HEWLETT*PACKARD. TECHNICAL INFORMATION FROM THE -hp- LABORATORIES. Absorption Modulators for Simple or Complex Microwave Modulation

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1 HEWLETT*PACKARD NAL TECHNICAL INFORMATION FROM THE -hp- LABORATORIES Vol. 16, ho. 3 PUBLISHED BY THE HEWLETT-PACKARD COMPANY PAGE MILL ROAD, PAL0 ALTO, CALIFORNIA NOVEMBER, 1964 Absorption Modulators for Simple or Complex Mirowave Modulation High-performane diode modulators permit mirowave modulation by signals ranging from fast waveforms to d ontrol voltages without frequeny reation Fig. 1. Waveguide model of P-I-N Modulator an be used on output line beyond oax-to-waveguide adapters or other refletion-produing fittings, and an modulate mirowave power with sine waves, omplex waueforms, or pulsed signals with extremely fast response. Modulator absorbs mirowave power in aordane with instantaneous bias applied to p-i-n diodes and is used here to level mirowave Dower in waveguide system. Fig. 2. Effet of modulation on klystron operation is shown by spetrum of RF in klystron avity. In photo at left, klystron output is modulated by P-I-N Modulator, whih shows no visible sidebands. In photo at right, klystron output is modulated by onventional diode swith; sidebands here show frequeny pulling. (Modulator pulse rate: 10 k; spetrum width: 10 klm; vertial sale: linear.) PRINTED IN U.S.A. AMPLITUDE and pulse modulation of mirowave osillators have always presented ertain problems to the mirowave engineer beause of the undesirable moding and inidental FM that an our when modulating signals are applied to an element in the mirowave tube. Beause of these problems, muh interest has been shown in modulation tehniques whih allow the mirowave osillator itself to operate unmodulated while a diode swith gates the power on the output transmission line. These systems, however, have required isolators or pads to prevent the frequeny pulling that is aused in tightly-oupled osillators by refletions from the swith. These limitations are irumvented by modulation methods whih absorb power on the transmission path* This tehnique not only allows the osillator to operate ontinuously with fixed but it also redues refletions below levels that ause frequeny pulling. In reent years, this kind of performane has been ahieved by ferrite modulators. The response time of these devies, however, has been limited by the swithing time of the magneti field that ontrols power absorption. More reently, p-i-n diodes have demonstrated a apability for the kind of perform- I J K. Hunton and A. G. Ryals Mirowave Variable Attenuators and Mddulators Using P-I-N Diodes, lre PGMTT Transations, Vol. 10, No. 4, July, OHEWLETT-PACKARD o.. is64 - ~

2 ~ OSCILLATOR- SYNCHRONIZER MODULATOR fl Fig. 3. P-I-N Modulators are symmetrial devies that aept RF at either end and supply modulated RF at opposite end. Modulating waveform is applied to BNC onnetor on side. - - Output Fig. 4. Highly stable pulsed RF is generated by CW osillator phase-loked to frequeny referene prior to modulation. ante desired. When plaed in shunt aross a transmission line, these devies behave as eletrially-ontrollable resistanes at mirowave frequenies and absorb mirowave power in proportion to the bias urrent supplied. This property enables p-i-n diodes to serve as variable attenuators for modulating mirowave power. Modulators that use p-i-n diodes are ompat, lightweight units that have as muh as 80-db maximum/ minimum attenuation ratios and response times of typially 30 nse. These apabilities show <great potential and have already led to several interesting appliations, to be desribed later in this artile. NEW MODULATORS Several new P-I-N Modulators have reently been designed in two series, an A series having a maximum attenuation of 35 db and a B series having a maximum of 80 db. The olletive frequeny overage of either series is from 0.8 to 12.4 G (see Table I). The new P-I-N Modulators are three-port devies with two of the ports serving as input and output for the mirowave power while the modulating signal is applied to the third port (Fig. 3). Up to 1 watt of RF power passing through an be modulated by any applied waveform with frequenies up to 10 M, or the RF power an be pulsed with rise and fall times of typially 30 nse. The P-I-N Modulator onsists of diodes installed in a setion of transmission-line or waveguide without biasing or ontrol iruitry. This allows plaement of the P-I-N Modulator at any onvenient point along the transmission path within a system irrespetive of the loation of ontrolling iruitry. Driving eletronis, for those appliations requiring it, have been designed into a separate unit. The Modulator-driver provides frequeny ompensation to extend the P-I-N Modulator frequeny response to 10 M. It also has d- oupled iruits to permit its use as the ontrol element in mirowave power leveling or in other appliations requiring ontrolled attenuation levels. The Modulator-driver is partiularly useful for pulse and square wave appliations requiring low pulse jitter and fast rise and fall times. The unit either generates pulses of suitable width and repetition rate itself, or it proesses externally-derived pulses to shape them for obtaining maximum rise time when driving any of the P-I-N Modulators. Spae is provided within the Modulator-driver enlosure for installation of the diode assembly when an all-in-one modulator is preferred for benh or field work. APPLICATIONS The new Modulators are apable of amplitude modulation under the ontrol of almost any time-varying signal. When a typial modulator was driven by a 300-ohm CW voltage soure to a modulation depth,? AMPLIFIER SIGNAL SOURCE P-I-N M 00 U L AT0 R Leveled Output FREQUENCY (G) Fig. 5. Mirowave power is leveled by modulator at any hosen point in system.. 2 =

