Design of Pulse Generator in 180nm Technology for GPR Applications

Design of Pulse Generator in 180nm Technology for GPR Applications
Design of Pulse Generator in 180nm
Technology for GPR Applications
A Thesis submitted in partial fulfillment of the Requirements for the degree of
Master of Technology
In
Electronics and Communication Engineering
Specialization: VLSI Design & Embedded System
By
Jitendra Kumar Mahanty
Roll No. : 212EC2137
Department of Electronics and Communication Engineering
National Institute of Technology Rourkela
Rourkela, Odisha, 769 008, India
May 2014
Design of Pulse Generator in 180nm
Technology for GPR Applications
A Thesis submitted in partial fulfillment of the Requirements for the degree of
Master of Technology
In
Electronics and Communication Engineering
Specialization: VLSI Design & Embedded System
By
Jitendra Kumar Mahanty
Roll No. : 212EC2137
Under the Guidance of
Prof. Subrata Maiti
Department of Electronics and Communication Engineering
National Institute of Technology Rourkela
Rourkela, Odisha, 769 008, India
May 2014
Dedicated to…
My Parents, Brother and
My Dear Friends
DEPT. OF ELECTRONICS AND COMMUNICATION ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA
ROURKELA – 769008, ODISHA, INDIA
Certificate
This is to certify that the work in the thesis entitled Design of Pulse Generator in
180nm Technology for GPR Applications
by Jitendra Kumar Mahanty is a record
of an original research work carried out by him during the year 2013 - 2014 under my
supervision and guidance in partial fulfillment of the requirements for the award of the
degree of Master of Technology in Electronics and Communication Engineering (VLSI
Design & Embedded System), National Institute of Technology, Rourkela. Neither this
thesis nor any part of it, to the best of my knowledge and belief, has been submitted for any
degree or diploma elsewhere.
Place:
Date:
`
Prof. S. Maiti
Dept. of ECE
NIT, Rourkela
Odisha-769008
i
ABSTRACT
In this work, we present a low-complexity and low cost pulse generator in 180nm
technology for ground penetrating ultra-wideband (UWB) radar system applications. Here I
have implemented an UWB pulse generator circuit. A UWB pulse generator is a method
introduced in communication system to simplify the data transmission and remove
disadvantages that occurs in other systems. This generator generates a Gaussian pulse for a
small period of time of the order of some nanoseconds. As UWB pulses are generated for a
short time, hence no carrier signal is required to send a base band or message signal. So
power loss due to carrier signal doesn’t exist at all. These pulses are very high in
frequency; hence it has very less chance to be got affected by noise.
This pulse generator uses a delay generator along with a Gilbert XOR cell for generating a
Gaussian pulse which can be shaped by using a FIR filter, and finally a Gaussian mono
cycle pulse is observed at the output which has a pulse width of 97ps thereby give rise to a
bandwidth of 10.3 GHz which meet the FCC requirements.
The pulse generator comprises of three cascaded delay blocks, a XOR block, and a FIR
filter. The interpolation delay blocks uses voltage for adjusting the delay time by the
control of the gains of each path. By adjusting the delay time, pulse generator can achieve
the required frequency. The XOR gate is implemented using a Gilbert cell. When the two
signals given as input have opposite voltage levels at a given time, the XOR gate creates a
pulse. After the XOR gate, a Gaussian pulse is generated and then it goes through the FIR
filter to shape it to a Gaussian mono cycle pulse. The design and simulation of the pulse
generator was performed using the Cadence UMC tool in 180nm CMOS process.
ii
CONTENTS
ACKNOWLEDGEMENTS .............................................................................................. I
ABSTRACT .................................................................................................................. II
CONTENTS ................................................................................................................. III
LIST OF FIGURES ..................................................................................................... VI
LIST OF TABLES ...................................................................................................... VII
1 INTRODUCTION ...................................................................................................... 1
1.1
Motivation .................................................................................................................. 2
1.2
Literature Review ...................................................................................................... 3
1.3
Overview of Thesis..................................................................................................... 4
2 GROUND PENETRATING RADAR ......................................................................... 5
2.1
Introduction ............................................................................................................... 6
2.2
History ........................................................................................................................ 7
2.3
Applications ................................................................................................................ 9
2.4
Principle .................................................................................................................... 10
2.5
Classification ............................................................................................................ 11
2.6.
Description of a Time-Domain GPR ...................................................................... 13
2.6.1 Transmitter ............................................................................................................. 13
2.6.2 Receiver .................................................................................................................. 13
2.6.3 Timing Circuit ........................................................................................................ 15
2.7
iii
Conclusion ................................................................................................................ 15
3 UWB COMMUNICATION SYSTEMS ..................................................................... 16
3.1
Definition of UWB ................................................................................................... 17
3.2
FCC Regulation ....................................................................................................... 18
3.3
Advantages ............................................................................................................... 20
3.4
Applications .............................................................................................................. 21
3.5
Conclusion ................................................................................................................ 21
4 PULSE GENERATOR ............................................................................................ 22
4.1
Gaussian Pulse Types .............................................................................................. 23
4.2
Pulse Modulation ..................................................................................................... 25
4.3
Pulse Generation ...................................................................................................... 25
4.4
MOS Current Mode Logic (MCML) ..................................................................... 26
4.5
Gilbert XOR Cell ..................................................................................................... 28
4.6
Conclusion ................................................................................................................ 28
5 FIR FILTER............................................................................................................. 29
5.1
Introduction ............................................................................................................. 30
5.2
Architecture ............................................................................................................. 31
5.3
Signed Digit Number System .................................................................................. 32
5.4
Redundant Number Systems (RNS) ...................................................................... 33
5.5
PPM Adder (Carry-Free Radix-2 Addition) ......................................................... 33
iv
5.6
MMP Subtractor (Radix-2 Subtraction) ............................................................... 35
5.7
Digit Serial SBD redundant Adder ........................................................................ 37
5.8
Conclusion ................................................................................................................ 37
6 DESIGN, IMPLEMENTATION AND SIMULATION OF PULSE GENERATOR ..... 38
6.1
Delay Generator ....................................................................................................... 39
