~ ~ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Efficient digital sweep oscillator with extremely low sweep rates A.A. Hiasat A.A. AI-Khateeb zyxwvutsrqpo Indexing terms: Pedersen digital sweep oscillator, Fractional addressing zyxwvutsrq Abstract: zyxwvutsrqpo The performance of the Pedersen digital sweep oscillator is improved using a new technique which is intended to improve the sweep rates so that they can reach extremely low ranges without the need of increasing look-up table width. The proposed technique is shown to be very efficient in increasing the effective table length, and hence, the number of generated samples during the sweep time. The sweep rate is improved by storing and reading the sine and cosine components of two sweep functions that are not harmonically related, and then evaluating the sweep signal of the phase difference. Better performance can be achieved by using the trigonometric identity method as demonstrated. There are no limitations on the time-bandwidth product of the proposed structure. Performance calculations and simulation results are given to support the analytically claimed results and demonstrate the performance. 1 Introduction Sweep signals are essential components in many appli- cations. They are widely used for system characterisa- tion, radar, sonar, acoustic imaging, target velocity estimation [ 11, time-delay spectrometery for calibration of ultrasonic transducers [2], determination of the sys- tem response with network analysers, permittivity measurements [3] and phase coding of sweep signals in communication applications. Although there are several applications which require a coherent sweep, i.e. a sweep signal where both the phase and frequency must be specified for all time, there are only a few methods for generating coherent sweep signals. The sweep signal can be generated by storing a predetermined waveform, such as a coherent sweep, in a high-speed digital memory. This method is limited by the time-bandwidth product. Other methods are limited by the maximum attainable frequency. A new method for generating digital sweep signals was proposed by Pedersen [4]. His approach is based on real-time digital evaluation of the phase function of zyxwvu 0 IEE, 1998 IEE Proceedings online no. 19982396 Paper first received 1st October 1997 and in revised form 2nd July 1998 The authors are with the Department of Electronics Engineering, Princess Sumaya University College of Technology, PO Box 1438, Amman 11941, Jordan (e-mail: aahiasat@rss.govjo) the desired swept signal, and then reading its value from a look-up table. Two integrators in cascade must be used to generate the quadratic phase function of the sweep signal. The sweep rate of the Pedersen sweep oscillator is determined by the parallel shifter on the address lines, by shifting the value of the total phase p bits to the left. The least significant zyxw p bits of the accu- mulator content are discarded when addressing the look-up table (LUT). The next signfficant K bits of the accumulator are used as an address pointer of the look- up table. Nevertheless, Pedersen described how an arbi- trary sweep rate can be achieved by varying the clock frequency. The main limitations of the Pedersen sweep oscillator are: the sweep rate is varied in steps of two only; and both the start frequency and start phase can be preset with integer values only. The second limitation causes harmonic distortion to be present in the generated waveform [5, 61. However, the performance of the Ped- ersen sweep oscillator was improved by implementing fractional addressing utilising interpolation methods [7]. Moreover, the sweep rates were improved [7] so that they can be varied with steps finer than that of the Pedersen system [4]. One more disadvantage of the Pedersen sweep oscillator is that the minimum sweep rate determines the necessary bit width of the accumu- lator and the counter. In this paper, a new technique is proposed to improve the sweep rates so that they can reach extremely low ranges without the need to increase the LUT width, or the accumulator and the counter bit lengths. The new technique is shown to be very effi- cient in increasing the effective table length. Moreover, the time-bandwidth product of the proposed sweep oscillator is increased by a factor of the table length over that of Pedersen’s sweep oscillator The low sweep-rate oscillator, proposed in this paper, can be efficiently implemented in applications that need a relatively long time measurement with varied fre- quency, such as earth-movement measurement, long time-delay spectrometry, weather-condition measure- ment, and determination of some system responses (radiation materials and their life time). These applica- tions justify the extra hardware requirements of the new oscillator. 2 New proposed sweep oscillator Coherent sweep signals can be generated by carrying out a digital integration twice, whereby the quadratic phase function of a linearly swept signal is produced and the modulus (24 is extracted from the total phase, 409 IEE zyxwvutsrqponm Proc.-Circuit.s Devices Syst., Vol. 145, No. 6, December zyxwvutsrqp I99X