RF source. In this article, the characteristicsof sub-nanosecond rectangular pulses in thefrequency domain are briefly presented. Withreference to a few types of devices suitable forshort pulse generation, typical impulse circuittopologies are discussed with a focus on thestep recovery diode. Using shunt short-circuit-ed transmission lines as a pulse-shaping net-work, the impulse can be further formed intoa polycycle Gaussian pulse.

PULSE CHARACTERISTICSCommonly used signals for UWB applica-

tions are step-like pulse, impulse, rectangularpulse, monocycle and polycycle pulse. Tocomply with the FCC power spectral maskfrom 3.1 to 10.6 GHz, these signals must beshaped with some kind of filtering techniques.Therefore, it is important to understand thetime response and frequency spectral contentof these signals. Generation of an ultra-shortpulse relies on the transition speed of a step(Ts) or the duration of the impulse (Td).2 If itis desired to fully utilize the 3.1 to 10.6 GHzfrequency band, the pulse duration should beless than 100 ps. To validate the relationshipof the pulse width with respect to the equiva-

Y.W. YEAPInfocomm Development Authority

of Singapore (IDA)Singapore

On 14 February 2002, the Federal Com-munications Commission (FCC) re-leased its First Report and Order1 to

permit the unlicensed use of ultra wideband(UWB) devices in the 3.1 to 10.6 GHz fre-quency band with an emission limit of –41dBm/MHz. Due to the prohibition of highpower amplification under the FCC Part 15regulations, the UWB devices are restricted tooperate at very low power. The announcementhas attracted worldwide interest in low powerUWB research and development as it opensmany opportunities in short-range and highspeed wireless communications, radio fre-quency identification (RFID), vehicular radarsystems, imaging systems, short-range posi-tioning system (geolocation) and handheld ap-plications, etc.

To address this emerging wireless technolo-gy, IEEE 802.15 WPAN Task Group 3 (TG3a)and Task Group 4 (TG4a) are currently draft-ing standards specifications for short-range,high rate communications and low rate posi-tioning systems. Proposed methods for imple-menting UWB solutions include time modula-tion UWB (TM-UWB), direct-sequencespread-spectrum impulse radio (DS-UWB),multi-band orthogonal frequency divisionmultiplexing (MB-OFDM) and multiband-im-pulse (MB-I) technologies. Regardless of thetype of technology employed for UWB com-munication, except for MB-OFDM, the fun-damental type of source required is a pulsed

ULTRA WIDEBANDSIGNAL GENERATION

Reprinted with permission of MICROWAVE JOURNAL® from the September 2005 issue.©2005 Horizon House Publications, Inc.

lent output spectrum, a simple exper-iment can be carried out. An Anritsupulse pattern generator is used togenerate two rectangular pulses withamplitude of 0.25 V referenced at 0V, with a period of 3.2 ns, and pulsewidths of 100 ps and 200 ps, respec-tively. The equivalent spectrum ofeach pulse is similar to a sinc (x)curve, as shown in Figures 1 and 2.The first null of the rectangular pulseoccurs at 1/Td, which is at 10 GHz fora Td of 100 ps and at 5 GHz for a Tdof 200 ps. It is also obvious that thesecond and third nulls of the 200 psrectangular pulse occur at 10 and 15GHz, respectively.

Depending on the type of applica-tion, the pulse repetition frequency

(PRF) is another design considera-tion as it affects the output frequencycontent. The pulse repetition rate isthe inverse of the pulse period.

The pulse with a period of 3.2 nshas a PRF of 312 MHz. For pulseswith duration of 100 ps, and periods of6.4 ns, 12.8 ns and 25.6 ns, the equiva-lent measured PRFs are approximate-ly 156, 78 and 39 MHz, respectively,as shown in Figures 3, 4 and 5. Theseparation between adjacent spectralpeaks will be narrower if the pulse pe-riod is longer. It is noticeable that thespectral peaks are very close to eachother at a pulse repetition frequencyof 39 MHz. Due to the lower PRF, theoverall power level of the spectra isproportionally low.

