US2459262A - Oscillation generator and frequency multiplier - Google Patents

Description

Jan. 18, 1949. A. P. BUFFiNGTON OEICITJII'JNI'ION GENERATOR AND FREQUENCY MULTIPLIER Filed Mafch 15, 1946 MULT/PL/ER do I - mama/var (MEG/VCYCLES) INVENTOR. HAHN P. BUF'F/NGTUN Patented Jan. 18, 1949 UNITED OSCILLATION GENERATOR AND FREQUENCY MULTIPLIER Alan P. Buflington, Philadelphia, Pa., assignor, by mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation .of Pennsylvania Application March 13, 1946, Serial No. 654,046

7 Claims. (01. 250-36) This invention relates to improvements in frequency multipliers, and particularly to improvements in frequency multipliers of the type driven directly by an oscillator. The invention has been found to be applicable but is not limited to the combination of an oscillator and a frequency multiplier employing a single dual-purpose vacuum tube, as for examplea double triode, which functions both as the oscillator and as the frequency multiplier.

It has long been conventional to use the superheterodyne principle in the amplification of received amplitude-modulated carrier wave signals, and it is equally conventional to. use this principle in the amplification of received frequency-modulated signals. As is well known, superheterodyne receivers customarily employ a local oscillator for supplying the heterodyning frequency used by the mixer or converter tube in the process of converting the frequency of the carrier signals to an intermediate frequency which is more conveniently amplified.

Under recently prescribed regulations, the broadcast of frequency-modulated signals in the United States is assigned to the frequency band extending from 88 to 108 megacycles. This band is very high in comparison with the amplitudemodulation broadcast band which extends from 550 to 1600 kilocycles, and it is also substantially higher than both the AM short wave band (upper assigned limit approximately 22 megacycles) and the heretofore employed FM band (42 to 50 mo.).

The generation of local oscillations over the range of frequencies required for heterodyning with the recently assigned very high FM carrier band presents difiiculties which were not present to any appreciable extent at the lower frequencies heretofore used' These difficulties may be reduced, but not altogether avoided, by the known expedient of employing an oscillator in combination with a frequency multiplier, the oscillator being operated at a sub-harmonic of the local frequency required for heterodyning, while the required high frequency oscillation is obtained from the output circuit of the frequency multiplier.

A frequency-modulation receiver employing an oscillator-multiplier in place of the more conventional fundamental-frequency oscillator is disclosed in the co-pending application of Milton L. Thompson, filed December 7, 1945, Serial No. 633,453, and assigned to the assignee of the present invention. While the arrangements therein disclosed are productive of very substantial improvements in the frequency stability of the locallygenerated oscillations, it has been found that frequently the voltage output of the multiplier section of prior, tunable oscillator-multiplier circuits is neither large enough nor uniform enough for satisfactory performance of the receivers frequency converter stage.

The output voltage obtainable from a frequency multiplier is known to be inversely proportional to the order of the selected harmonic. Consequently, in order to secure as high an output as possible, frequency multipliers are preferably employed to generate the second harmonic, rather than the third or higher harmonics, and as such arecommonly referred to as frequency doublers. In the case of a superheterodyne FM receiver, assuming that an intermediate frequency of the order of 9 megacycles is to be employed and that the local oscillation frequency is to be on the low frequency side of the received signal, it will be necessary for the oscillator section of the oscillator-doubler'to generate oscillations having frequencies tunable over the range extending from about 39 to 50 megacycles. The doubler section will accordingly deliver local oscillations whose frequencies are variable over the second harmonic range extending from '78'to me. The cause or causes of low and non-uniform doubler output voltage at these very high frequencies'are difiicult to determine. Among the factors involved is the fact that at high frequencies the inductive reactances of the leads between the various circuit components are appreciable in comparison to the reactance values of the circuit components themselves, and this is true even though the leads be kept as short as possible. Moreover, stray capacitances exist between the leads, the circuit elements, the tube electrodes, the chassis, etc., the values of which at these high frequencies are sufficient to affect seriously the operation of the apparatus. For these and other reasons, the operation of very high frequency apparatus is diflicult to analyse accurately.

