Application of digital oscilloscope in high frequency signal acquisition

Application of digital oscilloscope in high frequency signal acquisition

1. Collection of high-frequency signals

When a high-frequency signal (such as a radar waveform up to 100MHz) is to be collected and processed, a high-speed or ultra-high-speed hardware acquisition circuit is usually designed, including an amplification part, a filtering part, an A/D and a D/A conversion part, etc. The requirements for this circuit are very high, requiring simultaneous collection and storage, high circuit speed, and considering various radiation interferences, etc. At the same time, the price of finished products on the market is difficult to bear. And according to the sampling theorem, a continuous signal with a maximum frequency of / can be completely represented by a series of discrete sampling values ​​separated by a time interval of = 1/2f. Therefore, the sampling frequency F should be equal to or greater than twice the maximum frequency f of the sampled signal, that is, F ≥ 2f. Considering that the low-pass filter that actually recovers the waveform cannot have completely ideal characteristics, in order to correctly recover the signal, 9 = (2.5-5)f or higher is usually taken. When the sampled signal is as high as 100MHz, the sampling rate should reach 500MHz.

2 Communication between oscilloscope and computer

Tektronix's TDS series digital real-time oscilloscopes have long been widely used everywhere, and its supporting expansion modules TDS2CM and TDS2MM modules have the function of two-way communication with external devices, and can be directly connected to printers and microcomputers, making the storage and printing of waveforms very convenient. Among them, the TDS220 digital oscilloscope has a bandwidth of 100MHZ and a sampling rate of 1GS/s with a 10-fold scanning method. When the supporting TDS2CM module is connected to the computer using serial communication with an RS232 cable, the data and waveform of the oscilloscope can be directly read and processed using the corresponding software (such as Matlab, etc.).
Therefore, communication between the digital oscilloscope and the PC can be used. In terms of digital signal processing, the digital oscilloscope is equivalent to a high-speed signal collector. It transmits data to the computer and, in conjunction with Matlab, can realize the acquisition and processing of high-frequency signals. Moreover, compared with hardware processors in the general sense, there is no A/D and D/A conversion process, so the processing accuracy and speed are significantly improved. Moreover, the price of the oscilloscope supporting module TDSCM2 module is much cheaper than the hardware processors with corresponding functions.

3 Advantages and Disadvantages

The communication between the digital oscilloscope and the PC not only has the functions of a general desktop digital storage oscilloscope, but also gives full play to the powerful functions of the computer and the flexibility of software design. It has four significant features:
(1) It uses the programming language Matlab and object-oriented programming technology, with high software development efficiency, good operability and maintainability;
(2) It adds frequency domain analysis function to the digital storage oscilloscope;
(3) It makes full use of the computer's storage and peripheral connection capabilities, and the measurement results and waveforms can be directly printed out or shared through the network;
(4) Under the same hardware conditions, new instrument functions can be formed by modifying or adding software modules.
Since an oscilloscope, computer and RS232 connection are required, there is no simple and clear single hardware circuit, which is its shortcoming.

4 Application

4.1 Communication Principles

Matlab is an efficient numerical computing language launched by Math Works in the United States, which takes matrix as the basic programming unit. It is a highly integrated system that integrates scientific computing, image processing, and sound processing. It has powerful numerical computing functions, and the Instrument Control Toolbox provides the function of controlling ports such as GPIB, RS-232, VXI, and Centronics. When Matlab software is connected to the serial port of the TDS2CM module through RS232, data communication between the oscilloscope and the computer can be realized. At the same time, the numerical processing and matrix operation functions of Matlab can be used to perform various analyses and processes on the waveform data recorded by the oscilloscope.
Matlab creates a device object by calling the M file function and obtains the file handle of the device, so that the device can be operated like a file, and the relevant reading and writing of the peripherals can be performed. The schematic diagram is shown in Figure 1.

When hardware communication control is performed directly through RS232, the oscilloscope uses three control signals: DCD (Carrier Detect), CTS (Clear to Send), and RI (Ring Indicator) to indicate the current status, and uses RTS (Request to Send) to send data.

4.2 Data transmission

When Matlab reads and writes data to the waveform of the oscilloscope in binary format, the conversion between the read and write data and the actual data of the oscilloscope is given by formula (1):
Xn=Xzero+Xiner·n
Yn=yzero+Ymult(Yn－Yoff) (1)
Where: Yn is the data in the input and output buffers,
n is the number of data:
Xn, Yn are the actual sampling time and signal amplitude in the oscilloscope,
Xzero is the time of the first point of the acquired waveform,
Xiner is the sampling rate on the horizontal axis,
yzero is the amplitude at 0dB,
Ymul is the scale factor on the vertical axis;
Yoff is the vertical offset.
It can be seen from formula (1) that the transmitted data is completely in accordance with the sampling rate of the oscilloscope, and the waveform is real and reliable. The program flow chart is shown in Figure 2.

The commands to the oscilloscope are output in the form of a string by the fprintf function.
The setup and query commands are defined by the specific oscilloscope manufacturer.
The following is a partial program for reading data from the oscilloscope:
g=serial('coml');
g. InputBufferSize=10000;
g. timeout=10;
g. BaudRate=9600;
g. Parity='none'; g.
StopBits=1;
g. Terminator='LF'
g. FlowControl='hardware';
fopen(g);
fprintf(g,'select: chl on′);
fprintf( g ,'data: source chl′); fprintf (
g,'data: encdg srib′); fprintf(g,'data: start l′) ; fprintf (g,'data: stop 2500′); fprintf (g,' data: width 2 ′) ; 'wfmpre:xzero ? ′);

The reading of 100MHz radar waveform using the above program is shown in Figure 3.

4.3 Data Analysis and Processing and Examples

The data read from the oscilloscope is converted according to formula (1) to achieve the measured waveform data value, and the corresponding spectrum analysis can be performed in the computer.

5 Conclusion

Under the existing experimental conditions, the acquisition and processing of high-frequency signals by using the communication between the digital oscilloscope and the computer can fully meet the signal acquisition work in the general sense and can become an inexpensive experimental tool in the laboratory.