CN104034928A - Multi-path signal source based on PCI and FPGA - Google Patents

Abstract

The invention discloses a multi-channel signal source based on PCI and FPGA. The invention adopts FPGA as the central control unit, uses DDS technology to realize the change of sine wave frequency, and communicates with the upper computer through PCI. The signal source uses a high-precision D/A conversion chip as the core to form a waveform circuit, and uses electronic analog switches to realize multiple Signal output switching. The output signal frequency, amplitude and offset are adjustable, and the precision reaches 0.01%, which meets the design requirements.

Description

A Multi-channel Signal Source Based on PCI and FPGA

technical field

The invention relates to the field of digital technology, in particular to a multi-channel signal source based on PCI and FPGA.

Background technique

As an excitation source, the signal source can simulate various test signals to provide to the equipment under test, so as to meet the measurement and various practical needs. The early frequency synthesis technology uses a frequency reference to generate variable frequency signals from digital synthesis circuits. This way The work is unstable, the reliability is not high, and it is not easy to debug. Direct Digital Frequency Synthesis, DDS (DirectDigital Frequency Synthesis), as a brand-new frequency synthesis method has attracted the attention of the industry, and it can effectively overcome the above shortcomings. The combination of FPGA and DDS adopted in the present invention generates a signal source, and outputs 50 analog signals. Including 26 channels of low-voltage DC and sinusoidal signals and 24 channels of high-voltage DC signals, among which the output frequency of sinusoidal signals is 0-8kHZ, the output frequency of low-voltage DC is 0-6V, and that of high-voltage DC is 0-48V. The amplitude of the signal source can be adjusted, and the output Accuracy reaches 0.01%. Using the PCI bus to send commands and upload data to the host computer is more stable and faster than the USB bus.

Contents of the invention

The technical problem to be solved by the present invention is to provide a multi-channel signal source based on PCI and FPGA in view of the deficiencies of the prior art. The invention adopts FPGA as the central control unit, uses DDS technology to realize the change of sine wave frequency, and communicates with the upper computer through PCI. The signal source uses a high-precision D/A conversion chip as the core to form a waveform circuit, and uses electronic analog switches to realize multiple Signal output switching. The output signal frequency, amplitude and offset are adjustable, and the precision reaches 0.01%, which meets the design requirements.

Technical scheme of the present invention is as follows:

A multi-channel signal source based on PCI and FPGA, including power supply board, PCI card, backplane, analog quantity card, D/A conversion, signal amplification and conditioning, multi-channel switch selection module, following filter module; the power supply board will The input AC/DC is converted into the required voltage and output to the backplane, and then the backplane supplies power to the voltage analog card, and the backplane exchanges data with the PCI card on the computer through the optical fiber to complete the command download and data upload; The above-mentioned analog quantity card includes an analog quantity low-voltage card and an analog quantity high-voltage card; the analog quantity low-voltage card outputs a 0-6V DC analog signal and a sinusoidal signal with an adjustable frequency. The output of the analog high-voltage card is 0-48V, and the DC flow with adjustable amplitude; the host computer packs the board number, channel number, amplitude and frequency commands and sends them to the backplane through the PCI bus, and the backplane judges different boards The number is forwarded to different boards, and the FPGA parses the received host computer commands. The parsed commands will be used for reading and writing of FPGA internal ROM, conversion of high-precision D/A chip AD768 and strobe of multi-channel selection switch ADG708 , The low-voltage board signal is filtered and output, and the high-voltage board is followed to increase its drive current capability, and the OPA454 is used for secondary amplification and filtering before output.

The multi-channel signal source based on PCI and FPGA adopts the spartan-3 series chip XC3S400 of Xilinx Company as the main control chip of the analog quantity card to connect with the peripheral circuit

In the multi-channel signal source based on PCI and FPGA, the PCI card adopts a dedicated interface chip PCI9054 to realize the design of the interface module. The PCI9054 complies with PCI local regulation 2.2, and the data width is 32 bits.

For the multi-channel signal source based on PCI and FPGA, the optical fiber transmission is adopted between the PCI card and the backplane, and the optical module is selected as 84M/S OCM3443, which is respectively placed on the PCI card and the backplane to complete the data communication.

