US2287280A - Detection arrangement and negative impedance for same - Google Patents

Description

June .23, 1942; F TERMAN I 2,287,280

DETECTION ARRANGEMENT AND NEGATIVE IMPEDANCE FOR SAME Filed June 30, 1959 vvvvvvvvvv INVENTO R mam-mews TEAMA/V ATTORNEY Patented June 23, 1942 DETECTION ARRANGE IMPEDANC Del aware MENT AND NEGATIVE E FOR SAME Frederick E. Terman, Stanford University, Calii'., assignor to' International Standard Electric Corporation, New York, N. Y., a corporation of Application June so, 1939, Serial No. 282,055

' Claims. (Cl. 178-44) I The present invention relates to improved circuit arrangements for performing a detection, and particularly to such arrangements wherein a rectifier is used to detect the modulated carrier wave so as to produce therefrom a wave corresponding to the envelope of the modulatedwave.

7 The invention also relates to a novel type of circuit unit having the current/voltage characteristics of a negative admittance, which is employed in the improved circuit arrangement above mentioned but which is also suitable for other uses. I

It is an object of the present invention to improve the action of detectors, especially detectors of the rectifier type. Especially it is an object to reduce the occurrence .of distortion in such a detector and to enable such detector to be employed for detecting modulated waves having a depth of modulation 'approaching 100% without giving rise to distortion or over-modulation. It is a particular object of the present invention to raise the alternating current impedance presented to a. diode detector to a value substantially. equal to the value of' the direct current impedance presented to such detector, in order to cause the diode to faithfully detect modulated signals without alteration of the efiective modulation depth, and in order to reduce distortions and permit the detector to be used with modulated signals having a depth of modulation approaching 100%.

In accordance with one feature of the present invention the raising of the alternating current impedance presented to adetector of the rectifier type by the load circuit of such detector is effected by connecting a unit having the characteristics of a. negative admittance across a portion of such load circuit. It is a further object of the present invention to provide an improved type of unit which shallhave the characteristics of a negative admittance and which shall be suitable for use not only for raising the effective alternating current impedance of the load circuit of a detector in accordance with the principal object of the invention, but also generally forall other uses for which negative admittance. units are desired.

In modern radio receivers and other communication devices a common form of detector ar-- rangement for deriving the envelopewave from a modulated wave is the so-called diode detector employing a simple half wave or full waverectifier of the space discharge type for performwhich correspond to the direct This means that the output current tends'to ing detection. Such diode detectors are genwhich correspond to erally used ina circuit whose impedance' -with respect to direct currents is somewhat higher thanits impedance with respect'to audio frequency-currents. As a resultof such disparity between the direct currentand alternating cur- .rent impedances which form the effective load for the diode detector, it is usually found that the direct current output from 'the diode does not bear as great a ratio to the audio frequency current output from the diode as the mean carrier amplitude of the high frequency waves ied to the diode bears to the'audio frequencyamplitude variations of such high frequency waves fed to the diode; s

In other words, because of the smaller impedance offered-to alternating current in the output circuit of the usual diode detection arrangement, the changes of'carrler amplitude audio frequency signal waves are more effectively detected than the unchanging or mean value ofthe =carrier=waves current output from the detector. When, therefore, a wave whose depth of modulation is equal-toot close to is applied to such a detector, it results that the corresponding output wave of the detector tends to be a pulsating current whosedirect current component is smaller than its'altemating component as a result of thegreater efl'e'ctjve impedance of the detector circuit for direct current than for. alternating current.

Because of the unithe diode detector place, and therefore actually reverse inpolarity. directional characteristic of such a reversal cannot take a corresponding flattening of the troughsof the audio'frequency output wave necessarily takes place. H

One of the principal objects of 1 the present invention is to eliminate such distortion. Briefly this is accomplished by making use of a circuit element having the'characteristics of a negative admittance for raising the-effective alternating current impedance of the load circuit ofa diode detector so that this alternating current impedance shall become substantially equal to'the corresponding direct current impedance of the.-

load circuit. r

' The exact nature of the invention may best be understood by reference to the attached drawing, in whichr Fig. 1 represents a diode detectorconnected to a circuit arrangement whichserves both'asa two-stage audio amplifier to amplify the output of the detector,

and also simultaneously serves to present a negative admittance at points z-r of the diode output circuit; and

Pig. 2 represents an alternative arrangement wherein the negative admittance effect across terminals x's:' is produced by alternative means.

