“The main function of the telemetry signal source is to simulate missile-borne telemetry information. In terms of technical implementation, signal sources can be divided into analog signal sources, digital signal sources and DDS signal sources. Among them, DDS signal source is the development direction of modern signal source. DDS technology (Direct Digital Frequency Synthesis) is a new frequency synthesis method that has developed rapidly in recent years. It is programmable and easy to implement various digital modulations (such as PSK, FSK and other high-precision digital modulation), and has high frequency resolution. , The conversion speed is fast, the stability is high, the phase noise is low and the integration is high.In recent years, with the development of telemetry technology, telemetry products gradually show miniaturization, standard
The main function of the telemetry signal source is to simulate missile-borne telemetry information. In terms of technical implementation, signal sources can be divided into analog signal sources, digital signal sources and DDS signal sources. Among them, DDS signal source is the development direction of modern signal source. DDS technology (Direct Digital Frequency Synthesis) is a new frequency synthesis method that has developed rapidly in recent years. It is programmable and easy to implement various digital modulations (such as PSK, FSK and other high-precision digital modulation), and has high frequency resolution. , The conversion speed is fast, the stability is high, the phase noise is low and the integration is high. In recent years, with the development of telemetry technology, telemetry products have gradually presented application requirements such as miniaturization, standardization, and serialization. Therefore, in order to meet the application requirements, the telemetry signal source must be able to provide a variety of measured signal types, and adjust in real time according to changes in the parameters of the measured module to achieve one-to-one correspondence. However, the traditional telemetry signal source lacks flexibility and versatility in design, and the diversity and real-time performance of the measured parameters are poor, which cannot meet the development needs of telemetry products. Aiming at this point, this paper proposes a programmable telemetry signal source with FPGA (field programmable gate array) and DDS dedicated chip as the core.
1. The basic working principle of FPGA and DDS
Generally, the traditional signal source adopts the resonance method, that is, a frequency-selective loop is used to generate sinusoidal oscillation to obtain the desired frequency. The output waveform of this kind of signal source is single, and the frequency stability and accuracy are poor, so the traditional signal source has become more and more unable to meet the measurement needs of modern telemetry products. The telemetry signal source designed with DDS technology can meet the requirements of waveform diversification, flexible and configurable frequency and phase, and high frequency stability.
FPGA is a high-density programmable logic device. After more than 20 years of development, the logic scale of FPGA has grown from the initial 1000 available gates to the current 10 million available gates. The Verilog HDL language is used for design, which has great advantages in writing incentives and modeling. The basic components of FPGA are programmable input/output unit, basic programmable logic unit, embedded block RAM, abundant wiring resources, bottom embedded functional unit and embedded dedicated hard core. FPGA devices are arranged in an array by logic function blocks in structure, and these function blocks are connected through programmable internal wiring to realize certain logic functions. Due to the high integration of FPGA devices and the short development and listing cycle, they have been rapidly popularized and applied in digital design and Electronic production, and once dominated the field of high-density programmable logic devices.
Altera is one of the main suppliers of FPGA chips on the market, providing users with a complete development system and good after-sales support services, and has a mature series of products. The company’s programmable logic products can be divided into three categories: high-density FPGA, low-cost FPGA and CPLD. Compared with low-cost FPGAs, high-density FPGAs are mainly used in mid-to-high-end routers and switches, and the price is relatively high. Although CPLDs are low in price, they have limited wiring resources and cannot be applied to complex circuit timing function design. The Cyclone (hurricane) series is a low-cost FPGA introduced by Altera, which is mainly positioned in a large number of cost-sensitive designs. Cyclone EP1C6 is a cost-effective FPGA introduced by Altera, with an operating voltage of 3.3 V and a core voltage of 1.5 V. Its density is 5980 logic cells, including 20 128×36 b RAM blocks (M4K modules). , the total RAM space reaches 92 160 b, embedded 2 phase-locked loop circuits and a specific double data rate interface for connecting to SDRAM.
1.2 DDS and its chip
DDS adopts an all-digital structure that is different from traditional frequency synthesis methods. It was originally developed in the 1970s by the American scholar J. Tierncy et al. proposed that it is the third generation of frequency synthesis technology following the direct frequency synthesis and indirect frequency synthesis, with the rapid development of digital integrated circuits and microelectronics technology. DDS refers to the direct synthesis of the required waveform from the concept of phase quantization, which effectively solves many problems that cannot be solved by analog synthesis technology.
DDS is based on the sampling theorem. First, the waveform to be generated is sampled, the sampled value is digitized and stored in the memory as a look-up table, and then the data is read through the look-up table, and then converted into an analog quantity by a D/A converter. Resynthesizes the saved waveform. The basic principle block diagram of DDS is shown in Figure 1.
At present, AD Company is the largest supplier in the mainstream DDS chip market, and many DDS integrated chips it provides have been widely used due to its high cost performance. AD Company’s DDS products mainly include AD983X, AD985X and AD995X three series. For the AD985X series, although its performance is good, the power consumption is high, while the AD995X series has lower power consumption, but its price is higher than the AD983X series, which is a low-cost and low-power product. in the AD983X family. The maximum power dissipation of the AD9833 is only 20 mW. At the same time, AD9833 also has the characteristics of simple peripheral circuit, programmable frequency and phase. AD9833 is written through 3-wire SPI serial port, and there are 5 programmable registers inside, including 1 16-bit control register, 2 28-bit frequency registers and 2 12-bit phase registers. The user can set the desired function through the 16-bit control register. The analog output of the AD9833 is fout:
Where FREQREG is the frequency word in the selected frequency register.
The signal phase shift is pout:
Where PHASEREC is the phase word in the selected phase register.
