THE UNIVERSITY OF MICHIGAN RESEARCH INSTITUTE ANN ARBOR DESIGN CONSIDERATIONS FOR A CAPACITANCE-MODULATED VFO Technical Memorandum No. 60 Electronic Defense GroupDepartment of Electrical Engineering By: T. W. Butler, Jr. Approved by: C. B. Sharpe This is not a final report. Further investigation may make it desirable to have this report revised, superseded or withdrawn. Project 2262 TASK ORDER NO. EDG-4 CONTRACT NO. DA-36-039 sc-63203 SIGNAL CORPS, DEPARTMENT OF THE ARMY DEPARfTMENT OF ARMY PROJECT NO. 3-99-04-042 September 1958

ACKNOWLEDGEMENT The author wishes to acknowledge the assistance of Mr. Gordon A. Roberts in the design and testing of the capacity modulated VFO. ii

TABLE OF CONTENTS ACKNOWIEDGEMENT ii LIST OF ILLUSTRATIONS iv ABSTRACT v I. INTRODUCTION 1 II. THEORETICAL CIRCUIT DESIGN CONSIDERATIONS III. PRACTICAL CIRCUIT DESIGN CONSIDERATIONS 6 IV. PERFORMANCE CHARACTERISTICS 12 V. CONCLUSIONS 14 REFERENCES 16 DISTRIBUTION LIST 17 iii

LIST OF ILLUSTRATIONS FIGURE Page 1 Basic Wide Range Tuning Circuit 2 2 Basic FM Circuit 2 3 Capacity Variations vs Bias Voltage 4 4 Capacity Variations vs Temperature in ~C 5 5 FM Capacity Modulated VFO 7 6 Exterior View Showing Panel Arrangement 8 7 Interior View Showing Parts Layout 9 8 Exterior View Showing Outboard Modulator 10 9 Typical Curves of Frequency Deviation vs DC 13 Bias Voltage 10 Typical Curves of Frequency Deviations vs 15 Temperature in OC iv

ABSTRACT The use of ferroelectric capacitors in the design of a band switching capacity-modulated VFO which will meet FCC specifications regarding Narrow-Band frequency modulation (NFMI) is presented. The chief advantage of NFM is that it reduces or eliminates certain types of interference to broadcast reception and is quite simple and inexpensive to construct using capacitance-modulation techniques'.

DESIGN CONSIIDERATIONS FOR A CAPACITANCE-MODUIATED VFO I. INTRODUCTION The use of ferroelectric capacities in the design of an electrictuned capacitance-modulated oscillator has been presented in previous reports.l' The design of a band switching capacitance-modulated VFO which will meet F.C.C. specifications regarding narrow band frequency modulation (NFM) in the available Radio Amateur frequency bands is presented. Although the design was carried out with certain specific bands in mind the general design principles are flexible enough to allow their use in all VHF and UHF bands. II. THEORETICAL CIRCUIT DESIGN CONSIDERATIONS Since in the design of the capacitance-modulated VFO, electric tuning of the oscillator tank circuit is not a requirement (i.e., tuning over the range will be done by a variable air capacitor), and since it is desirable to maintain the ovrerall frequency stability of the basic VFO without resorting to thermostating techniques and close regulation of the bias voltage, the basic wide range tuning circuit of Fig. 1 may be modified somewhat as shown in Fig. 2.2 The tank circuit L C1 of Fig. 2 is shunted by the series arrangement of C2 and C* (voltage sensitive capacitor). Capacitor C2 C2 serves as a d-c blocking capacitor and also provides a means of multiplying the effective Q of the circuit. The Q of C2 and C* in series is approximated by:

Co DC BIAS ---- VOLTS RC L I~t 0 DC BIAS VOLTS _ —- VOLTAGE TUNABLE ELEMENT FIG. 1. BASIC WIDE RANGE TUNING CIRCUIT LIT Cl V-C2 R DC BIAS 1 0 * I C~MODULATION C"' - C = VOLTAGE TUNABLE ELEMENT C2 =SILVER MICA FIG. 2. BASIC FM CIRCUIT - 2