ULTRA-SHORT PULSEGENERATION

There are many techniques andapproaches for short pulse genera-tion, which are mainly developedfrom UWB radar applications. A cen-tury ago, a conventional method ofgenerating ultra-short pulses used

PRFpulse period

= 11( )

spark gaps with arc discharges be-tween carbon electrodes. With theevolution of technology, the variety ofdevices used for generating ultra-short steps or pulses has expanded toinclude gallium arsenide (GaAs) pho-toconductive switches,3–4 mercuryswitches, avalanche transistors, steprecovery diodes (SRD), tunnel diodesand avalanche diodes, etc. Table 1shows the typical characteristics ofsome of these devices.

In high power radar applications,pulse generation is usually accom-plished using avalanche diodes, GaAsphotoconductive switches or mercuryswitches. In this article, high powerdevices are ignored and the concen-tration is on low cost pulsers for lowpower UWB communications. Theprinciple of ultra-short pulse genera-tion is through charge-storage andthe discharging of these devices witha trigger input signal. The avalancheeffect in either a transistor or somekind of discharge/switching diodeproduces nonlinear characteristics.The nonlinearity results in very fastrise times for step or pulse genera-tion. The physics of conduction andintrinsic capacitive nonlinearity ineach device will not be discussed as

TECHNICAL FEATUREM

AG

NIT

UD

E(d

Bm

)

FREQUENCY (GHz)

−20−25−30−35−40−45−50−55−60−65

0 2 4 6 8 10 12 14 16

Fig. 1 Spectral content of a rectangularpulse of 100 ps duration and 3.2 ns period.

MA

GN

ITU

DE

(dB

m)

FREQUENCY (GHz)

−15

−25

−35

−45

−55

−650 2 4 6 8 10 12 14 16

Fig. 2 Spectral content of a rectangularpulse of 200 ps duration and 3.2 ns period.

MA

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ITU

DE

(dB

m)

FREQUENCY (GHz)

−30

−35

−40

−45

−50

−55

−60

−650 2 4 6 8 10 12 14 16

Fig. 3 Spectral content of a rectangularpulse of 100 ps duration and 6.4 ns period.

MA

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ITU

DE

(dB

m)

FREQUENCY (GHz)

−35

−40

−45

−50

−55

−60

−650 2 4 6 8 10 12 14 16

Fig. 4 Spectral content of a rectangularpulse of 100 ps duration and 12.8 ns period.

MA

GN

ITU

DE

(dB

m)

FREQUENCY (GHz)

−40

−45

−50

−55

−60

−650 2 4 6 8 10 12 14 16

Fig. 5 Spectral content of a rectangularpulse of 100 ps duration and 25.6 ns period.

TABLE ITYPE OF PULSE SOURCES AND THEIR CHARACTERISTICS5

Best Available RisetimeType Step/Pulse at Amplitude Notes

Mercury switch step 70 ps 300 V max PRF = 200 Hz

Avalanche transistor pulse 150 ps 12 V device selection necessary

Tunnel diode step 25 ps 0.25 V fastest transition time100 ps 1.0 V

60 ps 20 V commercially availableStep recovery step 100 ps 50 V specially ordered

200 ps 200 V four-stack

Avalanche diode impulse 400 ps 125 V MHz rep. rate

the focus here is on UWB signal gen-eration from a device and circuit per-spective. Interested readers can referto Reference 6 for a more compre-hensive understanding of the nonlin-ear behaviors of the devices. Basedon the best available rise time at am-plitude in Table 1, it is clear thatavalanche transistors, tunnel diodesand step recovery diodes are the bestchoices for low power ultra-shortpulse generation.

