A Tunable Ultra-Wideband Pulse Generator Using a

Variable Edge-Rate Signal

Erick Maxwell and Thomas WellerUniversity of South Florida, Dept. of Electrical Engineering

Tampa, Florida 33620, USAEmail: {emaxwel2, weller} d,eng.usf.edu

Abstract This paper presents a pulse-duration tunable Ultra-Wideband (UWB) generator that is developed using a variableedge-rate signal. Edge-rate variability is introduced byapplying a step recovery diode (SRD) to compress the edges ofthe source and then employing a simple RC network to adjustthe edge-rate. Afterwards, the compressed signal isdifferentiated using microstrip transmission lines in a shortcircuit stub configuration. The tunable generator resultingfrom this approach demonstrates Gaussian and monocyclepulses with: good symmetry and low distortion over thetunable range; pulse width variation from 800 to 1150ps over a1-2OpF capacitance range; and good agreement betweensimulated and measured results.

I. INTRODUCTIONUltra-Wideband (UWB) microwave systems are finding

application in the form of impulse radio, as well asrespiratory, cardiovascular and other sensing/monitoringapplications [1]. The Federal Communication Commission(FCC) defines UWB as an intentional radiator with aninstantaneous 10dB-fractional and total bandwidth of at least0.2 and 500MHz, respectively [2]. This bandwidth isachieved primarily by radiating short pulses that are derivedfrom a basic Gaussian pulse shape.

Tunable pulse generators are useful in UWB radar andradiometric measurement because they provide a platformfor optimizing the absorbed and reradiated power of anisolated target. Power optimization may be required forenhancing discrimination and evaluation of electricalcharacteristics associated with a target [3]. Consequently, atunable generator can also be used to achieve varyingpenetration depth, radiation intensity and range resolution bycontrolling the shape, bandwidth and center frequency of thespectrum this is due to a target's frequency-dependentelectrical properties. Tuning the spectrum shape and centerfrequency may be accomplished with pulse differentiation.However, bandwidth tuning is accomplished by adjusting thepulse-duration, which is a greater challenge and is presentedhere. Thus, pulse-duration tunable generators provide avaluable tool for research requiring UWB measurement ofthe electrical properties of materials.

This work was supported in part by the UNCF*MERCK ScienceInitiative and by the National Science Foundation IGERT Program underGrant DGE 0221681.

Jeffrey HarrowJames A. Haley VA Medical Center

Tampa, Florida 33612, USAEmail: Jeffrey.Harrow Wva.gov

vs LD .'c2 '

Variable Edge Rate Compressor Pulse Shaper RF.Ficrowave Differentiator

Figure 1. Schematic of a tunable UWB generator.

One approach for designing a pulse-duration tunablegenerator is based upon switching in sequential sections oftransmission lines to combine short pulses or varying thecircuit impedance. These systems often require a number ofdiscrete components in addition to power biasing for eachsection. Moreover, they often loose their Gaussian symmetryas more pulses are combined for increasing the width [4],[5]. A pulse-duration tunable generator using a variableedge-rate signal provides a simpler approach to pulse-duration tuning.

In this paper we report on the development of a newtunable pulse generator that is designed using a variableedge-rate signal, which operates by generating anddifferentiating a variable edge-rate rectangular pulse [6]. Theresulting generator produces Gaussian and monocycle pulseswith good symmetry and low distortion over the tunablerange, a tuning variation of 800 to 1150ps using a 1-2OpFcapacitance, and good agreement between simulated andmeasured results.

II. CIRCUIT DESCRIPTION AND DESIGNThe tunable pulse generator in Fig. 1 is implemented

with three sub-circuits, including: a variable edge-ratecompressor, pulse shaper and RF/microwave differentiator.The variable edge-rate compressor sub-circuit provides amechanism for producing a tunable pulse width by allowingslew rate control. This sub-circuit is constructed using aMetelics SMMD840-SOT23-OS step recovery diode torapidly charge up and snap back on the rising (SRI) andfalling (SR2) edges of the source. Although a sharp fallingedge is not typically used in the construction of a Gaussian

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waveform, the corresponding step recovery diode (SR2)contributes to the pulse shape, width and low distortionachieved in this circuit.