6.2
Gilbert XOR cell ...................................................................................................... 40
6.3
FIR Filter .................................................................................................................. 41
6.3.1 PPM Adder ............................................................................................................. 41
6.3.2 MMP Subtractor ..................................................................................................... 42
6.3.3 D Flip-Flop ............................................................................................................. 43
6.3.4 SBD Adder ............................................................................................................. 43
6.3.5 Proposed FIR filter ................................................................................................. 44
6.4
Proposed Pulse Generator ...................................................................................... 45
7 CONCLUSION AND FUTURE WORK ................................................................... 46
REFERENCES ........................................................................................................... 48
v
LIST OF FIGURES
Fig. 2.1 GPR Block Diagram .......................................................................................................................... 10
Fig. 2.2 Typical signal received by GPR ......................................................................................................... 11
Fig. 2.3 Block Diagram of Time-Domain GPR ............................................................................................... 13
Fig. 2.4 Block Diagram of Receiver ................................................................................................................ 14
Fig. 3.1 Frequency spectrum analysis on Bandwidth and Center frequencyt ................................................. 17
Fig. 3.2 Comparision on BW between UWB and NB systems ......................................................................... 17
Fig. 3.3 FCC part 15 for high frequency devicesr .......................................................................................... 18
Fig. 3.4 FCC spectral mask for UWB systems for indoor applications .......................................................... 19
Fig. 3.5 FCC spectral mask for UWB systems for outdoor applications ........................................................ 19
Fig. 4.1 Standard Gaussian pulse type and its PSD ........................................................................................ 25
Fig. 4.2 Gaussian mono cycle pulse type and its PSD ................................................................................... 26
Fig. 4.3 Gaussian doublet pulse type and its PSD .......................................................................................... 26
Fig. 4.4 Pulse generator block diagraml......................................................................................................... 28
Fig. 4.5 Basic MCML inverter/buffer circuit .................................................................................................. 30
Fig. 4.6 Basic Gilbert XOR cell....................................................................................................................... 32
Fig. 5.1 FIR Filter ........................................................................................................................................... 35
Fig. 5.2 Datapath ............................................................................................................................................ 35
Fig. 5.3 Pipelining ........................................................................................................................................... 35
Fig. 5.4 Transposed FIR filter ......................................................................................................................... 36
Fig. 5.5 PPM adder………………………………………………………………………………………………………38
Fig. 5.6 LSD PPM adder ................................................................................................................................. 38
Fig. 5.7 MMP subtractor ................................................................................................................................. 39
Fig. 5.8 LSD MMP subtractor ......................................................................................................................... 40
Fig. 5.9 SBD adder…………………………………………………………………..…………………………………..41
Fig. 6.1 Schematic of Delay Generator ........................................................................................................... 43
Fig. 6.2 Output of dalay generator .................................................................................................................. 44
Fig. 6.3 Schematic of Gilbert XOR cell .......................................................................................................... 44
Fig. 6.4 Gaussian pulse output of Gilbert XOR cell ....................................................................................... 45
Fig. 6.5 Schematic of PPM adder ................................................................................................................... 46
Fig. 6.6 Schematic of MMP subtractor ........................................................................................................... 46
Fig. 6.7 Schematic of D flip-flop ..................................................................................................................... 47
Fig. 6.8 Schematic of SBD adder .................................................................................................................... 48
Fig. 6.9 Schematic of FIR filter ....................................................................................................................... 48
Fig. 6.10 Schematic of proposed pulse Generator .......................................................................................... 49
Fig. 6.11 Gaussian mono cycle output of the proposed pulse generator ........................................................ 49
vi
LIST OF TABLES
Table 4-1: Comparison between OFDM and IR .............................................................................................. 27
Table 4-2: Comparison between CMOS and MCML ....................................................................................... 31
Table 5-1: Digit sets in addition of PPM adder ............................................................................................... 39
Table 5-1: Digit sets in addition of MMP subtractor ....................................................................................... 40
vii
Design of Pulse Generator in 180nm Technology for GPR Applications
1
INTRODUCTION
1
Design of Pulse Generator in 180nm Technology for GPR Applications
Recently, UWB technology is better option to replace the narrowband wireless
technologies. UWB transmitters operate with lower power ultra short pulses. UWB defines
transmission systems having spectral bandwidth over 500 MHz or having a fractional
bandwidth of more than 20% to 25%. This huge bandwidth provides a better opportunity
for number of lower power applications in UWB communication systems, wireless
networking and also in imaging of radar. These types of system depend on ultra-short
pulses and can be free of carriers. One of the technologies of UWB is the Pulse Generator.
Pulse Generator, modulating data in the time domain and improves data throughput with
lowest power consumption. A sinusoidal carrier signal is not required in a pulse generator
to move the generated signal to a frequency higher than the frequency of carrier signal, but
it can communicate with the message signal composed of ultra short pulses.
A series of pulses is used in UWB system by the method of PAM or PPM. It transmits
ultra-short pulses to make the frequency spectrum of the generated signal to several GHz
wider which leads to lower multipath fading because of the short duration pulse with a
wide fractional bandwidth. UWB system provides an excellent immunity to interference
from other narrowband systems even in propagation environments. Implementing Pulse
generator UWB transceivers with mostly digital circuits using no intermediate frequency
(IF) processing makes it easier and cheaper compared with the typical spread spectrum
transceivers – as used in Bluetooth and Wi-Fi.
1.1 Motivation
In UWB systems, a pulse generator is one of the key components which enable the UWB
communications. The PSD of the Gaussian pulses generated by the simple Pulse Generator
is not meeting the requirements of FCC.
2
Design of Pulse Generator in 180nm Technology for GPR Applications
For UWB communication systems, the FCC allocated power level is in the band between
3.1 to 10.6 GHz which is very small, i.e. about -41.3 dBm. For meeting FCC regulations,
we can pass the Gaussian pulse generated by the pulse generator through a FIR filter which