Figure 6 shows a typical impulsegenerator circuit using an avalanchetransistor. Short impulses will be gen-

TECHNICAL FEATURE

erated when the trigger source ex-cites the base of the transistor. Eventhough the best achievable rise timeis 150 ps (as shown in Table 1), a highDC voltage is required to power upthe transistor. Owing to the overheat-ing caused by avalanching, the pulserepetition rate is limited to the kilo-hertz to megahertz range. Since anavalanche transistor consumes a highDC power and has a low pulse repeti-tion rate, it is not a good choice forlow power and high data rate UWBsystems. A tunnel diode, which hasthe fastest transition time among oth-ers, is the best device to be used for

step-like pulse generation. However,the disadvantage of using a tunneldiode is its low output voltage of 0.25to 1 V for the best available rise time.The output voltage will be in tenthsof a millivolt (mV) after pulse shap-ing. As a result, a post-amplifier isnormally required to subsequentlyamplify the output voltage. Figure 7shows the schematic diagram of animpulse generator circuit with a tun-nel diode that produces output pulsesor oscillations. The circuit with a vari-able operating point, having a trans-fer function that is defined in Lye andJoe,7 is excited by cyclical input ana-logue waveforms to generate ultra-short pulses.

To achieve higher amplitude ofoutput voltage and satisfy rapid risetime requirements, an SRD could bethe most promising device for ultra-short pulse generation in low cost andlow power UWB applications. SRDsare widely used as impulse generatorsby UWB research groups as well as innovel circuits reported in recent pub-lications.8–11 Figure 8 shows typicalimpulse generator circuits with anSRD placed in series8–10 (a) to thetrigger source or (b) shunt across amicrostrip line.11 An input triggersource drives the series SRD thatgenerates a step-like pulse. It is fur-ther divided to deliver two equalstep-like pulses to the short-circuitedstub and load. The reflected step-likepulse from the short-circuited stub,which is 180° out of phase with theincident pulse, combines with thenext step-like pulse propagating to-ward the load to form a positive im-pulse at the output. For the shuntSRD, the energy is charged-stored inthe series inductor during the posi-tive half interval of the input trigger.In the negative half interval, thediode snaps off when the voltagedrops below a threshold value. Theconduction current falls rapidly tozero and an impulse is formed duringdischarging. Regardless of the place-ment of the SRD, the main designconstraint for a UWB pulse generatoris a wideband impedance matching.Poor matching of the output of thepulse generator to the load imped-ance will lead to a severe ringing phe-nomenon. To overcome this con-straint, some practical approaches us-ing impulse-shaping circuits arediscussed next.

RL=50Ω100Ω

10kΩ

AvalancheTransistor

C1

40V

C2

InputTrigger

Output

Fig. 6 Pulse generator circuit using anavalanche transistor.

RL=50ΩTunnelDiode

L

InputTrigger

Output

Fig. 7 Pulse generator circuit with tunneldiode shunted to the ground.

(b)

(a)λ/4

RL=50ΩL

SRD

SRD

C

InputTrigger

Output

RL=50ΩInputTrigger

Output

Fig. 8 Typical configurations of pulsegenerator circuits using step-recovery diodes.

TABLE IITYPICAL PARAMETERS FOR SRD SELECTION

Transition The transition speed of SRD establishes high frequency limit of operation. time/snap time, Tt Transition time has to be reasonably short, less than 100 ps for operation

up to 10.6 GHz.

The output frequency is governed by the junction capacitance. Lowercapacitance produces faster transition time and higher frequency content.To generate frequency spectra up to 10.6 GHz, the estimated junctioncapacitance should be less than 0.3 pF using Equation 2.

(2)wheref = output frequencyXc = output reactance (assume 50 Ω for optimum performance at f).

Carrier lifetime is closely related to the input frequency as shown in Equation 3. The lifetime has to be longer by at least 10 times or more of 1/input frequency so that the reverse current can reach a peak before the

Carrier lifetime, τ diode snaps back to high impedance state.

(3)

Low series Lower Rs provides better power efficiency due to lower ohmic loss.resistance, Rs

Breakdown Commercial SRD normally has VR of at least 15 V. As it is for low powervoltage, VB application, this is not a major concern.

