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Ultra-Wideband Pulse Generator with Cascode Impulse Shaping
Circuit
P.Pongsoon#\ K.Kaemarungsi*2, S.Siwamogsatham*3 and
D.Bunnjaweht#4
# Department of Electrical and Computer Engineering, Thammasat
University, Thailand '[email protected]
'[email protected] *
National Electronics and Computer Technology Center (NECTEC),
Thailand [email protected]
[email protected]
Abstract-U1tra-wideband (UWB) pulse generator circuit is said to
be the heart of any UWB systems. It is the fundamental circuit for
application and development of UWB technology. This paper presents
an UWB pulse generator with cascode transistors operating as a
pulse shaping circuit to generate Gaussian pulses. In this work,
the UWB pulse generator consists of three parts: an edge triggered
driver, an edge sharpener SRD, and a cascode for impulse shaping.
The proposed circuit allows the generator to create short and high
amplitude Gaussian-like pulses at low-cost. Our measured UWB pulse
was 1.0 ns pulse with 2.8 Vpeak.
I. INTRODUCTION Ultra-wideband (UWB) technology has recently
received
more interest due to a number of advantageous properties such as
high multipath resolution and material penetration [I]. Moreover,
the signal is transmitted as a pulse train with low duty cycle
which offers low average transmission power in the order of
microwatts. These characteristics make UWB systems attractive to
military, medical, communications, and ground penetrating radar
(GPR) applications.
In 2002, the US Federal Communications Commission (FCC) defined
a UWB system as a system that radiates a signal with a fractional
bandwidth greater than 20% or signals with an absolute bandwidth
greater than 500 MHz [2]. The FCC also imposed a set of rules of
spectral masks for maximum allowable transmission power for a
number of UWB applications. These spectral masks created challenges
in signal generation and transmission of UWB system in order to
avoid its potential interferences with existing communication
systems. Within those FCC's rules, there are a number of law
enforcement and security applications which allows UWB systems to
operate at frequencies below 960 MHz. The focus of this work is
aimed at a study of low cost and high performance signal generator
for UWB systems in this frequency band.
Traditionally, a low-power UWB pulse generator is a baseband
circuit which utilizes a technique of storing energy in a device
and releasing an abrupt energy pulse at a precise timing [3]. The
key semiconductor components which are often found in circuits for
UWB pulse generators are step recovery diodes (SRDs), avalanche
transistors and MESFETs. In the literature, SRD based pulse
generators were presented in [4] and [5]. A number of modifications
were made to the
978-1-4244-6908-6/10/$26.00 2010 IEEE
basic circuit to improve properties of UWB pulse such as in [6].
This paper proposed an application of cascode circuit to a
conventional UWB pulse generator circuit and studied the new
response of pulse generator circuit.
Generally, UWB pulse shaping can be achieved using a number of
devices and techniques such as GaAs, MESFETs, nonlinear
transmission lines, short-circuit stubs and resistivereactive
circuits [5]. However these pulse shaping techniques often have low
output amplitude. Therefore, in this paper we present a development
of an impulse shaping circuit using bipolar junction transistors in
cascode configuration. The proposed UWB pulse generator with
cascode impulse shaping circuit consists of three parts: an edge
triggered driver, a SRD edge sharpener, and a cascode for impulse
shaping circuit. Fig.1 illustrates a block diagram of the UWB pulse
generator where the cascode circuit is placed in the second part of
the diagram.
,-------:----,
4r T Fig. I Block diagram of the UWB Pulse Generator
II. CIRCUIT DESIGN AND DESCRIPTION Fig. 2 shows a schematic of
the UWB pulse generator used
in this work. It consists of three main parts: an edge triggered
driver, a SRD edge sharpener, and a cascode for impulse shaping
circuit. Note that shapes of signal at particular measurement
points are illustrated below the figure.
SRD Edge Sharpener
Coscode for Impulse Shaping Circuit
Edge Triggered Driver
1':01 A r;RlCR2' + 00' L ________ .: L ______
___________________ _________________ _
Fig. 2 Circuit of Ultra-Wide band pulse generator
A. Impulse Generator The impulse generator is the SRD sharpener
using a step
recovery diode (SRD). An important characteristic of this
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device is the transition time that determines the minimum
achievable pulse's transition [8]. Another key characteristic is
minority carrier lifetime of the SRD, which affects the energy
storage time of electric charge under reverse bias condition [8].
If a signal source is applied to the SRD pulse shaping circuit, the
rise time of the signal source should be less than minority carrier
lifetime of the SRD in order to obtain the maximum achievable pulse
amplitude [7].
