Design and Simulation of FM Signal Generator Based on DDS

introduction

DDS (direct digital frequency synthesizer) has many advantages such as continuous phase change, fast frequency conversion speed, high frequency resolution, low phase noise, high frequency stability, high integration, and easy control. It occupies an important position in modern frequency synthesis technology and is widely used in signal generators , radar systems, communication systems and other fields. MCU (microcontroller) has strong data processing and control capabilities, rich on-chip peripherals, high precision, low power consumption, and is widely used in electronic equipment. This article introduces a design method for a digital FM signal generator based on ADI's dual-channel DDS chip AD9958 to generate high-quality FM signals . The FM signal generated by this structure covers a wide frequency range, the carrier frequency and frequency deviation are digitally adjustable, the FM waveform frequency is highly accurate, and the cost is low and the reliability is high.

System structure principle

Frequency modulation (FM) systems use the variation of the frequency of a modulated signal to carry information. The instantaneous frequency of the FM wave is equal to the carrier frequency plus a time-varying frequency proportional to the modulating signal.

The expression of frequency modulation wave is:

Its instantaneous angular frequency is:

Where ω0 is the fixed angular frequency (carrier frequency); is the proportional constant (modulation constant), representing the sensitivity of the modulator [1].

The signal frequency of the DDS output can be given by the following formula:

Among them: is the reference clock, is the signal frequency resolution, is the output signal frequency, is the frequency control word, and is the number of bits of the phase accumulator. It can be seen that by setting the number of bits of the phase accumulator, the frequency control word and the value of the system reference clock, the output of any signal frequency can be generated [2].

It can be seen that, at that time, the lowest output frequency of the DDS, that is, the frequency resolution of this DDS is:

Using DDS to implement frequency modulation means that the frequency of the signal synthesizer output signal changes linearly with the amplitude of the modulation signal. The change of instantaneous frequency can be converted into the control of the change of the frequency control word [3]. Assuming that the modulation signal is converted into a B-bit digital signal by ADC, in order to meet the modulation frequency deviation requirement, it needs to be multiplied by an adjustable modulation constant in the MCU, and the product is used as the frequency control word of the modulation signal; assuming that the carrier frequency control word is, then the frequency control word of the frequency modulation wave is:

Substituting into (1) we get the instantaneous frequency sequence of the FM wave signal:

Considering the integrator function of the phase accumulator, assuming that the waveform memory stores a cosine wave, the frequency modulation signal sequence output by the DDS is:

After the FM signal sequence passes through the digital-to-analog converter and the low-pass filter, the analog signal obtained is the direct digital synthesized FM signal.

Hardware Implementation

Due to the full digital structure, the frequency band of the DDS output signal is limited by the device level and is generally within a few hundred megahertz. In this system, in order to meet the design index requirements of the modulation frequency band of 100 ~ 400MHz, considering the technical level and cost of the DDS chip, we use a dual-channel DDS combined with a mixer to improve the working frequency band of the FM signal, so that the maximum output frequency of each channel only needs to reach 200MHz. The same FM wave frequency control word is sent to the two channels of the dual-channel DDS, and the two channels DDS-1 and DDS-2 generate a completely synchronized FM signal sequence with a carrier frequency of , and then the FM signal sequence is mixed after passing through the D/A converter and the low-pass filter, and the DC component is removed. The analog signal obtained is the FM signal with a carrier frequency of . The hardware structure principle of the system is shown in Figure 1.

Signal generation module

The signal generation module is mainly composed of DDS chips. Here we use the dual-channel direct digital frequency synthesizer AD9958 from ADI, USA , with a maximum sampling frequency of 500 MSPS. It has two DDS cores and can provide two internally synchronized and independently programmed synchronous output channels. When the system clock operates at 500 MHz, the output frequency can be controlled in the range of 0 ~ 200 MHz[4], which can meet the requirements of system design.

AD9958 has four working modes, namely single frequency mode, modulation mode, linear sweep mode and amplitude output control mode. The single frequency mode is the default working mode after the chip is serviced. In this mode, the output is a sine wave with a certain frequency , amplitude and initial phase [5]. The AD9958 in this system adopts the single frequency mode. In this working mode, the two DDS channels share a common address. The frequency control word address is the register (0x04). By changing the frequency control word, the output frequency can be easily changed. Combined with the channel selection control word, the two channels can independently output two unrelated sine waves. The control function is completed by the MCU.

Since the maximum output frequency of DDS is limited by Nyquist sampling theorem, there is. At this time, considering the device factors, it is generally taken in actual use. In this system, the external reference clock uses a 50M high-stability crystal oscillator, and the DDS chip clock multiplier is set to 10, so that the system clock reaches 500 MHz. Therefore, the maximum output frequency of each channel that can ensure signal quality reaches 200 MHz.