The present invention utilizes FPGA and PCI bus technology, and by means of the principle of DDS (Direct Digital Synthesis), realizes the system design of multi-channel analog signal source, the frequency and amplitude of the host computer control signal, and the signal source has high precision and stability Good and multi-channel simultaneous output and other advantages.

Description of drawings

Figure 1 is a block diagram of the overall scheme of the system;

Figure 2 is a schematic diagram of the OCM3443 peripheral circuit;

Fig. 3 is a schematic diagram of a D/A conversion circuit;

Fig. 4 is a schematic diagram of a multi-channel output and signal holding circuit;

Fig. 5 is a schematic diagram of a secondary amplifier circuit;

Fig. 6 is a schematic diagram of a low-pass filter;

Fig. 7 is a flow chart of signal source program;

Figure 8 is the test result of high voltage card DC flow;

Figure 9 is the flow rate of the low pressure card;

Figure 10 (1.00009KHz) and Figure 11 (8.00001KHz) are sine waves with variable frequency.

Detailed ways

The present invention will be described in detail below in conjunction with specific embodiments.

1 overall design of the system

The present invention adopts a modular design, each module is a separate board, each board has clear functions and good overall coordination, and the system includes a power supply board (module), PCI card, backplane, analog quantity card, D/ A conversion, signal amplification and conditioning, multi-way switch selection module, following filter module. The modular design is convenient for designers to debug and expand. When there is a problem with the functional board, it is easy to troubleshoot and maintain.

The power board (module) converts the input AC/DC into the required voltage and outputs it to the backplane, and then the backplane supplies power to the voltage analog card, and the backplane exchanges data with the PCI card on the computer through the optical fiber to complete the command Download and upload data. The present invention uses two analog quantity cards (analog quantity low-voltage card and analog quantity high-voltage card) to realize the function, adopts Xilinx company spartan-3 series chip XC3S400 as the main control chip of the analog quantity card to connect with the peripheral circuit.

The PCI card adopts a dedicated interface chip PCI9054 to realize the design of the interface module. PCI9054 complies with PCI local regulation 2.2, the data width is 32 bits, and the burst transmission speed can reach up to 132MHz/s when the 33M clock is input. The overall system design is shown in Figure 1 shown.

In the present invention, the analog low-voltage card outputs 0-6V DC analog signals and sinusoidal signals with adjustable frequency. The output of the analog high-voltage card is 0-48V, and the DC flow with adjustable amplitude. The working principle of the system is: the upper computer packs the board number, channel number, amplitude and frequency commands and sends them to the backplane through the PCI bus, and the backplane forwards them to different boards by judging different board numbers (high voltage in the present invention) The card is 00, the low-voltage card is 01), the FPGA parses the commands received from the host computer, and the parsed commands will be used for reading and writing of FPGA internal ROM, conversion of high-precision D/A chip AD768 and multi-channel selection switch ADG708 Strobe, the low-voltage board signal is filtered and output, and the high-voltage board is followed to increase its drive current capability, and the output is output after secondary amplification and filtering by OPA454. The power module includes a power board (28V converted to 5V) and a customized DC power supply (output +55V, +28V, ±9V) to meet the voltage requirements of the entire system.

2. Hardware circuit design

2.1 PCI interface design

The PCI card is inserted in the PCI slot of the computer, and is the intermediate hub connecting the upper computer and the backplane. The connection between the local bus and the PCI bus is realized by using a dedicated interface chip PCI9054. Data width, transfer rate up to 132MB/S ^[1] . The local end of PCI9054 provides independent address lines and data lines, and the data width can be configured as 8 bits, 16 bits or 32 bits, allowing access to 0-50M local clocks. PCI9054 can be configured as a slave mode, and the computer controls the local bus through PCI9054 to complete the issuing of commands. It can also be configured as a DMA mode. In this mode, the participation of the CPU is not required, and the direct data transmission between the local bus and the computer memory is directly controlled by the PCI9054. In the present invention, the DMA burst transmission mode is used to meet the requirements of the upper computer and the PCI bus. high-speed data transmission.

