Upload
others
View
10
Download
1
Embed Size (px)
Citation preview
A Thesis for the Degree of Master
Single Chip CMOS Transmitter for UWB Impulse Radar
Applications
Chang Shu
School of Engineering
Information and Communications University
2009
Single Chip CMOS Transmitter for UWB Impulse Radar
Applications
Single Chip CMOS Transmitter for UWB Impulse Radar
Applications
Advisor: Professor Sang-Gug Lee
by
Chang Shu
School of Engineering
Information and Communications University
A thesis submitted to the faculty of Information and
Communications University in partial fulfillment of the re-
quirements for the degree of Master of Science in the
School of Engineering
Daejeon, Korea
Dec. 1st, 2008
Approved by
(Signed)
Professor Sang-Gug Lee
Major Advisor
Single Chip CMOS Transmitter for UWB Impulse Radar
ApplicationsChang Shu
We certify that this work has passed the scholastic standards re-quested by the Information and Communications University as a thesisfor the degree of Master
December 1st, 2008
Approved: Chairman of the CommitteeSang-Gug Lee, ProfessorSchool of Engineering
Committee MemberHyung-Joun Yoo, ProfessorSchool of Engineering
Committee MemberSeung-Tak Ryu, Assistant ProfessorSchool of Engineering
M.S.20062560
Chang Shu
Single Chip CMOS Transmitter for UWB Impulse Radar
Applications
School of Engineering. 2008, 63p.
Major Advisor: Professor. Sang-Gug Lee.
Text in English
i
Abstract
Ultra-Wide Band (UWB) technology has been studied intensively these
years. And transmitter plays an important role in the UWB transceiver sys-
tem since it has the function of generating the UWB impulses which is used
to transmit the information through propagation channels. Among the vast
applications of UWB technology, UWB radar is a hot spot since it is widely
used in the detection of moving object in the field of radar.
This thesis presents the design of an on chip transmitter for Ultra-
Wideband (UWB) Impulse Radar application. Short pulses with duration of
4.5 ns and bandwidth of about 500 MHz are generated by up-converting the
triangular shaped envelope to the center frequency by a specially designed
energy efficient mixer.
In this UWB Impulse Radar Project, two versions of transmitter test pat-
tern chips were designed, fabricated, and measured. Both versions of the
transmitters can work at pulse repetition frequency of over 1 MHz with over
ii
-2 dBm output power at 50 ohm load. Output pulse spectrum centered at 4.3
GHz can fit FCC spectrum mask with a side-lobe suppression of 20 dB. De-
signed in 0.18-µm CMOS technology, the two circuits have a chip area of
0.45 mm 1.2 mm.
The details of architecture decision and design process are discussed in
this thesis. The improvement of second chip is also analyzed comparing with
first version chip.
iii
Contents
Abstract ................................................................................................................. i
Contents...............................................................................................................iii
I. Introduction .................................................................................................. 1
1.1 UWB Definition and UWB Radar................................................... 3
1.2 UWB FCC Mask and Regulations................................................... 7
1.3 Structure of the Thesis ...................................................................... 9
II. UWB System Architectures ..................................................................... 10
2.1 Several reported UWB system Architectures ................................ 11
2.1.1 Sampling architecture .........................................................................12
2.1.2 Non-Coherent Architecture.................................................................15
2.1.3 Coherent Architecture .........................................................................16
2.2 UWB architecture used in this Project ......................................... 18
III. UWB Transmitter and Pulse Generator............................................... 19
3.1 Pulse Shape Decision and Analysis............................................... 23
3.2 System Requirement......................................................................... 26
3.3 System Architecture of TX............................................................. 27
IV. UWB Transmitter Circuit Structure...................................................... 29
4.1 Charge Pump .................................................................................... 30
4.2 Mixer ................................................................................................. 32
4.3 Single to Differential Converter ..................................................... 36
4.4 Ring Oscillator.................................................................................. 38
4.5 Drive Amplifier (DA)...................................................................... 40
4.6 Single to Differential Converter ..................................................... 42
iv
4.7 Amplifier for Template Pulse ......................................................... 43
V. Simulation Result of Transmitter and Layout .................................... 44
5.1 Simulation Result ............................................................................. 44
5.2 Overall simulation result ................................................................. 46
5.3 Layout of TX part and test block................................................. 47
VI. Measurement Results ................................................................................ 48
6.1 First Version Transmitter measurement.......................................... 50
6.2 Second Version Transmitter measurement ..................................... 53
VII. Conclusions ............................................................................................... 56
References .......................................................................................................... 57
Acknowledgement ............................................................................................. 60
Curriculum Vitae ............................................................................................. 62
v
Lists of Tables
Table I. The comparisons between the architectures in the following........... 11
Table II. Summary of different shape window ................................................ 25
Table III. System specifications ......................................................................... 26
Table IV. Summary of Post-Simulation in Version I chip .............................. 46
Table V. Summary of Post-Simulation in Version II chip .............................. 46
Table VI. Simulation and Measurement of first chip...................................... 52
Table VII. Simulation and Measurement of second chip................................ 55
Table VIII. Comparison of UWB transmitter.................................................. 55
vi
Lists of Figures
Fig. 1 Conventional Integrated Narrowband Transceiver................................ 4
Fig. 2 UWB “digitally” Radio .............................................................................. 4
Fig. 3 Waveform of pulse train used in UWB Radar......................................... 6
Fig. 4 UWB spectrum mask and FCC part15 limit ........................................... 7
