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Published in IET Microwaves, Antennas & Propagation

Received on 26th January 2010
Revised on 17th May 2010
doi: 10.1049/iet-map.2010.0030

ISSN 1751-8725

Efficient passive low-rate pulse generator

for ultra-wideband radar
K.K.-M. Chan1 K. Rambabu1 A.E.-C. Tan1 M.Y.-W. Chia2
1
University of Alberta, 2nd Floor, Electrical & Computer Engineering Research Facility (ECERF), University of Alberta,
Edmonton, Alberta, Canada T6G 2V4
2
Institute for Infocomm Research, 1 Fusionopolis Way, #21-01 Connexis (South Tower), Singapore 138632
E-mail: kkchan@ualberta.ca

Abstract: The design of a passive ultra-wideband (UWB) pulse generator using cascaded sections of shunt-mode
step-recovery diodes with decreasing lifetimes is presented. This pulse generator circuit generates 241 ps, 5.6 V
pulses at 10 MHz pulse repetition frequency without the use of an additional amplifier. This pulse generator can
be used in UWB radar systems that require sub-nanosecond pulses at low pulse repetition rates.

1 Introduction efficiency is related to the carrier lifetime of the diode used

in the circuit. An SRD with shorter transition time will
Ultra-wideband (UWB) radars are finding many applications in typically have a shorter lifetime and smaller ‘snap’ voltage.
the areas of medical imaging, non-destructive testing and in For an efficient harmonic generation, the time period of
security systems. One of the key issues of the UWB systems the driving signal should be less than the carrier lifetime of
is generation of ultra-short pulse with minimum ringing at the SRD [2]. The physical construction of the SRD
suitable pulse repetition frequency (PRF). Typical pulses used determines the conversion efficiency [5]. The scope of
in UWB radar systems are Gaussian pulse and Gaussian tuning the physical parameters of the SRD is beyond the
mono-cycle [1]. The most common way to generate such control of the system design engineer, so the conversion
pulses is by introduction of non-linear devices to a frequency efficiency of the generator has to be improved at the circuit
source. Harmonics of the source frequency will result from level.
the non-linearity of the circuit that has been realised using
passive or active devices. The choice of implementation is UWB systems have to meet regulatory mandates, for
determined by the specifications of the UWB pulses.
example the Federal Communications Commission mask
[6], so that they do not interfere with existing narrowband
Techniques for generating UWB pulses by using step- applications in the same spectrum. This limitation places a
recovery diode (SRD) have been extensively studied in the great challenge to UWB radar designers, who have
literature [2 – 4]. A commonly used scheme for UWB pulse detection range being a key system specification. For a
generation is the shunt-mode SRD impulse generator [3]. longer range, higher pulse amplitudes and lower pulse
The transition time of an SRD determines the minimum repetition rates (PRFs) are required. System designers can
achievable pulse width. A pulse width of 100 ps at 20 V reduce spectral emission levels by lowering the PRF, and
peak amplitude is possible with the SRD and the inadvertently, lowering the output pulse amplitude too.
maximum output voltage is limited by the reverse Thus, additional broadband amplifiers are needed.
breakdown voltage of the diode. The high breakdown Furthermore, a shorter transition time SRD is desired for
voltage and narrow pulse width make the SRD an ideal better range resolution. So, the desired UWB radar pulse
candidate for UWB pulse generation. generator has to produce short pulses efficiently at low PRFs.

The primary concern of the shunt-mode SRD circuit for This paper presents a novel method to produce
pulse generation is its conversion efficiency. The conversion short pulses at lower PRFs using cascaded sections of

2196 IET Microw. Antennas Propag., 2010, Vol. 4, Iss. 12, pp. 2196 – 2199
& The Institution of Engineering and Technology 2010 doi: 10.1049/iet-map.2010.0030
 www.ietdl.org

shunt-mode SRDs with decreasing carrier lifetimes. The C1 and load form a first-order high-pass filter. C1 is
pulse generator is able to generate 200 ps pulses at up to determined by the pulse width that sets the high-pass filter
73% efficiency (output voltage/input voltage) at 10 MHz cut-off frequency. The time constant of the circuit is the
PRF. The circuit is fully passive and does not require combination of C1 and the load. Pulse width of the system
biasing, and thus is inherently stable. is equivalent to time constant of the circuit

