A high-precision frequency signal generator developed using DDS chip AD9835

    Abstract: This article introduces a DDS special chip AD9835, and uses this chip to design a high-precision frequency signal generator. The basic principles and applications of the DDS chip and its interface with computers and microcontrollers are discussed. and analyzed the actual results.

    Keywords: frequency synthesis DDS signal source modulation

High-precision measurement often requires the use of high-precision, high-stability, and high-resolution frequency signal sources. A frequency synthesizer composed of multiple phase-locked loops has complex circuits, is expensive, has a long signal establishment time, and has poor dynamic characteristics. The direct digital frequency synthesizer (DDS) developed in recent years uses high-speed digital circuits and high-speed D/A conversion technology, and has advantages that were difficult to achieve with previous frequency synthesizers, such as short frequency conversion time (<20ns) and high frequency resolution. (0.01Hz), high frequency stability (10 -7 to 10 -8), the output signal frequency and phase can be quickly programmed to switch, etc., so the signal can be easily fully digitally modulated. Moreover, because DDS is a digital high-density integrated circuit product with a small chip size or low power consumption, DDS can be used to form a high-performance frequency synthesis signal source to replace traditional frequency signal source products.

We used Analog's AD9835 DDS special chip to design a synthetic signal source controlled by a microcontroller and computer. The main technical indicators are as follows:

Frequency range: 0.1Hz～10MHz

Frequency resolution: 0.1Hz

Frequency stability: 1×10 -7

Output amplitude: 0～±10V adjustable

Output waveform: sine wave, square wave (TTL level), PSK, FSK, frequency sweep

This signal source has two control methods that can be switched at will: one is to use the parallel port on the PC to transmit control instructions and parameters. For this reason, we used VB to write a control interface under the Windows 9x operating system. Through this program, it can be very easily Set various control parameters; the other is controlled by a microcontroller, setting parameters and selecting function menus through panel buttons, which is convenient for offline use in the field.

1 How DDS works

1.1 DDS technology

The DDS technology used in AD9835 starts from the phase φ of the continuous signal , samples, quantizes, and codes a cosine signal to form a cosine function table and store it in ROM. The phase increment is changed during synthesis. Because the phase increment is different, the number of sampling points in one cycle is also different, so the frequency of the generated sinusoidal signal is also different, thereby achieving the effect of frequency synthesis.

Here, the cosine wave signal itself is nonlinear, while its phase is linear (as shown in Figure 1). Therefore, every period of time Δt (clock cycle), the corresponding phase change ΔP , that is

ΔP=ωΔt=2πfΔt (1)

From equation (1), the frequency f of the synthesized signal can be obtained as:

f=( ΔP×fmc)/2π (2)

In the formula, fmc is the fixed clock frequency, fmc=1/ Δt , and the frequency f of the synthesized signal can be changed by changing the phase value ΔP .

The schematic block diagram of the DDS chip AD9835 is shown in Figure 2. Among them, the phase accumulator is 32 bits, and the high 12 bits are used as the key to read the cosine waveform memory. Each time, the clock increments the output of the phase accumulator, that is, the cosine ROM addressing address, by the frequency setting data K, and the corresponding waveform phase change is:

ΔP=2πK/2 32 (3)

Therefore, changing the phase accumulator setting value K can change the phase value ΔP , thereby changing the synthesized signal frequency f. After simplification, the synthesized signal frequency is determined by the following formula:

f=K·fmc/2 32 (4)

In the formula, fmc=50MHz, obtained with a high-stability crystal oscillator. The K value is between 1

1.2 Internal structure of AD9835 chip

The internal structure block diagram of AD9835 is shown in Figure 3. It has a 32-bit phase accumulator, two 32-bit frequency registers F0 and F1 (used to set the K value), and four 12-bit phase registers P0, P1, P2, and P3. When F0 and F1 are programmed to switch, FSK and frequency sweep functions can be realized; when P0, P1, P2, and P3 are programmed to switch, phase PSK modulation can be realized. The cosine function table is stored in ROM.