τ ≥10

input frequency

CfXj

c=

12π

Junctioncapacitance, Cj

TECHNICAL FEATURE

short transition time (Tt) and lowjunction capacitance (Cj) is requiredto generate a sub-nanosecond pulse.For instance, impulses with full widthat half maximum (FWHM) of approx-imately 200 ps have been generatedby using both an SRD that has a Tt of30 ps and Cj of 0.25 pF8 or an SRDthat has a Tt of 75 ps and Cj of 0.6pF.9–10 In both cases, the pulse repe-tition rate reported was 10 MHz.

The most important design consid-eration in an impulse circuit design isto minimize the distortion caused byringing in the output waveform. Thepractical solution is to improve theimpedance matching between theimpulse generator circuit and thepulse-shaping circuit. Techniques ap-plied in the design of an impulse-shaping circuit include simple match-ing using microstrip line stubs11 toreduce the signal reflections betweenthe circuits, the combination of aMESFET amplifier and a Schottkydiode,8 or a resistive matching net-work and Schottky diodes for rectify-ing and switching.9–10

It is obvious from Table 1 that anSRD with greater rise time will havea lower breakdown voltage. A devicewith extremely fast transition timewould have a slightly higher cost. Asemerging UWB communications ap-plications seek low cost as a goal, anovel impulse generator design tech-nique has been reported,12 using arelatively poor and thus low cost SRDwith a Tt of 150 ps and Cj of 1 pF.The pulse generator requires 0.5 V, <100 µA of the DC supply, and an in-put sinusoidal or rectangular wave-form trigger at 10 dBm. In fact, DCbias is an advantageous approach as ithelps to reduce the required inputtrigger power and provides a degreeof freedom for optimizing the outputwaveform. The circuit is capable ofgenerating ultra-short impulses withpulse repetition rates from 10 MHzto at least 200 MHz, depending onthe frequency of theinput trigger. Theimprovement of thisimpulse generator’sperformance is dueto a simple passiveimpulse-matchingnetwork, whichhelps to achievewideband imped-ance matching. The

pulse width, measured using an Agi-lent wide-bandwidth oscilloscope, isless than 200 ps, as shown in Figures9 and 10 . Figure 11 shows theequivalent frequency content of theimpulse with a PRF of 100 MHz. Thecost of all components used in thisprototype was less than US$0.60.

PULSE SHAPING The fractional bandwidth of UWB

transmission systems is defined as2(fH–fL)/ (fH+fL), where fH and fL arethe highest and lowest frequencies ofthe UWB bandwidth, respectively. TheUWB signal can be shaped to occupyonly a certain bandwidth. It was re-ported13 that the fractional bandwidthdecreases as the order of the derivativeof the Gaussian pulse increases. Fromthis point of view, the time-domainwaveform for a multi-band scheme willhave a higher order of derivative ascompared to a single-band scheme. Tocomply with the FCC power spectrallimit, the high order of derivative of aGaussian pulse, or polycycle Gaussianpulse, could be a more appropriate sig-nal because it has a bandpass frequen-cy spectrum and a smaller fractionalbandwidth. The challenge for thepulse-shaping circuit lies in convertingthe impulse to a polycycle Gaussianpulse. The conventional approach usesa wide-bandpass filter to truncate thespectra on a selected frequency band,which is 3.1 to 10.6 GHz in this con-text. UWB antennas can behave aswide-bandpass filters if properly de-signed. However, the design specifica-tions for UWB antennas will be verystringent to meet the specific require-ments of bandpass filtering, antennagain and the type of polarization simul-taneously. Therefore, it is recommend-ed to include a pulse-shaping networkprior to transmission.

