Construction of radar echo fluctuation model based on virtual instrument technology and Matlab software

1 Introduction

In recent years, with the rapid development of electronic technology and computer digital simulation technology, radar simulation methods have gradually become an indispensable means in the design, analysis and performance testing process of radar systems because of their economical, flexible and realistic characteristics. Debugging and performance evaluation of sub-systems, analysis and debugging of the rear stage of the system under conditions where the radar front-end is not available, and inspection of the overall performance and indicators of the radar. However, the current simulation of radar echo fluctuation characteristics is generally based on special radar simulators, and it is rare to use general-purpose instruments to study echo fluctuation characteristics.

Virtual instrument technology is the product of the combination of modern computer technology, communication technology and measurement technology, and represents the development direction of modern testing technology and instrument technology. The graphical virtual instrument programming software Lab VIEW itself is powerful and environment-friendly. It is widely used in testing and measurement, process control and processing, scientific research and analysis, etc. However, for some complex applications that require a large amount of data processing, LabVIEW seems to be somewhat inadequate. Matlab has strong engineering calculation functions and has become an indispensable basic software for computer-aided analysis, design, simulation, etc. in applied disciplines.

Agilent E8267D, E4438C, E443XB and other series of vector signal generators allow users to use Matlab software to write waveform files, and then download them through Agilent

The Assistant program downloads the waveform file into the signal generator, modulates the carrier wave, and transmits the modulated signal under the control of the trigger signal.

Therefore, the characteristics of the above-mentioned general instruments can be combined with Matlab on the LabVIEW software platform to simulate Swerling type I and II fluctuation signals.

2 hardware

Figure 1 shows the hardware composition. Matlab software is used to establish the radar echo fluctuation model, and then the I/Q digital waveform generated by Matlab is sent to the arbitrary waveform generator memory of the general instrument through the GPIB bus through the Agilent Download Assistant program. Select the appropriate Trigger source and trigger type to trigger the waveform file, so that RF output can be obtained.

Since the operating frequency of radar is high, if the frequency of E8267D is set to the radar operating frequency, the RF output signal of E8267D can make the radar work, but it cannot be observed on an oscilloscope. Therefore, for the convenience of observation, the synchronization pulse signal of the radar is connected. The PATTEN TRIG IN interface of E8267D serves as the external trigger signal of the signal generator. Set the frequency of the signal generator smaller and observe the RF output signal on the oscilloscope.

3 software

3.1 Swerling Ⅰ, Ⅱ type fluctuation mathematical model

Swerling type I and type II fluctuations refer to echo amplitudes fluctuating uniformly and independently according to the Rayleigh distribution respectively, and their probability density function is shown in Equation (1). The only difference between them is the amplitude fluctuation speed.

Among them, y is a random variable obeying a uniform distribution from 0 to 1. From formula (2), we can get the sequence whose amplitude obeys Rayleigh distribution.

3.2 Communication interface between Lab VIEW and Matlab

LabVIEW provides Matlab Script nodes and ActiveX function templates to enable communication between LabVIEW and Matlab.

Matlab Script nodes have the characteristics of multiple inputs and multiple outputs. In the node, users can edit Matlab programs, or directly load existing Matlab programs and run them in the Lab VIEW environment, so it is fast and convenient.

For ActiveX function templates, users can use a reference (Refnum, also called "reference") as the unique identifier of an object (file, device, network connection) in Lab VIEW. Since the reference is a temporary pointer pointing to an object (such as a Matlab file), it is only valid when the object is opened. Once the object is closed, Lab VIEW will automatically disconnect.

Since communication through Matlab Script nodes is easier to implement, this interface is used in this article.

3.3 Ups and downs envelope

Set the clock frequency smplclk of the arbitrary waveform generator in E8267D to l00. MHz. Each point within the pulse envelope waveform will occupy 1/smplclk or 10 ns of time. Suppose n is the number of points in the pulse rising, working, falling or cutting array, then the corresponding time of the pulse is, n*10 ns. Users can calculate the pulse width and pulse repetition period according to the following commands. As long as the appropriate n value is selected, the pulse with the required width and repetition period can be obtained.

Figure 2 shows the program using Matlab Script node to generate single pulse envelope. The amplitude A of the pulse envelope is obtained based on the previous principle of generating a Rayleigh distribution sequence.

Suppose the number of non-coherent accumulations is N, then within one observation time, for the Swerling I-type echo signal, the amplitude A of its N sequences all take the same value; in another observation time, the value of A is independent. For Swerling type II echo signals, the value of A is independent within each observation time.

3.4 Download fluctuation waveform

The Download Assistant program developed by Agilent can be downloaded for free on the company's website. He added Matlab keywords to the data control program and sent the data array to the signal generator through the public GPIB interface card. Therefore, it is easy to transfer Matlab waveform files to E8267D using this program. They are actually some Matlab subroutines, which can be called in the main program to control the instrument.

E8267D provides 4 trigger modes, select SegmentAdvance→Single here. Store each single pulse as a waveform segment (Seg-ment), and then combine the waveform segments into a sequence (Sequence). If the trigger polarity is selected to be positive and the trigger delay is turned off, there will be a fluctuating pulse output when the rising edge of each synchronization pulse arrives. The following are some SCPI commands.

The echo signal has a certain time delay t relative to the synchronization pulse. Since the synchronization pulse is used as the trigger input signal, the delay time f can be set through Trig Delay.

At the same time, you can also add a certain length of zero array before the single pulse to implement the delay according to the previous method of writing a single pulse. The delay time is calculated based on the number of array points and the clock frequency smplclk of the arbitrary waveform generator.

Figure 3 shows the code for downloading the fluctuation waveform. In order to facilitate the control of the signal generator amplitude, frequency, and trigger delay time on the front panel, set them as input variables. Since the Matlab Script node requires these three input variables to be string types, the numeric variables need to be converted into character variables through the "Number To FractionalString" function.

3.5 Reading oscilloscope data

The oscilloscope here mainly serves as a data acquisition function so that the computer can process the data.

In the Lab VIEW environment, you can use the Instrument I/O Assistant to complete communication with the oscilloscope, as shown in Figure 4.

Select Query and Parse in the Instrument I/O Assistant and enter the following query command in the Entera command text box:

Data:SOurce CH2;:DATA:ENCdg RIBINARY;WIDTH1;:CURVe?

Click the Run this step button to read the waveform data from the second channel of the oscilloscope and display it in the response window. Finally, the waveform data is analyzed to obtain the RF output waveform of E8267D, as shown in Figure 5 and Figure 6.

4 Conclusion

Using LabVIEW and Matlab for hybrid programming, on the one hand, you can apply Lab VIEW's powerful G language programming method to improve the efficiency of developing virtual systems; on the other hand, you can take advantage of Matlab's ability to perform complex numerical calculations to greatly enhance the efficiency of Lab VIEW, so that you can Better control the instrument and simulate radar echo fluctuation characteristics for subsequent automatic testing.