Fig. 5 UWB spectrum mask of Korea ................................................................. 8
Fig. 6 Sampling architecture UWB transceiver ............................................... 12
Fig. 7 Time-interleaved ADC in UWB receiver ............................................... 13
Fig. 8 Direct Sampling UWB Transceiver [8] .................................................. 14
Fig. 9 Direct Sampling UWB Transceiver [9] .................................................. 14
Fig. 10 Non-coherent UWB Transceiver [11] ................................................... 15
Fig. 11 Non-coherent UWB Transceiver [12] ................................................... 16
Fig. 12 Coherent transceiver used for ranging and communication [13] ...... 17
Fig. 13 UWB Impulse Radar Transceiver ........................................................ 18
Fig. 14 Burst-mode Pulse Generator [2] ........................................................... 21
Fig. 15 The operation principle of “carrier based UWB”............................... 22
Fig. 16 Gaussian shape envelope and frequency spectrum............................. 23
Fig. 17 Triangular shape envelope and frequency spectrum.......................... 24
Fig. 18 Comparison of several functions’ characteristics................................ 25
Fig. 19 Architecture of this transmitter ............................................................ 27
Fig. 20 Architecture of this transmitter in Transceiver System ..................... 28
Fig. 21 Charge Pump input clock, circuit and its output ................................ 31
Fig. 22 Charge Pump Output............................................................................. 31
vii
Fig. 23 Circuit of Mixer Proposed ..................................................................... 34
Fig. 24 Output of Mixer...................................................................................... 34
Fig. 25 With added switching pair, the outputs are “in phase”...................... 35
Fig. 26 Without added switching pair, the outputs are “differential”........... 35
Fig. 27 Differential to single converter.............................................................. 37
Fig. 28 Ring Oscillator circuit............................................................................ 39
Fig. 29 Oscillator Output.................................................................................... 39
Fig. 30 Drive Amplifier Circuit.......................................................................... 41
Fig. 31 Single to Differential Converter............................................................ 42
Fig. 32 Template Amplifier ................................................................................ 43
Fig. 33 Output of Drive Amplifier and its frequency spectrum ..................... 44
Fig. 34 Template Waveform............................................................................... 45
Fig. 35 Frequency Spectrum of Template......................................................... 45
Fig. 36 Layout of TX........................................................................................... 47
Fig. 37 Layout of TX........................................................................................... 47
Fig. 38 Die Photo of TX first version................................................................. 49
Fig. 39 Die Photo of TX second version ............................................................ 49
Fig. 40 Measured pulse time domain shape of first version chip.................... 51
Fig. 41 Measured pulse frequency spectrum of first version chip.................. 52
Fig. 42 Measured DA output of second version TX chip................................. 54
Fig. 43 Template output of second version TX chip (with buffer) ................. 54
1
I. Introduction
Ultra-Wide Band (UWB) technology has been a hot topic for a long time
since 1960s when a series of sample and hold receivers emerged and were
imported to UWB applications. Pulse generation, compression and
correlation and matched filter method has been developed since 1950s by
research centers. First UWB system for communication was built in 1970s
and it can also be used for Radar application [1]. And since late 1980s, Time
Domain Company was founded and has developed the first UWB chip used
in wireless communication, radar, and sensors. And in the new millennium
the well-known UC-Berkeley team has developed digitally UWB chipset
based on sampling after front-end amplifiers (FEA). UWB system also
evolutes from on/off keying [2] or pulse positioning modulation to more
complicate higher order modulations, such as BPSK and even FSK [3] .
Several pulse-based modulation schemes are found in literature such as
Pulse Amplitude Modulation (PAM), On–Off Keying (OOK), Pulse-Position
Modulation (PPM) or Bit-Position Modulation (BPM), Binary Phase-Shift
Keying (BPSK). However, due to the system of the UWB impulse radar,
there is no modulation involved in the transmitter part. The radar only needs
to measure the reflected pulses, so the important issue is the quality of the
pulse shape in time domain. And also, because of FCC regulation, in the
frequency domain, the FCC mask of Korea must be met as well.
2
It is somewhat interesting to look back and have some intuition of the
pulsed communication method and what is now called “Ultra-Wide Band”.
Although it seems like UWB has a short history comparing with other kind
of communication technology, it was actually used one hundred years ago
when the sparks were generated by Michael Faraday when he did the famous
experiment of discharging capacitor (transmitting pulse) and generating
sparks in another inductor (receiving the pulse). It is obvious that the sparks
are generated by manually turn on/off the switches which trigger the genera-
tion of sharp pulses and this is similar with the phenomenon of lightning
which, of course, has the voltage amplitude of millions of volts.
3
1.1 UWB Definition and UWB Radar
The UWB Signal Definition is: Fractional bandwidth is greater than 20%
of the center frequency, or the -10dB bandwidth occupies 500 MHz or more
of spectrum.
The intrinsic advantages of UWB can be seen from the famous Shannon
equation:
SNR)](1[logBW C 2 (1)
Where: C = Channel Capacity (bits/sec)
BW= Channel Bandwidth (Hz)
SNR= Signal to Noise Ratio
We can see that the channel capacity is proportional to the bandwidth and
thus, UWB has the potential to provide high data rate at moderate SNR. For
applications which face relatively large interferences or lower SNA, the
UWB can still provide enough data rate. This is the fundamental advantage
of UWB. The other advantages of UWB including: large bandwidth
potentially provides higher data transmission capability; resistance to
interference such as multi-path fading and noise; low EM environment
pollution and precise time resolution ability suitable for precise ranging and
timing [4]. Due to these tempting characteristics UWB technology has been
greatly used in RFIC tag [5], sensor networks, high data rate transceiver, etc.
From the system level comparison, UWB system has its own advantage of
4
simplicity, and low cost which can be seen from the comparison of Fig. 1
and Fig. 2. As the conventional narrowband transceiver needs mixer part and
PLL whereas in UWB system these parts can be omitted.
Fig. 1 Conventional Integrated Narrowband Transceiver.
Fig. 2 UWB “digitally” Radio.