A polycycle Gaussian pulse can berealized using multiple sections ofshort-circuited transmission lines, asshown in Figure 12. The impulse ar-

IMPULSE GENERATION USING SRD

The generation of sub-nanosecondpulses is mainly governed by thesnap/transition time and junction ca-pacitance of an SRD. To achievegood performance and minimize thecost of an impulse generator, the ma-jor criterion to be considered is theSRD selection. Typical parameters ofan SRD are listed in Table 2. Theperformance of the SRD relies oneach of the parameters described inthe notes. It has been shown previ-ously that the high frequency contentof a pulse is affected by the pulsewidth. Therefore, an SRD with very

TIME (ns) 0 2 4 6 8 10

0.5

0.4

0.3

0.2

0.1

0

−0.1

AM

PLI

TUD

E(V

)

Measured Simulated

Fig. 9 Simulated and measured impulsewith a PRF of 10 MHz.

TIME (ns)0 2 4 6 8 10

AM

PLI

TUD

E(V

)

0.90

0.75

0.60

0.45

0.30

0.15

0

−0.15

Measured Simulated

Fig. 10 Simulated and measured impulsewith a PRF of 100 MHz.

MA

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(dB

m)

FREQUENCY (GHz)

−15

−25

−35

−45

−55

−65

−750 2 4 6 8 10 12

Fig. 11 Measured frequency spectrum of an impulse with a pulse width of less than200 ps.

,1 ,2 ,3 ,n

RL=50Ω

Short-circuitedTransmission Lines

Output

Impulse

L1 L2 L3 Ln

Fig. 12 Typical passive pulse shaping circuit.

TECHNICAL FEATURE

Using a 100 MHz impulse source todrive the novel active pulse-shapingcircuit, which uses an amplifier,12 aGaussian pulse with a fourth-orderderivative is measured, as shown inFigure 13. Note that the measuredwaveform is shifted by –0.15 V forclearer distinction. Figure 14 showsthe equivalent frequency spectrum ofthe Gaussian pulse with a fourth-or-der derivative. The spectrum spreadsacross a wide frequency band from2.5 to 10 GHz with a PRF of 100 MHz.

CONCLUSIONA sub-nanosecond pulser is a key

enabling component for low powerUWB applications. The characteristicsof the UWB pulse should be carefullydesigned to fully utilize the powerspectrum efficiently. Depending onhow the technology is deployed, prop-er spectral filtering is required to com-ply with the FCC power spectralmask. Novel impulse generators andpulse-shaping design techniques mustbe further explored and developed toachieve more cost-effective, miniatur-ized UWB pulse sources. The mea-surements presented in this articlewere taken on the prototype devel-oped by the author to keep abreastwith the emerging technology.

ACKNOWLEDGMENTSThe author would like to thank An-

ritsu Co. (Singapore) for loaning thepulse pattern generator MP1763Cdemonstration unit and Agilent Tech-nologies (Singapore) for loaning thewide-bandwidth oscilloscope, Infini-ium DCA 86100B, and the spectrumanalyzer, E4407B (ESA-E Series), forprototype measurement.

References1. Revision of Part 15 of the Commission’s

Rule Regarding Ultra Wideband Transmis-sion Systems, FCC 02-48, First Report andOrder, Washington, DC, 22 April 2002,http://hraunfoss.fcc.gov/edocs_public/at-tachmatch/FCC-02-48A1.pdf.

2. J.R. Andrews, Picosecond Pulse Generatorfor UWB Radar, AN-9, Picosecond PulseLabs, Boulder, CO, May 2000.

3. J.S.H. Schoenberg, J.W. Burger, J. ScottTyo, M.D. Abdalla, M.C. Skipper and W.R.Buchwald, “Ultra Wideband Source UsingGallium Arsenide Photoconductive Semi-

conductor Switches,” IEEE Transactionson Plasma Science, Vol. 25, No. 2, April1997, pp. 327–334.

4. M. Fanbao, Z. Chuanming, Y. Zhoubing, J.Bingquan, W. Wentao and M. Hongge,“UWB Pulse Generation and RadiationUsing a Photoconductive Switching inGaAS,” IEEE 18th International Sympo-sium on Discharges and Electrical Insula-tion in Vacuum, Eindhoven, The Nether-lands, May 1998, pp. 785–787.

