【正文】 rger 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 output will toggle, indicating alternatively that the DDS frequency is higher or lower than the unknown. This is actually an acceptable and expected condition, because (as in a voltage parator) an equality indication could not exist. In our case this is not a problem because the circuit is embedded in a closed loop. The loop will act in a manner that after some short time, the hysteresis, the situation will be reversed and so on. The duration of hysteresis is variable. This situation is controlled, as will be explained later. Although an analog implementation of the frequency parator would look more robust to noise we insisted to the digital implementation for three reasons: ease of implementation in VLSI or Programmable Logic Devices (PLDs) with no need of analog ponents, wide frequency range of operation and shorter response time. Interaction between frequency parator and digital synthesizer After the successive approximation of the unknown frequency the Frequency Comparator realizes that the synthesized frequency is higher (lower) than the unknown one and produces a logic 0 (1) at the output which mands the up/down counter to count in the down (up) direction. As previously mentioned, the output of this counter is considered to be the FSW to the DDS stage. In the case when the DDS frequency was initially lower, the synthesized frequency will increase progressively to reach the unknown one. This will not be realized by the frequency parator and the synthesized frequency will keep on increasing for some clock cycles, until the parator detects the correct relation of it39。 k ? fin. Description of the prototype hardware For evaluation purposes two prototypes have been built and tested in the laboratory. The first approach was a low frequency instrument (operating up to 15 KHz) . The purpose of this implementation was to study the principles of operation of the proposed method. Next, a higher frequency prototype was built which will be described in more detail here. In order to implement the digital part of the prototype, (Frequency Comparator, Successive Counter, Correction Stage) two PLD devices from Altera (EPF 8064LC6812) were used. These devices are interconnected with the DDS, which is the Q2240I3S1 from Qualm. The DDS has a 32bit input and a 12bit output for the sine lookup table (LUT). The 12bit output of the LUT is fed into the D/A converter, the AD9713B from Analog Devices. Its analog output is connected to an I/V amplifier (currenttovoltage converter). The generated sinewave has upper harmonics, due to the DAC operation. These harmonics are removed from the filters that follow the DAC. The correction stage is implemented partially on the PLDs and partially on the microcontroller. Based on the updown mand of the frequency parator we store the two extreme values, FSW1 and FSW2, which are then transferred into the microcontroller (Atmel AT89C52), transformed into numerical representation and fed to the LCD Display. The microcontroller also controls the whole operation of the prototype. The behaviour of the instrument was according to the expected and was alike to a conventional bench frequency counter. The speed of measurement was checked using lower trace, obtained by the aid of a digital oscilloscope. Each state, high or low, of this waveform corresponds to the time required for one measurement. 4 Conclusion In this paper an alternative method of frequency measure