As for the UWB Radar, which is one of the main application of UWB
technology, because of the intrinsic characteristic of UWB signal, this kind
of Radar has its advantages over other kind of radar. Such as: penetration of
absorbers because of molecular resonances; resistant to multi-path; low
probability of intercept (LPI), and resistance to jamming, because short
5
pulse implies narrow range gates and therefore not susceptible to continuous
wave (CW) jamming.
UWB radars have become popular in recent years and have a vast
application in civil engineering and industry, such as family surveillance,
medical instrument, detecting living people buried under debris, movement
sensors, microwave imaging, collision avoidance [4]. There are several types
of UWB radar such as ground or wall penetration radar, UWB synthesis
aperture radar (SAR), medical imaging radar, UWB ranging radar [6]. All of
these UWB radars are based on the radar principle. For the positioning using
UWB radar, it is straightforward and the distance can be measured by
calculating the time delay during transmission. Although the principle of
detection is almost same among various reported radars: finding the
reflected pulse and compare the time delayed between the transmitted and
received pulses.
The operation of UWB Radar is like this. Send a pulse train (Fig. 3) from
the transmitter and after the pulse reflected back from the target, the receiver
detects the pulse by amplifying, correlating and digital sampling.
6
Fig. 3 Waveform of pulse train used in UWB Radar.
7
1.2 UWB FCC Mask and Regulations
Due to the FCC rule, a large slot of unlicensed band from 3.1 GHz~10.6
GHz is given. This band is suitable for the UWB radar application which
requires wide bandwidth. According to the low spectrum power density of
this FCC mask part 15 limits (Fig. 4), the WLAN application occupies the
band between 5 GHz and 6 GHz and the band above 6 GHz are reserved for
future usage. But for Korean domestic FCC rule, the FCC mask is more
stringent, which is shown in Fig. 5.
Fig. 4 UWB spectrum mask and FCC part15 limit.
8
Fig. 5 UWB spectrum mask of Korea.
From our design requirement based on both FCC rule of Korea and part 15
limit, we decided to use the bandwidth between 3.6 ~ 5.15 GHz, with
sidelobe rejection of 20 dB. Since the band between 3.1 GHz and 3.4 GHz
and above 5.15 GHz are reserved by Korean domestic FCC regulation.
We can see that the power spectrum density for the UWB pulse should be
below -41.3 dBm/MHz between 3.6 ~ 5.15 GHz, and below -69 dBm/MHz
at the border of the selected band. However, in the really application this is
not easy to realize, since the side-lobe rejection would be about 30 dB in this
case. Thus, we decide the pulse spectrum density should be below -65
dBm/MHz, which corresponds to side-lobe rejection of about 25 dB. This
FCC mask is very important for the choosing of architecture and system
specifications which will be discussed later.
9
1.3 Structure of the Thesis
This thesis is written in the following structure. In Section II, we propose
the UWB system architectures and decide the system we are using for this
UWB radar system. In Section III, we propose various types of UWB
transmitter architecture and introduce the core block of the UWB
transmitter— pulse generator (PG). In Section IV, the proposed transmitter
individual architecture and its pulse generator are introduced in operation
principle. In Section V, we present the circuits design of transmitter
individual blocks including simulation result of each block. In Section VI,
measurement results are presented and discussed for two versions of chips.
Final conclusion is in Section VII.
10
II. UWB System Architectures
FCC rule has given nearly 7.5 GHz (from 3.1 GHz to 10.6 GHz) of
unlicensed band for the indoor and outdoor application to UWB. Such wide
bandwidth makes various design of UWB system possible based on different
applications. For communication purpose oriented UWB, wider bandwidth
makes very high data rate possible, but that also requires transmitter and
especially the UWB receiver hardware to be fast enough to deal with such
high data rate (even as high as multi-GB/s).
Several architectures are introduced these years. And they can be put into
several categories: the first category is using high speed sampling method,
another way is using frequency domain processing method to relieve the
sampling speed requirement, and another category is using more analog
processing by coherent scheme and non-coherent scheme.
For radar applications, analog correlation method is always used since
analog correlation is suitable for the ranging detection since the pulse has the
wider pulse width which is suitable for detection. And such architectures
will be introduced in detail in the following parts.
11
2.1 Several reported UWB system Architectures
From literature study, we categorize the UWB system to the following
three kinds of architectures: sampling architecture, non-coherent architecture,
and coherent architecture. The first two architectures are widely used in
UWB communications and the last architecture are used for communication
with good timing ability thus that one is suitable for Radar application. And
direct sampling method can be aided with band-selected frequency domain
processing. These architectures are briefly introduced for the understanding
of this project’s system architecture and its advantages. And their
characteristics are summarized in Table I.
Table I. The comparisons between the architectures in the following
Architectures System
Complexity
Hardware
requirement
Power
Consumption
Timing
accuracy
Direct Sampling High Very high Very high Not good
Non-coherent Low Low Low Good
Coherent Moderate Moderate Moderate Very good
12
2.1.1 Sampling architecture
Sampling architecture has been reported as “Soft-ware Defined Radio” for
the receiver architectures because the front-end part is just a LNA and
amplifier with no other analog blocks, and a high speed ADC directly
samples the amplified signal and gives it to DSP for processing. The
advantage of this architecture is the ability to reconfigurate by changing
DSP part only with software, thus it can receiver any kind of modulation
with the same hardware, which is the future of UWB and any kind of
application needs high data rate. Fig. 6 basic shows this architecture and Fig.
7 shows the detailed part of ADC block since this is the core part of the
receiver. This ADC is using interleaved technique and can do very high
speed sampling with less power consumption [7].
Fig. 6 Sampling architecture UWB transceiver.
13
Fig. 7 Time-interleaved ADC in UWB receiver.