Referring more particularly to Fig. l, the diode detector l and its tuned input circuit 2 are connected in series with an output or load circuit which primarily includes the resistor I and the resistor l which is capacitatively coupled' through condenser i. The carrier frequency by-pass condenser t is also provided across the output circuit in order to by-pass the carrier frequency. The carrier frequency input to tuned circuit I is supplied inductively, in the usual manner. Such a circuit arrangement is in itself typical of the conventional circuits heretoforev impedances N, P. Q, 80 as to form with the impedance-of the detector circuit a sort of bridge circuit, and in the fact that the cathode lead of the second amplifier tube t! is returned not to ground but to a point intermediate to .the resistors N and Q.

The three impedances N, P. Q, which are shown as simple resistors in Fig. 1, are connected so as r to form with the external impedance M connected across the terminals ::a:, a sort of bridge circuit as shown. The symbol M-is used for conwhile the cathode is returned to the diagonally essentially capacitative impedance so that so far as the impedances presented to the diode are concerned this amplifier might be considered as equivalent to a further condenser shunted across the resistor I.

From inspection of the output circuit 1, l, I, I, it will readily be seen that the direct current impedance. of this output circuit .is necessarily higher than the corresponding alternating current impedance thereof. If it be assumed that resistors 8 and l. are both'of the same order of a magnitude and if thecapacity provided by the amplifier across terminals :s-cbe neglected, it would follow that the alternatingcurrentimpedance would be approximately one-half of the opposite corner of the bridge which is grounded.

The cathode of this tube tl is given a suitable positive direct current bias with respect to ground by means of resistor I; Since the control grid of tube tlis essentially at ground potential with respect to direct current by connection through resistors P and l or Q and N, this positive cathode bias provided by resistor I has the effect of making the grid more negative than the cathode as desired. With respect to all desired audio frequencies, however, the cathode of tube ii is substantially short circuited to ground through condenser l so that this cathode may be regarded as directly grounded with respect to signal frequencies;

direct current impedance. If any signal more than modulated were applied to input circuit 2, over-modulation and consequent distortion would result in the output of the diode.

The undesirableratio of direct to alternating current impedance could, it is true, be reduced byzmaking resistance lvery large in comparison with resistance 3, but in practice it is usually not possible-to make resistance 4 higher than a fewmegohms, and since resistance 3 is generally at least of the order of a fraction of a megohm.

it is ordinarly not practicable to obtain substantial equality of the alternating current and direct current impedances by simply raising resistance I. It should furthermore be noted that even if resistance 4 were made infinite the alternating current impedance would still be somewhat smaller than the direct currentimpedance because of. the capacitiveadmittance provided by the wiring and by the input circuit of the following audio amplifier.

In accordance with the present invention the audio amplifier not only does not present a capacitive admittance for increasing the disparity between direct current and alternating current impedances, but actually this amplifier is so con-' nected as to present at the terminals 2-: a negative admittance, thus serving to neutralize the positive admittance of resistor l. I

The connections of the negative admittance unit shown in Fig.1 to the right of terminals H differ from those of a conventional ampli fierprincipally in the connection of the three The suppressor grid of tube tl is directly connected to this cathode and its screen grid-is effectively short circuited to the cathode with respect to signal frequencies through condenser 8. Over high resistance II the desired direct current bias for the screen .grid of tube ti is sup-- plied by battery I I The plate;of tubetl is fed with the necessary 5 positive direct current from battery Hover feed-resistor II and coupled over condenser II and grid resistor It to the control grid of the next tube t2.

