08-10-2012, 02:08 PM
2-Channel-PC-Based Oscilloscope
2-Channel-PC.pdf (Size: 3.01 MB / Downloads: 75)
Background of the problem
Oscilloscopes traditionally are hardware based using a CRT (Cathode Ray Tube) designed to display voltage variations (periodic or otherwise); they are bulky, expensive and have difficultly displaying low frequency waveform.
The heart of the traditionally CRT oscilloscope is the display screen itself, the CRT. “The CRT is a glass bulb which has had the air removed and then been sealed with a vacuum inside. At the front is a flat glass screen, which is coated inside with a phosphor material. This phosphor will glow when struck by the fast moving electrons and produce light, emitted from the front and forming the spot and hence the trace. The rear of the CRT contains the electron „gun‟ assembly. A small heater element is contained within a cylinder of metal called the cathode. When the heater is activated by applying a voltage across it, the cathode temperature rises and it then emits a stream of electrons.”
Statement of the problem
More and more experiments are now „PC-assisted.‟ In addition, conventional acquisition systems are very expensive. Since portable PCs are today common and a USB link is a better solution than an old ISA bus, here we present an oscilloscope using USB port of the PC that operates at up to 10 kHz with ±16V input voltage. . The oscilloscope uses IC PIC18F2550 from Microchip as the main controller, which makes the oscilloscope compact, as there is no need of additional power supply for the entire circuit board.
VIEW OF PROJECT
This project attempts to achieve the same functionality as a traditional oscilloscope, using a PIC microcontroller for data acquisition (including appropriate analogue circuitry) which transfers the data to the PC through USB. A Microsoft Windows based software application will then display the waveform, as it would appear on a traditional CRT oscilloscope. This software application will have additional features not present on a traditional oscilloscope with greater flexibly as additional features can be added as they can be developed without the need for new hardware.
OPERATION
Figure 2- 1shows the circuit of the 2-Channel-PC Based Oscilloscope. MCP6S91 from Microchip Technology is an analogue programmable gain amplifier that is well suited for driving analogue-to digital converters (ADCs) and an analogue input to a PIC microcontroller. Two MCP6S91 programmable gain amplifiers (IC2 and IC3) make it possible to choose the input ranges for each of the two channels, by selecting a gain from 1:1 to32:1. The amplifiers are small, cheap and easy to use. A simple three-wire serial peripheral interface (SPI) allows the PIC to control them through its pins 5, 6 and 7. The MCP6S91 amplifier is designed with CMOS input devices. It is designed to not exhibit phase inversion when the input pins exceed the supply voltages. The maximum voltage that can be applied to the input pin is –0.3V (VSS) to +0.3V (VDD). Input voltages that exceed this absolute maximum rating can cause excessive current into or out of the input pins. Current beyond ±2mA can cause reliability problems. Applications that exceed this rating must be externally limited with a resistor to the input pin.
Vref (pin 3), which is an analogue input, should be at a voltage between VSS and VDD. The voltage at this pin shifts the output voltage. The SP Interface inputs are chip-select (CS), serial input (SI) and serial clock (SCK). These are Schmitt-triggered, CMOS logic inputs. The only disadvantage is that these amplifiers accept only positive signals .That‟s why voltage-shifting amplifiers LF353 (IC4A and IC5A) are used, one each for each channel input (see Figure 2- 1). The LF353 is a JFET input operational amplifier with an internally compensated input offset voltage. The JFET input device provides wide bandwidth, low input bias currents and offset currents. This voltage-shifting amplifier results in high input impedance and an attenuation factor of 1:4.5.A ±16V input signal is then shifted to the 0-5V range when the programmed gain is 1:1.
CONCLUSION
Monitoring of sound waves, which are difficult to monitor on a traditional oscilloscope due to the low frequencies involved.
Monitoring of an ECG signal, again because this is such a low frequency traditional oscilloscopes would have difficultly monitoring such a signal. ECG data could be logged and emailed directly to the doctor for diagnosis, or perhaps real-time TCP/IP internet communication so that the doctor could remotely monitor the ECG signal in real-time.
Monitoring of serial communications, for example RS485 works on the principal of differential voltages between two cables twisted together; hence the PC based oscilloscope could be used to view serial communications. Two oscilloscope channels would be used, and the PC software will automatically add the two channels together producing a virtual trace (A+B).
The PC based oscilloscope is ideal for demonstration purposes, for example using data projector a class of student could be introduced to the oscilloscope, with real waveforms being monitored (signal generator, or even a microphone for sound waves) and displayed on a large projector display.
