With the development of electronic technology, the stability and accuracy of the signal frequency are increasingly required in system design. General LC and RC oscillators cannot meet the requirements. The crystal oscillator's high accuracy and high stability just meet this requirement, but its frequency is single and the frequency value is not high. So there has been a method that takes this stable frequency as its core and can generate a large number of frequencies with the same accuracy and stability, which is called frequency synthesis. It uses a high-stability and high-accuracy standard frequency source as a reference. Adding, subtracting, multiplying, and dividing frequencies to generate a large number of different frequencies with the same stability and accuracy.A frequency synthesizer is an important part of an electronic device and one of the key factors that affect the performance of an electronic device system. Frequency synthesizers have developed quite well abroad and formed a series of monolithic integrated frequency synthesizer chips, including phase-locked integer/fractional frequency synthesizers, direct digital frequency synthesizers, dual-loop or multi-loop phase-locked frequency synthesizers. , DDS and PL hybrid frequency synthesizers. National Semiconductor's LMX243X phase-locked frequency synthesizer chip has a noise floor of -219 dBc/Hz, while Analog Devices' ADF4107 can operate at 7GHz. At present, there is no domestic capability to produce single-chip double-loop or multi-loop phase-locked frequency synthesizers. At present, the spurious of the DDS can generally be -70dB or so. The AD9858 monolithic integrated DDS chip produced by AD company has a sampling frequency of 1GHz and a maximum output of 400MHz. The ROM-LESS DDS method that appeared at the beginning of this century has reduced the circuit power consumption and is the most cutting-edge technology in direct digital frequency synthesis. Domestic use of 0.35Hm conventional complementary metal oxide semiconductor circuit (CMOS) C art, developed a new generation of ROMLESS DDS high-speed chip clock frequency up to 2GHZ. The technical indicators achieved by foreign hybrid frequency synthesizers are: In the DDS + DAS synthesis mode, the conversion speed is up to IlHs, the spurious component is -65dBc, the frequency range is [10MHz to 18GHz, and the step interval is 1F Iz, typical representative It is a WJ45 100 type frequency synthesizer of Wathin Johnson Company. In the DDS+PLL synthesis mode, the conversion speed can reach 100H, the spurious component is -75dBc, the phase noise is −15dBc/Hz@10 kHz, the frequency range is from 500MHz to 25 GHz, and the synthesis is segmented with a step interval of 1Hz. This is typical. The watch is a series of FSFR frequency synthesizers from Commun icat ion Techniques Inc. Frequency synthesis techniques are mainly divided into three categories: direct analog frequency synthesis (DAS), direct digital frequency synthesis (DDS) and indirect triphase frequency synthesis (PL). Synthesizers of various systems have corresponding characteristics: The direct analog frequency synthesizer technology is based on one or more highly stable reference signals, and is formed by multiple methods such as frequency multiplication, frequency division, frequency mixing, and switching filter amplification. A frequency synthesizer has the advantages of high frequency stability, fast frequency conversion speed, low phase noise, etc., but the system is complicated, bulky, and high in cost, and the high spurious levels are easily caused by the mixing links. Usually used in occasions with low frequency points and high phase noise requirements. Direct digital frequency synthesis technology is a new type of frequency synthesis technology that has just been developed in recent years. It has the advantages of fast frequency conversion, low phase noise, continuous phase and extremely high frequency resolution, small size, and light weight. The operating frequency band is relatively low and the frequency band is relatively narrow. It cannot be used directly in the wideband microwave band and is usually used for the generation of baseband signals. Indirect lock-in frequency synthesizing technology uses a phase-locked loop (PLL) to control the voltage-controlled oscillator to obtain the required frequency through a standard frequency source. It has the advantages of simple circuit, low phase noise, output frequency bandwidth, and flexible control, but Due to the existence of the loop filter, the conversion speed thereof is slow, and it is generally used in applications where there is a wide frequency band and a high frequency resolution requirement. 1, direct analog frequency synthesis Direct analog frequency synthesis is the earliest type of frequency synthesizer. The principle of its operation is to add, subtract, multiply, and divide one or several reference frequencies with a frequency multiplying and mixing circuit to generate the required frequency. Signals, according to their synthesis methods, are classified into two categories: related synthesis and non-related synthesis. The advantage of the direct analog frequency synthesizer is that the frequency conversion is fast (usually can be done 1~2Hs), the resolution is high, the phase noise is low, and the output frequency is high; the disadvantage is that the synthesis structure is complex, the size and weight are large, and the output harmonics, Noise and spurious frequencies are difficult to suppress and are suitable for electronic systems with large frequency steps and few frequency points. 2. Phase locked frequency synthesis A phase-locked frequency synthesizer is a frequency synthesizer that uses a phase-locked loop (PIL) for frequency synthesis. The principle of its operation is to divide the frequency of the RF signal generated by the voltage-controlled oscillator and the reference signal output from the crystal oscillator to obtain two frequencies respectively. The signals of approximately the same frequency are sent to the phase detector for comparison and the error signal is output. After filtering, the output of the voltage-controlled oscillator is controlled so that the RF signal output from the voltage-controlled oscillator remains stable. The working principle is shown in Figure 1. Show! The phase-locked frequency synthesizer is divided into an integer frequency synthesizer and a fractional frequency synthesizer. The frequency resolution of the integer frequency synthesizer is the reference frequency fr. In order to obtain a higher frequency resolution, fr must be reduced, and