2.2 Photoelectric conversion circuit design

Analog signals require high-precision, low-power transmission, so optical fiber transmission is used between the PCI card and the backplane. The advantages of optical fiber transmission are large communication capacity, low loss, long relay distance, and strong confidentiality. The use of photoelectric conversion can also resist interference and improve reliability. The optical module chooses 84M/S OCM3443 and places it on the PCI card and the backplane to complete the data communication. OCM3443 interface level standard is TTL, which is compatible with FPGA pin level, so it is directly connected to FPGA, designed with extremely low power consumption, and its peripheral circuit is shown in Figure 2.

2.3 Signal source module design

2.3.1 D/A conversion circuit design

The function of the D/A conversion circuit is to convert the digital quantity of the synthesized signal waveform into an analog quantity, and the higher the resolution of the D/A converter, the higher the precision of the output waveform. Usually, the quantization precision of 8 bits is basically satisfied. Accuracy requirements, and the actual DAC output needs to consider the influence of external conditions such as temperature on the signal accuracy, the actual output signal cannot reach the accuracy described by the DAC ^[2] chip. Therefore in order to reach the signal accuracy of index requirement, select the high-precision AD768 of 16 resolutions in the present invention, AD768 is the high-speed analog-to-digital converter of current/voltage mode output, and the full-bias current reaches 20mA when digital quantity input is all 1, satisfies To meet the requirements of the drive current, the conversion rate is as high as 30MSPS. Design circuit as shown in Figure 3, I _OUTA and I _OUTB are two complementary current output ends of AD768; According to chip data REFOUT is the reference voltage output value of AD768 is 2.5V, and reference current is 5mA, the present invention adopts the bipolar of AD768 sexual pattern. This mode makes the boundary value of the output voltage symmetrical by setting the value of I _BIPOLAR to half of the maximum feedback current I _FB . The relationship between the output current I _OUTA of the AD768 and the reference current I _BIPOLAR is:

I _OUTA ＝(DAC CODE/65536)×(I _BIPOLAR ×4) (1)

In formula (1), DAC CODE is a 16-bit digital input code, which changes between 0 and 65535. The value of I _BIPOLAR is 5mA. Therefore, the range of the output current IOUTA is between 0 and 20mA. After the output current is buffered and amplified by the operational amplifier AD811, the output voltage of the first stage is V _OUT1 which is -1~+1V, and the function of the second stage operational amplifier is an inverting amplifier circuit, and the final output voltage is V _OUT2 which is - 4~+4V, the function of the third-stage operational amplifier is an amplifying circuit with a reference voltage, and the final output voltage V _OUT3 is -0.7V~+7.3V, where the DAC CODE is determined by the input data value to be converted.

2.3.2 Design of analog switch and signal holding circuit

In the present invention, there are many analog quantities, reducing cost and space considerations, and using an analog switch to realize multi-channel selection and switching. The present invention selects ADG708 to complete 8-to-1 analog signal output. ADG708 is an 8-channel electronic switch. When it is enabled, The combination of 8 binary numbers of the 3-bit address lines A0, A1, and A2 controls the analog signal to be selected for output. It has been verified by experiments that there is a problem of weakening the signal through the analog switch, so a signal holding circuit must be designed to ensure the stability of the analog signal. And increase its driving current so that it can effectively drive the load during output. Figure 4 shows the analog switch (A) and follower circuit (B) for a low-voltage design. Figure 4 shows the circuit used by the low-voltage analog card. For the high-voltage card, the voltage value after D/A conversion and three-stage operational amplifier cascading is far from meeting the requirements. The signal output by the switch must be amplified twice. As shown in Figure 5, OPA454 is selected as the operational amplifier chip, and there are two power supply modes of 0-100V and ±50V. During the test, it was found that OPA454 is not a rail-to-rail operational amplifier, so its negative terminal voltage must be less than 0V. In this invention, -9V ～+55V power supply, the amplification factor is: 1+R119/R110, in which R119 uses a 12K resistor, and R110 is 2K, so the voltage gain is 7 times, and the theoretical maximum voltage after amplification reaches 49V, which meets the design requirements.