The cathode of thesecond tube H is returned to the lower corner of bridge M, N, P, Q, as shown. Thesuppressor grid of this; tube .t! is directly connected to the-cathode and the screen grid isefiectively short circuited to this cathode so'far as the'signal frequencies concerned over-condenser It, being fed with the required positive directcurrent bias over resistor II from battery II'. The plate of tube ii is fed over impedance II (which is shownas an inductive high impedance) from battery H. Output coupling condenser 18 couples the plate of tube t! to one of the output terminals o'-o and also to the other comerjof the bridge M, N,' P, Q. The

cathode of tube ii is correspondingly connected to the other of the output terminals 0-0 and to V the lower corner-of the bridge M,."N, P, Q. r

The operation of the negative impedance element shown to the'right of terminals H in Fig. 1, can best be explained by'assuming that an electromotive force 11- is produced by the action of the diode detector. Not all of this voltage will be effectiveacross theinput of tube tl, but the drop in N and the drop in Q will-both be effective to. form a signalapplied between ground and the control gridv of tube ti. This voltage between the grid oftube ti and ground will give rise to an amplified voltage acrossthe output of the ampllfler, which output voltage will serve to energize the loudspeaker or, other load (not shown) which is connected acrosso-o. This output voltage is'not -only eflective across the loudspeaker or other load, ybutisfal so fed back to the upper and lower corners of the bridge M, N, P, Q. Assuming for convenience oi. discussion that 7 NP 7 so that M:P=N:Q, then no voltage drop between the grid of tube ti and groundwillbe produced by such fed back voltage and therefore substantially no degenerationor regeneration will occur. 'A very considerable current, however, will flow a:'-:r which in this case is the diode detector circuit) Such current through M will cause a voltage drop in M so that the total voltage across :t-zc will be the original E. M. REE: plus the added drop due to the current in M.

It should be noted that the senseor polarityof this current through :c-c; isfso related to the total voltage existing across z :r, as to correspond to a negative impedance or more properly a'negative admittance on-the ri ht of terminals :ra:. Furthermore, the approximate magnitude of the negative admittance can .be estimated as followsr It can be seen vby inspection thatif the impedance of M- is substantially greater than NP/Q so that the bridge M, N, P,,Q, is considerably unbalanced, then clearly oscillations will occur if the gain of theamplifier is reasonably great. Since the occurrenceof oscillations indicates that the admittance of M issmaller in absolute magnitude than the negative admittance presented by. the unit-to-the right of, this means that for reasonably large values of voltage amplification the value of the negative admit-.

tance is only slightly smaller in absolute magnitude than the valueof positive admittancewhich Mjmust have to balance the bridge M, N, P, Q. In other words, the j admittance presented at 12-: by the back-co'nnected-ampliiler arrangement will be approximately -Q/NP if a is large.

The quantityia is the voltage amplification, i. e. the ratio of the'voltage across 00 to the voltage across grid-cathode of tube I under actual operating conditions,

tive admittance oilfered bythe unit the impedance M is assumed to be zero,,or NP/Q, or any other convenient value, and then the ratio of the current through x-a: to the voltage across For more accurate computation of the'nega- ,r-a: is computed for an assumed voltage of Eet If the impedance of I! be considered; infinite and t e impedances of age, J3, I5, late con-- through the external impedance M (i. e.- through the external circuit connected to theterminals pliclty.

the grid otptube ti and" ground arises by virtue of the feedbackv voltage from cathode of tube t2 tothe upper and lower corners ,of the bridge, still. under-these conditions a lcislight degeneration will be present because of' 'In the above discussion certain second order efiectshave' been neglected for thesakeo t sim Inthe-first place, it should be-noted.

that even if the bridgeM, N, P, Q, be perfectly balanced'so that no potential diilerencebetween the plate and the fact that both theigrid and 'fllamentoftube t2 7 will be made more positive; thus decreasing .fact that the plate very tube. the degeneration occurring between the output the; eilective -platevoltageof 'tube'tlh it should be noted that a slight degeneration-or negative feedback from the output of tube t2 to the input of this very tubeexists by virtue of the circuit of tube t2 includes resistor N while at the same time the input to this tube t2-is appliedfrom' tube I not between the grid and cathode, but" between the grid of 7 when 'and'ground; The drop in* resistor N f caused bythe output current of tube t2 is therefore degeneratively applied to the input of this It "should be noted, however, that and input of tube tZ- itself maybe considered as 3 equivalent merely to a-reduction in the effective amplification of this tube and thus is taken into account in the measured-value 'of a. Similarly the slight degeneration or negative' feedback existing around the whole amplifier from'the out- ,put of tube t2 to the input of'tube ti by virtue of the reduction of efiective plate voltage which occurswhen both theifilament and the grid 'become more positive is also automatically included in the calculations if a is measured under operating conditions;

