US2490448A - Frequency changer - Google Patents

Description

Dec. 6, 1949 s, A, OTT 2,490,448

FREQUENCY CHANGER Filed Nov. 9; 1945 ld Q Hu lo f f, I D 8 QV) u O C I kl 3 N g m.. k LS o QU) INVEN'I'OR. lg BYSTANLEYLOTT A TTORNEY Patented Dec. 6, 1949 FREQUENCY CHANGER y Stanley A. Lott, Sydney, New South Wales, Aus'- tralia, assignor to Amalgamated Wireless (Australasia) Limited, Sydney, New South Wales, Australia, a company of New South Wales Application November 9, 1945, Serial No. 627,751

(Cl. Z50- 36) 4 Claims.

This invention relates to modulators and like frequency changing systems in which it is required greatly to attenuate one or both of the original frequencies.

In small beat frequency oscillators of prior art construction, it has been common practice to employ multiple valves, (such as the well known GAS), in conventional frequency converter circuits in which a low pass filter is provided in the output circuit for rejecting frequencies except the required low frequency beats.

The satisfactory application of these prior art circuit arrangements involves many design problems which are difficult to overcome in an economical manner.

In order to secure frequency stability it is usual, for reasons well understood, to make the fixed and variable oscillators operate at as low a frequency as possible. If, in addition, the upper limit of the beat frequencies is required to be high, then the low pass filter in the output of the modulator or frequency changer' must be of elaborate design. For example, if the frequency of the fixed oscillator is 80 kc. and the frequency range of the variable oscillator 60-80 kc., in order to provide output beat frequencies up to 20 kc., then the low pass lter must transmit up to 20 kc. freely and then attenuate sharply at 60 kc.. Furthermore, inductances in the lter will require to be rather large, firstly because the impedance level is high. Large coils, in addition to occupying excessive space, tend to produce large fields which may interact upon the output.

From the foregoing it will be appreciated that the necessity for using a filter, in a circuit of the above mentioned type, for the purpose of rejecting the undesired frequencies in the output circuit, complicates design problems and adds greatly to the cost of the equipment.

In the past it has also been the practice, in order to suppress both of the input frequencies in the output, to employ various arrangements of the well known balanced modulator circuit.

These prior art modulator circuit arrangements require the use of balanced input and/or output transformers and eicient operation depends o n securing and maintaining a high degree of balance, a requirement that is not readily realizable in practice.

In a copending application Serial No. 635,098, filed December 14, 1945, a modulator circuit arrangement has been suggested which secures satisfactory suppression of the modulating frequencies in the output circuit without the use of balancing transformers.

Brieflythe arrangement adopted in the above mentioned copending application is as follows. Both modulating frequencies are applied simultaneously to the grid of one valve and to the cathode of a second similar valve which has its con.

trol grid effectively earthed for all operating frequencies and has an unbypassed resistive impedance in its cathode circuit. Due to this unbypassed impedance, so-called cathode phase inversion is achieved and the modulating potentials are effectively applied to the grid of this second valve degrees out of phase with the modulating potentials applied to the grid of the first valve.

The anodes of the two valves in this arrangement are connected -together and to an output circuit appropriately tuned to receive the desired beat frequency.

Although the invention set out in the above mentioned copending application overcomes most ofA the undesirable characteristics referred to above, it suffers from one serious disadvantage which prohibits its use in beat frequency oscillators, i. e., inability to prevent the modulating oscillators locking inf The object of the present invention is to provide a comparatively simple modulator or frequency converter circuit utilizing two valves in such a way that both of the modulating frequencies are greatly attenuated in the output, and in which the disadvantages outlined above in respect to prior art practices are substantially avoided.

The above objective is achieved in accordance with the present invention by an improved modulator arrangement comprising a pair of similar thermionic valves each having at least an anode, a cathode and a control grid, m'eans for applying modulating potentials to the control grid of one valve, means for applying modulation potentials of a different frequency to the control grid of the other valve, a cathode circuit common to both valves and having a high value impedance connected effectively in series therewith, and an output circuit for obtaining energy at a frequency equal to the sum and/or difference frequencies of said modulation frequencies.

For a more complete understanding of the invention and the manner in which it is to be carried out, attention is now directed to the following description in connection with the accompanying single figure of drawing, which illustrates one practical arrangement for performing the invention. y

Referring to the drawing, valves VI and V2 employed in the present example are shown as a source 'l of modulating potential-of x'e'd ire-A quency through a transformer A:Il "or other suit-v able coupling means in conventional manner.

Modulating potentials from a separate source trol grid 4 of valve V2 through a series resistor I and a shunt connected resistance-capacitynet- Work 25, 25. 1

The screen grid of each yof 'valvesVl Aland V2 is bypassed for all signal frequenciesby thecapacity Il and is supplied With .operating apo-1.y

tentials from the positive terminal I2 of a suitable high tension supply source (not shown) through the lead I3 an'd the series connected resistor I4. The suppressorgrid of Ieach`valve is connected to its associated cathode. .Y

The cathodes k2 of the valves 'VI and V2 ar connected together and to earth I5 through a series connected network comprising an impedance unit I6 and the self biasing 'resistance capacity combination I'l-I8.

The nature of the impedance I6 should be such that it maintains a. high impedance value for all frequencies to besuppressed.