Here we present two examples of UWB transceivers using this direct
sampling architecture. The first one is from UC-Berkeley team, the UWB
transceiver is shown in Fig. 8. It is using the time interleaved sampling
method and thus lowers the power consumption greatly. The system
complexity lies in the clock generation and ADC part.
The second example is using this similar time-interleaved sampling
method and can achieve 20 GB/s sampling rate. The digital FPGA is a
bottleneck is this case since a large amount of data needs to be processed.
14
Fig. 8 Direct Sampling UWB Transceiver [8].
Fig. 9 Direct Sampling UWB Transceiver [9].
15
2.1.2 Non-Coherent Architecture
The operation principle of non-coherent architecture is like this: in the re-
ceiver part, the incoming UWB pulse signal multiples with it self thus pro-
ducing an low frequency (near DC) signal, and this near DC signal can be
processed using analog blocks such as VGA and LPF and then fed into sam-
pling part. Since non-coherent system uses no synchronization between
transmitter and receiver, thus reduces complexity [10, 11].
Fig. 10 Non-coherent UWB Transceiver [11].
16
2.1.3 Coherent Architecture
Coherent system uses precise timing method such as delay lock loop
(DLL) to cope with the timing between the transmitter and receiver
(synchronization). It is more complex than its non-coherent counterpart but
can achieve higher data rate [12, 13]. For radar receiver, we can use both of
two schemes, but coherent scheme have better timing precision than the
Non-coherent architecture.
Fig. 11 Non-coherent UWB Transceiver [12].
17
Fig. 12 Coherent transceiver used for ranging and communication [13].
18
2.2 UWB architecture used in this Project
The basic architecture of UWB Radar is shown below, and this
presentation is about the transmitter (TX) part. Our whole radar system is
shown below which uses coherent scheme, and this work is a transmitter
which consists of the pulse generator and a drive amplifier (DA). The pulse
generator has two functions, one is to provide template for the correlator in
the receiver, and the other function is the transmitter: generate pulses to the
antenna. We will have a detailed discussion about the TX part of this
architecture shown in Fig. 13.
Fig. 13 UWB Impulse Radar Transceiver.
19
III. UWB Transmitter and Pulse Generator
From the architecture shown in Fig. 13, we can see that the main block of
the transmitter is the pulse generator (PG), thus we first introduce kinds of
PG so far and do the discussion and design own architecture.
There are many types of pulse generators so far, the earliest pulse
generator uses step recovery diode (not integrated), and other use analog or
digital method. Analog method focus on the generation of Gaussian shaped
pulses because it has good transfer ability and not much distorted during
transmission. According to the bandwidth requirement, several order of
derivative of Gaussian pulse is used, because higher derivative of Gaussian
pulse gives narrower bandwidth [14]. And these pulse generators with high
order of over seven derivatives usually have a much larger bandwidth
comparing with our 500 MHz target (barely meet the FCC mask).
In our system this derivative method is not suitable since we would need
very high order (nearly 30) of derivatives of Gaussian pulse. For the digital
generation methods, one way is to use the pulse pattern generation method
which combines many single pulses to generate desired pulse shape [15].
But the reported digitally controlled pulse generator operates in a bandwidth
of over 1 GHz and the pulse width is much smaller than 4ns so it is easier to
combine several edges into one pulse. Although the digital transmitter is
attractive in novelty, it needs about 32 edge combination cells to combine
into a single pulse which is 4 ns and center frequency is 4 GHz (similar to
20
our transmitter target), and the circuit is too complicated and power
consuming and hard to adjust each cell.
Another way for digitally generation of pulse reported in [16], which uses
very complicated architecture of using VGA to amplify each edge coming
from a register stack at different channel, and using integrator to sum up the
edge to form a single pulse. This approach is also not suitable for our target,
because we would need many channels since we have about 30 edges for
each pulse.
Another simple and intuitive way to generate a short high frequency pulse
is to using burst mode oscillator. By turn on and off the oscillator using gate
duration, the pulse which has center frequency of oscillator can be generated
[2, 17]. However, in [17] the gated pulse has the shape of rectangle and has a
high sidelobe power in spectrum which has to be filtered out, thus increasing
the system complexity and chip size. In [2], the author uses a modified way
to control the pulse and can achieve a side lobe rejection of over 20 dB by
using a burst mode LC oscillator. But the shape of pulse is fixed and not
optimal, and LC oscillator occupies large chip area. The circuit and output is
shown below in Fig. 14.
21
Fig. 14 Burst-mode Pulse Generator [2].
What we present here is a “carrier based modulated UWB pulse
generator”, which uses a triangular shaped envelope signal to modulate the
ring oscillator’s center frequency. Comparing with [18], which also uses a
triangular signal modulating with a frequency generated by PLL, this
transmitter has simplified the circuits of each individual block and proposes
a novel way of up converting the envelope to oscillator frequency. The
operation principle is illustrated in Fig. 15. The triangle envelope which has
a sidelobe rejection of over 20 dB is up-converted to the center frequency
(LO). There are several other papers using this scheme, but the envelopes
they used to modulate center frequency are different [19, 20]. In [19], a
Gaussian shaped envelope is used, but it is hard to generate a real Gaussian
envelope and the result turn out to be very different from the ideal Gaussian
shape, thus the side-lobe rejection is not good. In [20], a “tanh” shaped pulse
22
is generated as envelope, but the result is similar to the triangular shaped
envelope, although in theory the tanh does have better sidelobe rejection
ability. So, considering the system requirement and circuit complexity and
performance trade off, we decide to use the triangular shaped envelope and
use the carrier based UWB architecture, the operation principle of which is
shown in Fig. 15. And the detail discussion of pulse envelope selection is in
the following Chapter 3.1.
Fig. 15 The operation principle of “carrier based UWB”.
