Download High Voltage High Efficiency UWB Pulse Generator for Precision

High Voltage High Efficiency UWB Pulse Generator
for Precision Localization Wireless Sensor Network
M. Arif Hussain Ansari and Choi Look Law
School of Electrical and Electronic Engineering
Nanyang Technological University, Singapore
[email protected], [email protected]
Abstract—A high voltage, high efficiency ultra-wideband
(UWB) Gaussian pulse generator for the precision localization of
mobile objects in wireless sensor network system is presented. The
proposed microwave monolithic integrated circuit (mmic) for the
UWB pulse generator has peak output power of 25.7dBm with 0.9
mW total power consumption at 0.5 MHz pulse repetition
frequency. The proposed pulse generator efficiency is 19.9%. The
high output power is achieved by overcoming the breakdown
phenomena of the transistor for higher output voltage by using two
transistors in series. The 2μm HBT ADS model is used for the
circuit simulation. The power spectrum of the generated pulse
fully utilizes the FCC peak power limit spectrum mask. The
generated UWB pulse is centered at 4.2 GHz with peak-to-peak
voltage of 12.2 V into a 50 ohms load. The generated pulse envelop
has full width at half maxima of 0.85 nsec.
Keywords— Ultra-wideband impulse radio, UWB pulse
generator, UWB pulse former, sub-nanosecond impulse generator,
breakdown voltage of transistor.
I.
INTRODUCTION
The forthcoming decade is going to be the era of wireless
networking system due to its remarkable development and
advances [1]-[3]. One of the important examples of the wireless
networking system is wireless sensor network system (WSNs).
In WSNs, a number of wireless nodes with sensor are placed
over the network. They exchange their information among them
and may rely on each other to relay their information to a
gateway often over multiple hops. The processing of data from
distributed sensors can solve several problems. The wireless
node with the sensor in the WSNs requires longer battery life,
lower cost and should be smaller in size. In order to meet the
above-mentioned requirements, the low power Ultra-WideBand Impulse Radio (UWB-IR) technology is one of the most
prominent choices [4]-[5].
The Ultra-wide-band impulse radio (UWB-IR) technology
has various inherent properties that show its strong candidature
in the sensor network application. In addition to the low cost and
longer battery life, the UWB-IR enables ranging using the pulse
communication like radar [6],[7]. The fine timing resolution of
UWB-IR system provides for the precise locating and tracking
of moving objects in indoor cluttered environments. Due to this
amazing property of UWB technology, it can be applicable in
various industries like asset tracking and management,
healthcare, factory automation, real-time monitoring of
highways, bridge and other civil infrastructure [3],[8]-[9], etc.
The average power level of transmitted signal using UWB-IR
transmitter is very low like noise, which hardly interfere with
other signal. In the United States, the Federal Communication
The research was partially supported by the ST Engineering – NTU
Corporate Lab through the NRF corporate lab@university scheme Project
Reference C-RP10B
Commission (FCC) authorizes the use of UWB under
unlicensed spectrum driving the development of commercial
UWB based applications [10].
Low Duty
Cycle Clock
Pulse
Generator
Subnanosecond
Impulse
Generator
Ultra-WideBand Pulse
Former
Pulse Generator
Fig-1. Block diagram of UWB-IR Generator.
The block diagram representation of typical UWB-IR
generator shown in Fig-1. The UWB-IR generator consists of
low duty cycle clock generator, a sub-nanosecond impulse
generator, UWB pulse former and UWB antenna. The impulse
generator and pulse former together called pulse generator. In
these components, the pulse generator is the key component of
the system because shape of the pulse determines the frequency
characteristic of transmitting signal. It is necessary to generate
the pulse, which must satisfy the FCC spectrum mask regulation.
Under the FCC new regulation, the peak power of unlicensed
UWB emitter should not cross 0 dBm in a 50 MHz bandwidth
and the average power limit is -41.3 dBm/MHz [11]. The peak
output voltage of generated UWB pulse should be as large as
possible and pulse width as short as possible. The high output
voltage pulse can penetrate more, wide the range of localization
and tracking and short pulse width enhance the bandwidth.
There are mainly five categories of UWB pulse generation
method, namely 1) up-conversion – generate the base-band
signal and up-convert to the target frequency [12]. 2) Spectrum
filtering – generates the ultra-wide-band signal and use a filter
to shape the spectrum to required frequency range [13]. 3) Edge
combining – delay and combine the rising and falling edge of
digital pulse [14]. 4) Combine different delay edge to form short
digital pulse and use a filter to get UWB pulse [15]. 5) First,
generate a sub-nanosecond Gaussian impulse then delayed and
weighted by distributed network (pulse former) to get the UWB
pulse [16]-[17]. So far, many UWB pulse generator are reported
with different technology [7], [12]-[17]. Table-I indicates the
comparative study of peak-to-peak voltage, pulse width (Full
width at half maxima (FWHM)) and efficiency of pulse
generator with different technology. CMOS technology is good
for the narrow pulse width but the peak output voltage is limited.
