低功耗UWB雷达传感器技术在物联网中的应用

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"低功耗UWB雷达传感器是UWB技术在传感器领域的一种应用，主要关注于节能设计和微波雷达技术的结合。UWB（Ultra-Wideband）是一种无线通信技术，通过发送极短的脉冲来传输数据，具有低功率消耗、高精度定位、抗多径干扰等优点。这种传感器常用于监测、探测和跟踪等多种应用场景。在《LOWPOWERUWBCMOSRADARSENSORS》这本书中，作者Nuno Paulino、João Goes、Adolfo Steiger-Garção详细探讨了低功耗UWB CMOS雷达传感器的设计与实现。他们利用CMOS工艺技术，实现了集成了模拟电路和信号处理功能的低功耗雷达传感器，这在微电子学领域具有重要意义。CMOS（Complementary Metal-Oxide-Semiconductor）工艺因其高集成度、低功耗特性，是现代电子设备中的主流选择。 UWB传感器的工作原理通常涉及发射器、接收器和信号处理单元。发射器生成超宽带脉冲，这些脉冲在空间中传播，遇到物体后反射回来。接收器捕获这些回波，然后通过信号处理单元进行解码，从而获取目标的距离、速度等信息。由于UWB信号的低功率和宽频带特性，这种传感器在距离测量、运动检测、生物医学应用等方面表现出色。书中还提到了其他相关著作，如《THE GM/ID DESIGN METHODOLOGY FOR CMOS ANALOG LOW POWER INTEGRATED CIRCUITS》，该书由Paul G. A. Jespers撰写，深入探讨了低功耗模拟集成电路设计中的GM/ID方法，这对于理解UWB雷达传感器的电路设计至关重要。此外，《SUBSTRATE NOISE COUPLING IN RFICs》讨论了射频集成电路中的衬底噪声耦合问题，这是UWB雷达传感器在高频环境下需要考虑的关键因素。在UWB雷达传感器的设计中，还需要考虑诸如干扰抑制、频率选择性、动态范围等挑战。例如，《MULTI-BAND RFFRONT-ENDSWITH ADAPTIVE IMAGEREJECTION》探讨了多频段射频前端如何适应性地抑制图像干扰，这对于提高UWB雷达传感器的性能至关重要。而《SILICON-BASED RFFRONT-ENDSFORULTRAWIDEBAND RADIOS》则关注于硅基射频前端在超宽带无线电中的应用，这部分内容对于实现高效的UWB雷达传感器前端至关重要。低功耗UWB雷达传感器是UWB技术在传感器领域的创新应用，结合了低功耗设计和高级信号处理技术，以实现精确、节能的探测功能。通过深入研究这一领域的专著，我们可以了解到这一技术背后的复杂电路设计和优化策略，为未来的UWB应用提供重要的理论和技术支持。"

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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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