i For the purpose of eliminating distortion in the diode detectorcircuit,the best adjustment of the negative admittance presented by the unit is one which raises the 'alternating'current'im- I pedance of the diode load circuit to substantially the same valueras the direct current impedance of thisload circuit. It is'not desirable to overcompensate so as'to make the altematingcur-v rent impedance greater than the direct'current impedance because.when this is the :case theef- 'iective' input resistance oil ered by thediodejand its associated circuit .to the input wave applied to tuned circuit 2 is greater with respect to the sideband than with respect to the; carrier. The' result of this is that whna signal having'a:

depth of modulation-approaching 100% is apsidered zero the negative admittance at :r-:r-is I It will be noted that in actual operation M need not have the value zeronor the value NP/Q as assumed above, since the value of the negative admittance presented at H is independent of the external circuit at least if the impedance M of this external circuit is not so great asv to cause instability and oscillation. For very many purposes, however, it is desired that the negative admittance of the unit at :t-x should be nearly equal and opposite to the positive admittance of the connected circuit. In such cases M will be roughly equal to NP/Q.

T h to a the ,purely' resistive plied, for example,- by inductive coupling to the tuned circuit 2, the 'resultinglcarrierfrequency I current flow in tuned circuit'l conta n a larger percentage of. sideband and a smaller percentage of carrier than the originalmodulated wave, and I thus is likely to correspond tolan over-modulated condition in circuiti.

In the above discussion it-has beenassumedi for the sake of simplicity, that the impedances conductance.

but likewise the shunted capacitive admittance of the wiring, and also perhaps or the amplifier circuit itself; r

.I have found that the negative admittance .unit. shown to the right or the terminals :n-a: in Fig. 1, may be used-not onlyto produce the eiiect of a negative conductance, but generally to produce theiefiectof negative admittance or a any type.

For this-purpose it ismerely necessary to replace the resistor P by an impedance but also-stray capacities which may exist, across thisresistor. 1

Fig.- 2 represents an'alternative arrangement which may in some cases be preferred, especially when it is desired to improve the direct current ,to-alternating current impedance ratio of a diode detector which is already connected to an existing audio amplifier with-a minimum of alterations to the existing amplifier. Inthe circuitof Fig. 2, the parts corresponding to parts already described in Fig. l, are designated'by the same reference characters distinguished by the addition of primes. The apparatus to'the left of the terminals ,.r- :r' is illustrated, for the sake of generality, as comprising'a diode of-the s c-called full wave or duplex type rather than a simple half wave dlodejbut actually such apparatus may correspond exactly tothe apparatus to the left of the terminals :c-o: in Fig, 1.- The negative impedance and amplifier unit to the right'of the terminals :r'-a:' is, however, based upon a different principle from the negative impedance unit shown in Fig. l. The unit shown in Fig. 2 has a positive feedback resistor P' connected from the ungrounded side of the output o'o to the ungrounded side of the input :r'--a:'. The connection of such a feedback resistance P tends to produce a regeneration unless Q has zero impedance, and in order to overcome such an effect a negative feedback 'or degeneration is provided by means of the voltage divider M, N' which serves to make the cathode-oi tube H v more positive at those instants when thepla'te of sor grid is tied to the cathode and the screen grid is by-passed to the cathode.

The negative admittance across terminals inf-x of the negative admittance unit of Fig. 2

where a is large. a Although the inventionis'illustrated-as appliedto a diode rectifier of either the half wave or iull'wave type, it will be understood that it i equally applicable to other rectifiers such as, for example, dry disc rectiflers of the copper oxide or selenium type. It should also-be noted ,of desired character such as an-inductance, a

that m y be taken at amt-airt me fromthose indicated In the figures. In g 1,

for examp1e,'the output as shown, does, not have either side grounded; 1,11" desired the output could be taken between the upper ,term-ina'l o and ground in Fig, 1 although in suchcase the impedance of the'outputwould have to. be taken into consideration since it would be effectively across the terminals H.

