I. Introduction Signal generators are widely used in teaching experiments and scientific research projects. Direct Digital Synthesis (DDS) technology has advantages such as high frequency resolution, fast switching speed, continuous output signal phase, ability to output arbitrary waveform signals, and the ability to achieve fully digital automatic control, making it the preferred signal source for radar, communication, and engineering design systems. In spread spectrum and frequency hopping systems, digital broadcasting, high-definition television, linear frequency modulation, instrumentation, and electronic measurement, DDS has gradually become the core technology of high-performance signal generators. This paper proposes a design scheme for an alternating signal generator based on AT89S52 and AD9850. Its amplitude modulation circuit adopts TLC5615, simplifying the circuit design, improving the current amplitude-controllable signal generator circuit design, and improving control accuracy. 2. System Composition This system design uses the AT89S52 microcontroller as the controller, with microprocessor application technology and DDS AD9850 technology as the core. The microprocessor controls the AD9850 to realize functions such as frequency preset and control word setting. The AD9850 implements the signal generator function. The microprocessor controls the TLC5615 D/A converter, which in turn controls the AD534 multiplier to achieve adjustable sine wave amplitude. The system hardware circuit design consists of a microcontroller system control circuit, a sine wave generator functional circuit, an amplitude modulation circuit, a filtering circuit, and a power amplifier circuit. The system structure block diagram is shown in Figure 1. [img=580,223]http://cms.cn50hz.com/files/RemoteFiles/20081225/943925001.jpg[/img] 3 Functional Module Design 3.1 Signal Generation Module The AD9850 DDS device from Analog Devices (ADI) is used. The microcontroller acts as the controller to realize frequency synthesis and control. The AD9850 integrates a 32-bit phase accumulator, a sine wave lookup table, and a 10-bit high-speed digital-to-analog converter. The phase accumulator is the core. The device has a maximum clock reference frequency of 125 MHz and a minimum clock reference frequency of 1 MHz. When the system clock frequency falls below the minimum clock frequency, the system automatically enters sleep mode. The AD9850 contains a 40-bit data register, including a 32-bit frequency control word, a 5-bit phase control word, a 1-bit power-on sleep control word, and 2 manufacturer-reserved bits. The 40-bit control word can be input in parallel or serially. The AD9850's output frequency fout is determined by the input reference clock and the 32-bit frequency control word, i.e., fout = Δphase × fclk / 2^32. Here, Δphase is the 32-bit frequency control word, and fclk is the input reference clock frequency. Figure 2 shows the signal generation circuit. This system design controls the internal register through a parallel port, using an external 12 MHz reference clock input. The DAC's full-scale output current is 20 mA. The spurious-free range performance of the output signal is optimal when the full-scale current output from the IOUTB and IOUT pins is 10 mA. After weighing the options, a 0.1 kΩ resistor was connected to each of the IOUTB and IOUT pins, resulting in a peak-to-peak value of 1 V for the AD9850 output sine wave. The circuit output now produces the desired sine wave, but this wave needs adjustment to meet the actual design requirements. [img=580,326]http://cms.cn50hz.com/files/RemoteFiles/20081225/943925002.jpg[/img] 3.2 Amplitude Adjustment Module Since the AD9850 outputs a sine wave with only a fixed amplitude, it cannot achieve adjustable amplitude. Therefore, a programmable amplifier is used to implement amplitude control. However, this method can only achieve multiple adjustment and cannot achieve high-precision continuous adjustment. This design requires continuous and high-precision sine wave amplitude control; therefore, a TLC5615 D/A converter is used to control the input signal of the AD534 to achieve continuous amplitude modulation. The TLC5615 is a serial 10-bit D/A converter with a maximum output voltage twice the reference voltage. It features a power-on reset function and requires only three serial buses to complete 10-bit serial data input. It is easily interfaced with industry-standard microprocessors or microcontrollers, simplifying circuit design. The TLC5615's output function is VKZ = 2 × VREFIN × D / 2^10, where VREFIN is the reference voltage (2.5 V in this design) and D is the frequency control word, programmable in software as needed. The microprocessor controls the TLC5615 to achieve 10-bit amplitude adjustment with an accuracy of 0.005 V. The AD534 is a low-drift single-channel amplifier with a wide operating bandwidth and low error rate. Its input signal is differential (dual-ended), meaning only differential signals can enter the amplifier to filter out common-mode signals. Its transfer function is VO = (X1 - X2)(Y1 - Y2) / (10V) + Z2. In this system, X2, Y2, and Z2 are all grounded, and the two multiplication signals are changed to single-ended ground input, with linear control of the output voltage. The output voltage is: VOUT = VDDS × VKZ = 2 × VREFIN × (D/210) × VDDS. Where VOUT is the output of the amplitude adjustment module, and VDDS is the output of the AD9850. Since the AD9850 output amplitude is 1 V, VOUT is determined by the TLC5615 to achieve amplitude adjustment. Its circuit is shown in Figure 3. 3.3 Filtering Module The sinusoidal signal output by the AD9850 contains a DC component, while the system design requires no DC component output; therefore, a high-pass filter is needed. The AD9850 does not have an internal low-pass filter, and internal D/A conversion and system clock may generate high-frequency noise. Therefore, the sinusoidal signal output by the DAC inevitably contains high-frequency noise. To prevent high-frequency interference from causing magnetic field disturbances and measurement errors, a low-pass filter should be added to the signal output port to suppress high-frequency interference, thus forming a band-pass filter. During hardware circuit testing, the directly designed active bandpass filter exhibited significant amplitude fluctuations and poor consistency within the passband, failing to meet application requirements. Based on actual requirements, a filter circuit was designed consisting of a second-order active voltage-controlled voltage source high-pass filter connected in series with a first-order low-pass filter. The system's passband range is 50 Hz to 3 kHz, with a gain of 2 and a Q value of 1. The bandpass filter circuit is shown in Figure 4. [img=580,777]http://cms.cn50hz.com/files/RemoteFiles/20081225/943925003.jpg[/img] 4 Software Design The system software was written in C language. Compared to assembly language, C language offers easier operation of the underlying hardware, higher modularity, and better readability and portability. This software design manages all functions of the signal generator and consists of two main parts: an initialization module and a functional module. The initialization module is used to initialize various hardware registers, data registers, and display elements. The functional module consists of three parts: a display module, a keyboard input module, and a signal generation module. The keyboard module is mainly used to set the frequency, phase, and amplitude. The system software design flowchart is shown in Figure 5. [img=536,786]http://cms.cn50hz.com/files/RemoteFiles/20081225/943925004.jpg[/img] [align=left] 5 Conclusion This system design, based on the AD9850 and TLC5615, solves the problem of adjustable signal generator amplitude. In the application of alternating magnetic field measuring instruments, it generates relatively ideal waveform data with smoothness and no obvious glitches, and its amplitude adjustment accuracy can reach 0.007 V. Currently, signal generators have broad application prospects, but further improvements are needed in terms of accuracy. [tr][/tr][td] [/td][/align]