The design and analysis of the pulse generator are Time constant = RC (2)
presented in Section 2. In Section 3, circuit is optimised
for pulse amplitude and pulse width. In Section 4, a For a 200 ps pulse width and 50 V load, C1 is 4 pF.
prototype pulse generator has been built and results are
shown to verify the design and circuit performance. 3 Optimisation of pulse generator
Initial design obtained in Section 2 is optimised to achieve
higher output pulse amplitude and narrower pulse width by
2 Design of pulse generator practically tuning L1, L2 and C1 values using commercially
The schematic diagram of the proposed pulse generator is available components and changing SRD1. During the
shown in Fig. 1. Source is a 50 V waveform generator that manual optimisation, it is observed that the output pulse
generates continuous waves (CWs) with amplitudes greater amplitude is sensitive to the carrier lifetime of SRD1 and
than the turn-on voltages of SRD1 and SRD2. L1 and value of L1 whereas output pulse width is sensitive to the
SRD1 form the first stage of the shunt-mode harmonic values of L2 and C1. Pulse ringing can be controlled by
generator, whereas L2 and SRD2 form the second stage. careful selection of L1 and L2 values.
First stage is cascaded with second stage in series
configuration. 3.1 Optimisation for pulse amplitude
The design starts with the specification of the system pulse It is observed that the output pulse amplitude is sensitive to
width, which determines the selection of SRD2 that has a the carrier lifetime of SRD1 and value of L1. The choice
transition time capable to meet the system specification. for SRD1 will be the one that has a carrier lifetime longer
Pulse widths of each stage are, at minimum, twice the than that of SRD2. The longer the lifetime of the SRD,
transition time of the respective SRD. SRD1 is then the higher the charge it can store and so the greater the
chosen, to have a longer carrier lifetime than SRD2. ‘snap’ voltage because of a larger initial current. Therefore it
Equation (1) shows the relation between pulse width and generates pulses with higher output amplitude at lower
inductor value L in a single-stage shunt-mode harmonic PRFs where the pulse repetition period is greater than the
generator [2]. C is the reverse bias capacitance of the SRD. diode lifetimes. However, a larger difference between the
Values of L1 and L2 in Fig. 1 are determined by lifetimes of SRD1 and SRD2 will reduce the output pulse
amplitude for a given system pulse width. SRD1 should
√ have lifetime of at least twice the lifetime of SRD2.
Pulse width = p LC (1)
Increasing L1 value results in both higher output pulse
amplitudes and ringing. Fig. 2 shows the effect of L1 on
For UWB applications with 5 GHz centre frequency, the
the output pulse amplitude (L2 ¼ 12 nH, C1 ¼ 3.9 pF).
required system pulse width, ts , is 200 ps. For ts ¼ 200 ps,
Thus, L1 value can be optimised for pulse amplitude while
SRD2 (M-Pulse MP4023) is chosen with a transition time
of 50 ps. The reverse bias capacitance of SRD2 is 0.35 pF
and the calculated L2 is 12 nH. SRD1 (M-Pulse MP4043)
is chosen, as discussed in Section 3, for carrier lifetime four
times that of SRD2. It has a transition time of 120 ps and
reverse bias capacitance of 1.5 pF. For an expected pulse
width of 240 ps, L1 ¼ 3.9 nH.

Figure 1 Schematic diagram of the UWB radar pulse Figure 2 Effect of L1 on output pulse (L2 ¼ 12 nH,
generator C1 ¼ 3.9 pF, Vin ¼ 10 V CW)

IET Microw. Antennas Propag., 2010, Vol. 4, Iss. 12, pp. 2196 – 2199 2197
doi: 10.1049/iet-map.2010.0030 & The Institution of Engineering and Technology 2010
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Figure 3 Effect of L2 on output pulse (L1 ¼ 3.9 nH, Figure 4 Photograph of UWB radar pulse generator
C1 ¼ 3.9 pF, Vin ¼ 10 V CW) prototype

maintaining an acceptable level of ringing. The output The following results show that the proposed two-stage
voltage, V(t), of the single-stage shunt-mode pulse pulse generator managed to generate pulses of higher pulse
generator can be determined from [3] amplitudes (127% increase) as compared to single-stage
shunt-mode pulse generator, at the expense of longer pulse
 ⎛  ⎞ widths (51.5% increase). Efficiency (output voltage/input
L/C ⎝− z2
V (t) = −Io exp bt ⎠ sin(bt) (3) voltage) has improved to 56% from 25%.
1 − z2 1 − z2
Fig. 5 shows the output of the proposed pulse generator for
where Io is the initial current
 in the inductor√
√ during the a 10 V peak-to-peak CW input at 10 MHz PRF. It is
‘snap’, and b = 1 − z / LC and z = L/C /(2R).
2
compared with the output of the single-stage shunt-
Equation (3) explains the effect of L1 on pulse amplitude. mode pulse generator designed using fast transition time
An increase in L value, as shown in (3), increases the SRD (MP4023) based on the design method outlined in
amplitude of V(t), and reduces the damping factor of the [1], with L ¼ 12 nH and C ¼ 2 pF. The measured outputs
negative exponential term of V (t). in Fig. 5 are in black and simulated outputs in dark grey.
The simulation software used is Ansoft Designer from
Ansys with the Nexxim circuit transient simulator and
3.2 Optimisation for pulse width non-linear microwave diode models. The measured pulse
It is observed that the output pulse width is sensitive to the amplitude in Fig. 5, of single-stage circuit (dashed line)
values of L2 and C1. Reducing the value of L2 shortens is 2.47 V and full-width-at-half-maximum (FWHM)
the pulse width. Fig. 3 shows that shortest pulse width is pulse width is 159 ps. For the same input, the proposed
obtained for L2 ¼ 1.2 nH at L1 ¼ 3.9 nH and C1 ¼ 3.9 pF. pulse generator (solid line) produces a pulse of pulse
amplitude 5.6 V and FWHM pulse width of 241 ps.
Referring to Fig. 1, C1 and load form a first-order high- It should be noted that the current in the inductor before
pass filter. Reducing C1 shortens the output pulse width at the ‘snap’ is 115 mA. Simulation results predicted
the expense of pulse amplitude. comparable pulse widths, amplitudes and ringing levels
with the measurements.