When the SRD is forward-biased, the quantity of the storage
charge is controlled by DC bias and the charge is stored in the
intrinsic layer of the SRD. This allows the SRD to continue to
conduct current when the device is reversebiased. This reverse
conduction continues until the charge is swept out of the intrinsic
layer. When the charges are depleted, the diode abruptly stops
conducting and shuts off and the fall time of diode reverses which
is also equal to the rise time of the voltage on the load. Note
that this duration is also called the transition rise time. This
time is a function of diode design, circuit constraints, and diode
operating conditions [8].
B. Cascade Impulse Shaping Network The design of cascode impulse
shaping network is shown
in Fig.2. A cascode circuit is generally defined as a two-stage
amplifier consisting of a transistor in common-emitter (CE) mode
(transconductance amplifier) followed by a transistor in
common-base (CB) mode (current buffer) [12]. The key advantage of
the cascode configuration is that it is a wideband circuit [12].
Moreover, the circuit can reduce the Miller effect due to high
input-output isolation [12]. The common-emitter state is formed by
Qi and common-base state is realized by Q2 as depicted in Fig.
2.
First, the transistor Qi is used for clipping. Therefore, if the
peak-to-peak value of the input waveform drives the transistor into
a saturation region, a portion of the output waveform will be
clipped as shown at point B in Fig. 2. Typically, most transistor
circuits are kept operating in the active region so that the output
signal will not be distorted. However, in our case, the current
rather than the voltage of the waveform is of interest. When there
is a large signal excursion such as impulse waveform fed into the
cascade circuit, the transistor output current responds linearly to
the input current. However, the input-output voltage relationship
is non-linear [11].
Second, the transistor Q2 is used for clamping. In CB state of
this second transistor, we want to clamp the amplitude of the
positive input signal. The clamping effect of Q2 causes the
original signal to have only negative pulses as shown at point o in
Fig. 2. The usages of transistors for clipper and clamper purposes
were presented in [11].
In Fig. 2, the output pulse width and output amplitude are
controlled by resistor Ri. The resistor R2 and the capacitor C are
used as a high-pass filter for the transient response of the
cascode circuit. They also help reduce the ringing effect of the
signal. Analysis of mid-band frequency response of the cascode
impulse shaping network follows [12]. When using the parameters Ri
= 1 kn, C =100 pF, and R2 =100 n in this paper the mid-band
frequency is approximately 920 MHz.
III. EXPERIMENTAL SETUP We fabricated a prototype of the
proposed UWB pulse
generator circuit using a FR4 glass epoxy substrate with
relative dielectric constant of 4.4 and thickness of 0.8 mm. Fig. 3
shows a photograph of the printed circuit board of the UWB pulse
generator. Two main components used on the board were a step
recovery diode (SRD) and two wideband bipolar junction transistors
(BJTs). The SRD, MA44769 from MlACOM Technology, was in SOT23
package [13]. This SRD has pulse's transition time of 150 ps and
minority carrier life time of 20-50 ns. The two BJTs forming the
cascode circuit were BFG520 manufactured by NXP Semiconductors and
were also in SOT23 package. This BJT has a 9-GHz bandwidth
[14].
Fig. 3 Photograph of UWB pulse generator
To study responses of the circuit according to various input
signals, we measured the output signals based on the input signals
with parameters given in Table I. To drive the input of the
circuit, we utilized an Agilent 33250A which is an 80-MHz Function!
Arbitrary waveform generator to feed a train of pulse with
different characteristics according to Table I. Note that we used
the pulse function to generate different input signals with varied
edge time and pulse width. At the output, we used Tektronix
TDS5104B which is a 1 GHzJ5 GSample/sec Digital Phosphor
Oscilloscope to measure the output Gaussian monocycle waveform.
TABLE I INPUT PARAMETER SETUP
Amplitude Edge time Pulse Width (Vp-p) (os) (os)
1 5 10 2 10 30 3 15 50 4 20 70 5 25 100
In this work, the input parameters that we are interested in are
input pulse repetition frequency, edge (rise/fall) time, pulse
width, and amplitude. The output parameters of interest are the
output edge (rise, fall) time, linearity, output pulse width, and
amplitude.
IV. MEASUREMENT RESULTS AND DISCUSSION In this section, we
describe our measurement results and
discuss the implication of our proposed circuit.
A. Gaussian Pulse at Output Fig.4 shows the result of a Gaussian
pulse measured at the
output of our proposed UWB pulse generator. The generator
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produced 1.0-ns pulse duration and approximately 2.8 Vpeak (3.56
Vpeak-to-peak). This UWB pulse had a center frequency bandwidth of
approximately 1.0 GHz which was calculated by !c=lIT. Tek Stopped
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FaU(Cl)1 709.1ps II: 706.23921n m: 766.4p M: 1.034n 0": 39.33p
n: 34.74k
_ ft ,.. Rise(Cl)! 1.094ns &........,+-fl__+t-
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Microelectronics [16] can be used to reduce the rise time of
input square wave to less than 2 ns in our preliminary
experiment.
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16 --.r- 3MHz --e--- 5MHz 14
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