3 f "Leveled F '---- _-- To Reeiver SOURCE 3 OB P-I-N I COUPLER I p * I P-I-N MODULATOR P-I-N MODU LATOR DRIVER DRIVER DRIVER - Traking Signal TIMING CONTROL Fig. 6. P-I-N Modulators are useful for swithing antennas sequentially to reeiver input, as shown in this high-speed traking system. of approximately 50%, the RF envelope was found to have a half power point at a modulating frequeny of 1 M. Compensated driving iruitry an be designed to provide flat response to 10 M or more. Modulating about a level approximately 7 db down results in less than 5% envelope distortion for modulation depths approahing 60%. Depending upon the degree of linearity and frequeny response required, shaping iruits an be inorporated to ompensate for the typial transfer urves shown in Fig. 17. The versatility of the new family of.p-z-n Modulators adapts them to a variety of appliations. For instane, the Modulators an be used in appliations requiring pulsed signals that are phase-loked for frequeny stability. For this applia- tion, the osillator tube operates CW and the signal is modulated beyond the point where an RF sample is taken for the osillator-synhronizer, as shown in Fig. 4. Leveling of mirowave power to within a fration of a db over a 2:1 bandwidth is possible with a P-I-N Modulator, a flat detetor, and a suitable amplifier, as shown in Fig. 5. Flatness of leveling is limited almost entirely by the oupler and detetor, and an be improved by use of the ompensated oupler tehnique." The high on/off ratios available in the "B" series Modulators make them ideal for use as SPST swithes. One appliation is the traking of fast moving objets by swithing an array of antennas in sequene, as shown in Fig. 6. Sine traking is done eletrially, the mehanial in- Fig. 7. Two P-I-N Modulators funtion as balaned modulator by introdution of 180" phase-shift at arrier frequeny in one of two parallel RF paths. Swept-Frequeny Measurements with Osillosope Display" -hp- Appliation Note No. 61. Fig. 8. Output spetrum of balaned modulator shown in Fig. 7. (Carrier frequeny: 2200 M; spetrum width: 100 klm; vertial sale: linear.) ertia of a single traking antenna poses no problem. Unlimited possibilities exist when two or more modulators are used in a mirowave system. For example, suppressed arrier modulation is obtained with a balaned modulator using two 3-db ouplers and two Modulators, as shown in Fig. 7. A 90" phase shift is introdued by eah oupler in one RF path and when this signal is added to the signal from the other path, arrier anellation takes plae.. TABLE I -hp- P-I-N MODULATORS SOURCE I 35 I MODULATOR Coaxial Svn 0; t Svn X Fig. 9. Two P-I-N Modulators in series ahieve short-pulse widths by overlapping pulse "on" times.

