False signal generator

Basic working principle of DDS

Basic principle of DDS

Direct digital frequency synthesis is a frequency synthesis method that uses digital technology to directly generate various frequency signals by controlling the speed of phase change.

Direct digital frequency synthesis is a frequency synthesis method which uses digital technology to directly generate various frequency signals by controlling the speed of phase change.

The basic structure of DDS is shown in figure 1, which is mainly composed of phase accumulator, sine ROM table, D/A converter and low-pass filter.

The basic structure of DDS is shown in figure 1, which is mainly composed of phase accumulator, sine ROM table, D/A converter and low-pass filter.

The reference clock fr is generated by a stable crystal oscillator.

The reference clock is generated by a stable fr crystal oscillator.

The phase accumulator consists of an N-bit adder and an N-bit phase register, which is similar to a simple adder.

The phase accumulator is composed of n adders and n phase registers, which is similar to a simple adder.

Whenever the clock pulse arrives, the adder adds the frequency control data and the accumulated phase data output by the phase register, and sends the addition result to the data input end of the phase register.

Every time a clock pulse comes, the adder adds the frequency control data and the accumulated data output by the phase register, and the data of the addition result is sent to the input end of the phase register.

The phase register feeds back the new phase data generated by the adder after the last clock, so that the adder can continue to add with the frequency control data in the next clock.

The phase register feeds back the new phase data generated by the clock function on the adder to the input end of the adder, so that the adder can continue to add under the action of the next clock frequency control data.

In this way, the phase accumulator performs linear phase accumulation under the action of the reference clock. When the phase accumulator is full, it will overflow and complete a periodic action. This period is a frequency period of DDS synthesized signal, and the overflow frequency of accumulator is the signal frequency output by DDS.

Therefore, the phase accumulator performs linear phase accumulation under the action of the reference clock. When the phase accumulator is full, it will produce an overflow and complete a periodic action. This period is a frequency signal synthesized by DDS, and the periodic frequency of accumulator overflow is the signal frequency output by DDS.

Under the control of the reference clock fr, the frequency control word is accumulated by the accumulator to get the corresponding phase data, and this data is used as the sampling address to address the sine ROM table for phase-amplitude conversion, so that the output waveform amplitude at a given moment can be determined.

Under the control of the reference clock fr, the frequency control word of the accumulator accumulates the corresponding phase data, and this data is sent to the addressed sine ROM table as a sampling address for phase-amplitude conversion at a given time, so that the amplitude of the output waveform can be determined.

DAC converts the waveform amplitude in digital form into an analog signal with the required synthetic frequency, and filters out unnecessary sampling components with a low-pass filter, thus obtaining a continuously changing output sine wave determined by the frequency control word.

DAC converts the wave amplitude and frequency in digital form into the required analog synthesis form, and the low-pass filter used in signal sampling does not need to be weighted, so the frequency control word determines to output a continuously changing sine wave.

The relationship between the output frequency f0 of DDS and the reference clock fr, the length n of the phase accumulator and the frequency control word FSW is as follows:

The relationship between output parasitic frequency f0 and reference clock fr, phase accumulator length n and frequency control word is as follows: FSW；;

The frequency resolution of DDS is:

Stray frequency resolution of:;

Because the maximum output frequency of DDS is limited by Nyquist sampling theorem, the maximum output frequency of DDS is fr/2. However, in the actually designed DDS system, due to the non-ideal output filter, the highest frequency of the output signal can only reach about 40% of the reference clock frequency fr.

Because the maximum output stray frequency is limited by Nyquist sampling theorem, the maximum output frequency of DDS is fr/2. However, in the actual design of DDS system, due to the non-ideal output filter, the maximum frequency of output signal can only refer to the clock frequency of about 40%.

Traditional signal generator

Traditional signal generator

Signal generator, also known as signal source or oscillator, is widely used in production practice and scientific and technological fields.

Signal generator, also called signal source or oscillator, is widely used in production practice and science and technology.

Various waveform curves can be expressed by trigonometric function equations.

Various waveform curves can be obtained from trigonometric function equations.

A circuit that can generate various waveforms such as triangular wave, sawtooth wave, rectangular wave (including square wave) and sine wave is called a function signal generator.

It can generate various waveforms, such as triangular wave, sawtooth wave and rectangular wave (including square wave). This circuit is called sine wave function signal generator.

Function signal generators are widely used in circuit experiments and equipment testing.

Function signal generator is widely used in circuit experiment and test equipment.

For example, in communication, broadcasting and television systems, radio frequency (high frequency) emission is needed, and the radio frequency wave here is the carrier wave. In order to execute audio (low frequency), video signal or pulse signal, an oscillator capable of generating high frequency is needed.

For example, in communication, broadcasting and television systems, radio frequency (high frequency) is needed to emit radio frequency waves. Here, carrier waves, audio (low frequency), video signals or pulse signals are carried out, and only high frequency waves need to be generated.

In industry, agriculture, biomedicine and other fields, such as high-frequency induction heating, melting, quenching, ultrasonic diagnosis, nuclear magnetic resonance imaging and so on. An oscillator with high or low power and high or low frequency is needed.

In industry, agriculture, biomedicine and other fields, such as high-frequency heating, melting, quenching, ultrasonic diagnosis, nuclear magnetic resonance imaging, etc., oscillators with large or small power and high or low frequency are needed.

The function signal generator is usually implemented as follows:

There are usually the following ways to realize the function of signal generator:

(1) Function generator composed of discrete components: usually a single function generator, with low frequency, instability and difficult debugging.

(1) component with division: it is usually a function generator, and the frequency of a single function generator is not high, so its work is not very stable and it is not easy to debug.

(2) It can be made of transistors, operational amplifier IC and other general devices, and is more generated by special function signal generator IC.

(2) It can be made of transistors, operational amplifier IC, etc. The more general devices are generated by signal generator IC with special functions.

Early functional signal generator IC, such as L8038, BA205, XR2207/2209, etc. Less functions and low precision. The upper frequency limit is only 300kHz, so it is impossible to generate higher frequency signals, and the adjustment method is not flexible enough. Frequency and duty ratio cannot be adjusted independently, they affect each other.

Early functional signal generator IC, such as L8038, BA205, XR2207/2209, etc., had few functional functions, low accuracy, upper frequency limit of 300kHz, unable to generate signals with higher frequency, inflexible adjustment mode, large frequency and occupied space, and could not be adjusted independently, which influenced each other.

(3) Function generator using monolithic integrated chip: It can generate various waveforms, reach higher frequency and be easy to debug.

(3) Function generator using monolithic integrated chip: It can generate various waveforms, reach high frequency, and is easy to debug.

In view of this, Maxim Company of the United States has developed a new generation of function signal generator ICMAX038, which overcomes the shortcomings of the chip in (2) and can reach a higher technical index, which is beyond the reach of the above chip.

In view of this, American Xinmei Company has developed a new generation of ICMAX038 function signal generator, which overcomes (2) the shortcomings of the chip and can achieve higher technical indicators, which is the shortcoming of the chip.

MAX038 is called high-frequency precise function signal generator IC because of its high frequency and good accuracy.

MAX038 is called high-frequency precise function signal generator IC because of its high frequency and good accuracy.

MAX038 is the first choice in the design of PLL, VCO, frequency synthesizer and pulse width modulator.

In the circuit design of PLL, vco, frequency synthesizer, pulse width modulator and so on, the selected devices are all MAX038.