C + C* 02+0* where Qt is the Q of C*. If C* is very much larger than C2, as it is in the practical case, the effective Q of the circuit is greatly improved. The actual curve of capacity variation vs voltage of the modulating element C* is obtained from the e-T-E surface2 at the VFO nominal operating temperature (in this case 250C) and replotted as shown in Fig. 3, curve a. Curve b of Fig. 3 is a plot of the series combination of C2 and C*, where C2 has a constant value equal to 4.7 Elf under all variations of voltage while C* varies as shown. For the same voltage deviation (e.g., + 40 volts either side of the 125 volt bias point) it is seen that the series combination changes only about.0025 ywlf/volt, while for the tunable element alone the change is about.3 ~f/volt. To control the deviation even further, additional padding (e.g., 30 plf) can be shunted across C as shownby curve C, Fig. 3,. For the given t 40 volts deviation about the 125 volt bias level the series-parallel arrangement change is about.001 af/volt. The actual curve of capacity variation vs temperature of the modulating element (C*) may also be obtained from the e-T-E surface at the VFO bias voltage (in this case 125 volts) and replotted as shown in Fig.4, curve a. Curve b of Fig. 4 is a plot of the series combination of C2 and C*, where C2 remains constant with temperature at a value of 4.7 llf and C* varies as shown. Curve c is aplot of the series-parallel arrangement where the 30 pff padder is also considered temperature insensitive. From the use of a series-parallel arrangement, it is seen that: (1) the linearity is improved; (2) the stability of the VFO is improved (i.e., changes in frequency due to temperature and bias variations are reduced); and (3) control of the - 3 -

4.2,,F=~~,,L~-t-t~tttt-C____ ___ 00 4.74.: Y 4.C 30 ~f Zi: INITIB 5~~ 4.4 c 4.31 1 I I i I -: 4.2_ 0100 3.e~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~1 4.* 4. 4.0 3.9 t~~~~~~~ w a: o I. A -J Q *V0LTjrE TUNABLE CAPACITOR I] I __-__ _ z 00 100 200 300 400 ~ BIAS VOLTAGE -. CAPACITY VARIATIONS VS. BIAS VOLTAGE cn I (FERROELECTRIC NOMINAL OPERATING TEMPERATURE 250 C)

j 0 64 4.7/40fI 0 ____ _____ ~~~~~~~62 30 0 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~~6 4.51 r r 1 r I I I/ 1 I C I 1 1 1 1 I I I 1 150 4.4.4 4.3 G-0 4.2 42 4i1 44.0 VOLTAGE TUNABLE CA~PACITOR 8 0 5 to 15 20 25 30 35 40 45 50 ~ la TEMPERATURE 0C -. CIOQ U, CAPACITY VARIATIONS VS. TEMPERATURE IN 0C L cr (FERROELECTRIC BIASED AT 125 V)

carrier frequency deviation due to the modulating signal is vastly facilitated, especially under band switching operation where a constant band to band modulation index is desired. III. PRACTICAL CIRCUIT DESIGN CONSIDERATIONS To demonstrate the feasibility of NFM, a typical commercially available all-band VFO was modified as shown in Fig. 5 and the photos of Figs. 6, 7 and. 8. The basic circuit is the series-tuned Colpitts or "clapp" oscillator circuit. The tank circuit consists of two separate coils and two separate stator connections of a differential capacitor to develop the basic output frequencies. The basic output frequency in range 1 is 1.75 me, doubling to 3.5 mc.and quadrupling to 7 mc. The basic output frequency in range 2 is 7 me, doubling to 14 me, tripling to 21 mc and quadrupling to 28 me. Padder capacitors are placed in parallel with the sections of the differential tuning capacitor for adjustment. A series arrangement, consisting of a 7.5 Aif silver mica capacitor and a 200 Af ferroelectric capacitor are connected across range 1 of the tank circuit, while a second series arrangement, 3.7 Off silver mica capacitor and a 200 off ferroelectric capacitor, is connected across range 2 of the tank circuit. According to FCC regulations for NFM, the channel width should not exceed twice the highest audio frequency in the modulating signal; therefore, based on an upper audio limit of 3 kc, the channel width should not exceed 6 kc. In band switching operation the modulation index is multiplied by the same factor that the carrier frequency is multiplied by; therefore, if the basic output frequency is 7 me, for example, with a frequency deviation of 2 kc, then the output at 28 me will have a deviation of 4 x 2 kc or 8 kc which is far too great. -6