5. T.W. Barrett, “History of Ultra Wideband(UWB) Radar & Communications: Pio-neers and Innovators,” Progress in Electro-magnetics Symposium 2000, Cambridge,MA, July 2000, p. 9.

6. S.A. Maas, Nonlinear Microwave and RFCircuits, Second Edition, Artech HouseInc., Norwood, MA, 2003.

7. K.M. Lye and J. Joe, “Method and Appara-tus for Generating Pulses from AnalogWaveforms,” United States Patent Applica-tion Publication, US2002196065, Decem-ber 26, 2002.

8. J.S. Lee and C. Nguyen, “Novel Low CostUltra Wideband, Ultra-short-pulse Trans-mitter with MESFET Impulse-shapingCircuitry for Reduced Distortion and Im-proved Pulse Repetition Rate,” IEEE Mi-crowave Wireless and Components Letters,Vol. 11, May 2001, pp. 208–210.

9. J.S. Lee, C. Nguyen and T. Scullion, “NewUniplanar Sub-nanosecond MonocyclePulse Generator and Transformer forTime-domain Microwave Applications,”IEEE Transactions on Microwave Theoryand Techniques, Vol. 49, No. 6, June 2001,pp. 1126–1129.

10. J.W. Han and C. Nguyen, “A New UltraWideband, Ultra-short Monocycle PulseGenerator with Reduced Ringing,” IEEEMicrowave and Wireless Components Let-ters, Vol. 12, No. 6, June 2002, pp.206–208.

11. K. Madani, et al., “A 20 GHz MicrowaveSampler,” IEEE Transactions on Mi-crowave Theory and Techniques, Vol. 40,No. 10, October 1992, pp. 1960–1963.

12. Y.W. Yeap and C.L. Law, “UWB Impulseand Polycycle Pulse Generation with ON-OFF Keying Modulation,” United StatesProvisional Patent Application, July 16,2004.

13. M. Welborn and J. McCorkle, “The Impor-tance of Fractional Bandwidth in UltraWideband Pulse Design,” IEEE Interna-tional Conference on Communications Di-gest, Vol. 2, 2002, pp. 753–757.

Yeap Yean Wei received his B.Eng degree incommunications and electronics engineeringfrom Northumbria University, UK, in 2002. Heis currently working as an associate consultantin the Network Technologies Group, InfocommDevelopment Authority of Singapore. His jobresponsibilities include identifying, trackingand exploring emerging technologies inwireless network domains.

riving at the first junction will split intotwo equal impulses to L1 and ,2. Theimpulse propagates along L1 and is re-flected back to the junction from theshort-circuited stub. It combines withthe next incoming impulse and formsa monocycle pulse if the time delay(length of ,1) is properly optimized.Thus, a polycycle pulse can be formedfrom multiple reflections from theshort-circuited stubs, ‘L1’ to ‘Ln’,combining with the next incomingpulse. The lengths of ‘,’ and ‘L’ haveto be properly tuned to obtain a poly-cycle pulse with minimum distortion.Based on this principle, a monocycleGaussian pulse has been reported8–10

using only a single short-circuitedtransmission line.

The method explained above canbe realized easily using a microstripline but the physical dimension of thepassive pulse-shaping circuit em-ployed could be relatively large. Toovercome this constraint, a novel de-sign technique using an active pulse-shaping circuit has been reported.12

TIME (ns)12.2 13.2 14.2 15.2 16.2 17.2

AM

PLI

TUD

E(V

)

0.06

0

−0.06

−0.12

−0.18

−0.24

Measured Simulated

Fig. 13 Simulated and measured time-domain waveform using an active pulseshaping circuit.

MA

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(dB

m)

FREQUENCY (GHz)

−38

−43

−48

−53

−58

−63

−68

−73

−780 2 4 6 8 10 12

Fig. 14 Measured frequency spectrum of a Gaussian pulse with fourth-orderderivative.