4 In the balaned molulator, the two series resistors are hosen to equalize the sensitivities of the P-I-N Modulators, as disussed on page 6, and d bias voltages are set to provide approximately 7 tlb of attenuation. The arrier suppression obtainable is shown by the spetrogram of Fig. 8; the low level of additional side bands in the spetrogram gives some indiation of the low distortion. Extremely narrow pulses an be generated by two modulators in series. A single P-I-N Modulator driven by the Modulator-driver is limited to 100-ns pulse widths beause of internal reovery times in the driver. This limitation is overome in the set-up of Fig. 9, whih Fig. 10. Short RF pulse ahieved with tandem modulator shown in Fig. 9. RF arrier: 1000 M; sweep rate: 5 nse/m. Phase oherent pulse is obtained by using RF arrier as syn input to sampling osillosope; ounted-down syn from sope then triggers first modulating pulse generator. passes RF power only when both modulators are on: The leading edge of the RF pulse therefore ours when the seond Modulator turns on and trailing edge ours when the first Modulator turns of. Pulse width is determined both by the width setting of the first Modulator and the delay setting of the seond. The repetition rate and pulse delay of the omposite signal an be varied by the ontrols of the first Modulator. A speial Modulator-driver used in the generation of simulated DME-TACAN and ATC signals further demonstrates the flexibility of p-i-n modulation. A DME signal onsists of gaussian pulse pairs. The requirements on the Modulator system are to pro- /I vide an RF envelope gaussian over at least the top 20 db of the pulse and to attenuate the RF at least 80 db between pulses. TACAN signals are generated by superimposing a low frequeny sinusoid on the DME pulse peaks to a depth orresponding to about 50y0 modulation. The steps in generating this signal with P-I-N Modulators are depited in Fig. 11. A three-pulse ATC signal is pitured in Fig. 12. One A type and one B type modulator are required for the generation of this signal. An additional swithing iruit (suppressor) generates the driving signal for the A Modulator while the B Modulator is driven by the standard driver iruitry in the external pulse mode of operation. PRINCIPLE OF OPERATION The P-I-N Modulator onsists of a number of p-i-n silion diodes mounted as shunt elements between the enter ondutor of a strip transmission line and ground (or in ridged waveguide in the higher frequeny units). Charge storage in * The iruitry desribed above has been designed into a speial Modulator-driver, designated as the -hp- Model H A. I RF IflY-iRF MC PI N MODULATOR Output ----w SHAPING Current Wave Form CIRCUIT P I N MODULATOR TACAN (B) Input Voltage - Fig. 11. DME-TACAN Modulator superimposes TACAN sinusoid on DME pulse pairs. t k--* SPIKER TRIGGER GENERATOR SUPPRESSOR CIRCUIT \ L-r--- Suppressor Pulse Fig. 12. Tandem modulators ahieve 3-pulse Air Trafi Control operation by using 35-db modulator (87314) to redue power into swithing modulator (80-db) during enter portion of puke triplet...-

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6 2 0- I - I Rated Frequeny Range * / / 80 DE Attdnuation 1. d..-, FREQUENCY (G) Fig. 15. VSWR of typial P-I-N Modulator (8731B) while biased for full 80-db attenuation and also while biased for minimum attenuation (zero bias). moothed, but the plots show the naximum insertion loss that nor- Tally is enountered. In pulse and square-wave appliations, it is reommended that a lak bias of about 5 volts be apdied during the RF on time. In he higher frequeny modulators, his results in as muh as I-db imlrovement in insertion loss over ero bias. The insertion loss inreases with orward bias urrent almost linearly n db until diode saturation ours. The attenuation sensitivity varies omewhat from modulator to nodulator but any two P-I-N Modilators may be made to have om- )arable sensitivities in db per volt iy the simple addition of approriate series resistane. The range )f expeted sensitivities is shown AS he shaded areas in the plots of ypial 35 db and 80 db modulation harateristis in Fig. 17. The attenuation with forward bias is also a funtion of diode spaing and hene of frequeny. This variation is more notieable at higher values of attenuation. An 80- db attenuator might have 2 to 5 db variation aross the band at the 25- db level. At the 80-db attenuation level the variation might inrease another 5 db. Maximum attenuation generally is expeted at the enter frequeny of a partiular modulator. TEMPERATURE EFFECTS As pointed out in an earlier paper, the p-i-n diode has a small positive temperature oeffiient when driven by a voltage soure but a negative oeffiient when driven by a urrent soure. This suggests that temperature sensitivity an be ompensated for at one attenuation level by proper hoie of the soure impedane. Temperature stability See footnote referene 1, page FREQUENCY (G) Fig. 16. Insertion loss of B series (80-db) Modulators when bak-biased with 5 volts for minimum pulsed RF attenuation. an also be ahieved by use of a temperature sensitive element in the bias soure. MODULATOR ELECTRONICS The modulator-driver that has been designed for the P-I-N Modulator lines is idential to the eletroni portion of the earlier Model 8714A Modulatort exept for the inlusion of two outputs (Fig. 18). One output drives the P-I-N Modulator diretly and provides suitable ompensation for waveforms or pulses to obtain wideband response. Pulses, for instane, are spiked to sweep the stored harge into or out of the diodes quikly and thus obtain extremely fast, jitter-free RF pulses with RF rise times running from 15 to 40 nse t See footnote referene 2 on page 5. r -hp- SPECIFICATIONS SERIES 8730 P-I-N MODULATORS Model number 8731A A A A X8735A X8735B Frequeny Range (C) Dynami Range (db) Min. lnserlion Loss (db) ~1.5 <2.0 <2.0 <3.52 <2.0 <3.0 <4.0 <5.0 <4.0 <5.0 rypial Rise Time (nsep rypial Deay Time (nse)] EWR, Minimum Attenuation EWR, Maximum Attenuation Maximum Input Power, Peak or CW (watts) ias Limits (volts)s +20, , , , , , , -10 $20, -10 $20, , -10 rypial Forward Bias Input Resistane (0hms)b ?F Connetor Type N N N N N N N N Waveguide1 Waveguide7 rie $ $ $ $ $ $ $ $ $ $ I. $5 v bias 2. To 4.0 G; 4.0 db to 4.5 G 4. To 4.0 G; 2.0 to 4.5 G 5. Negative voltage applies forward bias to diodes 3. Driven by -hp- 8403A Modulator 6. At attenuation levels of 10 db or more 7. Fits 1 x % in. (WR 90) waveguide Pries f.0.b. fatory. Data subjet to hange without notie. 06.

