US2141750A - Frequency converter - Google Patents

Description

E. W, HEROLD Dec. 27, 1938.

FREQUENCY CONVERTER Filed June 19,

.#1 ma 1m rs INVENTOR E. mmf/20w ATTORNEY Patented Dec. Z7, 1938 FREQUENCY CONVERTER Edward W. Herold, Verona, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 19, 1937, Serial No. 149,060

7 Claims.

The present invention relates to a frequency converter circuit for use in a superheterodyne receiver, and more particularly to means for the cancellation or neutralization of space-charge coupling existing between the oscillator and radio-frequency portions of the converter tube.

A frequency converter, sometimes referred to as a pentagrid converter, widely used at present in superheterodyne circuits, consistsv of a multielectrode vacuum tube designed to perform simultaneously the functions of a mixer (first detector) tube and of an oscillator tube. During investigations made on frequency converters of the multi-grid type in which the oscillator voltage is impressed on or generated by the first grid (in combination with other electrodes) it was found that a peculiar form of coupling between the oscillator and signal circuits existed which could not be explained by ordinary electrostatic or electro-magnetic eiects. Further work indicated that the coupling was caused by a change in the space-charge in the neighborhood of the signal grid as the oscillator voltage Went alternately up and down.

An apparently adequate explanation of the reffect was given by W. A. Harris in Proc. I. R. E. 23, pg. 279-294, April 1935, where it was also stated that the effect could be neutralized by the connection of a small capacitance from oscillator to signal grids. Later developments have shown that this method of neutralization is inadequate, especially at high frequencies.

An important object of my invention therefore is to rprovide a frequency converter circuit wherein the space-charge coupling between the oscillator and signal grids may be neutralized more eifectively than hitherto possible.

A further object is to effect such neutralization `with comparatively simple means. Other objects are to improve generally the operation and efficiency of a frequency converter over a wide range of frequencies.

The novel features characteristic of my invention are set forth with particularity in the appended claims. The invention itself however will best be understood by reference to the following Y description taken in connection with the accompanying drawing in which, Y

Figs. 1a, 1b and 2 are certain characteristic curve measurements obtained on the signal grid of a converter tube, Y

Fig. 3a is a schematic circuit diagram used to explain the action of the space-charge coupling,

Fig. 3b 1s an equivalent circuit,

(Cl. Z50-20) Fig. 4 is a frequency converter circuit embodying my invention, and

Fig. 5 is a practical circuit ofa frequency converter according to my invention.

I was led to my discovery by an analysis of 5 the high-frequency input admittance data taken on an RCA-GAS pentagrid converter with various #l grid voltages. The data are shown in the form of curves in Figs. la, 1b and 2 of the accompanying drawing. Fig. la shows the change in #4 gridV capacitance from the cold value as the #l grid voltage is changed. This curve was changed but very slightly as the frequency was changed and so is approximately correct for all frequencies used (i. e., up to 50 mc.).

Fig. 1b shows the #4 grid conductance as a function of the #l grid voltage. The so-called cold and therefore constant conductance of about 12 frmhos was subtracted from the data which shows, therefore, only the conductance caused by electron flow. The curve shown in Fig. 1b was measured at 31.5 mc. At other frequencies, the shape remained the same but the ordinate scale expanded or contracted to a close approximation as the square of the frequency. This is more clearly shown in Fig. 2 which shows the variation with frequency of the #4 grid negative conductance at two of thev values of #l grid voltage used in Fig. 1b.

In Fig. 3a is shown a circuit of a converter tube of the type under consideration. In this diagram, an oscillator voltage V0 sin et is applied to the grid adjacent to the cathode. It may bementioned that in the majority of frequency con-I verters of this type it is immaterial as regards this invention whether the oscillator voltage is applied from a separate source as shown or whether it is self-generated by using the first two grid electrodes of the tube as part of a selfoscillating circuit. The reason for this is that in the majority of tubes the #2 grid is comprised of two, relatively small, electrodes not in the direct path of the main electron stream. Variations in potential of this grid are, therefore, of only minor importance inraffecting the currents considered. In Figure 3a, the #2 grid is shown connected to the normal steady d-c potential only. The #3 and #5 grids are screening electrodes and are also connected to a steady supply voltage. The #4 grid, which is the signal grid, is shown connected through an external circuit, consisting of a meter M, to its normally negative bias. If the screening action of the #3 and 5 grids is reasonably effective, the #4 grid circuit inside the tube does not include admittances to any 55;

