Improvement of Ultra-Wide-Band Signal Timing

Improvement of Ultra-Wide-Band Signal Timing

B. Vojnović, B. Medved Rogina

Laboratory for Stochastic Signals and Processes Research, Electronic Division,

Ruđer Bošković Institute

Bijenička 54, Zagreb, Croatia

Phone/Fax: +385 1 4680 090, E-mail: ;

Abstract – Ultra-Wide-Band (UWB) technology became in recent years one of very promising technologies worlwide. It shows growing application in many areas, especially in the two fields: high-resolution, low-power pulse radar sensor and wireless UWB communications. The question of precise time position of the pulse signal, which carry essential information is in the focus of designers interest. In the most UWB applications we need to generate low jiterred, ultra-short signal. A method for generating subnanosecond pulse signals with time-jitter as low as 100 ps was described and theoreticaly analyzed.

I. INTRODUCTION

Ultra-Wide-Band (UWB) wireless technology became in recent years one of very promising technologies after recognition of their commercial potential [1,2,3,]. World wide interest in UWB wireless increased greatly after year 2002, when US Federal Communication Comission (FCC) gave first authorization for it. In Europe research on UWB started later with some concerns about the interference it could cause. Other names for this techniques are somewhere in use: impulse communications, impulse radio, subnanosecond communications, as well as carrier-free , nonsinusoidal communications etc.

According to actual FCC statement, UWB is any signal that occupies at least of 500 MHz of bandwidth in the spectrum range between 3.1 and 10.6 GHz. Because this technology is developping very fast we could expect some changes in the regulation area. In the practice, signals covering the spectrum range from 1,5 GHz and higher are considered as UWB also. Additional requirement is that the fractional bandwidth, defined as

is higher than 0.25, where: fH is the upper and fL lower frequency limits, which define frequency range containing dominant part of total signal energy. It is evident that this definition avoids requirement for definition of the center frequency. In practice it is often defined by the relation

Speaking in the time-domaine, UWB technique involves transmitting, receiving and processing of stream of low power (in the region of microwatts), very short pulses in the order of 10 picoseconds to 1000 picoseconds.

UWB signal has usualy the shape the Gaussian monocycle (Fig. 1.), sinusoidal monocycle or "bunch" of few sinusoidals.We consider two important technologies, which are under intensive development: UWB communications and pulse radar rnging, imaging and sensing systems. In communications systems the main issue is how to optimaly transfer voice, video nad data informations. Most used method to carry the information is the pulse time-modulation, which, however, poses strong requirements on UWB pulse timing-jitter (Fig. 2.).

Fig. 1a-Gaussian monocycle

Fig. 1b. Frequency spectrum of Gaussian monocycle

Mostly considered types of modulations are: pulse position modulation, pulse amplitude modulation, on-of-keying and bi-phase-shift keying modulations, as well as pseudo-random and chaotic modulations. Special attention has been payed last years to apply chaos theory and practice in UWB communication technology [4, 5].

Pseudo-random (Fig. 3.) and chaotic time modulation are similar in some way. These are both very robust regarding signal interception and detection, because they "smoothes" the frequency spectrum and make it appear as a pure "white noise", as it can be seen comparing spectra on Fig. 2b. and Fig. 3b. This enables more secure communication and makes have virtually unlimited number of channels.

In estimating UWB technology one has to compare its advantages and draw-backs.

There are many important advantages of UWB techniques;

(1) In data communication applications the system is working with modulated subnanosecond pulse trains having duty-cycle of less than 10 %. As a result we can deal with broad, noise-like frequency spectrum, having very low average power, capable to work in noisy and crowded environment. The well designed system shows high degree of interference resistivity, becuse any interfering signal affects only a small portion of the signal. On the other hand, due to very low power of the UWB signal its interference to other systems could by practicaly neglected. Many factors have to be analyzed to estimate system interference performances regarding other systems: distance between devices, modulation techniques used and propagation loss in a channel.

(2) UWB signal has the high probability to get through obstacles (walls) or to penetrate in the material, which make it suitable for many applications.

(3) UWB subnanosecond signal can be used for very precise distance, displacement and profile measurements, important in technical and medical diagnostics.

(4) It would be possibble to inject UWB signals in existing cable systems, becuse its power beyond the noise level in these systems. Some companies are considering such possibility [6].

The main drawback of UWB technology is potential interference with other wireless communication systems such as Global Positioning System (GPS), as well as other systems using narrow band.

Fig. 2a. Pulse-position modulation (PPM)

Fig. 2a. Pulse-position modulation (PPM)

The afore mentioned characteristics of UWB make it suitable for many applications such as:

1.Short distance voice, video and data communications links for single or multichannel transmission. This technique is very suitable for indoor multichannel links that will not interfere with existing devices.

Fig. 2b. Frequency spectrum of PPM

2. Pulsed radar imaging systems that include: ground penetrating radar (GPR) for detecting underground infrastructure location and condition, through-wall radar for detecting the objects through structures, radar anti-intrusion systems, etc.

