【正文】 dure is repeated (restarted) for each new measurement. Another important advantage is the noise immunity of the system, due to its closed loop nature. A detailed study of the noise behavior has not been carried out in this paper. This is mainly because the aim of this text is to present an alternative principle of frequency measurement. Moreover, the final output of the system is taken after some further processing (measurement correction) which also contributes to the noise immunity. 借助 DDS 的精密頻率的一種替代方法 內(nèi)容 頻率測量的方法基于閉環(huán)組成 ,主要是一個頻率比較器 (FC)和直接數(shù)字合成 器 (DDS),對此在本文中進(jìn)行了介紹。s maximum. In addition, the frequency step of the approximation would equal the 1/4 of the DDS maximum frequency. On every approximation the frequency step is divided by two and added or subtracted to the FSW of the DDS, depending on the output of the Frequency Comparator. The approximation procedure stops when the step size decreases to one. After that, an up/down counter substitutes the approximation mechanism. The digital FSW, after the appropriate correction and decoding, is presented in an output device . an LCD display or any other suitable means. Alternatively, it can be digitally recorded or it can be read by a puter. As conclusion of this initial approach we could say that the proposed method is based on a Digital Controlled Synthesizer which is forced to produce a frequency almost equal to the unknown one. Frequency parison The frequency parator seems to be the most critical stage of the design. The implementation is based on a modified phase/frequency parator proposed by Philips in the 74HC4046 PLL device. It consists primarily of two binary counters, counting up to two and an RS flipflop. The function of the frequency parator is based on the principle that the lower frequency, . larger period, includes (embraces) at least one or more full periods of the higher frequency (smaller period). This means that two or more rising edges of the higher frequency waveform are included within the lower frequency period. Considering the above, the circuit operates as follows: When the first counter (1) encounters two rising edges of the unknown frequency in one period of the DDS, it sets the output of the RS flipflop. The logic 1 of the RS flipflop acting at the U/D control input of the Up/Down counter forces the DDS to rise its output frequency. On the contrary, when the second counter (2) counts two rising edges of the DDS output within a period of the unknown frequency it resets the RS flipflop39。s display are presented. 1 Introduction The most monly used frequency measurement technique adopts counters that count the pulses of the unknown frequency during a predefined time window (aperture). Apart from this, techniques where the pulses of a reference frequency are counted during one or more periods of the unknown one are also mon. In the latter case, the period instead of the frequency is estimated .Some papers in [1] in the literature deal with the problem of low frequency measurement and are focusing in the frequency range of cardiac (heart) signals (a few hertz) or in the mains frequency (5060 Hz). These techniques are actually measuring the period of the signals and use some way to calculate its reciprocal, the frequency. In [2], the frequency is calculated by the method of lookup tables. Others [46] are microprocessor or microcontroller based. The above methods can be characterized as openloop methods . digital counters are used to count during a predefined tinle interval and calculate the result afterwards. Its closedloop form characterizes the proposed method in this paper. By the term closedloop we denote some sort of feedback. A waveform with a known (controlled) frequency is produced within the circuit and is fed back to the frequency parison stage which consecutively forces it to approximate the unknown (input) frequency. The device that produces the above mentioned waveform of controlled frequency is a Direct Digital Synthesizer. 2 Direct Digital Synthesis A typical Direct Digital Synthesizer consists of a RAM containing samples of a sinewave (sine lookup table, LUT). These samples are sw