低功耗超宽带UWB雷达传感器技术解析

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"Nuno Paulino, Joao Goes, Adolfo Steiger Garcao的《Low Power UWB CMOS Radar Sensors》是Springer出版社在2008年出版的一本书，该书属于Analog Circuits and Signal Processing Series，由Mohammed Ismail担任咨询编辑。这本书详细探讨了低功耗超宽带UWB（Ultra-Wideband）雷达传感器的设计与应用，旨在提供有关UWB雷达射频微波技术的专业知识。" 《Low Power UWB CMOS Radar Sensors》这本书聚焦于利用互补金属氧化物半导体（CMOS）技术设计的低功耗超宽带雷达传感器。UWB雷达是一种使用极短脉冲或宽带信号进行探测的技术，它在无线通信、目标定位、生物医学成像、室内导航等领域有着广泛的应用。由于其具有高分辨率、抗多径干扰和低功耗的特性，UWB雷达传感器在近年来受到了越来越多的关注。书中可能涵盖了以下几个关键知识点： 1. **UWB雷达原理**：解释了UWB雷达的工作机制，包括脉冲雷达、连续波雷达等不同工作模式，以及如何通过超宽带信号实现高时间分辨率和距离分辨率。 2. **CMOS技术**：讨论了采用CMOS工艺制作雷达传感器的优势，如成本效益高、集成度高、功耗低等，并可能探讨了CMOS工艺对UWB雷达性能的影响。 3. **低功耗设计**：详细介绍了降低雷达传感器功耗的各种策略，包括电路优化、功率管理技术和能效比的提升方法。 4. **射频微波电路**：涵盖了用于UWB雷达的射频前端设计，如混频器、放大器、滤波器等，以及它们在系统中的作用和挑战。 5. **信号处理**：阐述了UWB雷达信号的处理技术，如脉冲压缩、多普勒分析和目标检测算法，以及这些技术如何提高雷达系统的性能。 6. **应用案例**：可能包含了一些实际应用示例，如无线通信、环境监测、人体健康监测等，展示了UWB雷达传感器的多样化应用场景。 7. **系统集成**：讨论了将所有组件集成到单个芯片上的挑战和解决方案，包括布局布线、噪声管理和互连设计。这本书不仅适合电子工程专业的学生和研究人员，也对那些希望深入了解UWB雷达传感器设计和应用的工程师和技术人员非常有价值。通过深入学习，读者可以掌握如何利用CMOS技术开发出低功耗、高性能的UWB雷达传感器。

8 2 UWB Signals and Systems

CLK

out

Fig. 2.2 Sub-Sampling of a repetitive fast input signal in order to produce a slow output signal

with the same shape as the fast input signal

Using pulses with very short duration it is possible to measure the impulse re-

sponse (through a sampling oscilloscope) of a distributed microwave network and

then calculate the frequency response using the Fourier transform. The maximum

and minimum frequency values of the analysis are determined by the pulse width

(the shorter the pulse the higher the maximum frequency value of the analysis)

and by the duration of the observation time (the longer the observation time the

lower the minimum frequency value of the analysis). Of course that increasing the

maximum and the minimum frequency of the analysis requires both a fast sam-

pling frequency in the oscilloscope and a large number of sampled points of the

impulse response. This procedure reduced the time needed to measure the fre-

quency domain properties of radar stealthy materials by a factor of 100 [8]. The

traditional method consists in applying a sine-wave signal, measuring the S pa-

rameters and then repeating the procedure for the different frequencies of interest.

This is the method of choice for narrow band systems. But for wide band systems,

such as today's digital systems (Rambus, USB 2.0, Firewire and others), it is diffi-

cult to interpret the measured frequency response and understand where the dis-

continuities in the transmission lines are physically located. TDR provides a more

intuitive and direct look into the signal integrity in digital systems [11], [12]. By

using a step generator to generate a fast edge into the transmission line and using a

sampling oscilloscope to monitor the reflected waves at different points in the line

it is possible to locate the line discontinuities and understand the nature (resistive,

inductive or capacitive) of each discontinuity. It is also possible to understand

whether losses in a transmission system (a circuit board trace, a cable, a connector

and so on) are series losses or shunt losses.