3. Pulse-radar measurement systems that covers distance and displacement measurements, material thickness measurements, and many others.

Fig. 3a. Pseudo-random time modulation

Fig. 3b. Frequency spectrum of pseudo-random time

Fig. 3b. Frequency spectrum of pseudo-random time modulation

4. Radar sensors including robotic sensors, liquid level sensors and vibration sensors.

5. Small portable (stethoscope) radar for medical applications (heart dynamics and lung diagnostics).

II. CHARACTERISTICS AND METHODS OF GENERATIONS OF UWB PULSE SIGNALS

Requirements regarding pulse signal characteristics depends on applications. For operations in the 3.1-10.6 GHz communication systems , pulse durations or rise-times need to be in the order of 50-100 ps. The pulse amplitude is several volts and pulse repetition rate in the range of 10-100 MHz.

In pulse radar systems pulse duration is extended to about 1ns, while the amplitude could be more than 100 volts.

To generate these various vaweforms one usualy starts with step-like pulse, having fast rise-time or with very short impulse. If insufficient power is generated frome these basic circuits, a wideband post amplifiers shoul be added in the system.

To obtain desired pulse shape, which is mostly monocycle, different pulse shaping filters are used. In addition, UWB antenna if properly designed, acts as band-pass filter, thus affecting output pulse shape.

To obtain 50-100 ps pulses,in communication systems, there are several possibilities. Step-recovery (snap-off) diode (SRD) circuits have been used to obtain pulses having 50- 200 ps rise-time with amplitude of some hundred volts on the antenna terminal, when combined in conjuction with avalanche transistor.

Newest type, known as drift-step-recovery diode (DSRD) is able to produce more than 500V, in several hundreds of picoseconds [7] The DSRD as against SRD is on using minority-carriers with long-life-time and its breakdown voltage exceeds500volts per p-n junction.

Some FETs and fast ECL logic circuits can generate of about 1V pulses with 50-100 ps edges.

For extremly short pulse signals (about 10 ps) a new technology, Nonlinear Transmission Line (NLTL) is a promising solution. This is a synthetic transmission line consisting of series inductors shunted by varactor diodes, acting as voltage variable capacitors. The time delay through NLTL is a function of input signal amplitude. The whole circuit acts as an pulse-edge compressor. It was reported a 6V, 4 ps rise-time pulse, obtained by commercialy available NLTL pulse generator [8].

For pulse radar applications there are broader requirements on pulse parameters. In low power radar sytems the required pulse characteristics are similar to these in communications except the pulse width, which is wider to few nanoseconds. In GPR systems pulse amplitude can achieve more than 100 V, while the pulse width is about 0.5-1 ns. Such signals are usualy generated using fast FET circuits, avalanche transistors or DSRD circuits. The drawback of avalnche transistors is ther relative short life-time.

One of the most important parameter is the UWB signal is the pulse time-jitter. Strongest requrements on it are for communication systems and some high precision pulse radars. It can be estimated that the time-jitter should be in the range of 20-200 ps.

Generation of UWB signals is achieved by two basic methods: (1) use of direct signal generator followed by pulse shaping circuits and pulse post amplifier if needed; (2) use of circuits for post sharpening the slower signal, previously obtained from signal generator, and followed by necessary shaping circuits.

It is important to emphasize the contribution of the antenna frequency characteristics to the whole system output pulse-shape [8].

III. A NEW METHOD TO LOWER THE UWB PULSE SIGNAL TIME-JITTER

The method [9] is suitable for various UWB pulse circuits. It uses the pulse post sharpening for pulse generation. In this manner several goals could be achieved regarding pulse parameters requirements: fast rise-time and low time-jitter, less sensitive to the temperature variations, noise, and input pulse amplitude, as well as rise-time variations. Although the process of pulse sharpening assure low time-jitter, the problem of the input signal jitter has to be solved by using very stable clock circuits.

Fig. 4. Basic circuit of SRD pulse sharpening generator

Proposed method is based on switching properties of step-recovery diode, which belongs to class of charge storage diodes (including P-I-N diodes and noise diodes). It is a P-I-N junction structure, normally silicon, whose static characteristics are similar to other p-n junction diodes, but dynamically it is quite different, acting as a charge controlled switch. Such a property is physically realized by a lightly doped region around the junction, giving rise to a built-in retarding electrical field for the carrier transport in the forward direction. Diode forward current, static or transient, injects the charge into the diode. When this charge is being removed by the reversepulse current, the diode continues conducting (low impedance state) until all the charge is removed. At the point when the total amount of charge in the diode becomes zero, it stops conducting (high impedance state) very abruptly, in less than 100 ps. In this manner the diode acts as an integrator and zero-crossing ultra-fast timing discriminator. The basic circuitis given in Fig. 4.

The input pulse current signal injects the charge into unbiased diode. The same pulse inverted and amplified by a factor k, starts after well-defined delay-time to remove the stored charge. At the moment Td (discrimination time), when the total charge becomes zero, we get the sharp edge pulse having rise-time less than 100 ps, which gives us the time reference relying very strongly on the input pulse arrival-time (Fig. 5.).