The input frequency of the signal is low. In order to effectively avoid the interference of high-frequency signals, a low-pass filter is used to process the output signal. The RC active filter composed of an integrated operational amplifier has high input impedance and low output impedance, which can provide a certain The gain and cut-off frequency are adjustable. As shown in the figure, the filter is designed for a low-voltage board, in which the first-stage capacitor is connected to the output terminal, and an appropriate amount of positive feedback is introduced to improve the amplitude-frequency characteristics. The difference between the high-voltage analog card and the low-voltage analog card is that the integrated operational amplifier chip is different, and the high-voltage card uses OPA454 to form a filter circuit (Figure 6).

3. System software design

The present invention utilizes the principle of DDS and uses VHDL language to complete the instantiation waveform ROM of IP Core inside the FPGA, wherein the waveform data of the sine wave is generated by the MATLAB program, and a complete cycle of the sine signal is discretized into ²⁸ or 256 sampling sequences. After quantization, it is stored in the waveform memory, and then the waveform data is sent to the D/A as 16-bit binary numbers to complete the digital-to-analog conversion by addressing. The upper computer clicks to open the device to start the signal source, and then the FPGA judges whether it is a DC flow or a sine wave according to the information of the internal register. If it is a sine wave, the ROM address corresponding to the frequency value is calculated by the DDS principle ^[4] , and the ROM address is read from the ROM The data is taken out from the middle and sent to DA for output. If it is DC flow, the binary data corresponding to the internal register is directly sent to D/A for output. Each channel of DC flow or sine wave corresponds to a different independent address, and the adjustment of frequency and amplitude is completed by addressing. The control flow chart is shown in Figure 7.

4. Analysis of test results

Use an oscilloscope and a high-precision multimeter to test the output waveform and DC voltage value of the system respectively, and obtain sine waves with different frequencies and DC values with different amplitudes. Figure 8 shows the DC output of the high-voltage board, and Figure 9 shows the DC output of the low-voltage board. Flow, Figure 10 (1.00009KHz) and Figure 11 (8.00001KHz) are sine waves with variable frequency. It can be seen from the figure that the obtained waveform is smooth and the resolution is high. Among them, the frequency accuracy reaches 0.01% in the sine wave output, and the amplitude accuracy reaches 0.1% in the DC flow output, meeting the design requirements.

It should be understood that those skilled in the art can make improvements or changes based on the above description, and all these improvements and changes should belong to the protection scope of the appended claims of the present invention.

Claims (4)

1. A multi-channel signal source based on PCI and FPGA, characterized in that it comprises a power board, PCI card, backplane, analog quantity card, D/A conversion, signal amplification and conditioning, multi-way switch selection module, following filter Module; the power supply board converts the input AC/DC into the required voltage and outputs it to the backplane, and then the backplane supplies power to the voltage analog card, and the backplane exchanges data with the PCI card on the computer through the optical fiber to complete the command transmission and data upload; the analog card includes an analog low-voltage card and an analog high-voltage card; the analog low-voltage card outputs a 0-6V DC analog signal and a sinusoidal signal with adjustable frequency. The output of the analog high-voltage card is 0-48V, and the DC flow with adjustable amplitude; the host computer packs the board number, channel number, amplitude and frequency commands and sends them to the backplane through the PCI bus, and the backplane judges different boards The number is forwarded to different boards, and the FPGA parses the received host computer commands. The parsed commands will be used for reading and writing of FPGA internal ROM, conversion of high-precision D/A chip AD768 and strobe of multi-channel selection switch ADG708 , The low-voltage board signal is filtered and output, and the high-voltage board is followed to increase its drive current capability, and the OPA454 is used for secondary amplification and filtering before output. 2. the multi-channel signal source based on PCI and FPGA according to claim 1, is characterized in that, adopts Xilinx company spartan-3 series chip XC3S400 to be connected with peripheral circuit as the main control chip of analog quantity card. 3. the multi-channel signal source based on PCI and FPGA according to claim 1, is characterized in that, PCI card has adopted special-purpose interface chip PCI9054 to realize the design of interface module, and PCI9054 meets PCI local regulation 2.2, and data width is 32 bits . 4. the multi-channel signal source based on PCI and FPGA according to claim 1, is characterized in that, adopts optical fiber transmission between PCI card and backboard, and optical module selects the OCM3443 of 84M/S, places PCI card and backboard respectively The board completes the data communication.