What Is a Frequency Synthesizer?

Frequency synthesis technology originated in the 1930s and has a history of more than 70 years. There are three ways to implement a frequency synthesizer: direct analog frequency synthesis, indirect frequency synthesis, and direct digital frequency synthesis. According to the chronological order, it can be divided into 3 generations.

Using a high-precision crystal oscillator as a reference, a series of high-definition frequency sources with a certain frequency interval can be generated through synthesis technology, which are divided into direct synthesis and phase-locked loop synthesis.

Chinese name: Frequency synthesizer
Foreign name: frequency synthesizer
Benchmark: High-precision crystal oscillator

Technology: Synthesis Technology
Classification: Direct Synthesis and Phase-Locked Loop
Applied discipline: Communication

Introduction to Frequency Synthesizer

Frequency synthesizer basic circuit A frequency synthesizer is a device that uses one or more standard signals to generate a large number of discrete frequency signals through various technical approaches. Direct digital frequency synthesis (DDS) technology is a third-generation frequency synthesis technology that has developed rapidly with the development of digital integrated circuits and microelectronics technology after direct frequency synthesis and indirect frequency synthesis. It is based on digital signal processing theory and performs frequency synthesis based on the amplitude and phase relationship of the signal. It has extremely high frequency resolution, extremely short frequency conversion time, wide relative bandwidth, continuous signal phase and arbitrary waveform during frequency conversion. Many advantages such as its output capability and digital modulation function are being widely used in various fields such as instrumentation, remote control telemetry communication, radar, electronic countermeasures, navigation, and broadcast television. Especially in short-wave frequency hopping communication, the signal is constantly changing over a wide frequency band, and it is required to quickly switch the frequency and phase within a small frequency interval, so the local oscillator signal source using DDS technology is an ideal choice. This method is simple and reliable, convenient to control, and has high frequency resolution and conversion speed, which is very suitable for the requirements of fast frequency hopping communication.

There are three ways to implement a frequency synthesizer: direct analog frequency synthesis, indirect frequency synthesis, and direct digital frequency synthesis.

According to the chronological order of occurrence, it can be divided into 3 generations ^[1] .

First generation: direct analog frequency synthesis technology. Using one or more different crystal oscillators as the reference signal source, directly generating many discrete frequency output signals through frequency doubling, frequency division, mixing, etc. is called direct frequency synthesis. The signal obtained by this method has the characteristics of high long-term and short-term frequency stability, and fast frequency conversion speed, but it is difficult to debug and difficult to suppress spurs.

The second generation: phase-locked frequency synthesis technology. A phase-locked frequency synthesizer appeared in the 1950s, also known as an indirect synthesizer. It uses one or several reference frequency sources to generate a large number of harmonics or combined frequencies through harmonic generator mixing and frequency division. On the combined frequency. The required frequency output is indirectly generated by the voltage controlled oscillator. The advantage of this method is that because the phase-locked loop is equivalent to a narrow-band tracking filter, it can well select the signal of the required frequency, suppress stray components, and avoid a large number of filters, which is beneficial to integration and miniaturization.

The third generation: direct digital frequency synthesis technology. Since the 1970s, with the development of digital integrated circuits and microelectronic technology, a new synthesis method has appeared-direct digital frequency synthesis (DDS) technology. It starts from the concept of phase for frequency synthesis and uses digital sampling storage technology. It has the advantages of precise phase, frequency resolution, fast conversion time and other conflicts.

Frequency synthesizer applications

Frequency synthesizer basic circuit The function of the frequency synthesizer is to provide a frequency reference signal with a certain resolution to the microwave frequency sweep signal and lock the output frequency of the microwave signal point by point to obtain a high frequency and stable frequency sweep output signal.

The output of point-frequency signals and frequency-sweep signals is the basic function of microwave synthesis source. The point frequency output is the basis of the frequency sweep output (the output of the frequency sweep signal can be realized by program control using the point frequency). The following is the algorithm for implementing the point frequency function.

(1) The user sets the frequency f0 to be set on the front panel.

(2) Determine which frequency band f0 belongs to, and find the output frequency fYTO of YTO. And preset the frequency of YTO.

(3) Calculate the reference frequency of the YTO phase detector according to fYTO and f0, and derive the frequency division coefficients of the sampling loop and the fractional frequency division loop, and place the frequency division coefficient in the corresponding data latch.

Frequency synthesizer principle

Frequency synthesis source is an important functional unit of microwave systems and is widely used in electronic equipment such as transceivers, radar detection, communication, and detection instruments.