It will be understood by those skilled in the art that the local oscillation voltage introduced into the mixer or converter tube of a superheterodyne receiver is preferably held approximately con-' plitude of the local oscillator wave with respect to that of the inlcQming'R-F. carrier wave. For

R.--F. carrier wave.

amplitude of the difference-frequency component 3 linear detection, the amplitude of the local oscillation should be much greater than that of the Where this is the case, the

of the mixer tube output is largely independent of the amplitudeof the strongerv local signal,.and nearly proportional to the strength of the weaker incoming carrier signal. And the magnitude of the difference-frequency output is largest when:

the amplitude of the stronger component differs from the amplitude of the weaker'signal by the greatest amount.

In a preferred form of the present invention, improvements are made in-high frequency oscillator-multiplier circuits which efi'ect"-a substantial increase in the amplitude of the multiplier -,V includes grid capacitor ;l2',. grid leak: l3, paravsiticsuppressingresistor-14, :a part of inductance coil 5, conductor 25, and the ground lug 8, the circuit to cathode H being completed through chassis.

1 The-signal developed at grid of the oscillator portion of tube V is applied directly to grid l6 output voltage and which also attain a substana tial improvement in the uniformity of the amplitude over the frequency range.

In another form of the present invention, improvements are made in high frequency oscillator-multiplier circuits which'cause the amplitude of the output voltage of the frequency multiplier to be more uniform in magnitude over the required frequency band without necessarily accomplishing increase in the'amplitude.

It is an objectof this invention to providein combination a high frequency oscillator and a frequency multiplier, the frequency multiplier having improved uniformity of output voltage over the operating frequency range.

It is another object of thisinvention to provide a high frequency oscillatorin combination with a frequency multiplier, where in the frequency multiplier has substantially increasedoutput volt age, as well as improved uniformity of output voltage over the operating frequency range.

It is a more specific object of this invention to provide a high frequency, oscillator-multiplier having external coupling arrangements between the output circuit of the frequency'multiplier and the circuits of the oscillator whereby :high fre-. quency energy may be-fed back from the .multi-. plier to theoscillator to thereby achieve increased multiplier output and improved uniformity thereof over the operating frequency range.

These and other objects'and advantages of the present invention, as well as the invention itself; will be best understood by reference to the fol-- lowing description and accompanying drawings wherein i Figure l is a schematic illustration of a preferred form of circuit organization for carrying my invention into effect; and

Figure 2 illustratesby means of a graph, the scope of the improvements which are realized from the invention. o

Referring now to Figure 1, there is schematically illustrated a high frequency oscillator-fre-. quency-multiplier in which theoscillator portion is tunable to a sub-harmonic of the desired localoscillation frequency by means of the oscillator frequency-determining circuit I comprised of variable tuning capacitor 2, trimmer capacitor ,3, and inductance coil or winding 4. The latter is inductively coupled to coil or winding 5 as shown. Vacuum tube V, which may be a commercial type TF8 tube, is comprised of two triode sections, one of which functions as .theoscillaton. .As shown in, the,.;figure, the external circuit connections of the oscillator include a plate circuit comprised of,

coupling capacitor 6, a part of i-nductance coil conductor (which, physically, is verys-hort), and groundlug 8 of frequencymultiplier tango circuit v I 0, the return to the =-cathode .;l.l being;

of the second triode-section of the tube' This secondsection of tube V functions as the frequency multiplier and is tunable to the desired harmonic of the oscillator frequency by frequency-determining circuit H) which is comprised of variable tuning capacitor I1, trimmer capacitor IB'and' inductance coil 9. For maximum multiplier output, circuit I0 is preferably tuned to the second harmonic of the frequency generatedin the oscillator portion of the apparatus, and for convenience of operation tuning capacitors 2 and "may be ganged together by known'mechanical means. 7

Assuming that the apparatus illustrated "in Figure 1 is to function as an oscillator-doubler and is to be employed to supply the heterodyning frequency in a frequency-modulation receiver iof the superheterodyne type, the operating frequency 'of-the oscillator may be made-variabla-by means of the tuning/capacitortZ over a range extending from about .39t'o' 50 megacyclesnThe frequency-doubler portionof the apparatusisis; accordingly, tunable by meanslof. the capacitor I! over a range extending; fromnabou't 718 toi 10 0 megacycles, thusproviding coverage. ofsaiband 7 extending from about 87*to 109. mega'cycleswhen an intermediate frequency of 9 mBg-ECYCIBSF-iSEm ployed. The tuned oscillator and doubler circuits should, of course, track accuratelythroughout their tunable ranges.