other electrodes but the said screening electrodes. As is shown by the data of Figs. 1a and 1b the internal portion of the #4 grid may be represented by a conductance and a capacitance. Calling these cgi and Cgi respectively, an equivalent circuit for Fig. 3a is, therefore, Fig. 3b Where the #4- grid circuit through the external meter M and applied steady Voltage Vdc (equal to screen voltage plus signal grid bias as shown inV Fig. 3a) is shown completed inside the tube by a conductance ggl and a capacitance Cgil.

If the meter M of Fig. 3b is used as an indication of the current flowing in the external signal grid circuit, it is clear that if Cgi and agi werel constant, no alternating current would ow. Sinceggi is a conductance present only to alternating frequencies (i. e., it is Zero at d-c or zero frequency) there would also be no d-c current. This, of course, must be true since the signal grid is biased negatively. In the actual case, however,Figs. la and 1by show that Cgi and gia-4 are not constant if the #l grid voltage is Varied. Application of the oscillator Voltage V0 sin wt to the #1 grid causes Cg4 and cgi to vary up and down periodically in synchronism with the oscillator voltage. As regards the external #4 grid circuit, an a-c current will flow of oscillator frequency as long as Vdc is greater than Zero. The behavior is analogous to one of a circuit in which Cg4 is a condenser microphone and ggi is a high resistance carbon microphone, both actuated at oscillator frequency. Y

In a complete converter circuit, a tuned input circuit is connected in the signal grid in place of the meter M of Figs. 3a and 3b; In this case, the a-c current flowing in this circuit due to the variation of ggf and Cg4 will set up an alternating voltage of oscillator frequency which'is impressed on the signal grid. The voltage set up is especially high when the signal grid circuit is nearlyr in tune with the oscillator, as it is when a low I. F. is used. The net result is equivalent to a form of coupling between oscillator and signal circuits which is detrimental to good operation.

In brief, it is seen from the above analysis that the variations of oscillator grid potential cause corresponding changes in the signal grid conductance 4and capacitance. The fluctuations of this conductance and capacitance, acted upon by the d-c potential between the signal and adjacent grids, cause an oscillator frequency voltage to be set up across the signal grid input circuit. The latter voltage causesdifficulties in operation including lower conversion gain and troublesome signal grid current due to a-c voltage exceeding the bias. The diiculties are most pronounced when the ratio of `signal frequency to intermediate frequency is high.

Quantitatively', inspection of Fig. la' shows that, for a typical tube, assuming a mean oper- Y ating #1 grid bias of m8, the #4 grid capacitance change is about 0.06Mifd per volt change on #l grid. At 31.5 megacycles, this corresponds to a susceptance change of 12mhos-per'volt change on #l grid. At the same operating point, Fig. 1b

shows that the conductance change, again at 31.15Y mc., is about lcmhos per volt change on #1 grid.

From therelative magnitudes of the rates Vof change of the susceptance and theV conductance,VV

it is clear that the conductance change isabout equal in importance to the susceptance'change. Previous investigators appearV to `have been unaware of thisV condition.

Since the rate of change of the susceptance of the #4 grid increases linearly with frequency while that of the conductance increases with the square of the frequency, as vshown by the data of Figs. la. and 1b, the importance of the conductance change becomes much greater as the frequency is increased. At veryV high frequencies, the conductance change is the predominant eiect, while at low frequencies the susceptance change is most important. Experimental data taken at 20 megacycles showedthat the conductance change alone was suicient to cause serious coupling of oscillator and signal circuits.

As a result of my work, I have found that the phenomenon which occurs in the tube can be completely compensated for by the connection of an admittance from oscillator to signal grids provided this admittance is so chosen as to satisfy` certain relations as explained hereinafter.

