate frequency ampliflers operating at relatively high frequencies is caused by an effective change in coupling impedance; This may be better understood by redrawing Flg. 1 as shown in Fig. 3, wherein the interelectrode capacitance of tube T considered as a triode, together with its external stray circuit capacitances, have been shown in the form of a delta-connected network comprising grid-cathode capacitance Cc, plate-cathode capacitance C? and plate-grid capacitance Coo. For the purposes of the. present description neutralizing in-' ductance L has been deleted. While in Fig. 1 the input and output circuits I and 0 were shown as comprising pure reactances. they. ini iflways have some resistance which has here been capacitance Crc.

shown as RI and R0 for the input and output circuits, respectively. Ordinarily resistances R1 and R0 will be practically equal and may be assumed so for the purposes of this explanation. So long as the common coupling impedance in the network formed. by capacitances Cre, Cu, Cr and C is greater than the resistance of R: or R0 the circuit will have a double peak resonance transmission characteristic.

A change in tube gain is equivalent to a corresponding change in the plate-grid interelectrode Thus while the circuit may have a single resonance peak for a low value of tube gain, two peaks may appear as the gain is increased thereby causing considerable confusion where the circuit is depended upon for frequency or transmission characteristic analysis purposes. The double resonance peaks may be eliminated by reducing the common coupling impedance with an inductance tuned to the resonant frequency with the coupling circuit capacitances as will be presently described.

For purposes of analysis Fig. 3 may be redrawn as shown in Fig. 4 wherein the delta network has been converted into an equivalent star or T network with the equivalent capacitance in the vertical leg combined with the capacitance C to form capacitance Cc. Now if inductance L is of such magnitude that it'is in series resonance with capacitance Cc atthe resonant frequency of circuits I and O, the coupling impedance formed thereby may be made very much less than the critical circuit resistance R1 or R0 whereupon the transmission characteristic will have but a single resonance peak.

Similar considerations for the circuit of Fig. 2 show that when inductance L thereof is so chosen as to be in series resonance with the vertical leg of the coupling capacitance of the equivalent T network this circuit will also produce a single resonance peak and for the same reasons given above.

Another way of explaining this phenomenon may help to better understand the action of the inductance. The double resonance peak is caused by feedback through the plate-grid interelectrode capacitance Cra- By' properly adjusting the magnitude of inductance L a neutralizing feed- .back is introduced into the grid circuit and of the same magnitude but 180 degrees out of phase with that caused by the plate-grid capacitance. This compensating feedback voltage is derived from the voltage drop across the inductance L. ,The frequency shift or' double resonance peak is thereby wholly eliminated.

While for the sake of 'a specific disclosure a variable gain electron discharge device has been embodied in the disclosure, it is apparent that such a device need not be used. Any vacuum tube capable of relatively high voltage gain and operating at high frequencies may produce a double resonance peak in its transmission characteristic when coupling two circuits tuned to the same frequency. eliminated in the same manner and by the application of the same principles as above described for the specific variablegaintube disclosed.

What is claimed is: v 1

1. A device for eliminatingdetunlng of tuned circuits coupled by a variable gain electron discharge device comprising two circuits tuned to the same frequency, a variable gain electron discharge device having inherent inter-electrode capacitances coupling said two circuits, an input and an output circuit therefor including a portion common to both circuits, and an inductance inserted in the common portion of such magnitude as to be in substantial series resonance with said inherent coupling capacitances at said resonant'frequency.

2. A device for eliminating detuning of tuned circuits coupled by an electron discharge device comprising two resonant circuits tuned to the same resonant frequency, an electron discharge device having inherent interelectrode capacitances coupling said two circuits, an input and.

an output circuit therefor including a portion common to both circuits, and an inductance inserted in said common portion of such magnitude as to be in substantial series resonance with said inherent coupling capacitances at said resonant frequency.

3. A device for eliminating detuning of tuned circuits coupled by an electron discharge device comprising two circuits tuned to the' same reso- -nant frequency and having an effective resistance, an electron discharge device coupling said two resonant circuits, an input and an output circuit therefor includinga portion common to bothcircuits, and an inductance connected in the common portion of such magnitude as to reduce the coupling impedance between said tuned circuits below the efiective resistance thereof.

4. A device for eliminating detuning of tuned circuits coupled by an electron discharge device comprising two resonant circuits tuned to the same resonant frequency, an electron discharge device having inherent interelectrode capacitances coupling said two circuits, an input and an output circuit therefor including a. portion common to both circuits, said common portion including a self-biasing resistor and a by-passcondenser in parallel, and an inductance inserted in said common portion of such magnitude as to be in substantial series resonance with said inherent coupling capacitances and said byepass condenser at said resonant frequency.

5. A device for eliminating detuning of tuned circuits coupled by an electron discharge device comprising two resonant circuits; tuned to the same resonant frequency, an electron discharge device having inherent interelectrode capacitances coupling said two circuits, an input and an output circuit therefor including a portion common to both circuits, means for supplying a separate bias to said input circuit, and an inductance inserted in. said common portion of such magnitude as to be in substantial series resonance iThese double peaks may be with said inherent coupling capacitances at said svENG. Jormnssou.

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