The output value of the 32-bit phase accumulator is intercepted and the high-order 12 bits are added to the value of the 12-bit phase register Pi to form a 12-bit phase address to address the cosine ROM table. The amplitude value obtained by addressing becomes a synthetic cosine signal after 10-bit high-speed D/A conversion. The ratio SNR of the output signal S to all DAC output noise N is mainly related to the number of D/A bits, that is, to the digital quantization noise. Theoretical analysis shows that the SNR of 10-bit D/A can reach 60.2dB. The actual SNR of AD9835 given by AD company information is better than 50dB. The ratio of the total harmonic component distortion of the output signal to the frequency of the two main signals is m=fmc/f. The larger the m value, the smaller the harmonic distortion; when the m value is small, the harmonic distortion is larger. In order to eliminate harmonic distortion with small m, an LC high-order low-pass filter is used at the output end to filter out high-order harmonics. In this example, a fifth-order Butterworth low-pass filter is used, which can reduce high-order harmonics above 50MHz to -60dB, fully meeting the requirements of high-precision signal sources.

The pins FSELECT, PSEL0, and PSEL1 in Figure 3 are external modulation signals, which can be used for direct position control modulation of DDS to achieve digital binary frequency modulation (FSK) and digital four-valued phase modulation (PSK). The pins FSYNC, SCLK, and SDATA are used to set the program-controlled working mode of DDS. The data transmission mode is synchronous serial mode. In Figure 3, AD9835 can be set to SLEEP and RESET working modes. In SLEEP working mode, the power consumption is only 1.75mW.

2 DDS signal source design

2.1 Signal source block diagram

Figure 4 is the system block diagram. When the switch SW cuts upward, the signal source is controlled by the microcontroller, and the working mode, frequency and phase parameters are set by the keyboard and displayed using an 8-bit LED digital tube. The frequency resolution is 0.1Hz, which can realize point frequency, frequency sweep, PSK, FSK four working modes. When the switch SW is cut to the lower end, the program is controlled by the PC through the computer parallel port, and the working mode is the same as when controlled by the microcontroller. In order to ensure the signal output frequency band of 0 ~ 10MHz, the filter uses a passive LC 5th order filter. The D/A output of AD9835 is only about 1.2V, and the signal is amplified nearly 20 times by a two-stage wideband high-speed operational amplifier before being output. To meet the requirement of large signal 10V amplitude output without distortion, the slew rate of the final amplifier should satisfy S ≥ ωVm . At 10MHz, after calculation, S≥600V/ μs .

2.2 Control program

Whether programming in VB on a PC or in assembly language on a microcontroller, the main program block diagram is basically the same, as shown in Figure 5. In Figure 5, "initialization" refers to setting the control word to the AD9835, including setting the SLEEP, RESET, CLR, SYNC, SELSRC and other bits, and also choosing whether to use pins or serial control bits to control the AD9835 in future modulation. . Once set, the AD9835 will remain set until reset.

Since the AD9835 control parameters require input in synchronous serial mode, when controlled by a PC, the PC parallel port output method is used. Use parallel port data bit lines to simulate frame synchronization FSYNC, synchronous clock SCLK and serial data SDATA respectively, serialize them according to parameter requirements and assemble them into parallel data for output from the parallel port. In addition, since VB itself does not have oral reading and writing functions, it is necessary to write oral reading and writing function functions in other languages ​​and then use a dynamic link library. Called in the form of DLL to achieve port output. This program can also be used in combination with virtual instruments to form a digital frequency signal generator for the virtual instrument interface.

2.3 Actual measurement results

After the design of this instrument was completed, it was put into use, and all indicators met the design requirements. Judging from the measurement situation, the frequency purity and stability of the DDS frequency synthesizer are quite high. Figure 6 shows the measured spectrum diagram when the synthesizer output frequency is 2MHz. In the figure, each vertical division is 20dB. It can be seen that the amplitude attenuation of one octave is about -45dB.

Figure 7 shows the measured waveform when PSK phase jumps. The phase jump value is 90 degrees. It can be seen from the waveform that the instantaneity and accuracy of the phase jump are very good. The ability to accurately control the phase is a prominent advantage of DDS, which is also difficult to achieve by other frequency synthesis methods.

Figure 8 shows the transient waveform when the synthesizer output frequency jumps from frequency register F0 to F1. The waveforms are connected very well, and there is no transition zone to control the offset in the middle. This is also a prominent feature of DDS. Figure 9 shows the FSK modulation waveform.

DDS synthetic signal source has outstanding advantages such as high stability, high precision, high resolution, and high-speed signal establishment. It is the development direction of signal sources and has significant application value in many aspects such as electronic countermeasures, communications, and measurement. Special ASIC signal source chips and micro-programmed arbitrary waveform signal source chips integrated with DDS and PLD chips are also about to come out. This will be a major revolution in signal source technology. The use of gallium arsenide and other high-speed materials and technologies can further extend the DDS frequency to the high end, making it of great significance in software radio.