HBT technology seems like an appropriate technology to get the
higher output voltage with higher efficiency and optimum pulse
width.
TABLE I.
PERFORMANCE COMPARISION ON UWB PULSE GENERATOR
Peak-to-peak
amplitude(V)
Efficiency
(η)
Pulse
width(ns)
Reference
0.13μm CMOS
0.2
0.05%
0.6
[15]
0.18μm BiCMOS
0.26
---
1.5
[12]
0.18μm CMOS
0.68
0.94%
0.25
[14]
0.25μm pHEMT
4.5
10%
1.0
[16]
[17]
Technology
0.25μm pHEMT
6.4
13%
1.0
2μm HBT
7.0
10.4%
1.1
[7]
2μm HBT
12.2
19.9%
0.85
This work
The impulse generation takes two-step, in the first step, the
low duty cycle clock turns on the base-emitter junction of Q1 in
forward bias. The space charge is getting accumulate in the baseemitter junction of Q1 due to minority carrier injection during
the turn on time. In the second step, at the falling edge of the
clock pulse, the removal of charge takes place, which delays the
turn-off process of the transistor. After the removal of all
charges, the base current quickly goes back to zero. The speed
up in base current reflects as collector current, which is
differentiated by the square inductor to convert it into voltage.
The amplitude and the width of the pulse depends on the
inductance value of differentiating inductor, as shown in Fig-3.
In this paper, firstly; an optimized impulse generator circuit
is presented to get 7V sub-nanosecond Gaussian impulse with
~90 psec pulse width. The generated impulse then feed to the
proposed pulse former network to get high voltage UWB pulse.
The breakdown phenomenon of Base-Collector junction at
higher output voltage has been resolved. The proposed method
successfully achieved UWB pulse of 12.2V peak-to-peak output
voltage with percentage efficiency of 19.9%. The corresponding
peak output power is 25.7dBm.
This paper is organized as follow, section II covers the
Analysis of impulse generator and pulse forming network.
Section III discusses the simulation results of the proposed pulse
generator, and lastly conclusion is presented in section IV.
II. CIRCUIT ANALYSIS AND DESIGN
A. Impulse Generator
Vc1
C1
LS
Q2
R2
C3
Vimpulse
C2
R1
Q1
Low duty cycle
digital data pulse
Fig-2. Schematic of sub-nanosecond impulse generator.
The schematic of sub-nanosecond impulse generator is
shown in Fig-2. The cascode pair of HBT (Q1 and Q2) is used for
the higher gain and large output impedance [18]. C1 and C2 are
the RF grounding capacitor. Ls is the on-chip square inductor
and Vc1 is the biasing voltage of impulse generator.
Fig-3. Variations of peak amplitude (black line) and pulse width (red
line) of impulse voltage with the variation of inductance (Ls).
It can be observe from Fig-3 that initially, the peak amplitude
of the impulse is increasing with the inductance (Ls) and then
saturate around 7V corresponding to supply voltage 3V, whereas
the pulse width (FWHM) is increasing linearly with inductance
(Ls). In order to get optimized output pulse, a trade-off between
amplitude and pulse width is required.
B. Pulse Former
A pulse former is used to convert the impulse into UWB
pulse. The pulse former concept has been used in [16]-[17] using
pHEMT and [7] using HBT. The pulse former output voltage is
limited by the B-C junction breakdown voltage. To rectify this
issue, two transistor is used which share the total output voltage.
The transistor QAn is biased in class C mode to reduce the power
consumption. It will conduct current only when the input
impulse voltage arrives at the base terminals. The base of
transistor QBn is biased with Vc1 (Impulse generator biasing
voltage) but it will only turn ON if the transistor QAn is
conducting. The base voltage of QBn is fixed and positive (never
goes zero or negative) so the range of output voltage without
breakdown is increased. In the case of one transistor, the base
voltage can reach zero and can go negative as well, so the B-C
junction breakdown at lower output voltage as compare to
proposed circuit.
