The theory of frequency synthesis originated in the 1930s, and it has a history of more than 80 years. The early frequency synthesis consists of a set of crystal oscillators. The number of output frequency points required is determined by the number of crystals. It is necessary to artificially implement frequency switching. The crystal determines the frequency accuracy and stability, and it is rarely related to the circuit. This frequency synthesis method has now been replaced by a non-coherent method of synthesis. Although non-coherent synthesis also uses crystals, it works by using a small number of crystals to generate multiple frequencies. Compared to earlier frequency synthesis methods, non-coherent synthesizers not only reduce costs, but also increase the stability of the synthesized frequency. However, the development of such a crystal consisting of a few crystals is a very complicated process, and the cost is high. Therefore, with the development of frequency synthesis technology, coherent synthesis has also been proposed by scientists.
The first coherent synthesis method is mainly direct frequency synthesis (DFS). This synthesis method is a method for generating a desired frequency directly by adding, subtracting, multiplying, and dividing the frequency of one or several reference sources by means of frequency multiplication, frequency division, and frequency mixing. This method has advantages such as short frequency conversion time and low phase noise, so it also occupies a certain position in the field of frequency synthesis. However, since the generated frequency is obtained by a large number of frequency multipliers, frequency division, and frequency mixing, direct frequency The synthesizer is large in size, spurious and difficult to suppress, the structure is complex, the cost and power consumption are high, so the DFS has been basically eliminated.
Indirect frequency synthesis (Indirect Frequency Synthesis) follows DFS. Indirect frequency synthesis mainly refers to PLL (phase-locked loop) frequency synthesis. This synthesis method uses phase feedback and phase-locking techniques for frequency synthesis. This synthesis method has the advantages of high output frequency, low phase noise, good spur suppression, low cost, and ease of integration, and therefore has a place in the field of frequency synthesis. . However, since the frequency synthesizer of the conventional PLL adopts closed-loop control, after the output frequency is changed, it takes a long time to reach the stability again. Therefore, it is difficult for the PLL frequency synthesizer to achieve higher frequency resolution and faster frequency switching time at the same time.
Frequency synthesis technologyFrequency synthesis technology is the key to achieve high-performance indicators for electronic countermeasures and electronic systems. Many modern electronic equipment and system functions directly depend on the performance of the frequency synthesizer used. The performance of the frequency synthesizer directly affects the radar, navigation, and communications. The performance of modern equipment, such as space electronics equipment and instruments, meters, etc.
What are the frequency synthesis technologies1, direct digital frequency technology, namely DDS technology.
2, PLL frequency synthesis technology, namely PLL.
3, DDS + PLL technology.
Frequency synthesis technology has a variety of technical indicators, and its technical indicators reflect the advantages and disadvantages of frequency synthesis techniques. The following will introduce a number of basic technical indicators.
(l) Frequency range. The maximum synthesis frequency fmax and minimum synthesis frequency fmin output by the frequency synthesizer determine the frequency range, and usually the relative bandwidth can be used to measure the frequency range.
(2) Resolution. The minimum interval between the two discrete frequency points of the frequency synthesis output is the resolution of the output frequency, and the requirements of the frequency resolution are different for different occasions.
(3) Switching time. Refers to the time required to switch from one frequency to another when it is stable and within its effective phase error range. The switching time is closely related to the circuit form of the frequency synthesizer.
(4) Spectrum purity. It refers to the purity of the spectrum of the output signal. It is usually measured by the phase noise and spurious components of the signal.
(5) Frequency stability and accuracy. Frequency stability refers to the degree of deviation between the output frequency and the nominal value within a specified time. The degree of deviation can be divided into long-term, short-term and instantaneous stability. Frequency accuracy refers to the error between the actual frequency and the standard frequency. The stability of the frequency is closely related to the accuracy, because only when the frequency stability is high enough, the corresponding frequency accuracy has practical significance.
Frequency synthesis technology advantages and disadvantages(1) Fast frequency switching;
(2) Extremely high frequency resolution;
(3) Maintaining phase continuity during frequency switching;
(4) The relative bandwidth is very wide;
(5) Full digital implementation facilitates monolithic integration.
The main disadvantages are limited working frequency, relatively high phase noise and spurious.
1. Real-time analog simulation of high-precision signals
By storing a large number of non-sinusoidal waveform data such as sine waveforms, square waves, triangular waves, and sawtooth waves in the waveform memory of the DDS, the waveform of the output signal can be arbitrarily changed by controlling the data manually or by computer programming. Using DDS's features of fast frequency conversion, continuous phase transformation and precise fine stepping, combining it with a simple circuit constitutes the best means and means for accurately simulating various signals. This is not comparable to other frequency synthesis methods. For example, it can simulate waveforms such as various neural pulses to reproduce waveforms captured by a digital storage oscilloscope (DSO).
2, to achieve a variety of complex methods of signal modulation
DDS is also an ideal modulator because the three parameters of the composite signal: frequency, phase, and amplitude can be accurately controlled by the digital signal, so the DDS can precisely control the phase of the synthesized signal by presetting the initial value of the phase accumulator. In order to achieve the purpose of modulation.
In modern communication technologies, there are more and more modulation methods. BPSK, QPSK, and MSK all require precise phase control of the carrier. The phase accuracy of the synthesized signal of the DDS is determined by the number of bits of the phase accumulator. A 32-bit phase accumulator can produce 4.3 billion discrete phase levels, and the phase accuracy can be controlled in the range of 8X10-3 degrees. Therefore, when the frequency is switched, simply pass the initial value of the preset phase accumulator. The phase of the synthesized signal can be accurately controlled and it is easy to implement various digital modulation methods.
3, to achieve fine frequency adjustment, as the ideal frequency source
DDS can effectively achieve frequency fine-tuning, which can replace multiple loops in many phase-locked loop (PLL) designs. Maintaining the proper frequency division ratio in a PLL can combine the high frequency resolution and fast switching time characteristics of the DDS with the high output frequency of the phase-locked loop, low spurious noise, and low clutter. More ideal DDS + PLL hybrid frequency synthesis technology.
When the frequency is coarse-tuned, the PLL is used to cover the desired operating frequency band. Selecting the appropriate frequency-division ratio can obtain higher phase noise, and DDS is used to cover those coarse-adjusted increments and achieve frequency fine-tuning within it. This scheme satisfies the stringent technical requirements of various systems for frequency sources with its superior phase stability and extremely low tremor effect. This is also the most widely used method for developing and applying DDS technology. Frequency synthesizers made using this scheme have been implemented at very high frequencies.
Of course, the application of DDS is not limited to these, it can also be used for nuclear magnetic resonance spectroscopy and its imaging, detection instruments. With the rapid development of DDS integrated circuit device speed, it has become an important and flexible design method that can be used to meet system frequency requirements.
4, frequency synthesis technology in the communication circuit application
Direct digital frequency synthesizer is the main tool of modern frequency synthesis. It has many advantages such as high frequency resolution and fast frequency conversion. This device is widely used in many fields. In the CORDIC algorithm, operations on data are only shifted and added/subtracted and are easily implemented in hardware. Moreover, the CORDIC algorithm is also easily implemented in the pipeline and can be performed at high speed in the computing system.
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