PC-based oscilloscope (PCO)

   Oscilloscope software running in Windows that uses the computer's sound card as a cheap ADC
Although most people think of an oscilloscope as a self-contained instrument in a box, a new type of "oscilloscope" is emerging that consists of a specialized signal acquisition board (which can be an external USB or Parallel port device, or an internal add-on PCI or ISA card). The hardware itself usually consists of an electrical interface providing insulation and automatic gain controls, several hi-speed analogue-to-digital converters and some buffer memory, or even on-board DSPs. Depending on the exact hardware configuration, the hardware could be best described as a digitizer, a data logger or as a part of a specialized automatic control system.


The PC provides the display, control interface, disc storage, networking and often the electrical power for the acquisition hardware. The viability of PC-based oscilloscopes depends on the current widespread use and low cost of standardized PCs. Since prices can range from as little as $100 to as much as $3000 depending on their capabilities, such instruments are particularly suitable for the educational market, where PCs are commonplace but equipment budgets are often low. The acquisition hardware, in certain cases, may only consist of a standard sound card or even a game port, if only audio and low-frequency signals are involved.
The PCO can transfer data to the computer in two main ways - streaming, and block mode. In streaming mode the data is transferred to the PC in a continuous flow without any loss of data. The way in which the PCO is connected to the PC (e.g. USB) will dictate the maximum achievable speed using this method. Block mode utilizes the on-board memory of the PCO to collect a block of data which is then transferred to the PC after the block has been recorded. The PCO hardware then resets and records another block of data. This process happens very quickly, but the time taken will vary according to the size of the block of data and the speed at which it can be transferred. This method enables a much higher sampling speed, but in many cases the hardware will not record data whilst it is transferring the existing block, meaning that some data loss will occur.

The advantages of PC-based oscilloscopes include:
• Lower cost than a stand-alone oscilloscope, assuming the user already owns a PC. Professional-grade PCO hardware (e.g. with bandwidth in the MHz rather than in the kHz range) tends to be more expensive than e.g. a typical PCI sound card, and some can even cost more than a new PC [1].
• Easy exporting of data to standard PC software such as spreadsheets and word processors.
• Ability to control the instrument by running a custom program on the PC.
• Use of the PC's networking and disc storage functions, which cost extra when added to a self-contained oscilloscope.
• PCs typically have large high-resolution color displays which can be easier to read than the smaller displays found on conventional scopes. Color can be utilized to differentiate waveforms. It can also show increased information including more of the waveform or extras like automatic waveform measurements and simultaneous alternative views.
• Portability when used with a laptop PC.
There are also some disadvantages, which include:
• Power-supply and electromagnetic noise from PC circuits, which requires careful and extensive shielding to obtain good low-level signal resolution.
• Data transfer rates to the PC, which are dependent upon the connection method. This affects the maximum sampling speed achievable by the PCO when streaming.
• Need for the owner to install oscilloscope software on the PC, which may not be compatible with the current release of the PC operating system.
• Time for the PC to boot, compared with the almost instant start-up of a self-contained oscilloscope (although, as some modern oscilloscopes are actually PCs or similar machines in disguise, this distinction is narrowing).
As more processing power and data storage is included in oscilloscopes, the distinction is becoming blurred. Mainstream oscilloscope vendors manufacture large-screen, PC-based oscilloscopes, with very fast (multi-GHz) input digitizers and highly-customized user interfaces. Software for a PC may use the sound card or game port to acquire analog signals, instead of dedicated signal acquistion hardware. However, these devices have very restricted input voltage ranges, limited precision, and very restricted frequency ranges. The ground reference for these inputs is the same as the ground for the PC logic and power supply; this may inject unacceptable amounts of noise into the circuit under test. However, these devices can be useful for demonstration or hobby use.
If a sound card is used, frequency response is usually limited to the audio range, and DC signals cannot be measured. The number of inputs is limited by the number of recording channels and the inputs can handle only audio line-level voltages without the risk of damage. If the game port is used as the acquisition hardware, the sampling frequency is very low, typically below 1 kHz, and the input voltages can only vary over a range of a couple of volts. In addition, the game port cannot easily be programmed for a specific sampling rate, nor can it be easily assigned a precise quantization step. These limitations only make it suitable for low-precision visualization of low frequency signals.

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