THE PIN DIODE AS A

MICROWAVE MODULATOR

a p p l i c a t i o nnote

1 AUG. 67

INTRODUCTION

The PEN diode has opened the way to a new method ofmodulating microwave signals. Placed across a trans-mission line, the PIN diode becomes an absorption-type attenuator which permits sine-wave, square-wave, and pulse modulation without frequency pulling.

The limitations of present amplitude modulation meth-ods are becoming more evident with the advance ofmicrowave technology. For example, it has been nearlyimpossible to amplitude modulate a klystron oscillatordirectly with anything but a square wave or pulse.Sine-wave modulation traditionally results in signif-icant frequency shifts with changes in amplitude. Inaddition, conventional klystron oscillators have rel-atively slow rise and decay times and poor frequencystability, and jitter in the rf pulse istoohigh for precisepulse measurements. Modulators for lower-frequencytriode master- oscillator-power-amplifier (MOPA) gen-erators have low on-off ratios, and speed of rise anddecay of the modulated signal is limited by the Q of themodulated stage.

PRINCIPLES OF OPERATION

The PIN Modulator is a high-speed, current-controlledabsorption type attenuator. A simplified illustrationof the hp Model 8730A''B series PIN Modulators isshown in Figure 1. Each PIN unit includes a low-passfilter, two high-pass filters, a number of PIN diodes,and a 50-ohm strip transmission line (ridged waveguidein the higher frequency units).

The PIN diode is a silicon junction diode whose PandN trace regions are separated by a layer of intrinsic(I) semiconductor (silicon). Thus the name PIN diode.

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At frequencies below about 100 Me the PIN diode rec-tifies as a junction diode. However, at frequenciesabove 100 Me, rectification ceases due to stored chargein the intrinsic (I) layer and the diode acts like aresistance by conducting current in both directions.This equivalent resistance is inversely proportional tothe amount of charge in the I layer. An increase inforward bias current (current at a negative voltage) in-creases the stored charge and decreases the equivalentresistance of the PIN diodes. When reverse bias isapplied, reverse current flows until stored charge isdepleted at which time equivalent resistance becomesa maximum, in the order of thousands of ohms.

To understand how a PIN Modulator works, considerthe following: the PIN diodes are mounted as shuntelements between the RF transmission path and ground.The transmission path has a characteristic impedanceof 50 ohms. When the PIN diodes are forward-biasedthe equivalent diode resistance is about 30 ohms andmost of the RF energy is absorbed by the diodes insteadof propagating down the 50-ohm transmission path.However, when the diodes are reverse- or back-biasedthe equivalent diode resistance is in the order of thou-sands of ohms and the microwave currents will flowdown the transmission path because diode resistancecompared to the 50-ohm path impedance is negligible.

GENERAL OPERATION

The PIN Modulators are three-port devices whichaccept RF power at either end-port and, depending uponthe BIAS signal applied, provide a modulated RF out-put at the other end-port. As an operating device, thePIN Modulator should be thought of as a variable RFattenuator whose attenuation level is controlled by DCcurrent and voltage. By applying a +5 volt DC poten-tial to the BIAS input connector, RF signals will bepassed through the PIN Modulator with only minimumresidual attenuation. By applying a current equal to thevalue stamped on the instrument label (at about 0.7volts DC) at the BIAS input, at least rated attenuationis provided at all points across the frequency band(i.e., for "A" models at least 35 db and for "B" modelsat least 80 db). The DC current at a negative voltageis forward-bias and typically is about 3 to 5 ma for"A" models and 6 to 7 ma for "B" models. By varyingthe forward bias current between the stamped valueand zero, any level below maximum rated attenuationcan be established. Figure 2 shows typical residualattenuation curves.

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The attenuation increases with forward bias currentalmost linearly with db until the diodes saturate. Theattenuation sensitivity varies from unit to unit and al-so with time for individual units. For this reason thePIN Modulators are not suitable for use as program-med attenuators. The range of expected sensitivityvariation between units, and for a single unit with time,is shown in the shaded areas of the typical 35-db and80-db attenuation plots in Figure 3.

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The attenuation with a constant forward-bias varies withfrequency. This variation is small at low levels of at-tenuation and more noticableat higher values of atten-uation (see Figure 4). In addition, it should be noted thatattenuation is a minimum at each end of the frequencyband. Thus, the frequency band can be exceeded pro-vided a degradation in attenuation range as well as otherspecifications is acceptable. Note: exceeding the bandlimits by a large degree is unpractical.

IMPEDANCE MATCH

The PIN Modulator has a good impedance match at alllevels of attenuation over its frequency range. Thismatch is illustrated in Figure 5, which shows the SWRof a typical PIN Modulator measured under both zerobias and maximum forward bias conditions. Hence,with this constant match, frequency pulling effects dueto the PIN Modulator is negligible. If the PIN Modulatoris used outside its specified frequency range, SWR char-acteristics will be degraded in addition toother operat-ing specifications. Also, spurious responses canresultwhen exceeding specified frequency range.