Figure 2. Photo of the single-stage tunable UWB pulse generator. Fromleft to right is the variable edge-rate compressor, amplifier, pulse forming

network and RF/microwave differentiator

The edge-rate associated with the rectangular pulse thatresults from the step recovery diode is controlled with asimple RC network, where RN is a 60-ohm chip resistor - avalue that was determined by optimizing the pulse shape inADS. The RC time constant that results from this networkprovides a means to vary signal rise time by modifying thecapacitance. The capacitance may be determined by firstrecognizing that the series inductance and shunt capacitanceforms a first order low-pass filter. The cut-off frequency forthis filter is obtained from the reciprocal of the step-functionrise time. Consequently, the low-pass filter inductance (L k)and capacitance (C k) are defined by the following equations:

(1)L-Ro LkRL.k -

Ck

C' Ct

k

where, R0 is the circuit resistance, Ck is the normalizedcapacitance, Lk is the normalized impedance and CoC is thecut-off frequency [7]. Since, this circuit is based onachieving pulse-duration tuning by modifying the step-function rise time, a minimum and maximum rise time isused to constrain the filter elements. The full rise timeassociated with the 70.Ops transition time for the Metelicsstep recovery diode is used to limit the minimum step-function rise time. However, this time does not account forthe diode junction capacitance since it is measured in a testfixture. In addition, the transition time is measured betweena 20-80% rise in amplitude. A 10 ns minority carrier lifetimeis used to limit the maximum step-function rise time becauseit affects the operational frequency of the diode. Theselimits are applied to equations 1 and 2 above, usingnormalized element values, which correspond to amaximally flat pass-band. This endeavor results in aninductance range of 4.0-0.35nH and capacitance range of0.17-14.5pF. Consequently, the LD and CN are implementedwith a 0.35nH chip inductor and a capacitor trimmer with a1-2OpF range, respectively (Sprague-Goodman SG2020).

(2)

The resulting variable edge-rate signal is then passed tothe pulse shaper sub-circuit, which forms a Gaussian pulse.This sub-circuit consists ofthe following: a Picosecond PulseLabs 5840A-107 amplifier with 21dB of gain and 35dB ofisolation over a 8OkHz-9.3GHz bandwidth; an attenuator of6dB that is used to help meet the OdB input powerrequirement for the amplifier; and a pulse forming networkthat is used to differentiate the incoming rectangular pulse.Since, the pulse-forming network includes a differentiator;the initial value for the lengths LI and L2 was set to aquarter wavelength of the maximum frequency [8]. Since,the maximum frequency is determined from the rise time, thelength may be expressed by the following equation:

(3)Len = 1.2 Z10-90%4 -J u

where £ is the electric permittivity, pt is the magneticpermeability, and the factor of 1.2 is applied to approximatethe full rise time from the 10-90% rise time (cl090%). Theresulting length was then optimized in ADS to achieve adesirable ripple and overshoot. As a result, the pulse-forming network is constructed from a short circuit stub with

020

0.15

0.10

0,150. -

(A)

D.15

D10

D.05

D.C)

a 0Q

D. 15

o 455A 0 55 60 6Time (ns)

(B

Figure 3. Simulated and measured waveforms at 1, 2, 5, 10 and 2OpFcapacitance values (from left to right); A: Simulated data at variable edge-rate compressor ; B: Measured data. Note: since SRDs are used for therising and falling edges, the duty cycle follows that of the source. Also,

both a sinusoid and square wave produce similar waveforms.