The output voltage of the multiplier may :be taken from coil 9,?as for example, from tap 19', and may if :desired be applied through conductor 20 directly to a control electrode of the converter tube of a superheterodyne receiver (not shown).

Ifthis is done; the most desirable. tap point' onz' coil 9 willbe determined byithe impedance-50f. the converter tube into which the multiplieno ute put voltage is injected. V

Coupling .zcapacitor 2| isolates the ,--multiplier tank circuit from the high D.-C. voltage applied to the anode of the multiplier triode, while pro viding-thernecessary R.-F. .couplinghgtw i anode andsaid tank circuit. Resistor 22 and choke coil 23,.are effective to isolate the tunedradio. frequency circuits; from B+. If desired; both 'of'these isolating elements maybe choke coils, or both may be resistors.

.In the conventional form of oscillator-multiplier, the low-potential end of inductance coil is either connected directly to the groundeds-ides; of capacitors 2 and/or 3, orisgroundeddirectly,

- as for example,-to thechassis. Similarly, inthe conventional circuit, the tapped connection-pf coil 5 is either connected directly ,to cathode ll. or grounded directly, as to the chassis.

I have found, however, thatthe output voltage. v of a high frequency doubleroramul-tiplier,canbe substantially increased, and in addition heldarelai ti-vely constant in amplitudepver the required high frequency range, :by connecting the low :po-u

tential end .ofcoil vll and also the tapped tonnes-- I tion 24 of ;coi-1;5 toa tapped poi t. go @519 potential endof coil 9. In Figure 1, these connections are made byway of conductor 25. The tapped connection 26 is preferably made very close to ground. Inthe actual construction of the particular high frequency oscillator-multiplier illustrated in Figure 1, the tapped connection 26 is made to the upper end of ground lug 8which connects coil 9 to the chassis. It will be understood that at the high frequencies involved the .inductance of lug 8 is appreciable and that lug 8 functions as a portion of coil 9. The physical dimensions of lug 8 were such that the ungrounded end thereof was approximately three-eighths of an. inch removed from-the chassis. I have found that the position of the tapped-connection is most. critical; for example, if tap 26 is moved to any appreciable extent either closer to or farther away from chassis (ground) than the end of lug 8, the improvements in the amplitude and uniformity of the multiplier output voltage rapidly diminish. Of'course the optimum adjustment will vary in different arrangements, depending upon-the physical size of the structures involved. v

I have also observed that the direction of winding of coil 4 has a bearing upon the results achieved. With the Winding in one direction, the optimum improvements in multiplier output voltage are obtained; if the winding be reversed the improvements may not be realized. Which direction of winding is productive of the most favorable results may be readily determined by trial.

Reference is now made to Figure 2 wherein curves representing the output voltage of the frequency multiplier are plotted against frequency, both for the conventional circuit and also for the novel circuit organization embodying the invention illustrated in Figure 1. Curve A of Figure 2 represents the output voltage characteristic of the frequency multiplier when the conventional form of circuit hereinbefore described is used. Curve B is the output voltage curve obtained when the novel arrangements of Figure 1 are employed. It will be observed that, with the exception of the extreme low-frequency end, the magnitude of the output voltage represented by curve B is much greater than that shown by curve A. It will be noted also that the voltage variations within the band '78 to 100 me. are negligible in curve B as compared with those of curve A. Curve B therefore represents a very important improvement over the conditions shown in curve A.

The precise theory of operation of the improved circuit has not been definitely established. It will be appreciated by those skilled in the art that, at the very high frequencies here under consideration, stray inductances and stray capacitances existing throughout the circuits substantially affect the operation of the apparatus. These effects may be quite different at different frequencies in the operating range. For example, a particular, physically small portion of circuit may be series resonant at one frequency, and parallel resonant at a different frequency within the same operating band, and in consequence substantially affect the output voltage of the multiplier, or of the oscillator, or of both.

The circuit organization illustrated in Figure 1, which substantially improves the output voltage characteristics of the oscillator-multiplier, provides coupling means for feeding back high frequency energy from the output circuit of the multiplier to the plate and/or grid circuits of the oscillator, the phase relations apparently being such that increased signal voltageon grid l6 of the multiplier is realized, thereby producing increased output.

. If desired, only tap 24 of coil 5 may be connected to tap 26 of coil 9, the connection of thelow potential end of coil 4 to tap 26 being omitted, and connection thereof made. directly to the grounded sides of capacitors 2 and/or 3, or to chassis (ground) in the conventional manner.