TEMPERATURE STABILITY

The attenuation variation with applied bias current andbias voltage at different temperatures is illustrated,over a limited range, in Figure 6. Attenuation in DBvaries almost linearly with current, while variationwith voltage follows an almost perfect exponentialcurve. If a constant voltage is maintained, the attenua-tion rises with increasing temperature. With a con-stant current, the attenuation drops with increasingtemperature. The fact that the attenuation variesoppositely with temperature for constant-voltage andconstant-current operation suggests that an optimumvoltage with a selected series resistor added betweenthe source and the PIN Modulator would give tem-perature compensation over a limited operating range.This compensation is not built into the PIN Modulator.

CLOSED LOOP LEVELING

The PIN Modulator can be used in a closed loop level-ing system as the power limiting device for maintainingRF power levels constant. A simplifier block diagramof a closed loop leveling system employing a PIN mod-ulator is shown in Figure 7. The system consists ofsome sampling device which samples the main lineRFpower level; a detecting device which provides a DCsignal proportional to the sampled signal; some com-parison device which compares the detected signal towhat it should be when main line power is at its lowestpoint and then provides a bias current sufficient to in-crease PIN Modulator attenuation and maintain a con-stant RF power level.

LEVELING CAPABILITY. In a closed-loop levelingsystem involving a PIN Modulator, leveling capabilitydepends entirely upon loop equipment. Hence, loopequipment should be selected on the basis of main lineRF power levels and leveling requirements. In theclosed-loop system shown in Figure 8 the flatness ofleveling is determined primarily by the coupling varia-tion of the directional coupler used. The Power Meteris used because it provides extra loop gain. In addition,

02120-3

Appl. Note 58 Page 3

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02120-2

Page 4 Appl. Note 58

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with output coupling known, the main line output powercan be read directly on the Power Meter allowing forcoupling variation. The Leveler Amplifier used has a0 to - 27 vdc output capability with output impedance ofabout 20K ohms, hence the 20K ohms series resistormakes it a bias current source providing a maximum1.35 ma DC current (referring to Figure 5, this meansthat the maximum control range possible using an "A"model 8730 is between 12 and 20db depending upon theindividual8730used). Typically, with the Power Meterand Leveler Amplifier combination, leveled power canbe held constant with ±0.2 db (plus coupler variation)for RF swept frequency rates equal to or longer than30 seconds/octave. In practice, slower swept frequencyrates always produce the best leveling. However, tcdetermine whether or not any system is leveling prop-erly an Oscilloscope should be used. Figure 9 comparesunleveled and leveled output of a klystron.

For faster leveled swept frequency rates, negativeoutput detector and directional coupler combinationssuch as the hp Model 786D and 787D Directional Detec-tors can be used instead of the Power Meter and

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Thermistor Mount combination described above. Typi-cally, for the Directional Detector and Leveler Am-plifier combination, leveled power can be held constantwith ±0.2 db (plus frequency response of DirectionalDetector). Note: the frequency response of the Direc-tional Detector includes coupling variation and RF re-sponse of the crystal. For the Model 786D (0.96 to2.11 Gc) this is ±0.2 db.

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AMPLITUDE MODULATION

The PIN Modulator can be used to amplitude-modulatethe RF signal with almost any time-vary ing signal.Modulation is accomplished with a DC bias current toobtain a specific attenuation level and then superimpos-ing a time varying current upon the bias current. Thespecific attenuation level upon which the modulatingsignal is superimposed must be equal to or greater thanthe peak amplitude of the modulating signal or peakclipping will occur. A control instrument designed foruse in modulating applications with the PIN Modulatorsis the hp Model 8403A Modulator which provides fre-quency compensation for extending the PIN Modulator

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02120-2

Appl. Note 58 Page 5

AM frequency response to 10 Me. However, a typicalsetup for modulating applications, without the use of thehp Model 8403A Modulator, is illustrated in Figure 10.

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MODULATION DISTORTION. When a PIN Modulatoris used in an AM system, some envelope distrotionoccurs. This distortion is a function of the peak at-tenuation and total attenuation range covered by themodulating signal. To minimize RF envelope distor-tion, the reference level upon which the modulatingsignal is superimposed should always be a minimum(i.e., reference level should be set at a point onlyslightly greater than the peak modulating signal am-plitude). Under these conditions, a 50% sinusoidalmodulating signal (9.5 db total swing) will result inless than 6.1% distortion. A plot of measured AMdistortion for a typical PIN Modulator is shown inFigure 11. Note: AM distortion was measured underthe optimum conditions described above. Since dis-tortion, as described here, is a function of the non-linear sensitivity relationship (shown in Figure 5),shaping circuits may be incorporated to limit overalldistortion.

AMPLITUDE MODULATION LIMITATION. In any AMsystem the modulating equipment limits RF modulatingcapabilities almost entirely. For example, the powersupply and modulation signal source must both be ca-pable of supplying sufficient current to the low imped-ance 87 30 BIAS input. See Figure 3 for typical currentsnecessary for desired attenuation levels. In addition,the PIN Modulator itself limits the total percent ofmodulation obtainable. With 3 5 db attenuation, AM per-cent of modulation is limited to a maximum of about96.5%; with 80-db attenuation the percent of modulationis for all practical purposes 100%.