23
3.1 Pulse Shape Decision and Analysis
From literature study of transmitter, we find that the pulse which has a
shape of Gaussian can provide the best side-lobe rejection [20]. The time
domain function for Gaussian pulse is like below, and has the time domain
shape of Fig. 16, and the pulse has the envelope of Gaussian is our target.
2
2
2t
pGauss eVV
. (2)
Fig. 16 Gaussian shape envelope and frequency spectrum.
From [20], the frequency spectrum side-lobe rejection can be over 45 dBr,
which can fit our transmitter FCC mask. So, we decide to generate a pulse
which has the envelope similar to that of Gaussian pulse. In this TX, we use
the triangular pulse as the envelope, which is shown in Fig. 17.
24
Fig. 17 Triangular shape envelope and frequency spectrum.
We can see that the side lobe rejection can be about nearly 30 dBr.
Because of the nonlinearity of the system operation, the triangular wave will
be look like a near Gaussian shape, so it can meet our target. What’s below
is the comparison of time domain and frequency domain characteristic of
several common functions as envelope. We can see that the triangular
envelope can meet our target and can be realized by circuit easily. The
summary of most of the pulse envelope is in Table. II, we choose the
triangular shape which can provide over 25 dB of side-lobe suppression
capability.
25
Fig. 18 Comparison of several functions’ characteristics.
Table II. Summary of different shape window
Window
type
Sidelobe
(dBc)
Blackman -57
Hamming -41
Hann -31
Rectangular -13
Triangular -25
Half-sine -22
Gaussian -45
26
3.2 System Requirement
From system link budget of the UWB-IR, the specification of the TX is
shown in Table III.
Table III. System Specifications
And also, the Radar system in our project requires the TX part to provide
the template pulse for the receiver part. So, a template of 800 mV is required
also, which has the similar characteristic as the TX output.
Parameters Value
TX output 0 dBm
Template output 800 mV
Center frequency 4.30 GHz
Side lobe rejection 25 dB
bandwidth 500 MHz
Pulse width about 4 ns
27
3.3 System Architecture of TX
The architecture of our TX is shown below (Fig. 19). The clock control
the generation of triangular envelope, and the triangular envelope which has
a bandwidth of 500 MHz is up-converted to our center frequency (4.30
GHz) by the Mixer; a differential to single converter is needed to get the
triangular pulse from a differential output of the mixer. And the pulse is fed
to the drive amplifier and an external PA for the Antenna; the pulse is also
sent into another branch same as the upper part, which also fed into the
single to differential converter for providing the template for the receiver.
Fig. 19 Architecture of this transmitter.
The TX part in the whole system has two functions as we have discussed
in Chapter II, one is to provide pulse to the antenna, and the other is to pro-
vide template for the coherent receiver. Thus, the whole architecture of TX
part in this UWB transceiver is shown in Fig. 20.
28
Fig. 20 Architecture of this transmitter in Transceiver System.
29
IV. UWB Transmitter Circuit Structure
We use the “carrier based modulated UWB” scheme for this transmitter
shown in Fig. 15. Same scheme is also used before by [18, 19, 20], but the
triangular envelope used in this transmitter is different from [18, 19], and the
circuits to realize each block are much different from [18]. The core of this
transmitter is the pulse generator, which consists of envelope generator, up-
converting mixer, and ring oscillator. As the output of mixer is differential, a
differential to single converter is also applied using a traditional current
mirror topology. Finally, the modulated single-ended pulse is amplified by a
two-stage drive amplifier which is matched with 50 ohm load and can
provide -2 dBm output powers. The goal of this transmitter is to generate
high amplitude (over 500 mV) pulse which has bandwidth of about 500
MHz and sidelobe rejection of over 20 dB. Because we are using the 3.1
GHz to 5.15 GHz band, which make our center frequency set to be 4.3 GHz,
in order to avoid the interference domestic regulation of Korea also. And as
a part of radar system, the power consumption should be small, using 1.8-V
supply voltage.
30
4.1 Charge Pump
The function of the charge pump is to generate a triangular shaped
envelope, which is used to modulate the “carrier frequency” to generate the
desired spectrum mask. Similar approach has been used in [18] and [20], but
in [18], the triangular shaped envelope is generated by a more complicated
circuit. And [20] use external triangular signal. The idea of using charge
pump to generate triangular comes from the “edge combination method”
which is used to form pulse using digital way in [15], but in [15] they did
not use this “carrier based UWB” and use edge combination approach
instead. The merit for this approach of generate triangular signal is that it
can generate high amplitude envelope with little power consumption. In Fig.
21, schematic of charge pump and its output is shown. The two input clocks:
CLK_N and CLK_P are generated using DTG 5334 data timing generator
for this transmitter test chip, while it is generated by the delay line for the
whole UWB radar system. By the tuning of bias for M1 and M4, the
capacitor which is about 2-pF will be charged when CLK_P is low and
discharged when CLK_N is high. If the current of charging and discharging
is same, the output of charge pump would generate a triangular signal with a
width of designed value 5.5 ns, and the amplitude is about 0.7 V with an
offset of 450 mV. This envelope will be directly connected with the mixer.
The output of this CP is shown in Fig. 22.
31
Fig. 21 Charge Pump input clock, circuit and its output of triangular
envelope.
Fig. 22 Charge Pump Output.