I 234567 AND SWITCH -I 1 160 M f SI 2 8 M 3 4O0 4 40 M 5 20 M 6 15 M / 10 M F LATOR /:l ~~~,___ ~ 81| =*RF OUTPUT C19 R5 R6 +250 VOC V c C15 2.2T CC2 RI Z Li L2 C16 1_ 22 C 22K~P 3 INPUT R4. POWER FIL. 1.3~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C 22 SILVB R. k v CONNECTOR C5 R2 R3 1S TCO P1 ii_ I t OR IUIID C R2 Cx... - \5 - 25 22 C C TRR 32 0, +7 A 12 t,~~~~~~ lb ~~,i~ 1, ) 0 p 1 /2v cl C2 7 4 Cl2 C C -_ " 3 14 C 56 22 i 2.2 2 02' C9 30 2022 811220 8220 POWER SWITCH 2 STAND -BY INPUT 116 1., 5-13 l 18 Cl7 cD3, C20 0.005 s 1I bR n52 cbs~cc3 o1 3 ON P2 TO C3, 1.2, 1. TUNING.~,.. L.-'~ PIN 2 CAPACITOR B+ 21 0.1 C22 PI PARTS LIT TO Cmwritors Rlfts~ts FIL. C, 4.7 d CERAMIC MO15. R/ 2.2 k k/ v C2 22 wd SILVER 0ICA R2. 3 330 k 1/2 2C3 20 r22d W,X 0 2 1 CAPACITORI R0 22 1/2 C25 0.c5 - 25 2 d CEAIZE C 2DIMR 2 P1 2 7 212 + c02, 1.23 252 *d 0 u2r RA P 82 r82017201. I2D 0 C2 PACIIOR 1fd 1.2f1. 2 1.5 k 20 13 0 2. 3al. 20el210 801 MO23 ULATOR6 AMPLId FIERLVER 2CA A 2.2 k 31/2 V-3 C7 k7 ~d SILVER ItlCA 100 k.2 v RI I R14 R11, RIC, C9 C25 c l C9 30 prpfd SILVER MICA R12 0.5 m; POT 75 SILVE ICA (215' I SERIES) DYNAMICG. C1z 10ad CERAMIC A750 IMndutors ( —- DEVIATION 13 35 wrd MAX TUNIFG CAPACTOR FM CP I M L VF MICROHONECONTROL R12 C1L.5 - 25 Od CERAMIC TRIMICR NPO LIR MICROPHONE C1-' C16 51o p.d SILVER )ICA L3 1 h C17' C18' C20 0.005 o~ d O]ERA PURPOSE CE:IRAMIC L 5O - 150ph Rg C19 10D maicd SILVER 1OILAER -ZCA + C21 0.1 mfd METALD PAPER 400 VOC C24 RI3 C22 1o60 Afd SILVER K[CA 6AUC C2k 50l jd 25 VDC szCrmoLrrlc v.AX7 5 -o0.05 pfd rETALZE PAPER kO0 VbC Y3 Ioa saOaPz.G ~zICH c2' 250 omfd CENRALAB PA-2013 -2 SPECIAL 3 POSTON SWITCH MODULATOR AMPLIFIER ii ~4AL'O-CRTPcm P2 JONS S-304-CCT P JONES P-p06-=CC FIG. 5. FM CAPACITY MODULATED VFO