TDR has also been used to locate various defects in electrical power distribu-

tion cables (such as: voids, shield protrusions, contaminants, etc.) by locating par-

tial discharge events in the cable [13], [14]. The cable under test, terminated with

an open end, is connected to an excitation voltage source at the other end. This

voltage is gradually increased until a partial discharge (PD) event occurs at a cer-

tain point in the cable; the pulses generated by the PD event travel in the cable and

are reflected by the ends of the cable until they disappear because of the cable

2.1 A Brief History of UWB Systems and their Applications 9

attenuation. By measuring the time between the arrivals of the various pulses at

one end of the cable it is possible to determine where the defects are located.

Another area of application for TDR techniques is liquid level measurement in

tanks [15], [16]. The level of the liquid (or liquids) in a tank can be determined us-

ing a probe line that acts as an open transmission line for the TDR pulse signal.

The tank contents that surround the probe line become the transmission line isolat-

ing dielectric. At each fluid interface, a change in the dielectric constant of the flu-

id surrounding the probe results in a change in the electrical impedance of the line

that reflects a portion of the input pulse amplitude to the TDR receiver. The Pulse

reflection amplitude and timing information can be used to accurately determine

the fluids levels and the location of each fluid interface within the tank (if more

than one type of fluid is present in the tank).

In the 1970's it became apparent that the sub-nanosecond pulse technology

could be employed in a variety of proximal sensing applications [8], [17], this is

essentially extending the time domain reflectometry principles to work in free

space without a transmission line, this is also know as free-space time-domain re-

flectometry. Since in those systems, the objective is simply to determine the pres-

ence or absence of a return signal from a possible target, the return sensing system

(receiver) can be designed to determine the threshold (location) information only,

leading to significant reductions in cost and complexity of the required electronics

for target signature analysis. The receiver senses, over a narrow time window,

when the input signal exceeds a given threshold value, indicating the presence of a

target and determines the time delay between this event and the initial transmis-

sion of the sub-nanosecond pulse. The distance to the target is directly related to

this time delay. This type of systems can be used to establish security “bubbles”,

where an alarm is triggered if something enters the protected area. In [8] two types

of receivers are described. The first, called single-hit receiver is based on a tunnel

diode that is triggered by the input very-fast low energy impulse. This receiver is

depicted in Fig. 2.3.

2.1 A Brief History of UWB Systems and their Applications 11

circuit is ready to detect another fast pulse at the input. In conclusion this circuit

works as a monostable circuit, that when a small amplitude short pulse is pre-

sented to its input, produces a large amplitude pulse with a long duration at the

output. This output pulse can be used to measure the time between the transmitted

pulse and the return pulse created by the target (echo) and therefore, determine the

distance of the target. The clock signal that triggers the transmission of the short

pulse can be applied to a transmission line with the necessary time delay for the

pulse to travel to target and back to the receiver; this delayed clock can be used to

sample the output of the receiver, to determine if a target is present at the prede-

termined distance. The sensitivity of this receiver is determined by the tunnel di-

ode, if this device is biased closer to its turning point, the amplitude value of the

input pulse necessary to trigger the diode is reduced. This means that the circuit

threshold has been lowered and it will be more sensitive to any input pulses, but at

the same time it will be more prone to trigger on unwanted signals, i.e. circuit

noise.