Previous explanations of the effect attributed it, correctly, to changes in space-charge in the vicinity of the signal grid caused by the Variations at oscillator frequency. By this explanation, high-frequency transit time elects are neglected. I have found, however, that at high frequencies, the conductance change mentioned is a transit timel phenomenon which is also present and modifies the behavior of the tube markedly so as to make the earlier explanation inadequate. At the same time, the use of `a capacitance connected from signal to oscillator grid to mitigate the eifect as was done byrprevious investigators has also been found to .be inadequate at high frequencies. Instead, it is found to be desirable touse Va more complexadmittance; in the usual case it is necessary to add a resistance, preferably in series with the capacitance, in this circuit. The net result is complete elimination of the coupling effect.

In particular, I have found that cancellation or neutralization of the coupling effect described can be accomplished by a connection of a capacitance and a resistance from #l grid to #4 grid. The C and R combination canbe either series or parallel but the former is preferred when operation is to occur over a band of frequencies. Othernetworks combining resistance, inductance and capacitance can also be used to advantage in particular instances.

To show the neutralization, I have taken data in aV circuit similar to Fig. 4. V1 and V2 are two vacuumtube voltmeters, the rstreading the applied oscillator voltage and the second reading the voltage induced on the #4 grid circuit L2, Cz which is always adjusted so as to give maximum deflection` on V2 for oscillator fundamental frequency.Y At 2O megacycles with'the circuit L2, C2 havingY a resonant impedance of about 8000 ohms, the following data were taken:

Neutralization Vl Vg N0ne T .V 1. 5 3.60 Capacitance only (approx. 2 mrnid.) 6.0 3. 30 Capacitance and series resistance (approx. 2 mmfd. and

455 ohms) 6.0 0

Y oscillator voltages, however, a slightly different value of neutralizing admittance was needed. The behavior was qualitatively as predicted vby Fig 1 when it is considered that the #1 grid voltage Vis altered periodically by the oscillator; a

very'large V1 wouldv'swing-the tube to theV upward .converter curving parts of the AC and conductance curve and reduce the neutralization necessary somewhat.

It is clear from the data given above that the use of capacitance neutralization, only, was decidedly unsatisfactory. 4With the correct resistance-capacity arrangement, however, it was found that the LzCz circuit was completely independent of the oscillator circuit; tuning C2 through resonance gave no change in any of the electrode currents or any reaction upon the oscillator voltage. This is in marked contrast to the behavior with capacitance neutralization only.

The explanation of the means by which cancellation is effected by the connection of an admittance from oscillator to signal grid involves the sending of a current through this admittance into the signal grid external circuit which cancels the current in this circuit due to the internal changes of Cg4 and ggl. It is found that the action of the varying Cg4 may be counteracted by a constant capacitance connected from oscillator to signal grid. An analysis shows further that the action of the varying gg4 may be counteracted at any one frequency by a constant conductance connected from oscillator to signal grid. As the data of Figs. `1 and 2 show, however,`the conductance change of gg4 varies with the square of the frequency. To adequately cancel the induced effects, the cancelling conductance would have to have a value rising with the square of the frequency. Although more complex circuits can be devised which approximate this characteristic, the simple series connection of a capacitance and resistance can be shown to approximate the condition sufficiently closely, provided the frequency is not too high. As is well known, the effective conductance of a series capacitance C and resistance R is "1+w2c2R2 which for small values of @CR is closely approximated by g=w2C2R. Thus a conductance whose value depends approximately (very closely for small values of wCR) on the square of the frequency is obtained.`

At the same time, the effective capacitance of such a series circuit of C and R is approximately constant (again assuming small values of wCR). The combination, therefore, will adequately cancel the coupling effects over a wide range of frequencies, essentially from frequency to an upper limit imposed by the approximation that wCR be not too large. In the practical case described by theV data of Figs. 1 and 2 this upper limit is about 40 megacycles.

In Fig. I have shown a practical frequency system embodying my invention. Briefly, the converter comprises the multi-electrode vacuum tube l0 having the grids marked #1, #2, #3, #4 and #5 interposed in the `order named between cathode il and anode l2. High frequency signal voltages are impressed on the tunable input/circuit I3, the high potential end of which is connected to the #4 grid or signal control grid. Bias for this grid may be obtained from a source of potential connected' to the minus side of the grid resistor lli or from an A. V. C'. potential, as Well known in the art. The oscillator portion of the converter is constituted by the cathode H, oscillator grid #1, oscillator anodegrid #2 and the circuits associated therewith..