Or, theabove connections may be reversed, that is, the low potential end of coil 4 may be connected to tap 26 of coil 9, tap 24 of coil 5 being connected in the conventional manner directly to chassis (ground) or directly to cathode H.

I found that if the preferred circuit illustrated in Figure 1 is modified in either of the above manners, the output voltage characteristic of the multiplier is appreciably improved over that of the conventional circuit insofar as uniformity of output over the frequency band is concerned; but substantially increased amplitude of output is not realized.

I have described and illustrated my invention by means of an oscillator-multiplier which uses a single dual purpose vacuum tube, but it will be understood that the invention is also applicable to the combination of an oscillator and frequency multiplier in which separate vacuum tube envelopes are employed. Moreover, the invention is not limited to the particular form of oscillator circuit shown in the illustration; any of the oscillator circuits known to be suitable for the generation of high frequency oscillations, of the order and for the purposes here involved, may be employed.

Having described my invention in a preferred form, but intending to be limited only by the scope of the claims, I claim:

1. In combination, a high frequency oscillator comprising a tube having cathode, plate and grid electrodes, said oscillator including a tunable frequency-determining network and plate-cathode and grid-cathode circuits, a frequency multiplier having an output circuit tunable to a harmonic of the oscillator frequency, means for applying the oscillator output to the multiplier, and means for applying a portion of the output of said multiplier to the grid-cathode circuit of the oscillator.

2. In combination, a high frequency oscillator comprising a tube having cathode, plate and grid electrodes, said oscillator including a tunable frequency-determining network and plate-cathode and grid-cathode circuits, at frequency multiplier having an output circuit tunable to a harmonic of the oscillator frequency, means for applying the oscillator output to the multiplier, and means for applying a portion of the output of said multiplier to the plate-cathode and gridcathode circuits of the oscillator.

3. In combination, a high frequency oscillator comprising a tube having cathode, plate and grid electrodes, said oscillator including a tunable frequency-determining network inductively coupled to the grid-cathode and plate-cathode circuits of said oscillator, a frequency multiplier having an output network tunable to a harmonic of the oscillator frequency, means for applying the oscillator output to the multiplier, and means for applying a portion of the multiplier output to the frequency-determining network of the oscillator.

4. In a high frequency oscillator-multiplier having an oscillator section which includes a tube having cathode, plate and grid electrodes swe tie and-a. tunable frequency-determining network inductively coupled to the grid-cathode and plate-cathode circuits of said oscillator andhaw ing: a multiplierv s'ectionwhich includes .an. output network tunaloletov a harmonicofthe oscillator frequency; means for applying a portion of; the multiplier outputto the grid-cathode circuitoi the oscillator and to. the oscillators frequencydetermining network, said means comprisinga connection from the multiplier. output. network to the oscillators frequencyedetermining network and grid-cathode circuit.

5; An oscillator-multiplier including two triodes in a single vacuum tube envelope, the multiplier portion comprising one of said :triodes. and a frequency-determining output circuit including an inductance, the oscillator portioncomprising the other of said triodes and plate-cathndev and grid-cathode circuits including a tunable frequency-determining network, said grid-cathode cathode-and grid-cathode circuits incIudinefl/a tunable; frequcnw determining;networkgrssaid grid-cathadecircuit: including. a tanned nortiQ of .saiduzdouhler output.inductance;saidetapped portionwheing av portion: of the: low potential tvid of said doubler, inductance.-

'7. An. voscillator-doublenz including a double triodez vacuum tube; wsaid doublet; portion nris ng ne ofvsaid ,triodes.;.a-nd-a -tunabl quency detemliningputnut circuit includin' inductance" coil; aid oscillator: pontionscompris ingrthe. other :ofzsalid-etriodes za platereathflqe circuit, a -griclrcathode circuitpandwa tunable irequencyeidetermining network-inductively cou-A v pledto said plateecathode: andv gridecathodeacire:

.cuits', saidlgridecathode circuitincludinge tapped portion of-.1 said doubler: output: inductance c'oil said tapped-- portion comprising a portion-loft the low potential end. of-::said: doubler. output induct-s ance coil.

I PJ'BUFFINGTGNi BEFEBENGEMIIE a The following references are oi record rinfthe' Number I Name I 2,193,491 Reinartz; e- Marl Ii2,"-'-194Q .17.

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