2. Programmable telemetry signal source based on FPGA and DDS chip
The traditional telemetry signal source has poor programmability in design, which affects its flexibility and versatility to a large extent, and also causes a serious waste of resources. The design of this scheme has strong programmability, can be flexibly configured, and has strong versatility, which greatly saves resource costs.
The hardware circuit of the telemetry signal source is mainly composed of low-cost FPGA and DDS dedicated chips, and the software is programmed in Verilog language. For the software part, the control interface and control word programming of the signal source is an important part of software programming. The FPGA control interface realizes the serial communication protocol through programming. The preset control word must be serially output to the DDS special chip according to the communication protocol of the control interface, and the DDS chip can receive the control word information and output the required waveform according to the received control word information. .
2.1 Hardware composition of telemetry signal source
The telemetry signal source mainly includes the following three components.
(1) Button circuit. It mainly conveys control information to the FPGA part. Some keys provide waveform selection information, and another part provides frequency information of the waveform to be output.
(2) System FPGA control core. FPGA is the core control part of the system. When the FPGA receives the key information, it sends the corresponding control information to the DDS chip. Through programming, FPGA chip EP1C6T144 can realize flexible configuration.
(3) DDS circuit. This part mainly uses AD9833 chip to build the peripheral circuit. Generate the required waveform signal according to the received FPGA control information and output it.
The overall block diagram of the system is shown in Figure 2.
In this system, the user can output the default frequency sine wave, triangle wave, square wave and other waveforms through the waveform selection key. In the FPGA control module, when the FPGA receives the data or state change information, the corresponding variable assignments set will change accordingly, and then the corresponding control word will be output to the AD9833 chip. AD9833 receives the control word by directly Digital frequency synthesis, and finally output the desired waveform.
2.2 Control interface of telemetry signal source
DDS chip AD9833 is a 3-wire SPI interface, which can be directly connected to some microprocessors, but for FPGA, it must be realized by programming the SPI protocol. Therefore, in the FPGA control, the SPI is modularized. Whether it is the output of the phase control word or the output of the frequency control word, it needs to go through the SPI module and output according to the SPI protocol. The block diagram of the FPGA control principle is shown in Figure 3.
In the FPGA, the external key information is first received, and the key state or data module is triggered. According to the information provided by the module, the corresponding registers (preq0, fdreq0, fhreq0) are assigned in the phase and frequency control module, and the completion is completed. The configuration of the phase and frequency control words is input to the SPI module, and the SPI protocol is output to the AD9833 through the SPI module. The control output must meet the timing control of the AD9833. The timing sequence is shown in Figure 4.
Under the control of the serial clock input SCLK (spiclk), when SCLK is high, and the enable signal FSYNC (spics) is low, SDATA (spido) starts to input data, and the data is written into the AD9833 in the form of a 16-bit word. FSYNC can remain low during multiple groups of 16 SCLK pulses, transmit a continuous stream of 16-bit words, and turn high on the 16th SCLK falling edge of the last word after the data transmission is complete.
2.3 Software control word of telemetry signal source
For flexible, configurable, and highly versatile telemetry signal sources, real-time changes in parameters such as frequency and waveform are essential. To realize the real-time change of these parameters, the control word must be changed accordingly. For example, the control word of sine wave is hexadecimal number 0008, the control word of triangular wave is hexadecimal number 000A, and the control word of square wave is hexadecimal number 0028.
From the calculation formula of AD9833 analog output frequency (refer to formula (1)), it can be known that if a 20 MHz crystal oscillator is used as the main frequency clock of AD9833 to output a 10 kHz sine wave signal, the hexadecimal of the frequency word FREQREG can be calculated. The number is 20C49. If the frequency register 0 and phase register 0 of AD9833 are selected in the software design, after adding the register identifier, the high-order hexadecimal number of the frequency word written by the FPGA to AD98 33 is 4008, and the low-order hexadecimal number is 4008. 4C49. Before writing data to the frequency register, if the hexadecimal number 2000 is written to the control register, the frequency register can be set to a complete 28 bits for use; if the hexadecimal number 0000 is written to the frequency register Can be used as two 14-bit registers. The phase word can be calculated according to equation (2). When the phase offset is 0°, the phase word PHASEREC is the hexadecimal number D000 (the phase register is marked as 1101); when the phase offset is 180°, the phase word PHASEREC is the hexadecimal number D800.
3. Simulation verification
Simulation is to use its own simulation software to simulate the function of the entire project under the Quartus II environment.
The simulation uses a 20 MHz crystal oscillator as the main frequency clock of the AD9833 to output a sine wave, square wave, triangle wave with a phase offset of zero and a frequency of 10 kHz, and a 5 kHz sine wave with a phase offset of 180°. The results are as follows: 5 to 8.
When the frequency is 5 kHz, the hexadecimal number of the frequency word FREQREG can be calculated by the analog output frequency formula.
This paper proposes a telemetry signal source based on FPGA and DDS chip. The signal source is mainly composed of Cyclone EP1C6 and AD9833 chips to build the hardware circuit, uses Verilog language to realize programming, and controls the FPGA to make it output data to the DDS chip, and finally realize the output of the required waveform. Simulation shows that the telemetry signal source can output sine wave, triangle wave and square wave signal with frequency and phase adjustable in the frequency range of 0-12.5 MHz flexibly and conveniently. The parameterization of this scheme greatly facilitates the modification of the required waveform data and enhances the flexibility of the signal source. Although the signal source can output sine wave, square wave and triangle wave with flexible frequency and phase, it does not realize the output of arbitrary waveform. Therefore, the future research direction is to realize the design of arbitrary waveforms to increase the flexibility and configurability of the signal source and further enhance its versatility.