PULSE MODULATION

The PIN Modulator can be used as a pulsing or switchingdevice for RF power levels. Modulation is accom-plished by applying rated forward bias (see label at-tached to underside of PIN Modulator) for a maximumattenuation. Once maximum attenuation level is estab-lished, the RF power may be pulsed "on" by applying aconstant +5 to 6 volts to the BIAS input (voltage mustbe referenced to PIN Modulator ground). At the endof desired pulse width, the +5 to 6 volt DC potential mustbe switchedtoa -0.8 volt level with rated bias currentso that RF power level is pulsed "off".

To obtain pulsing with rise and fall times in the orderof 15 to 40 nanoseconds, the PIN Modulator must bebiased with the hp Model 8403A or from a speciallyshaped impulse waveform such as is illustrated inFigure 12. However, if rise and fall times in the orderof 100 to 300 nanoseconds are satisfactory, a setupsuch as is shown in Figure 10 may be used. Note: inany modulation system, modulation capability dependsupon the modulating waveform at the BIAS input con-nector. Hence, lead lengths should be as short aspossible to avoid capacitive cable effects. Figure 13shows typical pulse waveforms.

SIGNAL SEPARATION (GATING)

Figures 14 and 15, respectively, illustrate use of thePIN Modulator as an RF gating device for eliminatingspurious or "ghost" signals in antenna range receivers(Figure 14) or multiple signal separation in a spectrumanalysis system (Figure 15).

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Page 6 Appl. Note 58

A 100-nsec RF Pulse(Sweep Time: 20 nsec/div)

RF Pulse Expanded to Show Rise Time(Sweep Time: 5 nsec/div)

Detected 100-nsec Pulse Detected Pulse Expanded to Show Rise Time(Sweep Time: 20 nsec/div) (Sweep Time: 5nsec/div)

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02120-2

Appl. Note 58 Page 7

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Analyzer 2Gc-IF path so that the Frequency of theRF input need never affect the PIN Modulator. Forthis setup either a Model 8731B or a Model 8732Bmay be used since they both cover the 2 Gc fre-quency range of the IF path.

Figure 15. Multiple Signal Separation for Spectrum Analysis

MULTIPLE FUNCTION

For certain applications, it may be desirable to leveland amplitude modulate a given RF power level withthe same PIN Modulator. This can be accomplished bysimply setting up a closed loop leveling system andadjusting the reference attenuation level to a pointequal to the peak amplitude of the desired modulatingsignal. For this type of dual application the modulatingsignal can be applied directly to the leveler amplifierin the system shown in Figure 8.

The high on/off ratio available in the "B" series Mod-ulators make them ideal for use as SPST switches.One application is the tracking of fast moving objectsby switching an array of antennas in sequence, as shownin Figure 16. Since tracking is done electrically, themechanical inertia of a single tracking antenna posesno problem.

Unlimited possibilities exist when two or more mod-ulators are used in a microwave system. For example,suppressed carrier modulation is obtained with abalanced modulator using two 3-db couplers and twoModulators, as shown in Figure 17. A 90° phase shiftis introduced by each coupler in one RFpath and when

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this high-speed tracking system.

02120-2

Page 8 Appl. Note 58

Figure 17. Two PIN Modulators function as balancedmodulator by introduction of 180° phase-shift at car-

rier frequency in one of two parallel RF paths.

this signal is added to the signal from the other path,carrier cancellation takes place. In the balanced mod-ulator, the two series resistors are chosen to equalizethe sensitivities of the PIN Modulators, and dc biasvoltages are set to provide approximately 7 db ofattenuation.

Extremely narrow pulses can be generated by twomodulators in series. A single PIN Modulator drivenby the Modulator-driver is limited to 100-ns pulsewidths because of internal recovery times in the driver.

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This limitation is overcome in the set-up of Figure 18,which passes RF power only when both modulators are"on". The leading edge of the RF pulse therefore oc-curs when the second Modulator turns on and trailingedge occurs when the first Modulator turns off. Pulsewidth is determined both by the width setting of the firstModulator and the delay setting of the second. Therepetition rate and pulse delay of the composite signalcan be varied by the controls of the first Modulator.

Figure 18. Two PEST Modulators in series achieveshort-phase widths by overlapping pulse "on" times.

Figure 19. Short RFpulse achieved with tandem mod-ulator shown in Figure 18. RF carrier: 1000 Me; sweeprate: 5 nsec/cm. Phase coherent pulse is obtained byusing RF carrier as sync input to sampling oscillo-scope; counted-down sync from scope then triggers

first modulating pulse generator.

CONCLUSION

The PIN diode as a microwave modulator overcomesmany of the drawbacks of present modulation systems.The diode reduces reflections by absorbing power andimproves spectral purity by permitting the source tooperate continuously. Thus the PIN diode is the heartof a true advance in the technique of microwavemodulation.

02120-2