4 Prototype and measurement

A prototype of the proposed pulse generator has been built
using Rogers 4003 substrate (1r ¼ 3.38, h ¼ 32 mil).
M-Pulse Microwave MP4033 is used as SRD1 and
MP4023 as SRD2. L1 and L2 are 10% tolerance multilayer
ceramic chip inductors with values of 22 and 1.2 nH,
respectively. A 3.9 pF porcelain multilayer chip capacitor is
used as C1. The choice for these components is for an
optimised pulse with trade-off among the amplitude, pulse
width and ringing considerations.

Fig. 4 shows a photograph of the prototype housed in a

metallic chassis. The dimensions of the prototype board are
25 mm × 25 mm. Figure 5 Outputs for 10 MHz CW input

2198 IET Microw. Antennas Propag., 2010, Vol. 4, Iss. 12, pp. 2196 – 2199
& The Institution of Engineering and Technology 2010 doi: 10.1049/iet-map.2010.0030
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predicted quite accurately in the simulations. Efficiency has

improved from 51% to 73%.

5 Conclusions
A novel pulse generator design, using cascaded sections of
shunt-mode SRDs with decreasing lifetimes has been
presented for UWB radar applications. The proposed pulse
generator is ideal for systems requiring high-amplitude,
narrow pulses at lower PRFs. The conversion efficiency of
the generator has improved to 56% for CW input and 73%
for square wave input.

Figure 6 Measured outputs for square wave input 6 References

[1] TAYLOR J.D.: ‘Introduction to ultra-wideband radar
systems’ (CRC Press, 1995)
Higher-amplitude ringing has been observed for the
proposed pulse generator as compared to the MP4023-
[2] ‘Pulse and Waveform Generation with Step Recovery
based single-stage pulse generator. The pulses will be
Diodes’. Hewlett-Packard Application Note #918, http://
differentiated when radiated from the transmit antenna [7]
www.hp.woodshot.com/hprfhelp/lit/diodelit.htm, accessed
and pulse ringing is inevitable because of the band-limited
July 2010
frequency response of filters, amplifiers and antennas.
However, the system’s range resolution can be preserved in
[3] HAMILTON S., HALL R.: ‘Shunt mode harmonic generation
the presence of ringing with the use of pulse shaping filters
using step recovery diodes’, Microw. J., 1967, pp. 69– 78
or differentiators together with a correlation-based receiver
architecture.
[4] HALL R., HAMILTON S., KRAKAUER S.: ‘Impulse shunt mode
harmonic generation’. Dig. ISSCC, February 1966
In the second measurement, the pulse generators are fed
with 10 MHz PRF square wave input with 5 V peak-to- [5] MOLL J., KRAKAUER S., SHEN R.: ‘P-N junction charge storage
peak amplitude and rise/fall time of 4 ns. The measured diodes’. Proc. IRE, January 1962, pp. 43– 53
and simulated pulse generator outputs are shown in Fig. 6.
For the single-stage pulse generator, the measured pulse [6] FCC 02– 48: ‘FCC first report and order: in the matter
amplitude is 2.54 V and FWHM pulse width is 159 ps. of revision of part 15 of the commission’s rules regarding
For the proposed pulse generator, the pulse amplitude is ultra-wideband transmission systems’, 2002
3.67 V and FWHM pulse width is 211 ps. The current in
the inductor before ‘snap’ is 75 mA. The predicted [7] RAMBABU K., TAN A.E.-C., CHAN K.K.-M., CHIA M.Y.-W.: ‘Estimation
amplitudes for the simulator are slightly more than the of antenna effect on ultra-wideband pulse shape in
measured because of sharp rise/fall times of 0 ns for the transmission and reception’, IEEE Trans. EMC., 2009, 51,
square wave generator. Pulse widths and ringing levels are (3), pp. 604– 610

IET Microw. Antennas Propag., 2010, Vol. 4, Iss. 12, pp. 2196 – 2199 2199
doi: 10.1049/iet-map.2010.0030 & The Institution of Engineering and Technology 2010