shapes the Gaussian pulse to a Gaussian mono cycle pulse. Thus, we will focus on
designing a pulse generator which fits the power spectrum density (PSD) of the FCC
regulations in this paper.
1.2 Literature Review
• David J Daniels, in 1988, published a tutorial paper by giving a good technological
analysis of the GPR technology. He himself published his book on “Surface Penetrating
Radar” on the year 1996 which presents all the key elements of this technique for the nonspecialist users and engineers.
• Bart Scheers of Royal Military Academy from the Department of Electrical Engineering
and Telecommunication, Brussels, in March 2001 published a thesis on comparision of
different types of GPR systems and found GPR as one of the challenging technologies for
detection of buried objects of various types. He showed that the range resolution is directly
related to the bandwidth of the system which led to development of UWB GPR.
• Shin-Chih Chang of University of Texas at Arlington in December 2005 published a
thesis on UWB Pulse Generator producing higher derivative Gaussian Pulse for GPR
applications. He claimed that the higher the derivative of the Gaussian pulse, higher is the
chance to meet the FCC requirements of bandwidth between 3.1 to 10.6 GHz.
• Kevin M. Marsden of Virginia Polytechnic Institute and State University in December
2003 published a thesis which describes the Low Power CMOS Pulse Generator as a better
option for Ultra Wideband Radios. In his thesis he designed a rectified cosine generator by
modifying the pulse generator for lower power consumption.
3
Design of Pulse Generator in 180nm Technology for GPR Applications
• Prabhat Kumar Barik of National Institute of Technology, Rourkela in June 2011
published a thesis entitled “FIR Filter IC Design Using Redundant Binary Number
Systems”. In his thesis he designed a FIR filter using RNS algorithm, which comprising of
different blocks like SBD adder, PPM adder, MMP adder, D-FF etc.
1.3 Overview of Thesis
The UWB transmitter generates and transmits very short duration pulses as signals without
a carrier. The proposed pulse generator consists of a pulse generator along with an FIR
filter. Implementing the FIR filter on the Gaussian pulse from the pulse generator can form
the Gaussian mono cycle pulse which meets the requirement of the PSD for UWB systems
by maximizing bandwidth. In chapter 2, we briefly discuss about the Ground Penetrating
radar (GPR). The principle and applications of the UWB communication system is
discussed in chapter 3. The structure and function of the pulse generator is introduced in
chapter 4. In chapter 5, we focus on the pulse shaper FIR filter. The simulation result of
the proposed pulse generator is discussed in chapter 6. Finally, in chapter 7, the conclusion
and future work are introduced.
4
Design of Pulse Generator in 180nm Technology for GPR Applications
2
GROUND PENETRATING RADAR
5
Design of Pulse Generator in 180nm Technology for GPR Applications
This chapter gives an overview of Ground Penetrating Radar (GPR). In this chapter we
focus on the GPR history, applications, principle, and classification. In Section 2.5 we
discuss about the different types of GPR and did a comparative study on both time-domain
and freq-domain GPR. Finally, we ended up with a result that the time-domain GPR is
better than a freq-domain GPR as it doesn’t require a carrier to transmit a message signal.
Hence in the next Section 2.6 we describe the main components of a time-domain GPR;
like transmitter, receiver and timing circuit. In this thesis, we implemented a Pulse
Generator circuit for generating a Gaussian pulse and pass it through a FIR filter for
shaping the pulse to a mono cycle pulse which is the key component of the transmitter
block of a time-domain GPR.
2.1 Introduction
Ground Penetrating Radar belongs to the family of radar systems that image the
subsurface.GPR is also called as Surface Penetrating Radar (SPR). Nowadays, GPR is a
fast growing technology and the number of its applications is still growing. Locating pipes
and cables, civil engineering (bridge inspection, finding voids), security, archeology
investigation, geophysical surveys are few examples of its application. The operating
principle of the GPR is based on Maxwell’s equation of EM wave propagation in the
inhomogeneous sub surface medium. It sends EM waves into the ground and samples the
backscattered signals which are based on the electrical parameters. The special property of
GPR is that it can detect signals due to all three types of electrical parameters, i.e. electric
resistivity, magnetic permeability and conductivity. Hence a GPR system has the potential
for locating and identifying both metallic and non-metallic buried targets based on the
signal characteristics.
6
Design of Pulse Generator in 180nm Technology for GPR Applications
2.2 History
English scientist J. C. Maxwell proposed EM theory of light in the year 1865. In 1886, H.
R. Hertz proved Maxwell theory to be right, who discovered EM waves.
In 1904, Hulsmeyer first used the EM signal to determine the presence of metal objects by
measuring the travelling time of EM waves to a metal object like a ship. This is the first
practical radar testing and he also registers his invention in Germany and United Kingdom
patents. But six years later descriptions of their use for locating buried objects come into
sight in one German patent presented by Leimbic and Lowy. The technique consists of
dipole antennas in an array of vertical boreholes and comparing the amplitude of the
received signals when one pair of antenna is used to transmit and receive at the same time.
In this way, an image could be generated on any region within the array which with its
higher conductivity than the surrounding medium absorbs the radiation.
In 1917, the super heterodyne receiver was invented by the French Engineer Lucien Levy
and he uses the abbreviation of “Intermediate Frequency”. In 1921, the US American
physicist Albert Wallace Hull invented the magnetron as an efficient transmitting tube. In
1922, the electrical engineers A. H. Taylor and L. C. Young of the Naval Research Lab,
USA located a wooden ship for the first time. In 1926, Hulsenbeck used pulsed techniques
for determining the structure of buried targets. He noted that any dielectric variation would
also produce reflections and the pulsed technique uses an easier realization of directional
sources, which had advantages over seismic methods. In 1929, Stern does the first ever
GPR survey for determining the depth of Glacier. For the first time in the year 1930, an
aircraft is located by L. A. Hyland of Naval Research Laboratory in USA. Also in the same
year Cook, Roe and Eller Bruch investigated the probing of rock and coal.
7
Design of Pulse Generator in 180nm Technology for GPR Applications
In 1931, radar is mounted on a ship with some antennas which are used with some
parabolic dishes with horn radiators. In 1936, two electrical technicians named G. F.
Metcalf and W. C. Hahn developed klystron and uses it as an oscillator tube in radar for
amplification.
From 1930 onwards, Pulsed techniques were developed for determining the depths of ice,
fresh water, salt deposited in water, sands of desert and rock formations. Probing of rock as
well as coal was also investigated by Cook, Roe and Eller Bruch. In 1940, different
equipments of radar are developed in USA, France, Russia, Japan and Germany.
A researcher, Gerald Ross from Sperry Rand Corporation, in late1960’s introduced UWB
technologies and his first patent for UWB technologies was granted in the year 1973 and
this UWB technology was used in a GPR system in 1974, and became a KEY success for
Geophysical Survey Systems, Inc. (GSSI). In US, the Department of Defense starts using
the UWB technology to image through walls and into the ground. A more extended history
of GPR and its growth up to the mid-1970s is given by Nilsson.
In 1980, the 1st geotechnical applications are done in Denmark and Sweden and reports
showed that GPR was a very promising tool especially in surveys in peat areas. In 1988, a
tutorial paper with the title “Introduction to Subsurface Radar” was published by Daniels
which gives a good overview of the GPR technology. In the year 1996, a book entitled
Surface Penetrating Radar was published by D.J. Daniels. From now then the range of
applications is increasing very much day-to-day.
8
Design of Pulse Generator in 180nm Technology for GPR Applications
In the year 1998, FCC realized that the UWB technology has got the potential to
investigate the possibility of allow it for the commercial use. When FCC allocates a
defined spectrum for UWB system, the research and development on this field has taken
the pace on February 14, 2002. UWB has several advantages such as higher data rate, low
power radiation, best immunity to multipath, and simplicity in hardware compared to NB
systems. Many of these advantages are directly related to the huge bandwidth of a UWB
device.
2.3 Applications
GPR has been used in the following applications:

Archaeological investigations

Borehole inspection

Analysis of Bridge deck

Assessment in Building conditioning

Investigation of Contaminated lands

Anti-tank mines detection

Investigation of reinforced concrete

Investigation on Forensic data

Investigation of Geophysical data

Detection of cables and pipes

Medical imaging

Planetary investigation

Tunnel linings

Thickness (soil) determination, etc.
9
Design of Pulse Generator in 180nm Technology for GPR Applications
2.4 Principle
Now-a-days, GPR is one of the most capable technologies for close detection and
identification of buried Anti-Personnel (AP) Landmines, due to its ability of detecting nonmetallic objects in the sub-surface medium. The operating principle of Ground Penetrating
Radar is that it sends the EM signals into the ground and receives the backscattered signals.
The EM signal will be back scattered when any one of the electrical parameters change in
the ground. The property of the GPR is that it can detect signals from all three types of
electrical parameters like conductivity, resistivity and susceptibility.
This property of the GPR system has the potential for locating and identifying both
metallic and non-metallic buried targets on the echo characteristics. Fig.1.1 shows a block
diagram of a basic GPR system. In GPR the antennas are normally scanned over the
surface in close to the ground. An EM wave sent into the ground will backscatter on any
electrical parameter discontinuity. The backscattered echoes that reach the receiving
antenna are sampled and processed by a receiver. Figure 1.2 shows a typical time
representation of a signal, received by the GPR at a given fixed position.
Figure 2.1: GPR block diagram.
10
Design of Pulse Generator in 180nm Technology for GPR Applications
Figure 2.2: Typical signal received by GPR
Normally the first and the largest echo are due to the air-ground interface and other echoes,
appearing later in time are reflections on target or clutter present in the subsurface. The
GPR can produce two or three-dimensional images by moving the antennas on a line or a
two dimensional grid. The main difference between the GPR and metal detector is, the
GPR has the potential to detecting non-metallic targets also in the application of AP land
mine detection.
2.5 Classification
Ground Penetrating Radar can be classified as below:
• Time Domain GPR
1. amplitude modulated GPR
2. carrier free GPR
• Frequency Domain GPR
1. Linear Sweep GPR
2. Stepped Frequency GPR
11
Design of Pulse Generator in 180nm Technology for GPR Applications
The working principle of time domain GPR is that, a Gaussian pulse is sent through the
transmitter at a given PRF into the ground and the backscattered signals is received by the
receiver. A pulse is sent by the amplitude modulated GPR with a carrier frequency which
can be modulated by a square envelope. If the duration of the pulse is very short of the
order of some picoseconds, then the depth of resolution is good. So a mono cycle pulse is
used and central frequency of the mono cycle is equal to its 3dB BW. The needs for huge
bandwidth led to the development of carrier free GPR. In carrier free GPR a pulse is sent
without a carrier and the pulse width of the carrier signal is of the order of some
picoseconds. In this GPR we can use any type of pulse but generally a standard Gaussian
pulse is used. The carrier free GPR is also known as Ultra Wide Band GPR.
The principle of freq-domain GPR is that the freq. of the continuous signal is either
modulated with the help of a linear sweep or changes in fixed steps. In the linear sweep or
FMCW GPR system, the system continuously transmits a carrier frequency which is
changing by means of a Voltage Controlled Oscillator over a wide frequency range. The
frequency sweep is carried out due to a saw-tooth or a triangular signal within a certain
dwell time. The receiver receives the backscattered signal when the carrier signal emitted
by the transmitter strikes a buried object. The depth of the target determines the frequency
difference between the transmitted and received signal. The poor dynamic range of FMCW
radar is a major disadvantage for this type of systems. The FMCW radar simultaneously
receives the signals and transmits the signals. A frequency synthesizer is used in stepped
frequency GPR to step through a range of frequencies equally spaced by a given time
interval. Here a carrier signal is radiated and mixed with the received signal at each
frequency with a higher stability by using a quadrature mixer. In this thesis, we focus on
the design of Pulse Generator block of the time-domain GPR.
12
Design of Pulse Generator in 180nm Technology for GPR Applications
2.6. Description of a Time-Domain GPR
In this section, we briefly describes the various blocks of a Time-Domain GPR; like,
 Transmitter
 Receiver
 Timing Circuit
2.6.1 Transmitter
The transmitter consists of a pulse generator which produces short Gaussian pulses with
certain periodicity which is known as PRF. In this GPR the pulse generated is typically a
mono cycle or a standard Gaussian pulse, but Gaussian doublet can also be used. Due to
the fast discharge of energy stored in the capacitor, the pulse generation takes place.
Figure 2.3: Block diagram of a time domain GPR.
2.6.2 Receiver
To design the receiver circuit in the hardware is the biggest challenge. The performance of
the receiver circuit can change the over-all performance of the system. The receiver should
possess a large fractional bandwidth with a large dynamic range and a better noise
performance with high sensitivity.
13
Design of Pulse Generator in 180nm Technology for GPR Applications
Figure 1.4: Block diagram of receiver.
2.6.2.1 A/D converter
The signal received by the GPR systems is in the frequency range of 3.1 to 10.6 GHz.
Hence the use of a standard ADC is impossible to sample the received signals in real time,
which should satisfy the Shannon’s theorem. The sequential sampling technique should be
used to avoid this complexity.
2.6.2.2 Sample and hold circuit
The input to the ADC has to be very stable for a specific time for a correct A/D conversion.
This circuit changes the signal variation to maximum extent which must be smaller than
the quantization step of the ADC during the conversion time. To provide a constant value
to the ADC we must use this circuit. When the capacitor Cs is charged to a voltage which is
directly proportional to the input signal, the sample and hold circuit works. Hence the
samples correspond to a specific portion of the input signal.
2.6.2.3 The low noise amplifier (LNA)
The signal conditioning is carried out by using a LNA before the RF signal enters through
the S/H circuit for the maximum use of the dynamic range of the ADC. Here the Low
Noise Amplifier with a very less noise figure is the part of the signal conditioning element.
The LNA is not put as first element, but it can be used after the TVG.
14
Design of Pulse Generator in 180nm Technology for GPR Applications
2.6.2.4 Time varying gain (TVG)
In GPR systems, the signals transmitted by the antenna and the backscattered signals from
the object are both subjected to spreading loss. In lossy medium, the objects are buried. The