The second type of receiver described in [8], called a constant false-alarm rate

receiver, improves the sensibility of the single-hit receiver by processing several

hits before signaling the detection of the target. The receiver is essentially the

same, except that the bias current of the tunnel diode is adjusted by a feedback

loop. The objective is to bias the diode as close as possible to the threshold of the

negative resistance region, to increase the sensitivity of the receiver. The feedback

loop is controlled by the number of hits detected when no signal is present. This is

archived by radiating the pulse on the rising edge of a clock and sampling the out-

put of the single-hit receiver on the falling edge of the clock. As the sensitivity of

the receiver is increased the number of false hits increases, the feedback loop will

adjust the bias current of the tunnel diode until the number of false hits is equal to

3 over 32 observation time instants. This means that the probability of a false hit is

around

(1/16) , the probability of 32 consecutive false hits would be equal to

(1/16)

which is a very small number. The output of the single hit receiver is

sampled when the clock is present and the number of hits obtained is added, a tar-

get is considered to be present when this sum exceeds 25, this will eliminate most

of the false hits. A proximity radar for detecting incoming ammunitions, using this

receiver, is described in [18].

In the early 1990's, McEwan [19] developed a low cost, low power impulse ra-

dar system, using discrete components mounted on a small PCB; this sensor is

called Micropower Impulse Radar (MIR). This radar sensor uses impulses with a

duration between 1 and 2 ns. These pulses are generated by applying a step signal

to a step recovery diode. The radar transmitter emits these pulses with a random

interval to reduce the probability of interference between different radar sensors.

Each sensor has a random noise generator that modulates the period of the square

wave that is used to generate the short pulses. A simple block diagram of the im-

pulse transmitter is shown in Fig. 2.4

12 2 UWB Signals and Systems

Random Interval

generator

Fig. 2.4 Block diagram of the impulse transmitter used by the MIR radar sensor

The short pulses are generated using a step recovery diode [20]. Normal diodes

have a slow transition when the current is switched from forward to backward, the

step diode is designed to have an abrupt transition when the switch occurs, thus

producing a short pulse.

The pulse transmitter has nominal pulse repetition interval (PRI) typically equal

to 1

Ps, meaning that on average 1 million impulses per second are radiated. The

receiver is constituted by two single ended integrating samplers. These circuits are

constituted by a diode switch followed by an analog integrator. This integrator av-

erages the input samples, if no signal is present the average output of the integra-

tors will not change, if a signal is present the average output of the integrator will

increase. This configuration integrates several impulses to improve the circuit sen-

sitivity, taking advantage of the large number of available impulses. The input

signal is sampled into an integrating capacitor when a strobe signal is applied.

This strobe signal is generated from the random interval generator using a circuit

equal to the one depicted in Fig. 2.4. The strobe signal determines the range at

which a target might be located, thus a delay must be inserted between the random

interval generator and the short pulse generator circuit. In a simple application,

such as intrusion detection this delay can be constant and its value determines the

value of the security perimeter.

Another area where UWB signals have been applied is the field of subsurface

radar. The objective is to detect and locate buried artifacts and structures within

the upper regions of the earth's surface [21], [22]. Any dielectric variation in the

buried materials, produces a reflection from an incoming electromagnetic signal,

this reflection can be used to determine the depth and type of buried material.

Since the materials that constitute the earth structure introduce a high signal at-

tenuation, the maximum operating depth of these radars is usually limited to a few

meters (the signal attenuation ranges from 10 dB per meter to 200 dB per meter,

depending on the material). The signal attenuation is dependent on the type of ma-

terials of the soil and, in general, it increases with the signal frequency. The max-

imum useful frequency is below 1 GHz, for example wet clay soil has an attenua-

tion of 30 dB per meter for a frequency value of 100 MHz and an attenuation of

100 dB per meter for a frequency value of 1 GHz [21]. In most cases, subsurface

radar systems are forced to work with signal frequencies below 1 GHz but, need to

have a high-resolution to detect buried artifacts at shallow depths; therefore base-

band pulses with a short duration that inherently have a large bandwidths are nec-

essary. Typically, pulses with an amplitude of 100 V and a duration between 1 ns

and 5 ns are used. The received signal is down-converted using a sampling cir-

cuit and digitized, so that complex signal processing algorithms can be applied.

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