Select the appropriate oscillator from ordinary VCO, dielectric oscillator (DRO), coaxial oscillator (CRO), yttrium iron garnet oscillator (YIG) and various synthesis technologies to provide you with the most cost-effective frequency synthesis source. Coupled with advanced open-loop presets and closed-loop real-time correction technologies developed by Ocean Microwave, it can provide you with extremely short frequency conversion time.

Frequency synthesizer

Frequency synthesizer synthesis classification

Direct analog synthesis, phase-locked loop synthesis and direct digital synthesis. The direct analog synthesis method uses frequency doubling, frequency division, mixing, and filtering to generate multiple required frequencies from a single or several parameter frequencies. This method has fast frequency conversion time (less than 100ns), but has large volume and large power consumption, which has basically not been adopted. The phase-locked loop synthesis method uses the phase-locked loop to complete the addition, subtraction, multiplication, and division of frequencies. This method has a simple structure, is easy to integrate, and has high spectral purity and is widely used, but there is a contradiction between high resolution and fast conversion speed, and it can only be used in large step frequency synthesis technology.

Performance Index of Frequency Synthesizer

Frequency synthesizer is an important part of modern electronic systems, and has been widely used in communications, radar, navigation, electronic countermeasures and test equipment. The main performance indicators of the frequency synthesizer include the following aspects.

(1) Output frequency range

The frequency range refers to the range of change between the lowest frequency and the highest frequency of the output of the frequency synthesizer, including the meaning of the center frequency and the bandwidth.

(2) Frequency stability

Frequency stability refers to the deviation of the output frequency of the frequency synthesizer from the calibration value within a specified time interval. It is divided into three types: long-term, short-term, and instantaneous.

(3) Frequency interval

Frequency interval refers to the minimum interval between two output frequencies, also called frequency resolution. Frequency synthesizers for different purposes have different requirements for frequency separation, ranging from a few hertz to as large as a megahertz.

(4) Frequency conversion time

Frequency conversion time refers to the time that the output frequency changes from one frequency to another.

(5) Spectrum purity

Spectral purity is measured by stray components and phase noise. Spurs are also called parasitic signals and are divided into two types: harmonic components and non-harmonic components. They are mainly generated by nonlinear distortion during frequency synthesis. Phase noise is a measure of output A parameter for the magnitude of phase jitter.

(6) Modulation performance

Modulation performance refers to whether the output of the frequency synthesizer has functions such as amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM).

With the continuous development of electronic technology, the requirements for the performance of frequency synthesizers are getting higher and higher. Since the 1980s, various countries have been developing and developing their own DDS products. The most popular DDS products are Analog Devices, which has more than ten kinds of chips, such as AD7008, AD9830-9835, and AD9850-9854, forming 0 120MHz. Series of wide output frequency range. In addition, Qualcomm also has Q2334, Q2368 and other products. The clock frequency of these DDS chips ranges from 30MHz to 125MHz. The chip has integrated D / A converter and quadrature modulator from general functions. With the development of digital signal processing devices, DDS can be implemented in software on FPGA, DSP and other programmable devices.

Introduction to frequency synthesizer direct digital frequency synthesis

1. Principle of direct digital frequency synthesis

Direct digital frequency synthesis (DDS) technology is relative to direct analog frequency synthesis (DAS) technology. DDS directly generates signals in a discrete-time manner, and generates modulated signals by changing the amplitude, frequency, and phase, so it can generate arbitrary waveforms for digital communication systems. Digital link up and down conversion in the software radio system, generation of local carriers, and voltage-controlled oscillators can be implemented with DDS technology.

Direct digital frequency synthesizers (DDFS, DirectDigitalFrequencySynthesizer) using DDS technology can be divided into different categories such as sinusoidal output DDS, pulse output DDS, and phase insertion DDS, among which the application of sinusoidal output DDS is the most common.

For sinusoidal output DDS, there are two main methods for waveform generation. The first is a real-time calculation method. This method requires real-time calculation of special trigonometric function values. If it is processed by DSP, the implementation is more complicated, so this method is less used. The second is a table lookup method. The main idea is to store the sine value corresponding to each phase in a lookup table (usually implemented by ROM). In the actual waveform generation process, you only need to perform a corresponding lookup according to the phase, and then you can directly Generate a sine waveform value.

2. Advantages of direct digital frequency synthesis

(1) Accuracy

(2) Wide relative bandwidth

(3) High frequency resolution

(4) Ability to quickly switch

(5) Ability to output arbitrary waveforms

(6) Small size, low power consumption, easy to integrate

3 Shortcomings of direct digital frequency synthesis

(1) Restrictions on the operating frequency band

(2) Poor clutter suppression performance

(3) Low phase noise performance