32
4.2 Mixer
Mixer block is specially designed for this transmitter. It is basically
designed out of a single balance mixer with an added switch pair. As we
know, single-balanced mixer is always used in the receiver part, and for a
transmitter which operate at 4.3 GHz center frequency, if we use single
balanced mixer as the up-conversion mixer, there would be large LO leakage
to the output, because the single balance mixer does not have the LO
suppression ability which double balanced mixer has. But here we still could
use a modified version of single balanced mixer to up-convert the envelope,
because this transmitter has some special characteristics. The input IF, which
is the triangular envelope generated by charge pump, has a big amplitude
and small time duration. Using this characteristic, a novel mixer idea is
implemented in this transmitter: we add another differential switch pair to
the single balanced mixer, as shown in Fig. 23. Because the input envelope
has high amplitude and the offset is as small as 450 mV, this mixer has
modulated pulse output only when there is a big envelope coming in. For the
rest of time, it does not have DC current since the Gm stage is biased at such
low voltage (same with offset). Thus when there is no DC current, this mixer
is the same as a double passive balanced mixer and with no DC current
consumption, and thus it has LO suppression function. And the merit here is
that we do not need to add DC current source for the added switching pair,
which is the novelty of this up-converting mixer. The output is shown in Fig.
33
24 which consists of two differential output of the mixer. As we can see,
when there is no pulse envelope coming into input, the two differential
output is “in phase” (Fig. 25), which means it works like a double balanced
passive mixer with LO suppression ability. But in the contrast, without this
switching pair (single-balanced up-conversion mixer), the two different
output is “differential” as it should be (Fig. 26). The simplest way to explain
this LO suppression functioning is to consider this mixer as an open circuit
at both pairs’ tail current source when there is no input envelope, thus of
course Out+ and Out- are symmetrical and identical or “in phase” as the load
is same for the differential outputs which is exactly like double balanced
passive mixer. When we take the output of mixer differentially, they cancel
each other. And when there is big envelope coming in, the envelope is
modulated and we have differential output which can be converted to single
ended pulse with a triangular shaped envelope. Thus we need a differential
to single converter after this mixer.
34
Fig. 23 Circuit of Mixer Proposed.
Fig. 24 Output of Mixer.
35
Fig. 25 With added switching pair, the outputs are “in phase”.
Fig. 26 Without added switching pair, the outputs are “differential”.
36
4.3 Single to Differential Converter
The role of differential to single converter is to make the differential
output of mixer into single-ended. As we can see that the output of the mixer
is like “V” shaped, that is because of the triangular shape of envelope shown
in Fig. 23 is modulated by the LO signal which comes from the ring
oscillator. In order to turn them into a symmetrical pulse like Fig. 15, a
subtract function circuit is needed. We can use an on chip transformer to do
this function, but it is not easy to integrate and consumes big chip area. Here,
we decide to use the simple topology of differential to single converter [21],
which is shown in Fig. 27. The PMOS transistors are used at the input
because of the input in Fig. 27 has the shape of “V”, and the NMOS
transistors are used as at the current mirror thus the phase delay of this
subtract circuit can be minimized. All of the transistors are using the
minimum size in order to reduce the phase delay during subtract process and
make this to be nearly an ideal subtraction circuit.
37
Fig. 27 Differential to single converter.
38
4.4 Ring Oscillator
A three-stage ring oscillator is designed using the typical circuit topology
[22]. Because there-stage ring oscillator has smaller stage delay, we can
achieve high oscillation frequency without consume much current. The
circuit is shown in Fig. 28. This type of oscillator oscillate at lower
frequency when the bias is smaller, thus the current to charging and
discharging the capacitance of transistors is smaller, which lead to longer
stage delay, thus lower oscillation frequency. Current consumption of this
ring oscillator is about 5 mA using 1.8 V supply voltage and the layout area
is less than 0.01 mm^2. The tuning range of oscillation frequency is from 3.7
GHz to 6.7 GHz from measurement result by changing the bias of tail
current source. And the time transient response is shown in Fig. 29 with the
frequency spectrum.
39
Fig. 28 Ring Oscillator circuit.
Fig. 29 Oscillator Output.
40
4.5 Drive Amplifier (DA)
In order to amplify the pulse from the output of differential to single
converter and get -3 dBm output power which is matched to 50 ohm, a two-
stage drive amplifier is designed. The topology of drive amplifier is using
very popular cascode topology at each stage. The first stage of the drive
amplifier can be turned off by disable the CG stage transistor using a clock.
The clock goes through two inverters to control the bias of CG stage and
these inverters act as a buffer to increase the loading ability of the clock. The
reason to turn it off is for the whole radar system application in the future. In
this test pattern, we enable it all the time. The gain of two-stage DA is about
13 dB, and consumes 7 mA DC current from simulation and it has an IIP3 of
6 dBm when the input is using 125 ohm signal source which is equal to the
output impedance of the block before DA. The total circuit is shown in Fig.
30.
41
Input
Bondingwire
Output matched 50ohm
DA turn on/off control
Fig. 30 Drive Amplifier Circuit.
42
4.6 Single to Differential Converter
From the system architecture of the TX, we can see that the output of the
differential to single converter also goes to another branch besides the Drive
Amplifier. The function of this branch is to provide the Pulse Template to the
receiver, and it will be given to the correlator (which is a multiplier) in the
receiver. We use the simple CS topology to get the two differential output.
The topology is shown in Fig. 31.
Fig. 31 Single to Differential Converter.
43
4.7 Amplifier for Template Pulse
After the Single to Differential converter, we get the differential template,
and since the receiver need about 800 mV of each template, we need an
Amplifier for the template. Here we use the differential amplifier for the
differential template. The interface issue between the correlator and this
amplifier is a problem. We manage to use the inductor peaking for high
output template swing. Thus the inductor resonate with the receiver
correlator input capacitance at our center frequency for inductor peaking.
The size of the inductor is about 3.5 nH. The current consumption of this
Amplifier is about 3.3 mA at 1.8 V supply voltage. From system level
simulation we get the differential template each is 800 mV peak to peak. The
circuit topology is shown in Fig. 32.
Fig. 32 Template Amplifier.