higher the object is buried into the ground, the higher is the losses that will be introduced
by the ground. The TVG is an attenuator whose attenuation changes when we change the
time, but sometimes we wrongly consider it as an amplifier. The PIN diodes are used in
TVG as the time varying attenuator which is having a variable resistance dependent to
voltage and they also have lower junction capacitance.
2.6.3 Timing Circuit
The receiver used in a time domain GPR is based on a non-coherent reception of the
backscattered signals from the object. A timing circuit is used to control the reception of
the signals which synchronizes the work between the various parts of the GPR system. The
timing circuit is used for three operations. First, it has to trigger the pulse generator.
Secondly, it has to generate the timing signals which are needed for the sequential sampler,
i.e. a trigger for the ADC at the intersection of the slower as well as faster ramps. Lastly, it
has to control the timing for the TVG.
2.7 Conclusion
GPR is one of the most challenging technologies for detection of buried objects of various
types. The range resolution of a GPR is directly related to the BW of the system. The need
for higher BW is the reason for the development of UWB GPR. Generating and receiving
UWB pulse is one of the most challenging areas to be addressed by GPR research
community to realize cost effective reliable UWB GPR system.
15
Design of Pulse Generator in 180nm Technology for GPR Applications
3
UWB COMMUNICATION SYSTEMS
16
Design of Pulse Generator in 180nm Technology for GPR Applications
Ultra-wideband (UWB) is a challenging technology having caliber to make a revolution in
wireless communications. It operates using lower power ultra-short pulses. In this section,
we will introduce the UWB system including its definition, advantages, applications and
the FCC regulation.
3.1 Definition of UWB
The most frequently use of the term “Ultra-wide bandwidth (UWB)” comes from the UWB
radar family and refers to electromagnetic waveforms. According to the FCC definition,
Ultra-Wideband transmission systems must have either an absolute bandwidth equal to or
greater than 500 MHz or the fractional bandwidth equal to or greater than 20%. The
fractional bandwidth is defined as B / fC , where B = fH-fL denotes –10dB bandwidth and
center frequency fC = (fH + fL) / 2.
Figure 3.1 Frequency Spectrum Analysis on Bandwidth and Center frequency
Figure 3.2 Comparision on Bandwidth between UWB and NB systems
17
Design of Pulse Generator in 180nm Technology for GPR Applications
3.2 FCC Regulation
In 1998, FCC starts the process of UWB technology regulatory analysis. In February 2002,
FCC announces the rules which permit UWB for operating under certain indoor and
outdoor PSD specifications. In this thesis, the indoor power spectral mask will be
discussed. The Figure below shows the operating and FCC acceptable frequency range of
some high frequency services. Most high frequency cases are regulated by FCC Part 15. Of
course, UWB systems including wireless communications are defined in this. According to
the Figure, UWB wireless communication systems must operate at above 3.1 GHz and with
less than -40dBm of EIRP, which stands for Equivalent Isotropically Radiated Power,
which means the signal power supplied to the UWB antenna.
Figure 3.3 FCC Part 15 for high frequency devices
As shown in Figure 3.4, for indoor applications, the emission power of UWB devices
should be less than -41.3 dBm between 0 to 0.96 GHz, less than -75.1 dBm between 0.96
to 1.61 GHz, less than -53 dBm between 1.61 to 1.99 GHz, less than -51.3 dBm between
1.99 to 3.1 GHz, less than -41.3 dBm between 3.1 to 10.6 GHz, and less than -51.3 dBm
from 10.6 GHz above. Same type of analysis can be carried out for outdoor applications
presented in Figure 3.5.
18
Design of Pulse Generator in 180nm Technology for GPR Applications
Figure 3.4 FCC spectral Mask for UWB systems for indoor applications
Figure 3.5 FCC spectral Mask for UWB systems for outdoor applications
19
Design of Pulse Generator in 180nm Technology for GPR Applications
3.3 Advantages
Since UWB technology is based on the transmission of pulses with a small amount of
power, UWB communication system have certain advantages over narrow band
communication systems. According to Shannon’s communication theory,
CC = B log2 (1+SNR)
Cc: channel capacity
B: bandwidth
SNR: ratio between the signal power to the noise power.
Since Ultra wideband has wide bandwidth, it is suitable for higher data rate communication
which is defined by IEEE 802.15.3a standards and can get a speed limited to 480Mbps.
This data rate is far beyond the existing speed of 1 Mbps of Bluetooth, 11 Mbps of
802.11b, and 54 Mbps of 802.11a/g. In multipath environment, the huge transmission BW
allow us for fine time resolution for multipath arrivals, which got the potential to reduce the
fading compared with the narrow bandwidth. Since the transmitter and receiver work in
high resolution time domain, each multi path signal can be detected as an individual signal,
i.e. without fading. UWB technology can be applied for locating wireless networks because
of its high range of resolution. In addition, very lower power and higher processing gain
will enable overlay and ensure minimum mutual interference between UWB and other
applications. Another advantage of UWB is low cost. Since impulse radio does not require
a carrier, and it only has a message signal for processing, hence no intermediate frequency
processing is needed for the IR UWB systems. That is, resulting in simpler circuitry. As
UWB devices do not require Local Oscillators and up- or down- converters, hence they are
cheap.
20
Design of Pulse Generator in 180nm Technology for GPR Applications
3.4 Applications
Based on the above advantages, the potential of UWB systems is vast. The four major
applications of UWB are:
 Wireless personal area networks
 Sensor networks
 Vehicular radars
 Imaging systems
3.5 Conclusion
Hence an UWB communication system is preferably used to simplify the data transmission
and remove disadvantages that occur in other narrowband systems. As UWB pulses are
generated for a shorter duration, hence no carrier signal is required to send a base band or
message signal. The pulses generated by UWB systems are very high in frequency; hence it
has very less chance to be got affected by noise.
21
Design of Pulse Generator in 180nm Technology for GPR Applications
4
PULSE GENERATOR
22
Design of Pulse Generator in 180nm Technology for GPR Applications
An UWB pulse generator is a method introduced in communication system to simplify the
data transmission and remove disadvantages that occurs in other systems. This generator
generates a Gaussian pulse for a small period of time of the order of some nanoseconds. As
UWB pulses are generated for a short time, hence no carrier signal is required to send a
base band or message signal. So power loss due to carrier signal doesn’t exist at all. These
pulses are very high in frequency; hence it has very less chance to be got affected by noise.
Initially we discuss about the different types of pulses and pulse modulations in this
chapter. After that in section 4.3 we discuss the principle of pulse generation and next to it
