【正文】 正確的讀數(shù)。 觸發(fā)器的邏輯 “1”在向上 /向下計數(shù)器的 U / D 的控制輸出中起 RS 作用，強(qiáng)制 DDS 升高輸出頻率。這種情況被控制，也將在后面解釋。下跟蹤的 U / D 命令（輸入）到計數(shù)器上，而跟蹤是一個假設(shè)的 “調(diào)頻 ”波形 被不同的規(guī)定。這些諧波在 DAC 之后將從過 濾器刪除。一個詳細(xì)的噪音行 為的研究已經(jīng)在本文中指出。每個狀態(tài)，波形的高或低，相當(dāng)于一個 測量所需的時間。 原型硬件的描述 用于評估的目的， 兩個原型在實(shí)驗 室已建成。在相反（降低）的情況下，同樣的 現(xiàn)象也將會被觀察到。該電路需要一些時間來實(shí)現(xiàn)正確的 頻率的關(guān)系。在此之后，向上 /向下計數(shù)器替代逼近機(jī)制。一個在 DDS 系統(tǒng)的經(jīng)驗法則是可以 。在限定相位跳躍的頻率設(shè)置字的控制方式下來搜尋這些樣本。 當(dāng)循環(huán)沉淀， 頻率設(shè)定字給出了未知的高頻數(shù)字估計。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。計數(shù)器的輸出頻率設(shè)定字 （ FSW） 代理指示的 DDS 產(chǎn)生一個新的正弦波頻率接近未知之一。 2 直接數(shù)字頻率合成器 一個典型的直接數(shù)字頻率合成器包含一個正弦波（正弦查找表 LUT）樣品的 RAM。一個（已知）頻率源，即 DDS，采用于一個閉環(huán) 并且被迫逐步產(chǎn)生頻率等于未知輸入輸出。在步長下降到一時逼近過 程停止。一個充滿活力的機(jī)制代替了。這不會通過頻率比較器 “實(shí) 現(xiàn) ”和合成頻率將會在一些時鐘周期繼續(xù)增加，直到比較器檢測出它的兩個輸入 頻率的正確關(guān)系，未知的一方和 DDS 輸出。 k ? fin。 在數(shù)字示波器 的幫助下，測量采用較低速度跟蹤檢查。 這主要是因為本文的目的是要提出一個頻率測量的 替代原理。這次調(diào)整階段一部分實(shí)施在 PLD 一部分在微控制器。很明顯，使用 “假設(shè) ”是因為沒有一個可用的波形在電路（除輔助 DAC）中。雖然模擬執(zhí)行頻率的比較將產(chǎn)生更 加強(qiáng)勁的噪音，我們堅持?jǐn)?shù)字實(shí)現(xiàn)，原因有三：在超大型積體電路或可編程邏輯 器件（ PLD）實(shí)現(xiàn)容易，沒有模擬組件，頻率范圍寬的操作和更短的需要響應(yīng)時 間。相反，當(dāng)?shù)诙€計數(shù)器（＃ 2）在一個周期內(nèi) 記錄兩個未知的頻率的上升的 DDS 輸出的邊緣， 它又恢復(fù)成 RS 觸發(fā)器的輸出 的。 電路的操作 該電路工作在一個新的測量 DDS 的 輸出頻率會在一開始以逐次逼近的方法控制 這樣一種方式。設(shè)置 FSW 為二，計數(shù) 器的結(jié)果間隔數(shù)為二，以時鐘周期來完成一個周期的正弦波輸出。其閉環(huán)形式刻畫了本文提出的方法。 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 measurement has been proposed. It has been pointed out that in most cases this method is faster than conventional methods for the same frequency resolution. On the other hand, the precision of the method can be very high due to the inherent high frequency resolution characteristic of the DDS that is employed. This synthesizer, which can be thought as an oscillator, is driven to oscillate in the region of the unknown input frequency. A parison with conventional methods has been given and two prototypes have been built and tested in the laboratory. The second major advantage of this method is that if repetitive frequency measurements are to be taken, the instrument remains locked and the frequency measurement does not restart from the beginning, but instead is automatically driven to lower or higher values. In other words, the loop has the capability to follow the changes in the frequency of the input