IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 61, NO. 1, JANUARY 2012 261 A High-Performance Low-Ringing Ultrawideband Monocycle Pulse Generator Tian Xia, Senior Member, IEEE, Anbu Selvam Venkatachalam, Student Member, IEEE, and Dryver Huston Abstract—In this paper, a performance-enhanced ultrawide- band monocycle pulse generator circuit using a step recovery diode is designed and tested. Factors that could cause pulse wave- form distortions are analyzed. By applying an attenuator and a simple yet effective pulse-shaping circuit, the impedance matching between the pulse generator and the oscillator source is consider- ably improved, and a quality monocycle pulse is generated with small ringing and leveraged amplitude. Measured results validate the design. Index Terms—Gaussian monocycle pulse, pulse generator, ripple, step recovery diode (SRD), ultrawideband. I. I NTRODUCTION U LTRAWIDEBAND impulse radar has emerged as a valu- able sensing technology. By transmitting high-intensity pulses and analyzing the reflection signals, the impulse radar can identify and characterize subsurface and hidden objects [1], [2], [11], [13]. To achieve a reliable sensing performance, it is necessary to develop a pulse generator circuit that can produce quality pulse signals with high amplitude and low levels of ringing. Most impulse radar systems adopt Gaussian or monocycle pulses to achieve wideband sensing spectra and superior timing resolutions. Compared with a Gaussian pulse, a monocycle pulse has the advantage of containing less power in the dc and low-frequency bands that cannot be radiated through antenna. Therefore, an impulse radar system using monocycle pulses can achieve higher power efficiency. There are various pulse generator designs. In [8]–[10], customized complementary metal–oxide–semiconductor inte- grated circuits are developed to attain flexibility in controlling the pulse shape and interfacing with other on-chip digital and radio-frequency components. However, the main issues for the customized chip designs are long development time and high design cost that are not suitable for most low-volume application-specific radar systems. Therefore, designs using off-the-shelf components become attractive due to a lower development cost and a shorter development time. Among many off-the-shelf designs, the step recovery diode (SRD) has been used as a key device [3]–[7] due to its unique Manuscript received December 29, 2010; revised May 26, 2011; accepted June 8, 2011. Date of publication July 22, 2011; date of current version December 8, 2011. The Associate Editor coordinating the review process for this paper was Dr. V. R. Singh. The authors are with the School of Engineering, University of Vermont, Burlington, VT 05405 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2011.2161022 characteristics that can be used to sharpen pulse transition edge. In [3] and [4], shunt-connected SRDs are adopted to produce subnanosecond-wide Gaussian pulses. In [5], an SRD is con- nected in series with a pulse-shaping network that employs two Schottky diodes, and then, a shunt filter produces a monocycle pulse. However, experimental results illustrate that the pulse signal generated with the first method has a high level of ring- ing. The monocycle pulse produced from the second method (see [5]) can achieve low-level ringing and good symmetry. Nevertheless, the main drawback is the relatively small signal amplitude, i.e., the peak-to-peak pulse amplitude is slightly above 400 mV. In [6] and [7], monocycle pulses are generated using an SRD and short-circuited stubs by combining two opposite-phase Gaussian pulses. The downsides of this method are that the monocycle pulsewidth is large, which is equal to twice that of the single Gaussian pulse, and the monocycle pulse amplitude can be only as high as, or even lower than that of the incident Gaussian pulse. In [12], the SRD monocycle pulse generator employs a pair of resistive loaded stubs and a pair of short-circuited stubs to suppress the ringing tail of the im- pulse waveform. The obtained pulse peak-to-peak amplitude is 550 mV, the pulsewidth is about 320 ps, and the ringing level is below -22 dB. In this paper, we first analyze the factors in an SRD pulse generator that could degrade pulse signal integrity to cause large ringing and amplitude reduction. Correspondingly, simple yet effective solutions that can leverage circuit performance are proposed. Experimental circuits are then implemented for design validations. II. ANALYSIS OF PULSE DISTORTION FACTORS An SRD is a highly nonlinear device. When it is used in a pulse generator design, its nonlinear characteristic can affect pulse generation quality and cause waveform distortions. A. Impedance Mismatch Fig. 1 shows the circuit model of an SRD, where Cp, Ls, and Rs represent the parasitic package capacitance, inductance, and series resistance, respectively. D1 is the p-n diode, and Cj represents the junction capacitance. Cj is the dynamic capacitance whose value changes upon SRD bias voltage Vs. When SRD is in a forward-bias state, ma- jority carriers (electrons on the n-side and holes on the p-side) are injected across the junction and become minority carriers (electrons on the p-side and holes on the n-side) in the intrinsic i-layer. With the maximum amount of minority carriers is 0018-9456/$26.00 © 2011 IEEE