The tuned circuit I 3 connected between oscillator grid #l and cathode ll determines the frequency of the locally-generated oscillations. Suitable positive potentials are impressed on the oscillator anode-grid #2, screening grids #3 and #5, and the anode I2.` Included in the connections from the potential source to said electrodes are the filters l5, I6 and I l, respectively. The interaction within the tube between the locallygenerated oscillations and the received signal oscillations results in the production of an intermediate frequency in the output or anode circuit which is tuned to that frequency by the resonant circuit I8. As well known, the I. F. is further amplified and then detected, the resulting audio frequency being fed to a sound reproducingl device.

As heretofore explained, in order to compensate for the space charge coupling effects between the oscillator grid and the signal grid, an admittance comprising the series connection of a condenser C and a resistance R is connected between said electrodes as shown. Although I have shown the invention as applied to converter of. the type having a self-excited oscillator, it is to be understood that the invention is equally applicable to a converter system wherein an external oscillator is employed in which case the locally-generated oscillations will be applied to the #l grid, the #2 grid then serving as an additional screening grid which may be connected to the cathode or placed at some suitable d--c potential.

Having thus described my invention, what I claim is:

1. A frequency mixing system comprising a multi-electrode vacuum tube, a signal frequency acting on one of said electrodes and a local oscillator frequency acting on another of said electrodes, said last mentioned electrodes having by reason of their'proximity and the potentials impressed thereon undesired interaction, and means for cancelling said interaction of one electrode upon the other, comprising an admittance connected between said electrodes, the admittance consisting of both conductive and susceptive components.

2. In-a frequency converter, the combination of a vacuum tube provided with a cathode, an anode and a plurality of control electrodes interposed between said cathode and anode, means for causing a control electrode in proximity to the cathode to oscillate at a definite frequency, means for impressing a signal modulated carrier wave on another of said control electrodes remote from the first control electrode, there being undesired space-charge coupling effects between said control electrodes, and means for compensating for said coupling effects comprising a complex admittance having conductive and susceptive components connected between said control electrodes. Y

3. In a frequency-converter, the combination of a vacuum tube provided with a cathode, an anode and a plurality of control electrodes interposed between said cathode and anode, means for causing a control electrode in proximity to' the cathode to oscillate at a definite frequency, means for impressing a signal modulated carrier wave on another of said control electrodes remote from theflrst control electrode, there being undesired space-charge coupling effects between said control electrodes, and means for compensating for said coupling effects comprising a circuit including capacitance and resistance connected between said control electrodes, the resistance having a value of approximately 450 ohms and the capacitance a value of approximately 2 micromicrofarads which are effective to compensate for said coupling effects in the frequency range from about 10 to 25 mega cycles.

4. In a frequency converter, the combination vof a vacuum tube provided with a cathode and sisting of both conductive and susceptive components connected between signal and oscillator grids.

5. The invention defined in the preceding claim wherein said admittance is constituted by a series-connected resistance and capacitance, the resistance having a value of approximately 450 ohms and the capacitance a value ofV approximately 2 micromicrofarads which are effective to compensatefor said coupling eiects in the frequency Vrange from about V10 to 25 megacycles.

6. In a frequency converter, the combination of a vacuum vtube provided with an oscillator` section and a signal amplifying section, said sections having undesired coupling therebetween within the tube which increases with frequency, and means for nullifying the effects due to the undesired coupling over a comparatively wide frequency range, said means comprising an external circuit connected between said sections, said circuit including a resistance and a seriesconnected capacitance.

7. In a frequency converter, the combination of a vacuum tube provided with an oscillator 'section and a signalamplifying section, said sections having undesired -couplng therebetween within the tube, said coupling arising by reason of the signal grid capacitance variation which increases with frequency and the signal grid conductance variation which increases with the square of. the frequency,and means for compensating for said capacitance and conductance variations, comprising an admittance connected between said sections, said admittance having both conductive and susceptive components.

EDWARD W1 HERO-LD.

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