Electronic projects

Digital USB oscilloscope

Digital USB oscilloscope

Nowadays, the use of various measuring devices built on the basis of interaction with a personal computer is quite numerous. A significant advantage of their use is the ability to save the obtained values ​​of a fairly large volume in the device’s memory, with their subsequent analysis.

 

The digital USB oscilloscope for a computer, the description of which is given in this article, is one of the variants of such measuring instruments for a radio amateur. It can be used as an oscilloscope and a device recording electrical signals into the RAM and hard drive of a computer.

The circuit is not complicated and contains a minimum of components, as a result of which it was possible to achieve good compactness of the device.

Main characteristics of the USB oscilloscope:

  • ADC: 12 bits.
  • Time base (oscilloscope): 3…10 ms/division.
  • Time scale (recorder): 1…50 sec/sample.
  • Sensitivity (without divider): 0.3 Volts/division.
  • Synchronization: external, internal.
  • Data entry (format): ASCII, text.
  • Maximum input impedance: 1 Mohm in parallel with 30 pF capacitance. 

Description of the operation of an oscilloscope from a computer

To exchange data between the USB oscilloscope and the personal computer, the Universal Serial Bus (USB) interface is used. This interface operates on the basis of the FT232BM (DD2) microcircuit from Future Technology Devices. It is a  USB-COM interface converter . The FT232BM microcircuit can operate both in the BitBang direct bit control mode (using the D2XX driver) and in the virtual COM port mode (using the VCP driver).

The AD7495 (DD3) integrated circuit from Analog Devices is used as the ADC. This is nothing more than a 12-bit analog-to-digital converter with an internal reference voltage source and a serial interface.

Digital USB oscilloscope

The AD7495 also has a frequency synthesizer that determines the speed at which information is exchanged between the FT232BM and the AD7495. To create the necessary data exchange protocol, the USB oscilloscope software fills the USB output buffer with individual bit values ​​for the SCLK and CS signals, as shown in the following figure:

Digital USB oscilloscope

The measurement of one cycle is determined by a series of nine hundred and sixty consecutive conversions. The FT232BM chip with a frequency determined by the built-in frequency synthesizer sends electrical signals SCLK and CS, in parallel with the transmission of conversion data along the SDATA line. The period of the 1st full conversion of the FT232BM ADC, which sets the sampling frequency, corresponds to the duration of the period of sending 34 bytes of data issued by the DD2 chip (16 data bits + CS line pulse). Since the data transfer rate of the FT232BM is determined by the frequency of the internal frequency synthesizer, then to modify the sweep values, you only need to change the frequency synthesizer values ​​of the FT232BM chip.

The data received by the personal computer, after a certain amount of processing (scale change, zero adjustment) are displayed on the monitor screen in graphic form.

The signal under study is fed to the XS2 connector. The OP747 operational amplifier is designed to match the input signals with the rest of the USB oscilloscope circuit.

The DA1.2 and DA1.3 modules are used to build a circuit for shifting a bipolar input signal to the positive voltage zone. Since the internal reference voltage source of the DD3 microcircuit has a voltage of 2.5 volts, then without using dividers the input voltage coverage is -1.25..+1.25 V.

In order to be able to study signals with negative polarity, with actually unipolar power supply from the USB connector ( USB connector pinout A), the voltage converter DD1 is used, which generates a voltage of negative polarity to power the OP747 op-amp. To protect the analog part of the oscilloscope from interference, the components R5, L1, L2, C3, C7-C11 are used.

The uScpoe program is designed to display information on the computer monitor screen. With this program, it becomes possible to visually evaluate the magnitude of the signal being studied, as well as its shape in the form of an oscillogram.

Digital USB oscilloscope

The ms/div buttons are used to control the oscilloscope sweep. The program allows saving the oscillogram and data to a file using the corresponding menu items. The Power ON/OF buttons are used to virtually turn the oscilloscope on and off. When disconnecting the oscilloscope circuit from the computer, the uScpoe program automatically switches to the OFF mode.

In the electrical signal recording mode (recorder), the program creates a text file, the name of which can be specified in the following path: File->Choice data file. Initially, the data.txt file is formed. Then the files can be imported into other applications (Excel, MathCAD) for further processing.

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