the comparision between MCML and CMOS logic is carried out; and we conclude that on
designing a delay generator block MCML is a better choice. In section 4.5 the operation of
Gilbert XOR cell is discussed because it is used to generate a Gaussian pulse.
4.1 Gaussian Pulse Types
The most popular UWB system is IR-UWB system because it does not require a sinusoidal
carrier signal to shift the signal frequency to a higher level. A standard Gaussian pulse is
one of the signals generated in UWB IR systems given by:
𝑦(𝑡) =
𝐴
√2𝜋σ2
exp(−
𝑡2
)
2𝜎 2
If the transmitter produces a Gaussian pulse, the output of the transmitter antenna will be
the first derivative Gaussian pulse, given by:
𝑡2
𝑦(𝑡) = −
exp(− 2 )
2𝜎
√2𝜋 𝜎 3
𝐴𝑡
If the transmitter produces a first derivative Gaussian pulse, the output from the antenna
will be a Gaussian doublet pulse, given by
𝑡2
𝑡2
𝑦(𝑡) = −
(1 − 2 )exp(− 2 )
𝜎
2𝜎
√2𝜋 𝜎 3
𝐴𝑡
23
Design of Pulse Generator in 180nm Technology for GPR Applications
Figure 4.1 Standard Gaussian Pulse type and its PSD
Figure 4.2 Gaussian Mono cycle Pulse type and its PSD
Figure 4.3 Gaussian Doublet Pulse type and its PSD
24
Design of Pulse Generator in 180nm Technology for GPR Applications
4.2 Pulse Modulation
In UWB system there is variety of modulation types used. In some modulations, the
information bits are directly modulated to very short pulses. Since there is no IF, the base
band processing of the signal is done. Well-known modulations types include TH-PPM,
TH-PAM, and DS- PAM. On the other hand, some UWB systems use carriers.
4.2.1 IR Vs OFDM
Table 4-1 Comparision between OFDM and IR
Parameters
OFDM
IR
Band
Multi
Single
Bandwidth
Sub bands of approx. 500 MHz each.
Uses all available bandwidth
Speed
480Mbps
1Gbps
Carrier Based
Yes
No
Power Consumption
More
Less
Complexity
More
Less
Up-Down Converter
Yes
No
DAC
Yes
No
PAR
High
Low
4.3 Pulse Generation
In the UWB communication systems, the pulse transmitting block has to generate short
duration pulses before modulating the message signal. For this kind of function, this block
can be composed of a digital pulse generator and a pulse shaping circuitry. In this thesis,
the digital transmitter consists of a delay generator and a XOR cell, and the impulse
shaping circuitry uses a band pass FIR filter. The final output shape must be a Gaussian
Monocycle pulse.
25
Design of Pulse Generator in 180nm Technology for GPR Applications
To design the pulse generator, delay circuits and XOR circuits have been used. Firstly a
reference clock signal is sent into the delay circuit, and the delayed signal at the output is
applied to the XOR gate with the reference input clock signal. In the delay circuit, the delay
time is controlled by the voltage. Using the control voltage to adjust the delay time in the
delay circuit; the pulse generator can change the pulse width (1/f) to achieve a required
frequency. The Gilbert XOR Cell is used as an XOR gate to create short transient pulses.
When the two different input signals have opposite voltage levels at the same time into the
XOR, short pulses will be generated out from XOR gate. Those short pulses are Gaussian
pulses, and then become the input signals to the pulse shaping FIR filter. The proposed FIR
filter performs a convolution of the impulse response with a sequence of input sampled
values and produces the output values with an equally numbered sequence.
Figure 4.4 Pulse Generator Block Diagram
4.4 MOS Current Mode Logic (MCML)
MOS Current Mode Logic (MCML) is studied for low power, high speed and mixed signal
environment. Since it has lower output swing than CMOS, MCML has lower power
consumption and high speed. Since the MCML is a differential logic style, the PDN is fully
differential. The basic CML structure contains three mail blocks: differential pull down
network (PDN), pull up resistor, and a constant current source.
26
Design of Pulse Generator in 180nm Technology for GPR Applications
The total propagation delay is given by:
𝐷𝑒𝑙𝑎𝑦 = 𝑁 ∗ 𝑅 ∗ 𝐶 =
𝑁 ∗ 𝐶 ∗ ∆𝑉
𝐼
Therefore, the voltage swings ∆V along with the bias current I control the delay. These two
parameters can be adjusted to obtain the necessary delay in my design. The static power
consumption of the MCML circuit can be calculated as follows:
𝑃𝑜𝑤𝑒𝑟 = 𝑁 ∗ 𝐼 ∗ 𝑉𝑑𝑑
Figure 4.5 Basic MCML inverter/buffer circuit
Table 4-2 Comparision between CMOS and MCML
Parameter
CMOS
MCML
Power Supply Voltage
High
Low
Power Dissipation
Dynamic (High)
Static (Low)
Noise Margin
High
Low
Voltage Swing / SwitchingCurrent
High
Low
Threshold Voltage
High
Low
Speed
Low
High
Input Capacitance
High
Low
Rate of Charge and Discharge
Different
Same
Transistors
May be ON/OFF
Always ON
Transistor Size
Varying
Identical
Area
More
Less
27
Design of Pulse Generator in 180nm Technology for GPR Applications
4.5 Gilbert XOR Cell
This Gilbert cell is very famous circuitry in RF, and it is being used as a standardized
structure. So in this thesis, the Gilbert cell has been applied for XOR directly. Also, on this
Gilbert cell, all passive components have been replaced by all active components. When
the two input signals have opposite voltage levels at the same time into the XOR, short
pulses will be generated out from XOR gate. Each input voltage values can be decided by
DC common mode simulation. In this thesis, in place of the pull-up resistors RFP is used
where as RFN is used in place of current source.
Figure 4.6 Basic Gilbert XOR cell
4.6 Conclusion
In the UWB communication systems, the pulse transmitting block has to generate short
duration pulses before modulating the message signal. For this kind of function, this block
can be composed of a digital pulse generator with an impulse shaping circuitry. In this
chapter, we discuss about the digital transmitter block which consists of a delay generator
and a Gilbert XOR cell which is responsible for generating Gaussian pulse can be
generated. But to meet the FCC mask, we should shape it to a Gaussian mono cycle pulse
with the help of a FIR filter which is discussed in the next chapter.
28
Design of Pulse Generator in 180nm Technology for GPR Applications
5
FIR FILTER
29
Design of Pulse Generator in 180nm Technology for GPR Applications
A filter is used to modify some characteristics of a signal when passed through it. The
multiplication of two spectral sequences in frequency domain is the linear convolution of
two sequences given in the time domain stated by Fourier transform. In this chapter,
initially we discuss the basic principle of FIR filter along with its architecture.
5.1 Introduction
A FIR filter performs a weighted average of a finite number of samples of the given input
sequence. The basic input output structure of the FIR filter is a time-domain computation
based on a difference equation. Figure 5.1 shows a signal flow diagram of a standard 4-tap
FIR filter. Since the FIR filter coefficients are similar to the values of impulse response, the
general format of a standard FIR filter can also be represented as the equation given by:
𝑀
𝑦[𝑛] = ∑𝑘=0 h[k]x[n − k]
The relation of the input and output of the designed FIR filter is expressed in terms of the
input and the finite impulse response which is also known as finite convolutions sum. We
can find the output by obtaining the convolution of the two given sequences x[n] and h[n].
