【正文】 tion decreases the frequency of the DDS. At a first glance one could think that the synthesized frequency could reach the measured one (fin) and then the operation of the counter stops. Unfortunately this is not the case. A dynamic mechanism takes place instead. The circuit needs some time to realize the correct frequency relation. We will refer to this time as hysteresis. Hysteresis depends on the initial timing relation of the DDS output and on the unknown frequency. Initially, during the hysteresis period, the indication regarding the larger frequency is ambiguous . it can be erroneous. The ambiguity settles when two rising edges of the higher frequency waveform occur during one period of the lower one. If we consider the case of the DDS frequency to be equal to the unknown one, we will find that the parator39。s maximum count. The major blocks have been shown . Among them are the Frequency Comparator and the DDS. To overe some disadvantages of the specific frequency parator a correction stage has been incorporated. This stage is also used for the measurement extraction in order to display the correct reading. Operation of the circuit The circuit operates in such a way that at the beginning of a new measurement the DDS output frequency would be controlled in a successive approximation way. The initial DDS frequency would be half of it39。s output instead of the filtered and hard limited waveform because significant jitter will be encountered. The frequency of the output signal for an nbit system is calculated in the following way。s 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 instrument39。An alternative method of precise frequency by the aid of a DDS Contents A method of frequency measurement based on a closed loop posed 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 FC39。s inputs. The FC accepts the hardlimited waveform of the DDS as well as the unknown frequency. From the parison of the two signals a logic output that controls an up/down counter is produced. The counter39。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 swe