Control Module

The functions of the control module are mainly composed of MCU chips. The MCU integrates a wealth of peripheral devices and has excellent processing capabilities. The use of MCU to complete the peripheral circuit can make the entire system structure simple and easy to use. In this system, MCU, as the core control module, completes the acquisition of the modulation signal frequency control word, the reception of the carrier frequency control word, and the control of DDS.

Modulation signal frequency control word: This system uses the ADC integrated in the MCU chip to obtain the modulation signal frequency control word. According to actual needs, the modulation signal of this system is a voice signal with a frequency range of 50 ~ 3400 Hz. According to the Nyquist sampling theorem, the ADC sampling frequency should be no less than 6.4 kHz. Considering that high-speed intensive sampling can reduce frequency deviation, the ADC sampling frequency is set to 100 kHz. The voice modulation signal is converted by A/D to obtain a 12-bit digital signal, which is multiplied by the modulation constant as the modulation signal frequency control word.

Carrier frequency control word: MCU communicates with the external control module through the on-chip UART interface, and the external control module uses asynchronous communication to send instructions such as carrier frequency control word to MCU. MCU processes the received instruction signal and extracts the carrier frequency control word required by the user.

Frequency control word of FM wave: MCU adds the modulation signal frequency control word and the carrier frequency control word as the frequency control word of FM wave and processes it according to the frequency control word format of DDS before sending it to DDS.

Low pass filter

DDS uses digital technology. The final synthesized signal is obtained after D/A conversion. Its spectrum contains rich high-order spectrum components. In order to obtain a signal output with pure spectrum, they must be filtered out with a low-pass filter. The attenuation characteristics of the filter are required to be steep and the delay time is required to be short.

Software Design

The whole system adopts modular programming and is written in C language, which is easy to transplant and has strong readability. It is mainly based on the frequency control word format of AD9958 , and writes these control words into the internal register of AD9958 through MCU to generate the corresponding frequency. The software mainly realizes two functions: system initialization and frequency control word writing.

System initialization: includes the initialization configuration of the MCU itself, and writing configuration instructions such as system clock, working mode, and channel selection to the AD9958 according to the register configuration method of the AD9958 chip .

Frequency control word writing: To complete a frequency control word replacement, the MCU needs to send a channel instruction according to the frequency word writing format of AD9958 , which is 40 bits in total. The upper 8 bits are the register address (0x04), and the lower 32 bits are the frequency control word. Within an ADC sampling cycle, the channel instruction must be sent completely so that the output frequency can be continuously updated according to the ADC sampling frequency, thereby realizing digital frequency modulation . The instruction writing method of AD9958 can be divided into serial loading and parallel loading. This system adopts serial loading, and the MCU directly sends the frequency control word to AD9958 . At the rising edge of each system clock (SCLK), a control word is shifted in from the data input port SDIO_0. After 40 consecutive SCLK cycles, the 40-bit control word can be loaded into the buffer register. After the rising edge of the update signal (IO_UP) arrives, the control word is transferred to the control register, and the two channels of AD9958 update the output frequency at the same time.

Simulation Results

In practical applications, in order to improve the resolution and break through the limitation of waveform memory capacity, phase truncation technology is usually used. The existence of phase truncation will produce phase truncation error, thus causing spurious spectrum components. Since the analysis of spurious is relatively complicated, this paper builds the corresponding Matlab simulation platform according to the above hardware design method without considering the phase truncation error, and simulates the method of digital synthesis FM signal . The result is shown in Figure 2.

According to the system design requirements, it is assumed that the carrier frequency of the FM wave to be generated is 200 MHz and the modulation frequency deviation is 45 MHz. For the convenience of analysis, it is assumed that the modulation signal is a 30 MHz sinusoidal signal. According to the previous analysis, the carrier frequency control word is 0x66666666, and the modulation signal. As shown in Figure 2, (a) and (b) respectively give the waveform and spectrum of the FM wave with a carrier frequency of 100 MHz output by the two channels DDS-1 and DDS-2 of the DDS; (c) and (d) give the waveform and spectrum of the FM wave with a carrier frequency of 200 MHz obtained after mixing and filtering of the two channels. It can be seen from the spectrum diagrams of (b) and (d) that new spectrum components with an interval of 30 MHz appear symmetrically on both sides of the carrier component, which is consistent with the spectrum structure of the theoretical single-tone FM wave.

Conclusion

This paper discusses a method for synthesizing FM signals based on the dual-channel DDS chip AD9958 . It makes full use of the existing DDS chip to generate cross-band FM signals, with programmable output carrier frequency and frequency deviation, high minimum frequency resolution of up to 0.116Hz, and a maximum output frequency of 400MHz. It has very high accuracy and frequency stability, while reducing costs, power consumption, and size. The hardware circuit of the FM signal generator is simple and versatile. In fact, it can also be configured with MCU control instructions to enable DDS to generate other arbitrary waveform signals. At present, the digital FM signal generator design scheme proposed in this paper using the combination of AD9958 and MCU has been put into practical use in a certain engineering project.