the loop bandwidth must be reduced at the same time. (In order to ensure the stability of the loop, the loop bandwidth is generally required to be smaller than the reference frequency fr. One-tenth of the time, resulting in longer conversion times. Therefore, in practical applications, multi-loop frequency synthesis techniques and fractional frequency synthesis techniques are commonly used in order to obtain high frequency resolution, fast conversion time, and broadband characteristics. The multi-loop frequency synthesizer consists of a plurality of phase-locked loops. The high-order loop provides a relatively high-frequency output with a relatively large frequency interval. The low-order loop provides a relatively low-frequency output with a relatively small frequency interval. The two-part output can be added together. Get composite signal output with high frequency and frequency resolution and fast conversion speed. The advantages of the PLL frequency synthesizer are its simple structure, small size, easy integration, convenient debugging and low spurious, so it is widely used. The disadvantage is that the frequency conversion time is relatively long, and it is difficult to meet the electronic system that requires fast frequency hopping time. . 3, direct digital frequency synthesis Direct digital frequency synthesis (DDS) technology is a fully digital implementation. In recent years, with the improvement of integrated circuit process technology, DDS technology has developed rapidly. The DDS is mainly composed of a phase accumulator, a sinusoidal waveform table, a D/A converter, and a low-pass filter. The working principle is shown in FIG. Under the control of the system clock fc, the frequency control word K is sent to the phase accumulator, the data of the phase accumulator is updated, and then the data N in the phase accumulator is used as an address to query the sine ROM table, and the phase increment is changed by You can change the output frequency of the DDS. The waveform quantized data taken from the waveform ROM memory is converted into an analog voltage/current by an A/D converter, and then the high-order harmonics in the waveform are filtered by a low-pass filter to obtain the output frequency of the DDS. (3) Scrambling code injection technology The most fundamental reason for phase spurism caused by phase rounding is that $( n ) is a periodic sequence, so destroying this periodicity becomes the main research method to reduce DDS spurs. Several kinds of methods for dithering the main input; can add jitter to the frequency control word; can increase the addressing address of the ROM, namely the phase jitter main input; can also shake the data before the DAC conversion, that is, the amplitude jitter main input, such as Figure 5 shows. DDS is - a fully digital system, with the advantages of easy integration, extremely fast frequency hopping speed, extremely high frequency resolution and phase continuity in frequency switching. The disadvantage is that the spurious is relatively large and the output frequency is low. 4, hybrid frequency synthesis Hybrid frequency synthesis is a very popular synthesis technique in recent years. There are several synthesis methods such as DDS-excitation PLL.DDS interpolation PLL.DDS+multiplication (DAS) and DDS+ mixing. DDS excitation PLL frequency synthesis block diagram shown in Figure 6. This scheme utilizes the high resolution of DDS to increase the frequency resolution of the PLL output and achieve a higher frequency output, while the low-pass filter in the PLL loop plays a good role in suppressing the out-of-band DDS spurious. However, the frequency conversion time depends on the PIL. DDS interpolation PLL frequency synthesis block diagram shown in Figure 7. Since the DDS output does not undergo PL multiplication, the phase noise and spur introduced by the DDS will not deteriorate the output. With low phase noise and good spurious performance, the spectral purity is much better than the DDS excitation PL. This scheme is still based on PL and the frequency conversion time is still very slow. The block diagram of the DDS+ frequency synthesis is shown in Figure 8. By replacing the PLL with an analog frequency multiplier, the weakness of the long frequency conversion time of the DDS+PLL is overcome. At the same time, the output frequency of the frequency synthesizer is increased by the frequency multiplier chain composed of multiple frequency multipliers and filters, and the output bandwidth of the frequency synthesizer is expanded. . However, as the output frequency expands and widens, the output filter becomes more and more complex, and even a single filter cannot be used but a switch and filter component are used to realize the ultra-high-speed frequency synthesizer developed at home. This program. The disadvantage is that the output bandwidth is not wide, generally only tens of megahertz. The block diagram of the DDS+ mixing frequency synthesis is shown in Figure 9, using a mixer to move the DDS output frequency directly to the microwave band. The first-level BPF filters out the wide-band spurs of the DDS, and the second-stage BPF filters out the local oscillator frequencies, unwanted sideband frequencies, and other unwanted mixing products. The advantage of this solution is that the spurious level and phase noise of the DDS itself does not increase, and the agile speed and resolution of the frequency remain unchanged, that is, the DDS has good performance; the disadvantage is that the output frequency range is narrow (when the DDS output frequency When the range exceeds an octave, the second harmonic of the low frequency end becomes spurious at the output of the mixer and cannot be filtered out), and there are double sideband outputs, local oscillator leakage, and other mixing products. From the 1970s onwards, frequency synthesizer technology has gone through three stages: direct analogue (DAS), indirect phase locked, and direct digital (DDS). With the continuous improvement of the performance requirements of electronic equipment systems, the frequency synthesizer technology has also been rapidly developed. For example, small-sized, high-performance, highly-integrated frequency synthesizers and hybrid frequency synthesizers combining DDS and lock-in have gradually become development. trend. At the same time, the miniaturization, low-cost, high-performance, and multi-species characteristics of frequency synthesizers have further promoted the replacement of various military/civilian electronic systems. Easy Electronic Technology Co.,Ltd , https://www.yxpcelectronicgroups.com
