电子信息工程 外文翻译 外文文献 英文文献 借助DDS的精密频率的一种替代方法.doc

An alternative method of precise frequency by the aid of a DDS Contents A method of frequency measurement based on a closed loop composed mainly of a Frequency Comparator (FC) and a Direct Digital Synthesizer (DDS) is presented in this paper. The DDS serves as reference sinewave signal generator acting at one of the FCs inputs. The FC accepts the hard-limited waveform of the DDS as well as the unknown frequency. From the comparison of the two signals a logic output that controls an up/down counter is produced. The counters output acting as the Frequency Setting Word (FSW) instructs the DDS to produce a new sinewave closer in frequency to the unknown one. When the loop settles, the FSW gives the digital estimate of the unknown frequency. Advantage is taken from the inherent high resolution of the DDS and noise immunity of the loop, to design an equally precise and immune frequency meter. All the additional associated stages up to the instruments display are presented. 1 Introduction The most commonly 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 common. 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 (50-60 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 look-up tables. Others [4-6] are microprocessor or microcontroller based. The above methods can be characterized as open-loop methods i.e. digital counters are used to count during a predefined tinle interval and calculate the result aft