In the present example the impedance I6 is constituted by a parallel tuned circuit L, C. The value of the bias1resistor I'l-is selected torprovide such biasing potentials on the grids 4 as will cause the valves'VI and V2 to operateon a square law characteristic. It is vto beunderstood that biasing potentials may be applied 4to :the-grids l in any known manner.

The anodes 3 of the valves `VI and V2 are joined together and connectedto the ioutput terminal I 9 through theA coupling condenser E2l) and.

a filter network shown in a dotted rectangle. Operating potentials from the positive terminal l2 of the H. T. supply sourcefn'otrshown) are supplied to the anodes 3 through the'anodefload resistor 22.

With the circuit arrangementthus described modulating potentials, applied'between` earth I5 and the grid 4 of valve VI, from the-'source 1, will appear across the'cathodegload impedance in I5. As the grid -4 of -the-valve V2 is connected to earth I5, the potentials appearing across 'the cathode circuit will-vbe-appliedtofthe grid -4 of the valve V2 Witha phase4 'dier'encef of180.de= grecs as compared to' the -fpotentials applied'to the grid 4 of valve VI'.

The theory Yof cathode phase'inversion is so well known-that a detailed `510 9 of variable frequency are applied to -the conf Y.

explanation of the 'operation Vis fconsidere'd'unnecessary. In like mannermodulatingpotentials applied to the grid 4 "of valve V2,`from the source 9, appear across the cathode loadimpedan'ce-'IG and are applied to the gri'dli vofthe'valveVl `in trol grids of both valves, in phase=op'pos`ition,l without the'necessity of lusing push-.pull tra-ns# formers and in circumstanceswhichiprevent, to`

a argc extent, undesirable ylocking in of the modulating potentials. With the plates 3 connected togetherbthelcir- 1 cuit functions in known 'manner-'tofp'r'oduce in the output circuit, "modulation @products-corresponding to the sum and difference frequencies of the modulating potentials applied to the grids 4 Whilst, at the same time, greatly attenuating the original modulating frequencies from the sources 'I and 9.

In a practical embodiment of the invention as incorporated in'albeat frequency oscillator, the fixed modulation-source "l was ladjusted to supply an output frequency of 88 kc. whilst the variable frequency modulation source 9 was adapted .to Yprovide-output oscillations over a frequency range of'from 68 to 88 kc. With this selection fof modulating frequencies, beat frequencies withinrthe range of from 0 to 20 kc. may be obtained across-the output of the output filter and fed to any desired utilization circuit either di- Y -rectlyorin controllable amplitude through a resistance potentiometer 22 or other convenient attenuator network in known manner.

The impedance unit I6 in the common cathode circuit rcomprlsesl a parallel resonant L, C circuit tuned, in the present example, to 78 kc. whichiinaintains a high impedance for all frequencies to be suppressed.

Thevalue of the resistor 25 and the condenser 2B 4in `the vshunt 'connected resistance-capacity network inthe inputcircuit of the valve V2 is chosen to provide an .impedance which is low to harmonics generated in the variable oscillator 9 buthighenough to supply suiicient grid voltage at the fundamental frequency. In conjunction withalarge resistor at I5, this circuit also eliminates interaction due to voltage from the fixed oscillator 'I reacting with the variable oscillator 9 via the grid capacity of valve V2.

The'circuit of the present invention permits thezoutput filter tube 'of simple construction, one section beingzsulicient tof'attenuate harmonics and any fundamentalappearing in the output.

The outputlter inthe present example comprisesfa'sin'glepi section networkin which the impedance in theseries arm constituted by a parallel tuned circuit L1, C1, resonant at the same-frequencyas the tuned circuitL, C in the Vcommon cathode lead, and the shunt arms are capacities fasindicated at 23, 24 Ain the drawing.

'Ihe'i-rrvention could vbe applied to single sidebandor -suppressed 'carrier modulation, in which casethe cathode load Would'betuned to the :carrierffrequency.

Although, inthe present example, the valves VI an`d=V2have been Ashown as pentodes it is to be clearly understoodthat performance of the invention'is not restricted to the use of valves orthistype iSimilar'itriodes or other convenient types may Vbe employed without affecting the scope -of .the invention.

V4I claim:

1. In a modulationlsystem, twolelectron discharge devices each having electrodes including a-'control grid, a cathode and an anode, a first sourc'eofoscillatory energy of carrier wave frequency connected between the control grid and cathode of one device, a second source of energy v'of 'lower 4frequency connected between the control grid and cathode ofthe other device, airequencyfvariable impedance which is low at frequencies 4materially higher than the frequency of the energy in said last named source coupling the 1fgrid of said last'named device to its cathode, -an impede-nce -e`fhich is high at the frequencies `of saidftvllol sources common to the connections between the control grids and cathodes of both of said devices, whereby voltage variations on the -control grid of said other device -are set -up in 3. A system as recited in claim 1, wherein said l0 Number last-named impedance includes a resistor in series with a parallel inductor and capacitor and a capacitor in parallel with said resistor.

4. A system as recited in claim 1, wherein said iirst-named impedance comprises a resistor and capacitor in parallel.

STANLEY A. LOTT.

REFERENCES CITED The following references are of record in the i'lle of this patent:

UNITED STATES PATENTS Name Date 2,218,524 Chaiee Oct. 22, 1940 2,240,450 Wolfskill Apr. 29, 1941 2,240,452 Wolfskill Apr. 29, 1941 2,349,933 Mueller et a1. Feb. 12, 1946

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