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In order to maintain a constant deviation for a given modulation voltage, additional padding is shunted across the ferroelectric capacitor as the band is switched to higher frequencies. Since NFM is not allowed in the 1.75 mc band the basic frequency output was considered to be 3.5 mc and the deviation adjusted at this point to be t 2 kc for a change in bias of t 40 volts. For 7 mc operation a 30 Staf padder is switchacd across the ferroelec~tric capacitor as shown in the schematic. With this arrangement, deviation in the 1.75 mc band would be 1/2 of the deviation obtained in the 3.5 mc band or t I kc. The basic output frequency'in range 2 is 7 mc., and. the deviation is adjusted at this point to 2 kc for a change in bias of t 40 volts. For operation at 14, 21, and 28 mc, additional padding capacitance is switched across the ferroelectric as shown. The ferroelectric capacitor bias voltage is 150 volts and is taken from the plate of the 0A2 regulator tube. The regulated voltage also represents the center of the linear portion of the voltage capacitance curve of the ferroelectric capacitor. Decoupling of the r-f from the bias is obtained by means of a one meg-ohm resistor, R7., connecting the plate of the regulator tube to the bias line and the 400 Stxf capacitor C22. The 330 K resistors, R2and R3, and the 400 [i~if capacitor C22, serve as a f ilter to prevent r-f from getting back'into the audio circuit. At the same time, the filter allows the audio variations at the plate of the audio amplifier to appear virtually unattenuated across the modulating capacitors. The modulator section is a two stage audio amplifier utilizing a 12AX7 tube in a straightforward circuit. The control, R 12, at the input to the second audio section, serves as a deviation control. Since the amplitude

the deviation of the oscillator. The optimum setting of this control will vary somewhat from band to band and is best determined by operational checks. The final setting should correspond to a deviation of somewhat less than 3 kc. The modulator was constructed in a 2-1/4"i x 2-1/4"i x 4" mini -box and mounted outboard fashion on the rear of the VFO. Audio leads were brought into the VFO through shielded cables. Since this particular VFO utilizes two tuned circuits in order to obtain greater bandspread, it was necessary to replace the existing bandswitch with a four-section, 7-position, non-shorting type ceramic switch. In most VFO modifications, where the tank is a single-tuned circuit, band switch modifications will be much simpler. In any event, all modifications can be made by following the ideas presented in the foregoing sections. Recalibration of the VFO should follow standard procedures as outlined in most Radio Handbooks3. IV. PERFORMANCE CHARACTERISTICS The oscillator frequency deviation on each band was measured by using a frequency counter in conjunction with a very stable frequency meter. The d-c bias on the ferroelectric capacitor was varied 40 volts either side of the 150 volt bias level and the deviation was noted. Two sample curves are shown in Fig. 9. Curve A (Fig. 9) shows the deviation in the fundamental frequency range (i.e., 7.2 mc) and curve B shows the deviation at the 4th harmonic (28.8 mc). Note that the modulation index in the two ranges is almost identical and that the modulator is linear over the entire range.

3000- 2000 _ B 1000 -40 -.35 -30 -25 -20 -5 -1O -5 0 5 1O 402 5 0 3 V- VOLTS DC F IG. 9. 1000- / 100TYPICAL CURVES OF FREQUENCP EITO VS. A X/ t l~~oo o - YIA UVSO RQEC EITO e/o -|e ~:::~IDC BIAS VOLTAGE -2000T OPERATING FREQUENCIES: A) 7.2 MC B) 28.8 MC FUNDAMENTAL FREQUENCY 7.2 MC - 3000 INITIAL BIAS 150V