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30 35 40 45 50Time (ns)

(A)

45 50 55 60 6!Time (ns)

Figure 4. Simulated and measured Gaussian waveforms at 1, 2, 5, 10 and2OpF capacitance values (from left to right); A: Simulated Gaussian pulse

at the output of the pulse shaper ( 1 st derivative of the edge-ratecompressor output); B: Measured pulse over capacitance. Note: There isgood amplitude agreement in measured and simulated data. However, theamplitudes associated with 5, 10 and 2OpF show increased attenuation

a length LI of 103mm and a width of 2.5mm. Since, theresulting Gaussian contains both positive- and negative-going pulses, an Agilent Technologies Schottky barrier diodepackage (HSMS2862) is used to clamp the negative goingreflections by providing a ground path through DNI as well asblocking through DN2. The resulting Gaussian is then passedto the RF/Microwave differentiator sub-circuit where amonocycle is formed. This sub-circuit contains a DC-18GHz 3dB attenuator to minimize circuit reflections. It isimplemented using a 100-ohm resistor for matching and ashort circuit stub L2 that is 80mm long having a width of1.25mm. Each sub-circuit has been developed on a separateprinted circuit board for the prototype, as shown in Fig 2.

III. FABRICATION AND MEASUREMENTThe variable edge-rate compressor and pulse forming

networks are fabricated on a Rogers Corporation highfrequency laminate (RT5870) with a relative dielectricconstant of 2.33 and a board thickness of 0.787mm. TheRF/Microwave differentiator is fabricated on a 0.787mmFR4 glass epoxy substrate having a relative dielectricconstant of 4.2. The setup used to test this generator includesthe use of an Agilent 33120A arbitrary waveform generatorto produce the 14MHz lOVpp sinusoidal stimulus required at

the input of the circuit and an HP 54750A digitizingoscilloscope with an HP54715A 20GHz module to capturethe output. All measured data obtained from this setup wascompared to that simulated using Advanced Design System(ADS) 2003A.

The simulation and measurement data were taken oneach sub-circuit above. Measurement data was collectedusing an HP54750A oscilloscope with an HP54753A,20GHz TDR plug-in. Figure 3 demonstrates the tuningrange of the compressor as the capacitance is varied from 1-2OpF. The amplitude associated with the waveforms, in thisfigure, is a function of the amount of charge that is availablefrom the step recovery diodes at the time of the snap. Sincevoltage amplitude may be expressed mathematically as theequivalent charge over the total capacitance, it is expectedthat the signal amplitude would decrease with an increase inthe circuit's capacitance. Consequently, as the capacitance istrimmed the signal rise time decreases while the amplitudeincreases. Good agreement between simulated and measureddata may be observed in this figure.

Likewise, the measured and simulated data at the outputof the pulse forming sub-circuit (Fig. 4) demonstrates lowdistortion and good agreement. The waveform increases inwidth as the capacitance is increased from lpF, where it is800ps wide, to 2OpF where the waveform is 1150ps wide.

u~

Q~

35 40 45 50 55 60 65 7Time (ns)

(A)

35 40 45 50 55 60 65 7Time (ns)

(B)

Figure 5. Simulated and measured monocycle waveforms at 1, 2, 5, 10and 2OpF capacitance values (from left to right); A: Simulated monocyclepulse resulting from a 2nd derivative ofthe pulse forming sub-circuit; B:Measured monocycle pulse resulting from differentiating the Gaussian.

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The shape ofthe waveform remains Gaussian throughout thetunable range and has a distinct peak-amplitude as well as aslope that varies as a function of pulse width. However, theamplitude decreases sharply with an increase in capacitancefor the measured results, i.e., the simulated amplitude at2OpF is about 25mV, whereas the measured amplitude isabout 5mV. Consequently, the measured amplitude at 2OpFis very close to the ringing noise in the circuit. However, theGaussian shape may be restored by increasing the gain oftheamplifier at the input ofthe pulse forming sub-circuit.