The characteristics of the filter are controlled by the filter coefficient.
Fig. 5.1 FIR Filter
30
Design of Pulse Generator in 180nm Technology for GPR Applications
5.2 Architecture
The speed of the filter is defined as the rate at which input samples can be processed. To
increase the speed it is necessary to reduce the critical path between the input and output.
The critical path is defined as the path with the highest computation time among all
available paths that contain no delays. Pipelining reduces the effective critical path by
introducing pipelined latches along the data path. In this arrangement while the left adder
initiates the computation of the current iteration, at the same time the right adder is
completing the computation of the previous iteration result.
Fig. 5.2 Datapath
Fig. 5.3 Pipelining
Another FIR filter structure known as the transposed FIR filter does not require any
pipelining to reduce the critical path.
31
Design of Pulse Generator in 180nm Technology for GPR Applications
Fig. 5.4 Transposed FIR filter
In this thesis, we implemented a FIR filter by using RNS because the conventional number
system is a fixed positive radix number system having different weight for each bit, where
as a sign is used as a symbol followed by the number in r’s complemented form (where r is
the radix or base of the number system) in signed number. Addition of conventional
number systems requires carry propagation (serial signal propagation) from LSD to MSD
and the addition time depends on word-length, which is the main limitation of the
conventional number systems.
But Redundant number systems (RNS) is used to allow addition of two different numbers
in which no serial carry propagation is required because the time duration of the operation
is independent of the word-length of the operands and the time required for the addition of
two digits. This is the advantage of RNS over conventional number systems. For
implementation of the FIR filter, the structural blocks are to be designed such as PPM
adder, MMP subtractor, D- FF, SBD adder etc.
5.3 Signed Digit Number System
32
Design of Pulse Generator in 180nm Technology for GPR Applications
In conventional radix-r number system, a digit can take on values {0, 1, 2…, r-1} and the
digit set is S = {-(r-1), -(r-2) …, -1, 0, 1…, (r-1)}. For example, the digit set {-1, 0, 1} is
used for radix-2 (r =2) number system. A signed-digit is represented by the digits zi and
has the algebraic value
𝑍 = ∑ 𝑧𝑖 𝑟 𝑖
(5.2)
In this case, the number 3 can be represented as 0011 or 0101-1. Hence every number
allows multiple representations in signed-digit format and these numbers are known as
Redundant Number Systems. Signed-digit representation limits the carry propagation to
one bit position to left during the operation of addition and subtraction in digital computers.
5.4 Redundant Number Systems (RNS)
The purpose of redundant number representations is to allow both addition and subtraction
of two different numbers in which no serial carry propagation is required. The signed-digit
representation must have an unique representation of zero algebraic value of a number. The
redundant number is represented by n+m+1 digits zi (where i=-n… -1, 0, 1 …m) has the
integer value
𝑖
𝑍 = ∑𝑚
𝑖=−𝑛 𝑧𝑖 𝑟
(5.3)
5.5 PPM Adder (Carry-Free Radix-2 Addition)
RNS limits the carry propagation to a fewer bit positions, which is not dependent on the
word length. Hence faster addition results due to the carry propagation-free feature. Using
two unsigned binary numbers; positive and negative both, the radix-2 SBD number is
coded as X = X+ - X- . Hence each signed bit is represented using two bits as xi = xi+ - xiwhere xi+ and xi- is either 0 or 1; xi belongs to 1-, 0 or 1.
33
Design of Pulse Generator in 180nm Technology for GPR Applications
Figure 5.5 PPM Adder
In this adder a signed digit number xi is to be added to an unsigned digit yi which can be
solved using two steps. To generate intermediate sum pi in first step, all bits are added
simultaneously which lies in the range {1-, 0, 1, 2} and is expressed as:
pi = xi + yi = 2ti + ui
where ti denotes the transfer digit having the value of either -1 or 0 and is denoted as ti+ and
ui is the interim sum having the value of either 0 or 1- and is denoted as -ui-.
In second step, si is generated at the output by addimg ti-1+ and ui- as 1 digit given by:
si = ti-1+ + uiThen the arithmetic operation performed by the PPM adder (otherwise known as
Redundant Binary Full Adder- RBFA) is expressed as:
xi+ - xi- + yi+ = 2ti+ - ui-
Figure 5.6 LSD PPM Adder
34
Design of Pulse Generator in 180nm Technology for GPR Applications
Table 5-1 Digit Sets in Addition of PPM Adder
Digit
Digit Set
Binary Code
xi
-1,0,1
xi+ - xi-
yi
0,1
yi+
pi
-1,0,1,2
2ti + ui
ui
-1,0
- ui-
ti
0,1
ti+
si
-1,0,1
si+ - si-
5.6 MMP Subtractor (Radix-2 Subtraction)
This subtractor can subtract an unsigned digit number from a signed binary numbers. Using
two unsigned binary numbers; positive and negative both, the radix-2 SBD number is
coded as X = X+ - X- . Hence each signed bit is represented using two bits as xi = xi+ - xiwhere xi+ and xi- is either 0 or 1; xi belongs to 1-, 0 or 1.
Figure 5.7 MMP Subtractor
In this subtractor a signed digit number xi is to be subtracted to an unsigned digit yi which
can be carried out in 2 steps.
35
Design of Pulse Generator in 180nm Technology for GPR Applications
For all bit positions, the first step is carried out in parallel and an intermediate difference pi
is produced, which lies in the range {2-, 1-, 0, 1} and is expressed as:
pi = xi - yi = 2ti + ui
where ti denotes the transfer digit having the value of either -1 or 0 and is denoted as -tiand ui is the interim difference having the value of either 0 or 1 and is denoted as ui+.
In second step, si is generated at the output by addimg ti-1- and ui+ as 1 digit given by:
si = ti-1- - ui+
Then the arithmetic operation performed by the MMP subtractor is expressed as:
xi+ - xi- - yi- = -2ti- + ui+
Figure 5.8 LSD MMP Subtractor
Table 5-2 Digit Sets in Addition of MMP Subtractor
Digit
Digit Set
Binary Code
xi
-1,0,1
xi+ - xi-
yi
0,1
yi-
pi
-2,-1,0,1
2ti - ui
ui
0,1
ui+
ti
-1,0
-ti-
si
-1,0,1
si+ - si-
36
Design of Pulse Generator in 180nm Technology for GPR Applications
5.7 Digit Serial SBD redundant Adder
In Digit-serial SBD adder, two redundant binary numbers xi (xi+ - xi-) and yi (yi+ - yi-) can
be added simultaneously and gives the result si (si+ - si-) as a redundant binary digit sum.
This adder consists of PPM adder, MMP subtractor and D-FF (delay). This adder behaves
as pipelining architecture, by which critical path will be reduced and hence reduction of the
propagation delays.
Figure 5.9 SBD Adder
5.8 Conclusion
A FIR filter performs a weighted average of a finite number of samples of the given input
sequences. In this thesis, we use the FIR filter as a band pass filter to shape the Gaussian
pulse to a Gaussian mono cycle pulse to meet the FCC requirements of bandwidth between
3.1 to 10.6 GHz. This chapter shows the design of a FIR filter with the help of RNS by
implementing different SBD adder blocks with D flip-flops. Here each SBD adder is
designed with the help of one LSD PPM adder and one LSD MMP subtractor.