7 ~ ~ WEIGHT: Douglas A. Gray onsz&atyoris; see graph. ~ TRIGGER OUT SYNC OUT: 0.1 to 100 Bse in advane of output pulse, as set by DELAY ontrol. Net, 14% Ibs. (6,5 kg). PRICE: -hp- Model 8403A. $ Pries f.0.b. fatory. Data subiet to hange without notie. Y F 100 KC 50KC /---I I 3 lokc IKC < 01 I O IOOUS z DELAY OR WIDTH (Whihever is Greater) * 7 *

8 )WAVE SIGNAL SOURCES GENERATOR CAPABILITIES r I I E EO z 40 L I - EO h r 40 e a 20 0 I P FREQUENCY (G) rig. 3. Maximum output power us. frequeny of new -hp Mirowave iignal Soures (attenuator probe was adjusted for peak power before eah measurement). ins of a frition luth that als the readout to be set to any venient referene point. he new Signal Soures ollely over a frequeny range from to 4500 M in overlapping ds, from 800 to 2400 M in the - Model 8614B and from M in the Model 8616B. ximum output power through main RF output is at least 15 liwatts (3 milliwatts in the high- requeny instrument) and over,t of the band, 20 milliwatts is ilable, as shown in Fig. 3. An iliary output, useful for power!i monitoring or phase loking klystron, provides a fixed mnt of RF power whih is at t M milliwatt at the low power nt. ACKNOWLEDGMENT he design group for the new sigsoures inluded Alan L. Seely OURCE POWER -hp-i96d \ -hp- 418A OlRECllONAL COUPLER THERMISTOR MOUNT 4. Instrument set-up for sensitivity lsurements uses -hp- Model 431B ier Meter to alibrate signal soure nuator. Measured power level in L, modified -20 db beause of ou-., is dialed into attenuator readout i thumbwheel on front panel. At- Lator may now be readjusted and will ontinue to indiate power level at oupler auxiliary arm FREOUENCY (Gel and Raymond H. Spoelman, and William W. Nelson made valuable ontributions to the mehanial design. Industrial design was by Thomas C. Lauhon. -Douglas A. Gray SPECIFICATIONS -hp- MODELS AND 0616B SIGNAL SOURCES OUTPUT FREQUENCY RANGE: 86148: 800 to 2400 M 86168: 1800 to 4500 M Single, linearly alibrated ontrol, diret reading within 2 M. VERNIER AF ontrol has 1.5 M range for fine tuning. FREQUENCY CALIBRATION ACCURACY: 86148: f 5 M or f0.5%, whihever is greater : k10 M FREQUENCY STABILITY: Approximately 0.005% / C hange in ambient temperature; less tha,n 0.003% hange for line voltage variations of t 10%. Residual FM: less than % peak; 8616B. less than 6 k peak. RF OUTPUT POWER 86148: At least 15 mw max ontrolled by attenuator : At least 15 mw max, 1800 to 3000 M. and at least 3 mw max, 3000 to 4500 M, ontrolled by attenuator A seond, fixed RF output (% mw minimum) also is provided. ATTENUATOR RANGE: At least 130 db. ATTENUATOR ACCURACY: db/lo db from.-lo dbm to -127 dbm; diret reading linear dial, 0.2 db inrements. Baklash is less than 0.2 db. MODULATION: Internal 1 k square wave, external pulse, external FM. PRICE: -hp- Model 86148: $ hp- Model 86168: $ Pries f.0.b. fatory. Data subjet to hange without notie.