The RF/microwave differentiator sub-circuit produces amonocycle pulse from the Gaussian that results from thepulse forming sub-circuit. Figure 5 shows the simulated andmeasured waveforms. As demonstrated, a monocycle havinga 1.6ns width, results from differentiating the 800psGaussian. The differentiator used in this sub-circuit producespulses that have closely matched amplitudes at the positiveand negative levels.

0

E -10

= 20

30

40

50

-6oq)

X 70

-800 1 2 3 4 5 6 7 8 9 10

Frequency (GHz)

0 1 2 3 4 5 6Frequency (GHz)

(B)

Figure 6. The shaped spectrum for the FCC maskand the normalized frequency response of wavefor

lpF and 1OpF capacitance value; A: Response ofGoResponse ofMonocycle wavefori

The normalized spectrum associated with the Gaussianand monocycle waveforms above are shown in Fig. 6. Thesefigures show that the tuning range and bandwidth decreasesupon the application of a second derivative in theRF/Microwave Differentiator sub-circuit. The tuning rangemeasures 300 NMHz and 160 NMHz, for the Gaussian andmonocycle, respectively. Likewise, the bandwidthassociated with the 1OpF capacitance measures 1.4 GHz forthe monocycle spectrum (Fig. 6B) and 1.7 GHz for theGaussian spectrum (Fig. 6A). A comparison of theseresponses against the shaped spectrum of the FCC mask formedical imaging demonstrates that pulse durationmodifications alone may not provide sufficient shaping tooptimize the mask. However, better optimization may beobtained by a combination of methods including pulse-duration tuning and pulse differentiation [9].

IV. CONCLUSIONWe have developed a tunable pulse generator based on a

novel mechanism of utilizing step recovery diodes forvariable edge-rate compression. This approach simplifiesUWB generator design by allowing a focus on generating asmooth slope for the step in a rectangular pulse and thendeveloping RF/microwave differentiators. The waveformsthat result from this approach demonstrate good Gaussiansymmetry throughout a tuning range of 800ps to 1150psusing a 1-2OpF capacitance trimmer. In addition, thesecircuits require only an AC input and a DC supply for theamplifier, do not require biasing, and contain only 8 discretecomponents. This approach should be useful for applicationsthat require a tunable UWB source.

REFERENCES[1] R. Fontana, "Recent system applications of short-pulse ultra-

wideband (UWB) Technology," IEEE Trans. Microwave TheoryTech., vol. 52, no. 9, pp. 2087-2104, 2004.

[2] Federal Communications Commission, Code of Federal Regulation,Title 47, ch. 1, part 15, "Radio Frequency Devices," Sub-Part F,Ultra-Wideband Operation, Sec. 503, pp. 767-768, 2003.

[3] J. Taylor, Ultra-Widband Radar Technology, Boca Raton: CRC Press,2001.

[4] J. Han and C. Nguyen, 'Ultra-Wideband Electronically Tunable PulseGenerators," IEEE Microwave And Wireless Components Letter, vol.14, no. 3, March 2004.

[5] J. Han and C. Nguyen, 'Microstrip Impulse Generators with TunablePulse Duration for Ultra-Wideband Applications," 2003 Asia PacificMicrowave Conference, Seoul, Korea, Nov. 2003.

[6] C Hsue, T. Cheng and H Chen, "A second-order microwavedifferentiator," IEEE Microwave And Wireless Components Letter,

7 8 9 10 vol. 13, no. 3, March2003.[7] D. M. Pozar, Microwave Engineering, 2d Ed., New York:John Wiley

& Sons, Inc., 1998, pp 422-496.[8] C Hsue, T. Cheng and H Chen,, "Implementation of First-Order and

Second-Order Microwave Differentiators," IEEE Transactions on>for medical imaging Microwave Theory and Techniques, vol. 52, no. 5, May 2004.ms generated using a [9] I Oppermann, M. Hamalainen, and J. Linatti, UwB theory andaussian waveforms; B: applications, West Sussex:John Wiley and Sons, 2004 pp.2-8.'ms.

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