Vc2
L
Cs1
Cs2
τ2
Cs3
R1
Vc1
C1
R01
R2
QB1
C2
R02
QA1
C4
R04
QA2
Cc2
Cc1
Vimpulse
QB2
Vpulse(UWB)
QB4
QA4
Cc4
τ1
τ1
R4
τ1
RL = 50 Ω
Fig-5. High voltage sub-nanosecond impulse voltage signal
Fig-4. Circuit diagram of improved UWB pulse former
The schematic diagram of the proposed pulse former is
shown in Fig-4. Four stages of transistor pair are used to
generate the four different negative amplitude impulses. The
input impulse at every stage of the transistor is delay by
τ1 (τ1 = τP; τP is the total impulse width). Realization of time
delay is done by off-chip strip-line. Τo generate the positive
amplitude impulse, a short-circuited stub with time delay τ2
(τ2 =τ1/2) is used. The short-circuited stub is connected to the
output of transistor pairs, which converts the negative amplitude
impulse into positive amplitude impulse. At the output stage, the
negative and positive impulse added together to form a UWB
pulse. To get the Gaussian shape UWB pulse, an amplification
factor of all the four stages must be define. To set the
amplification factor of each stage, the input coupling
capacitance “Ccn” (1 ≤ n ≤ 4) of the transistor is to be calculated
by the formula (1) given below [7].
A( n +1)
An
Cc( n +1)
= exp( −α l )
Ccn
Cc( n +1) + Cπ
Fig-6. High voltage ultra-wide-band pulse
2
V peak
* teff * PRF
Efficiency (η ) = 2 R
DC _ power
(1)
Ccn + Cπ
An is the amplification factor, α is the attenuation constant in
the effective medium of strip-line and l is the length of the stripline. Ccn is the input coupling capacitance of the transistor. Cπ is
the input capacitance of the transistor. The effective base
capacitance of the HBT transistor is given by the series
combination of Ccn and Cπ.
TABLE II.
(2)
TOTAL DC POWER CONSUMPTION IN PULSE GENERATOR
Impulse
Generator
Pulse Former
Total Power
(µW)
Voltage (V)
3.0
10
Current (µA)
95
65
935
III. SIMULATIONS AND RESULTS
The Advanced Design System (ADS–2016.01) is used for
the simulations of proposed circuit. The ADS model of 2μm
HBT process is used for the circuit simulation. WIN
semiconductor provided the ADS model of 2μm HBT process
with beta (β) value 75 and common emitter breakdown voltage
(BVCEO) 15V. The biasing voltage Vc1 = 3Vand Vc2 = 10V are
chosen after several simulations and optimization. The low duty
cycle clock for the input of impulse generator is realized by the
“Vtpulse” voltage source of ADS library.
The output of impulse generator after optimization is shown
in Fig-5. The impulse has peak value ~7V with full width at half
maxima (FWHM) ~90 psec. The output of the pulse generator
with 50Ω terminating impedance is shown in Fig-6. The
Gaussian UWB pulse, peak to peak amplitude is recorded 12.2V
and the FWHM (teff) is 0.85nsec.
Fig-7. Power spectral density plot of generated UWB pulse with FCC
peak power and average power spectrum mask.
Table II shows the power consumption of impulse generator
and pulse forming network separately whereas the total power
consumption of the pulse generator is 0.9 mW. The peak power
output is 25.7 dBm at the PRF of 0.5MHz. The efficiency of the
proposed pulse generator is 19.9% when evaluated using the
formulae given in (2). Fig-7 shows the power spectral density
plot of the generated UWB signal with the FCC peak power and
the average power spectrum mask. The solid blue line showing
the FCC peak power limit whereas the dash blue line represents
the FCC average power limit. The peak power plot of generated
UWB pulse with the 50 MHz bandwidth (dBm/50MHz) is
depicted in Fig-7 with solid black line. The average power plot
(dBm/MHz) of generated UWB pulse depicted with dash black
line. From Fig-7, it can be observed that generated UWB pulse
is under the FCC peak and average power limit. The center
frequency of generated UWB is 4.25 GHz and the 10dB
bandwidth is 2GHz. At the GPS band (0.96 to 1.61 GHz), the
power spectral density crosses the limit of spectrum mask
provided by the FCC. To overcome this issue, high pass filter at
the output terminal of pulse former can be easily integrated to
suppress the power level in the GPS band.
IV.
CONCLUSIONS
An integrated high output voltage and high efficiency UWB
pulse generator is presented for the precision localization in the
wireless sensor network system. First, a sub-nanosecond
impulse generator has been optimized to get the high voltage
impulse. After that, the UWB pulse former has been designed so
that it can resolve the breakdown issue of the B-C junction to get
the higher output power. The simulation of the proposed pulse
generator has been carried out in the ADS-2016.01 circuit
simulator. The generated output pulse has peak to peak voltage
of 12.2V with the FWHM 0.85 nsec into a 50 ohms load. The
output power of pulse generator is 25.68 dBm at 0.5 MHz PRF.
The power consumption from both the sources is 0.9 mW and
the calculated efficiency of the proposed pulse generator is
19.9%.
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