'Thenegative admittance units of the present inventionareuseiu-l notonly in connection with a detectionarrangement as shown, but also generally Iorall' otherv applications ,where anegative admittance o1 stable characteris desired.

The negative admittance unit of Fig.- 1 (shown to the right of the terminals1:r) may, for example, be'connected directly across. tuned L. 0. unit such as the circuit ljfor th'e purpose or neutralizing the resistive admittance of this tuned circuit so as. to raise its effective reactance resistance ratio. It is well known that an anti-resonant circuit may be considered equivalent to a pure inductance and pure capacitance and a shunted high resistances Thus the connection or a negative conductance almost equal to the positive conductance of the equivalent shunt resistor will almost neutralize this equivawhole unit will be correspondingly reactive or complex 50 that the impedance of the unit at :c-a: will be like the impedance at P but with a negative sign. If, for example, P or P is induc tive while the other quantities remain resistive, the negative-admittance presented will correspond to the admittance of a negative inductance; that is, it will be similarto a capacitance in its phase characteristic, but will become smalleriin' magnitude with rising frequency, thus corresponding to an'inductan'ce' in its frequency characteristic. Although itis preferredto employ P or P as the generalized impedance for determining theycharacter of the negative admittance of the units the impedance N or N may be used instead of P or? a the controlling impedance. The impedance Q'and M'"also exert control but in inverse fashion. j

Although I prefer to'use, one of the types of negative admittance units shown in Figs. 1 or 2, for equalizing the alternating current and direct current impedances of a diode detector, other known types of negative, admittance units may be employed. Furthermore, as above mentioned, it is ordinarily sufiicient' to employ a negative conductance rather than a more complex negative admittance and, therefore, known types of devices'suitable forgiving negative conductance may be employed. V i

Although certain particular embodiments. of my invention have been shown and described for the purposes of illustration; it will be understood that various adaptations, alterations and modifications, thereof occurring to one skilled in the 7 art may be made without departing from the scope of my invention as defined in the appended claims.

What I claim is:

'1. A device for presenting to an external circuit a current voltage characteristic corresponding to a negative admittance, which comprises an amplifier having an input including a grid lead and a cathode lead and an output, a first imj pedance joined'in series with said external circuit to form a first voltage divider, a connection from the junction between said external circuit and said impedance to the grid lead, a second and a third impedance joined in series with each other to form a second voltage divider, a connection from the junction between said second and third impedances to the cathode lead, and means connecting said two voltage dividers in parallel across said output.

2. A device for presenting to an external circuit a current voltage characteristic corresponding to a negative admittance, which comprises a twostage resistance coupled amplifier having an input and an output, three impedances connected to each other and to said external circuit to form a bridge, connections including a blocking condenser extending from said output to one diagonal of said bridge, and direct connections from the other diagonal of said bridge to said input.

3. A device for presenting to an external circuit a current voltage characteristic correspond- :0

ing to a negative admittance, which comprises a first and a second space discharge tube each having a. cathode and a grid and an anode, means for applying operating potentials to said elec-' trodes, three impedances connected to each other and to said external circuit to form a bridge having first and second corners diagonally opposite one another and third and fourth corners also diagonally opposite one another, a connection including a blocking condenser extending from the anode of said second tube to the first corner of said bridge, a direct connection from the cathode 01 said second tube to the second corner of said bridge, two further direct connections from the third and fourth corners of said bridge to the grid and cathode of said first tube, respectively, and a connection including a second blocking condenser extending from the anode of said first tube to the grid of said second tube whereby the anode and cathode of said first tube are coupled to the grid and cathode of said second tube over said second blocking condenser and oneor said impedances.

4. A device according to claim 3, wherein said first and fourth corners of said bridge are adjacent said external circuit.

7 5. A device according to claim 3, wherein said second and third corners of said bridge are adjacent said external circuit.

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