In the analysis it is assumed that the diode is "ideal" current integrator and charge zero-crossing discriminator, because the following requirements are met:

(1)Injected charge is completely removed during reverse recovery phase;

(2)The leakage charge due to reverse diode capacitance is negligible;

(3)The minority carrier life-time could be chosen long compared to diode conducting time;

(4) The diode does not exhibit charge (energy) triggering sensitivity because it does not belong to the class of regenerative circuits. That is important especially for slower input pulses , because there is no time-walk due to different signal slope at the point of discrimination.

(5)The diode minority-carrier life-time is one order of magnitude higher than the discrimination time. This condition enables additionally, discrimination time be insensitive on temperature variations.

The discrimination time Tdis defined (if we suppose that the time scale begins at the moment of the pulse arrival) by the equation:

where: I0 is pulse current amplitude, k is amplification factor of reversed and delayed pulse, td is pulse delay time, and s(t) is the shape function of the pulse. For the approximation of linear rise of the signal we get:

In this case discriminator time Td does not depend on pulse signal parameters, but only on circuit parameters k and td which are constants. If there is no correlation between random changes of circuit parameters k and td, and k is sufficently large, the time-jitter depends only on the quality of delay-line [9].

We have tested SRD circuit intentionally with very slow input pulse signals, having rise times between 100 and 500 ns. Pulse repetition frequency was les than 1 KHz. Dynamic of input pulse amplitudes was 1:15. Measured standard deviation of the time-jitter was in the range of 110-350 picoseconds. Obtained results show high degree of robustness of the method against input pulse amplitude and rise-time variations.

The results of the analysis of the noise influence on timing error [11] have shown that the standard deviation-to-rise-time ratio was less than 1/100 for the noise-to-signal ratio less than 1/10.

Comparing these results, we see that the proposed method of UWB signal sharpening and timing is dominantly sensitive on of the noise influence. In real systems we can easily assure signal-to-noise ratio of input pulse higher than 100, as well as stable amplitude and rise-time of the pulse. This enables estimate the timing-jitter of the UWB signal far below 100 picoseconds.

IV. CONCLUSION

Accepted range of UWB pulse time-position jitter in manyapplications is 10-100 picoseconds. Timing improvement of UWB pulse was achieved using the new method, based on step-recovery diode switching properties. This enables to generate UWB signal to meet requirements on timing-jitter as well as pulse amplitude and width stability.

Obtained results indicate that the proposed method is suitable for generating UWB signals, having timing-jitter far below 100 picoseconds.

.

id(t)

Td t

td

5a. Step-recovery diode injecting curent

Fig. 5b. UWB current pulse-signal

LITERATURE

[1] G. R. Alello; Ultra Wide-Band Wireless Systems, IEEE Microvwe, June 2003, pp. 36-47.

[2] H. V. Poor; Signal Processing for Wideband Communications, IEEE Information Society Newsletter, June 1992.

[3] J. D. Taylor; Ultra-Wideband Radar Overview, Ch. 1 in Introduction to Ultra-Wideband Radar Systems, Ed: J. D. Taylor, CRC Press, 1995.

[4] N. F. Rulkov, L. S. Tsimring; Synchronization Methods for Over Band-Limited Channels, International Journal on Circuit Theory and Applications, Vol.27, No. 6, December 1999, pp. 555-567.

[5] M. Ssushlik, n. Rulkov, L. Larson, L. Tsimring, H. Abarbanel, K. Yao, A. Volkovski; Chaotic Pulse Position Modulation: A Robust Method of Communications with Chaos, IEEE Communications Letters, Vol. XX, No. Y, Januar 1999.

[6] S. Stroh; Ultra-Widwband: Multimedia Unplugged, IEEE Spectrum, September 2003, pp.24-27.

[7] I. V. Grekhov et al. ; High Power Subnanosecond Switch, Electronics Letters Vol.17, No.7, 1981, pp. 422-423.

[8] J. R. Andrews; UWB Signal Sources, Antennas and Propagation, Application Note AN-14a, Picosecond Pulse Labs., Boulder, CO, August 2003, pp.1-11.

[9] B. Vojnović; A Subnanosecond Timing Circuit Using Snap-Off Diode, Compte rendus du Colloque Internationale sur l'Electronique Nucleaire, Versailles,France, 1968, pp. 59-1 to 59-7.

[10] B. Vojnović; Picosecond Timing with Slow Optoelectronic Pulse-Echo Signals, Proceedings of Topical Meeting on Optoelectronic Distance/Displacement Measurements and Applications, ODIMAP III, Pavia, Italy, 2001, pp. 399-404.

[11] B. Vojnović; Error Minimization of Sensor Pulse Signal Delay-Time Measurements, Proceedings of 23rd International Conference on Microelectronics, MIEL2002, (IEEE Sponsored), Niš, Yugoslavia, IEEE-Electron Devices Society, USA, 2002, pp. 265-268.