The deviation is within allowable limits on all band~s 3.5 through 30 mc. As shown in Figure 10, the f requency stability of the VFO vs temperature, with and. without the ferroelectrics'in the circuit, is compared.. The d~egradation of the VFO close to its normal operating temperature (390C) is not serious. It should. be noted. that neither thermostating techniques nor ad~ditional comapensating capacitors were used. in this modification. if increased. temperature stability is a requirement either of the above id~eas could. easily be incorporated.. V. C ONC LUS IONS The use of ferroelectric capacitors as the mod~ulating elements in a narrow-band. FM syst~em have proved. to be quite useful in the VHF-UHF region where conventional reactance tube and. phase-modulation methods are unwield~y. The chief advantage of narrow-band FM is that it reduces or eliminates certain types of interference to broadcast reception and. is quite simple and. inexpensive to construct using capacitance-modulation techniques. Detection of NFhM on a typical AM receiver is quite easily accomplished. by means of slope detection,'i.e., moving off frequency slightly. Use of this type of VFO in portable eand~mobi2le operation, where equipment must be restricted in size, should prove very helpful.

2500r200 FIG. 10. TYPICAL CURVES OF FREQUENCY DEVIATIONS.. vs. 1500 TEMPERATURE IN 0C 0 ~~~~~~~~~~NORMALIZED TO 390 C w 0~~~~~~~~~~~~~~~PERATING FREQUENCY 7.2 MC 01000 ~~~~~~~~~~~~~~FUNDAMENTAL FREQUENCY 7.2 MC -0-WITH FERROELECTRIC CAPACITOR IN TANK CIRCUIT -A-WITHOUT FERROELECTRIC CAPACITOR IN TANK CIRCUI *500NA 0 ~~T -* 25 ~~~~30 35 40 45 5 500 0% 10000

REFERExC.NCES, 1. M. Apstein and. H. H. Wileder, "Capacitor-.Mod~ulated Wide Range FM System," Electronics, Vol. 26, p. 190; October, 1953. 2. T. W. Butler, Jr., "Electric Tuned, Capacitor-Modulated. VHF Oscillator, Electronic Defense Group,, Technical Memorandtun No. 59, University of Michigan Research Institute. 3. "Radio Amateurs Handbook,." pp. 145, 34th Edition, 1957,, A.R.R.L., West Hartford,, Connecticut.

DISTRIBUTION LIST 1 Copy Document Room Stanf ord. Electronic Laboratories Stanf ord. University Stanf ord., Calif ornia 1 Copy Command *ing Ge ne ral Army Electronic Proving Ground. Fort Huachuca, Arizona Attn: Director, Electronic Warfare Department I Copy Chief, Research and. Development Division Office of the Chief Signal Officer Department of the Army Washington 25, D. C. Attn:* SIGEB 1 Copy Chief, Plans and. Operations Division Office of the Chief Signal Officer Washington 25, D. Co Attn: SIGEW 1 Copy Countermeasures Laboratory Gilfillan Brothers,, Inc. 1815 Venice Boulevard. Los Angeles 6, California 1 Copy Command~ing Officer White Sand~s Signal Corps Agency White Sand~s Proving Ground. Las Cruces, New Mexico Attn: SIGWS-EW 1 Copy Comiand~ing Officer White Sand~s Signal Corps Agency White Sand~s Proving Ground. Las Cruces, New Mexico Attn:* SIGWS-FC 1 Copy Connand~ing Off icer

65 Copie s Comnanding Officer USASR~DL Fort Mornmouth, New Jersey FOR - USASRDL Accountable Officer Inspect at Destination File No. 22824-PH-54-91(1701) 1 Copy H. W. Farris University of' Michigan Research Institute Ann Arbor, Michigan I Copy Technical Documents Services Willow Run Laboratories University of Michigan Ypsilanti, Michigan 11 Copies Electronic Defense Group Project File University of Michigan Ann Arbor, Michigan I Copy University of Michigan Research Institute Project File Ann Arbor, Michigan