44
V. Simulation Result of Transmitter and Layout
5.1 Simulation Result
Fig. 33 is the output of drive amplifier from simulation of first chip. The
output pulse has about 700 mV peak-peak-voltage at 50 ohm load, which is
about 0 dBm. The side lobe rejection is about 29 dB, which is between the
25 dB and 45 dB and it shows that the system architecture is reasonable. The
bandwidth of the pulse is about 500 MHz and it can be tuned by the
envelope of Charge Pump.
Fig. 33 Output of Drive Amplifier and its frequency spectrum.
45
The template output for the correlator in the receiver is shown in Fig. 34.
It has a near Gaussian shape so the side lobe rejection is 34 dB (Fig. 35).
Fig. 34 Template Waveform.
Fig. 35 Frequency Spectrum of Template.
46
5.2 Overall simulation result
The simulation result is shown in Table IV. The second version chip has
different result because of some modifications in the layout and we use
separated ground and voltage supply for the oscillator part.
Table IV. Summary of Post-Simulation in Version I chip
Table V. Summary of Post-Simulation in Version II chip
Required Postsim (TT) Corner(1.8V)
Para. Value unit 1.7V 1.8V 1.9V SS TT FF
DA amplitude >660 mV 546 660 970 470 660 1.0 V
Bandwidth 500 MHz 500 500 500 475 500 525
Side-lobe >20 dB 26 30 24 26 30 26
Frequency 4.35 GHz 4.4 4.35 4.3 4.4 4.35 4. 4
Template Vpp >800 mV 960 1200 1900 700 1.2V 1.7V
47
5.3 Layout of TX part and test block
Layout of this TX chip version one is shown in Fig. 36. The size of it is
about 1.4 mm^2. Layout of TX test pattern version two is shown in Fig. 37.
The size of it is about 2.1 mm×0.88 mm.
Fig. 36 Layout of TX.
Fig. 37 Layout of TX.
48
VI. Measurement Results
Two versions of transmitter test blocks have been fabricated in 0.18 µm
CMOS technology. Fig. 38 and Fig. 39 show the die photograph of two
chips. Both of chips have the active circuit area of 0.45 mm×1.2 mm for
transmitter part which is small for a whole transmitter. The whole circuit is
tested with 1.8 V supply. The second version chip was modified in some
blocks according to the analysis of the first version chip. And from the
measured wave forms, we can see some improvement of second chips.
49
Fig. 38 Die Photo of TX first version.
Fig. 39 Die Photo of TX second version.
50
6.1 First Version Transmitter measurement
Fig. 40 shows the measured output pulse wave form of the first version
transmitter. The input signals are two clocks of charge pump generated form
data timing generator DTG5334. The two clocks have 3 ns duration and
delayed by 3 ns.
From the measurement result of Fig. 40, we can see that the amplitude of
the pulse is over 300 mV, which is -6 dBm at 50 ohm load and meets our
target. The time domain pulse is symmetrical and the envelope is almost
triangular shaped. The width of the pulse is about 4.5 ns, which corresponds
to 460 MHz bandwidth in frequency domain. The slope of the pulse can be
tuned by changing the duration and delay of clocks and by the changing of
current source bias at charge pump. Here we adjust to an optimal value to let
the pulse has the best shape. Fig. 41 is the measured frequency domain
output of the pulse at PRF of 1 MHz. The spectrum has a sidelobe
suppression of about 20 dB.
Total current consumption is 17 mA for the transmitter and the drive
amplifier consumes 9 mA. The Ring oscillator consumes about 6 mA and
differential to single converter consumes 2 mA. The mixer does not
consume static DC power as we designed. For a whole transmitter, the
power consumption of 30.6 mA at this output power is acceptable for
impulse radar.
From the test, we find that the PRF of this transmitter can increase up to
51
100 MHz, thus makes it also capable of communication purpose application
using modulation of PPM or OOK. In this case we do not need the drive
amplifier and external power amplifier which are needed for radar
application. Table below summarizes the transmitter’s performances.
The comparison between the simulation and measurement is shown in
Table VI.
Fig. 40 Measured pulse time domain shape of first version chip.
52
Fig. 41 Measured pulse frequency spectrum of first version chip.
Table VI. Simulation and Measurement of first chip
Simulation Measurement
DA power 660 mV (0 dBm) 330~540 mV (-6 ~ -2 dBm)
Template amplitude 280 mv NA
Center frequency 4.3 GHz 4.3 GHz
Bandwidth 480 MHz 460 MHz
Power Consumption 30.6 mW 30.6 mW
Sidelobe rejection 27 dB 20 dB
53
6.2 Second Version Transmitter measurement
Fig. 42 shows the measured output pulse wave form of the second version
transmitter. The condition setup is same with the first version chip.
From the measurement result of Fig. 42, we can see that the amplitude of
the pulse is over 540 mV, which is -2 dBm at 50 ohm load and meets our
target. The time domain pulse is more symmetrical which means more
sidelobe suppression ability. The width of the pulse is about 4.5 ns, which
corresponds to 500 MHz bandwidth in frequency domain. From this time
domain figure we could see that it has improved amplitude and pulse shape
comparing with the first version chip.
The template amplitude is about 150 mV after the buffer, which
corresponding to over 500 mV of on chip template since buffer decrease the
amplitude of template a lot ( shown in Fig. 43).
Table VII below shows the simulation and measurement result of second
version TX chip. From the comparison with the simulation result and with
the Table VI, we could see the improvement in DA output amplitude by
nearly 4 dB and the pulse shape improvement by comparing the time domain
pulse shape and their FFT spectrum power density from Fig. 42 with Fig. 40.
54
Fig. 42 Measured DA output of second version TX chip with FFT spectrum.
Fig. 43 Template output of second version TX chip (with buffer).