37
Design of Pulse Generator in 180nm Technology for GPR Applications
6
DESIGN, IMPLEMENTATION AND
SIMULATION OF PULSE GENERATOR
38
Design of Pulse Generator in 180nm Technology for GPR Applications
As mentioned in chapter 4, this pulse generator consists of interpolation delay blocks and
an XOR block for the Gaussian pulse generation, and the FIR filter is used as a pulse
shaping circuitry as mentioned in chapter 5. The design and simulations of various circuits
were performed with Cadence in conjunction with UMC 180nm technology.
When the input signal is passed into the three delay blocks through an XOR gate and
compared with the original input signal through an XOR gate, approximate Gaussian
pulses are generated. The purpose is making the pulse in the UWB frequency range; i.e. in
between 3.1 to 10.6 GHz. So the generated Gaussian pulse is passed through a FIR filter
for generating the Gaussian mono cycle pulse in the time domain to meet the FCC mask.
6.1 Delay Generator
At first, a delay generator is implemented with three MCML delay blocks to generate a
delay of approx. 140 picoseconds; i.e. each MCML delay block approx. generates a delay
of 47 picoseconds.
Figure 6.1 Schematic of Delay Generator
39
Design of Pulse Generator in 180nm Technology for GPR Applications
Figure 6.2 Output of Delay Generator
6.2 Gilbert XOR cell
When the original input signal is passed through three delay blocks a delay of 140ps is
observed in the output of the delay generator. After that the generated delay signal is
passed through the XOR gate as one input and at the same time the original input signal is
sent as another input with the help of a Gilbert XOR cell, and at the output of the XOR cell
approximate Gaussian pulses are generated when the two given input signals have two
opposite voltage levels.
Figure 6.3 Schematic of a Gilbert XOR cell
40
Design of Pulse Generator in 180nm Technology for GPR Applications
Figure 6.4 Gaussian Pulse output of Gilbert XOR cell
6.3 FIR Filter
In this thesis, we implemented a FIR filter by using RNS because the conventional number
systems is a fixed positive radix number systems having different weights for each bit,
where as a sign has to be used for every signed number in r’s complemented form.
Addition of conventional number systems requires carry propagation from LSD to MSD
and the addition time depends on word-length. But RNS is used to allow addition of two
different numbers in which no serial carry propagation is required because the time needed
for the operation is not dependent on the word-length of the operands and the time required
for the addition of two digits. For implementation of the FIR filter, the structural blocks are
to be designed such as PPM adder, MMP subtractor, D- FF, SBD adder etc.
6.3.1 PPM Adder
Redundant number system limits the propagation of the carry to a fewer bit positions,
which is not dependent on the word length. Hence a faster addition results due to the carry
propagation-free feature.
41
Design of Pulse Generator in 180nm Technology for GPR Applications
Figure 6.5 Schematic of PPM Adder
6.3.2 MMP Subtractor
This subtractor can subtract an unsigned digit number from a signed binary numbers. In
this subtractor, using two unsigned binary numbers; positive and negative both a radix-2
SBD number is to be coded .
Figure 6.6 Schematic of MMP Subtractor
42
Design of Pulse Generator in 180nm Technology for GPR Applications
6.3.3 D Flip-Flop
In this thesis, the D flip-flop is used to generate delays while propagating the carry from
LSD to MSD. Hence it is used along with PPM adder and MMP subtractor both for
designing a SBD adder. It is also used along with SBD adder for designing a FIR filter
which is used as a pulse shaper in this thesis.
Figure 6.7 Schematic of D Flip-flop
6.3.4 SBD Adder
In Digit-serial SBD adder, two redundant binary numbers xi and yi can be added
simultaneously and gives the result si as a redundant binary digit sum. This adder consists
of PPM adder, MMP subtractor and D-FF (delay). This adder behaves as pipelining
architecture, by which critical path will be reduced and hence reduction of the propagation
delays can be seen. In this thesis, the SBD adder is used as a key component for designing
a FIR filter which is used for pulse shaping.
43
Design of Pulse Generator in 180nm Technology for GPR Applications
Figure 6.8 Schematic of SBD Adder
6.3.5 Proposed FIR filter
For implementation of the FIR filter, we use the structural blocks of SBD adders along
with D flip-flops. After generating a Gaussian pulse at the output of the Gilbert XOR cell,
the pulse is passed through this designed FIR filter for pulse shaping; i.e. at the output we
can a observe a Gaussian mono cycle pulse which meet the FCC requirements.
Figure 6.9 Schematic of FIR filter
44
Design of Pulse Generator in 180nm Technology for GPR Applications
6.4 Proposed Pulse Generator
In this thesis, a low-complexity and low cost pulse generator is designed in 180nm
technology for ground penetrating ultra-wideband (UWB) radar system applications. Here I
have implemented an UWB pulse generator circuit which is used in communication system
to simplify the data transmission and remove disadvantages that occurs in other systems.
This pulse generator uses a delay generator along with a Gilbert XOR cell for generating a
Gaussian pulse which can be shaped by using a FIR filter, and finally a Gaussian mono
cycle pulse is observed at the output which has a pulse width of 97ps thereby give rise to a
bandwidth of 10.3 GHz which meet the FCC requirements.
Figure 6.10 Schematic of Proposed Pulse Generator
Figure 6.11 Gaussian Mono cycle pulse output of the proposed pulse generator
45
Design of Pulse Generator in 180nm Technology for GPR Applications
7
CONCLUSION AND FUTURE WORK
46
Design of Pulse Generator in 180nm Technology for GPR Applications
For an IR-UWB wireless communication system, a pulse generator with a FIR filter was
designed. When the adjustment in the control voltage was done, an ultra short duration
Gaussian pulse was generated from the XOR gate which uses the Gilbert cell. The FIR
filter is designed as a pulse shaping circuitry to give a shape to the Gaussian pulse to a
Gaussian mono cycle pulse using CADENCE UMC tool in 180nm technology. By the
convolution of the input signal with the coefficients of the FIR filter, the Gaussian mono
cycle pulse has been generated. As an IR UWB signal source, the generated pulse should
meet FCC emission requirements. According to the simulation result, the PSD of the
standard Gaussian pulse coming from the XOR gate can’t meet the FCC mask. However,
after shaping the pulse using designed FIR filter, the PSD of the pulse is in between the
BW requirements of 3.1 to 10.6 GHz.
If the derivative of the Gaussian pulse is higher, then the center frequency is also moving
higher subject to a smaller bandwidth which meets the FCC requirements for GPR
applications. Hence generating a higher derivative Gaussian pulse is the future work.
47
Design of Pulse Generator in 180nm Technology for GPR Applications
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[10] Musicer, Jason: “An analysis of MOS Current Mode Logic for Low Power and High
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