2-Channel-PC.pdf (Size: 3.01 MB / Downloads: 75)
Background of the problem
Oscilloscopes traditionally are hardware based using a CRT (Cathode Ray Tube) designed to display voltage variations (periodic or otherwise); they are bulky, expensive and have difficultly displaying low frequency waveform.
The heart of the traditionally CRT oscilloscope is the display screen itself, the CRT. “The CRT is a glass bulb which has had the air removed and then been sealed with a vacuum inside. At the front is a flat glass screen, which is coated inside with a phosphor material. This phosphor will glow when struck by the fast moving electrons and produce light, emitted from the front and forming the spot and hence the trace. The rear of the CRT contains the electron „gun‟ assembly. A small heater element is contained within a cylinder of metal called the cathode. When the heater is activated by applying a voltage across it, the cathode temperature rises and it then emits a stream of electrons.”
Statement of the problem
More and more experiments are now „PC-assisted.‟ In addition, conventional acquisition systems are very expensive. Since portable PCs are today common and a USB link is a better solution than an old ISA bus, here we present an oscilloscope using USB port of the PC that operates at up to 10 kHz with ±16V input voltage. . The oscilloscope uses IC PIC18F2550 from Microchip as the main controller, which makes the oscilloscope compact, as there is no need of additional power supply for the entire circuit board.
VIEW OF PROJECT
This project attempts to achieve the same functionality as a traditional oscilloscope, using a PIC microcontroller for data acquisition (including appropriate analogue circuitry) which transfers the data to the PC through USB. A Microsoft Windows based software application will then display the waveform, as it would appear on a traditional CRT oscilloscope. This software application will have additional features not present on a traditional oscilloscope with greater flexibly as additional features can be added as they can be developed without the need for new hardware.
OPERATION
Figure 2- 1shows the circuit of the 2-Channel-PC Based Oscilloscope. MCP6S91 from Microchip Technology is an analogue programmable gain amplifier that is well suited for driving analogue-to digital converters (ADCs) and an analogue input to a PIC microcontroller. Two MCP6S91 programmable gain amplifiers (IC2 and IC3) make it possible to choose the input ranges for each of the two channels, by selecting a gain from 1:1 to32:1. The amplifiers are small, cheap and easy to use. A simple three-wire serial peripheral interface (SPI) allows the PIC to control them through its pins 5, 6 and 7. The MCP6S91 amplifier is designed with CMOS input devices. It is designed to not exhibit phase inversion when the input pins exceed the supply voltages. The maximum voltage that can be applied to the input pin is –0.3V (VSS) to +0.3V (VDD). Input voltages that exceed this absolute maximum rating can cause excessive current into or out of the input pins. Current beyond ±2mA can cause reliability problems. Applications that exceed this rating must be externally limited with a resistor to the input pin.
Vref (pin 3), which is an analogue input, should be at a voltage between VSS and VDD. The voltage at this pin shifts the output voltage. The SP Interface inputs are chip-select (CS), serial input (SI) and serial clock (SCK). These are Schmitt-triggered, CMOS logic inputs. The only disadvantage is that these amplifiers accept only positive signals .That‟s why voltage-shifting amplifiers LF353 (IC4A and IC5A) are used, one each for each channel input (see Figure 2- 1). The LF353 is a JFET input operational amplifier with an internally compensated input offset voltage. The JFET input device provides wide bandwidth, low input bias currents and offset currents. This voltage-shifting amplifier results in high input impedance and an attenuation factor of 1:4.5.A ±16V input signal is then shifted to the 0-5V range when the programmed gain is 1:1.
CONCLUSION
Monitoring of sound waves, which are difficult to monitor on a traditional oscilloscope due to the low frequencies involved.
Monitoring of an ECG signal, again because this is such a low frequency traditional oscilloscopes would have difficultly monitoring such a signal. ECG data could be logged and emailed directly to the doctor for diagnosis, or perhaps real-time TCP/IP internet communication so that the doctor could remotely monitor the ECG signal in real-time.
Monitoring of serial communications, for example RS485 works on the principal of differential voltages between two cables twisted together; hence the PC based oscilloscope could be used to view serial communications. Two oscilloscope channels would be used, and the PC software will automatically add the two channels together producing a virtual trace (A+B).
The PC based oscilloscope is ideal for demonstration purposes, for example using data projector a class of student could be introduced to the oscilloscope, with real waveforms being monitored (signal generator, or even a microphone for sound waves) and displayed on a large projector display.