“Modern communication technology, radar technology, Electronic measurement and some optoelectronic applications all require high-precision, high-stability, high-resolution RF sine wave signals. Different from the traditional analog RF oscillator, the direct digital frequency synthesizer DDS (Direct Digital Synthesizer) has significant advantages: high frequency stability, high frequency precision, and easy control.
Modern communication technology, radar technology, electronic measurement and some optoelectronic applications all require high-precision, high-stability, high-resolution RF sine wave signals. Different from the traditional analog RF oscillator, the direct digital frequency synthesizer DDS (Direct Digital Synthesizer) has significant advantages: high frequency stability, high frequency precision, and easy control.
2 How the system works
Direct Digital Synthesis (DDS) is a new digital control technology that is different from the traditional analog sinusoidal signal generation technology. Its basic principle is shown in Figure 1.
The sine wave signal y=sinωt is a nonlinear function. To directly synthesize a sine wave signal, the function y=sinx should be digitally quantized first, and then take x as the address and y as the quantized data, and store them in the waveform memory in turn. The DDS uses a phase accumulation technique to control the address of the waveform memory. At each reference clock cycle, a phase increment is added to the current result of the phase accumulator. The output of the phase accumulator is the address of the waveform memory. According to the output address of the phase accumulator, the waveform quantized data is taken out from the waveform memory, converted into an analog current by a D/A converter, and then converted into an analog voltage by an operational amplifier. The phase increment can change the output frequency value of the DDS.
Signal generators designed with Direct Digital Synthesis (DDS) have the following advantages over traditional signal sources:
(1) High frequency stability. The frequency stability depends on the stability of the reference frequency source used. The stability of common crystal oscillators can reach several ppm levels;
(2) High frequency accuracy The frequency resolution of the current common DDS devices can reach 32 bits;
(3) Easy to control Frequency, amplitude, phase, etc. can be controlled directly through the digital interface.
The working principle of the RF sine wave generator system is based on the DDS signal generation method. The low-pass filter and amplifier are used to improve the frequency characteristics and driving ability of the RF signal, and the interface control of the DDS device is completed through the controller and some peripheral supporting devices.
3 System Hardware Design
The hardware design of the system mainly includes direct digital synthesizer (DDS), low-pass filter, adjustable gain amplifier, controller and power supply. The system structure block is shown in Figure 2.
3.1 Direct Digital Synthesis Sine Wave Generator
According to the system design index, the DDS device AD9951 is selected. Its internal clock can reach 400 MHz, the frequency adjustment word is 32 bits, and the D/A converter output of 14 bits has lower phase noise and higher dynamic range. The phase-locked loop circuit is included to realize the frequency multiplication function of ×4 to ×20. Through the SPI interface, the writing of the control word and the frequency adjustment word can be realized, and the signal frequency and amplitude can be adjusted in real time.
3.2 Low-pass filter
According to the working principle of DDS, the directly generated D/A converter output signal must include the required frequency, its mirror frequency and harmonic frequency, etc. Therefore, a low-pass filter must be used for filtering at the output end. According to the Nyquist principle, usually Only frequencies within the bandwidth of half the device operating clock frequency are not affected by the image frequency.
Filters are mainly divided into active filters and passive filters. Due to the limitation of amplifier bandwidth and gain, active filters are mainly used in low-frequency filter design; high-frequency filters mainly use passive filters. Passive filters are mainly divided into Butterworth filter, Chebyshev filter and ellipse filter. Select Elliptic Filter in .
According to the requirements of the system index, a 170 MHz elliptical low-pass filter is added after the DDS device, and simulated by the GENESYS software, the results are shown in Figure 4.
3.3 Digital Gain Adjustable Amplifier
Digitally controlled gain, broadband amplifier MAX2055. The device is composed of impedance matching network, digital attenuator and amplifying unit. Its frequency range is 30 ~ 300 MHz, -3 ~ 20 dB gain is adjustable, and it is suitable for 50 Ω network. The gain can be adjusted through the B0~B4 digital control signals.
The controller mainly realizes the communication with the computer, and controls the frequency and amplitude of the sine wave generator and the digital gain of the amplifier. Using RS232 to communicate with the computer, it is convenient to set and read the current frequency value and control the output power. Choose 80C51 microcontroller P89LV51RD2, this device includes 64 KB Flash and 1 KB RAM, also includes SPI, UART interface and abundant I/ O port line. Support ISP (on-line programming), and can easily program through the serial port. Figure 5 is a functional block diagram of the controller in the entire system.
3.5 Power supply part
The power supply section supplies power to the digital power supply and analog power supply of each device.
4 System Testing and Analysis
After completing the system design and production and debugging, the performance test of the RF signal output was carried out. The measuring instrument model is Anistu MS2034A, its spectral resolution RBW is 10 Hz, and the frequency span is 200 kHz. The frequency of the external quartz crystal oscillator connected to the DDS is 25 MHz (accuracy is 10 ppm), the PLL is 14, the system clock is 350 MHz, and the digital programmable amplifier gain is 18 dB. (Note: The absolute amplitude accuracy power level of the instrument (≥-50 dBm, ≤-35 dB input attenuation, pre-off, 10℃～55℃): 100kHz～≤10 MHz, ±1.5 dB; >10 MHz～ 4 GHz, ±1.25 dB) Table 1 is the actual measured frequency value and power value corresponding to each set frequency point.
It can be seen from Table 1 that:
(1) Signal power characteristics With the increase of the output signal frequency, the signal power decreases. The general trend is consistent with the sinc function. Considering the high-frequency attenuation of amplifiers and transformers and the influence of high-frequency characteristics of discrete components, it can be seen that the measured values basically conform to the law.
(2) Signal frequency stability The stability and precision of the output frequency are mainly determined by the stability and precision of the crystal oscillator. The system uses a passive quartz crystal oscillator with a frequency of 25 MHz (with an accuracy of 10 ppm), and the frequency stability of the corresponding output signal is f/25×10 ppm, the frequency stability is better than 50 ppm. The high-precision and high-stability clock reference source can be used to improve the output frequency accuracy and stability of the RF signal of the entire system.
(3) The signal bandwidth is measured by Anistu MS2034, the spectral resolution RBW is 10 Hz, and the frequency span is 200 kHz. It can be found from the signal spectrogram that the signal output frequency is about several Hz.
A radio frequency sine wave signal generator based on DDS device AD9951 is designed. Through the design, manufacture and debugging, the experimental results are good. After analysis, suggestions for improvement are put forward. The system has reference value for the design of high-performance RF signal generator.