55
Table VII. Simulation and Measurement of second chip
Simulation Measurement
DA power 0 dBm (670mV) > -2 dBm (540 mV)
Template amplitude 250 mv 170 mv
Center frequency 4. 3GHz 4.3 GHz
Bandwidth 480 MHz 500 MHz
Power Consumption 34 mW 36 mW (without buffer)
Sidelobe rejection 28 dB 22 dB
We use the second version chip to compare with other works, and we can
see that for the transmitter, this work has medium power consumption and
high amplitude. And above all, this transmitter proves best pulse shape
among all. The comparison is below, Table VIII.
Table VIII. Comparison of UWB transmitter
References Technology Amplitude Bandwidth Power
[20],2006 BiCMOS .18μm 200 mV 550 MHz 31.3 mW
[18],2005 CMOS 0.18μm 200 mV 528 MHz 2 mW
[15],2007 CMOS 0.18μm 640 mV 1.4 GHz 29.7 mW
This Work CMOS 0.18μm 540 mV 500 MHz 25.2 mW
56
VII. Conclusions
A “carrier based UWB” transmitter is designed after system level
discussion, comparison and analysis in Section II and III. Circuit blocks are
explained in detail in Section IV. The simulation and measurement result of
two version chips are shown in Section V and VI. From these works, we can
see that our output is what we expected although there are some differences.
The novelty of this work lies in two aspects. The first one is the system
architecture in Fig. 20. The other novelty is the design of this up-conversion
mixer shown in Fig. 23. Although the mixer can only be used in this typical
architecture, we have some invention indeed and which is proved by
simulation and measurement.
57
References
[1] Terence W. Barrett, History of Ultra Wideband (UWB) Radar
Communications: Pioneers and Innovators
[2] Tuan-Anh Phan et al, “A 18-pJ/Pulse OOK CMOS Transmitter for
Multiband UWB Impulse Radio,” IEEE MICROWAVE AND
WIRELESS COMPONENTS LETTERS, VOL. 17, NO. 9, Sep. 2007
[3] D. Barras et al., “A Robust Front-End Architecture for Low-Power UWB
Radio Transceivers”, IEEE Trans. Microwave Theory and Techniques,
vol. 54, no. 4, pp. 1713-1723, Apr. 2006.
[4] Robert A. Scholtz, “Ultra-Wideband Radio”.
[5] L. Stoica, S .Tiuraniemi, A. Rabbachin, and I. Oppermann, “An ultra-
wideband Tag circuit transceiver architecture,” in Proc. Joint Ultra-
Wideband Syst. Technol. Conf., Japan, May 2004, pp. 258-262
[6] Time Domain Company, “Ultra-Wide band Radio”.
[7] Payam Heydari, “Design Considerations for Low-Power Ultra Wideband
Receivers”.
[8] Ian D. O’Donnell, Robert W. Brodersen, “A Flexible, Low Power, DC-
1GHz Impulse-UWB Transceiver Front-end”, University of California,
Berkeley
[9] Smaini, et al. “Single-Chip CMOS Pulse Generator for UWB Systems “,
JSSC 2006
[10] L. Stoica, A. Rabbachin, and I. Oppermann, “A low-complexity
58
noncoherent IR-UWB transceiver architecture with TOA estimation”,
IEEE Trans. Micro. Theory Tech., vol. 54, no. 4, part II, pp. 1637–1646,
April 2006
[11] Yuanjin Zheng , “A Low Power Noncoherent CMOS UWB
Transceiver ICs”, 2005 IEEE Radio Frequency Integrated Circuits
Symposium
[12] Y Zheng,” A CMOS Carrier-less UWB Transceiver for WPAN
Applications”, ISSCC2006
[13] Takahide Terada, et al. “A CMOS Ultra-Wideband Impulse Radio
Transceiver for 1-Mb/s Data Communications and 2.5-cm Range
Finding”, IEEE JOURNAL OF SOLID-STATE CIRCUITS
[14] Thilo Sauter, “Gaussian pulses and superluminality”, J. Phys. A:
Math. Gen. 35 (2002) 6743–6754
[15] T. Norimatsu, R. Fujiwara, M. Kokubo, M. Miyazaki, A. Maeki,
Y.Ogata, S. Kobayashi, N. Koshizuka, and K. Sakamura, “A UWB-IR
transmitter with digitally controlled pulse generator,” IEEE J.Solid-State
Cicuits, vol. 42, no. 6, pp.1300–1309, Jun. 2007.
[16] Mi-Kyung Oh, Jae-Young Kim, and Kwang-Roh Park, “Digitally-
Controlled UWB Pulse Generator for IEEE 802.1 5.4a systems,” IEEE
2007
[17] Y.H.Choi, Gated UWB pulse signal generation,” in IEEE Joint
Int.Workshop UWBST IWUWBS, May 2004, pp.122-124.
[18] J. Ryckaert, et al. “Ultra-Wideband transmitter for low-power
Wireless BodyArea Networks design and evaluation,” IEEE
59
Trans.Circuits Syst.-Part I, vol. 52, no. 12, pp. 2515–2525, Dec. 2005.
[19] Y. Zheng, Y. Zhang, and Y. Tong, “A novel wireless interconnect
technology using impluse radio for interchip communications,” IEEE
Trans. Micro. Theory Tech., vol. 54, no. 4, part II, pp. 1912–1920, April
2006.
[20] D.D. Wentzloff and A.P. Chandrakasan, “Gaussian pulse generators
for subbanded Ultra-Wideband transmitters,” IEEE Trans. Micro. Theory
Tech., vol. 54, no. 4, part II, pp. 1647–1655, April 2006.
[21] Behzad Razavi, “Design of Analog CMOS Integrated Circuits”.
[22] Asad A. Abidi, “ Phase Noise and Jitter in CMOS Ring Oscillator,”
Journal of Solid-State Circuit, Vol.41, NO.8, August 2006