CartUSB-Connected Modules and PC Software Convert PCs into Oscilloscopes and Spectrum Analyzers
By Alex Mendelsohn, eeProductCenter
Copyright 2004 © CMP Media LLC
With all of the news lately about leading-edge multi-GigaHertz-bandwidth oscilloscopes breaking the sound barrier, it's energizing to see that Pico Technology Limited (Cambridge, England) continues to address the lower end of the lab instrumentation spectrum with its range of new PC-hosted oscilloscopes
Selling for between about $700 and $1400, the company's PicoScope 3000 Series are intriguing instruments with features you'd expect on conventional bench scopes---and then some. For example, you can view quite a few multiple displays of the same signal, each with different settings or showing different parts of the same signal. Try that on a conventional bench scope.
As we've seen with quite a few PC-centric instruments of late, the 3000 Series modules derive their operating power from your PC's USB (Universal Serial Bus) ports. In this case, it's high-speed USB 2.0, which is now pretty much a standard for interfacing peripherals, printers, cameras, etc. to PCs.
USB All The Way
In its press statement Pico Technology doesn't mention that some PC oscilloscopes are USB 2.0 compatible, but only run at USB v1.1 speeds. In contrast, Pico's PicoScopes use the 480-Mbit/s speed of USB 2.0 to give these systems fast screen updates that make them resemble realtime scopes. Regardless of that, because USB 2.0 is backward compatible with USB 1.1, a PicoScope 3000 can still be used on older PCs that have slower USB 1.1 ports. It's just that the data transfer rate will be slower. Nonetheless, Pico points out that even at USB 1.1's rate, most transfers will be faster than if a parallel port was used for the scope interface.
What's more, when a 3000 Series module is plugged in, it becomes a plug-and-play peripheral, automagically announcing itself to the host PC and configuring itself. Finally, thanks to USB's networked topology, you can still hot-plug more USB peripherals into your PC while a PicoScope is operating.
Big Buffer
In the press release, Pico Technology's Technical Director mentions the system's high sampling rate and large memory buffer. With 1 Mbyte of memory, Pico's 3206 model, for example, can capture a lot of data, letting you zoom in on a signal area of interest.
While some low cost scopes collect only thousands of samples, a PicoScope 3000 can collect as many as million samples. That large buffer lets complex signals be captured in great detail over long periods of time. The scope's zoom functions can then be called into play to look at captured signals in detail. There's also the issue of sampling rate versus memory depth. At high sampling rates, a scope's buffer can be filled quickly, so the only way you can collect data for long periods of time is by throttling the sampling rate. If you have a scope with a high sampling rate but a small buffer, you can only collect at the fastest rated sampling rate at the top few timebases. With a PicoScope 3206 sampling repetitive signals, you can also use an equivalent-time sampling mode to boost the sampling rate to 10 Gsamples/s.
When viewing noisy signals, the combination of a high sampling rate and large buffer can also be used to oversample. PicoScope oscilloscope software lets you filter oversampled data to increase the resolution of the signal and remove random high frequency noise.
Frequency Domain Analysis
As the press statement mentions, these scopes also have spectral analysis capabilities. Oversampling can also be used in the spectrum analyzer mode, effectively increasing the resolution of the acquired data, which is equivalent to increasing the spectrum analyzer's dynamic range.
These scope's FFT-based (Fast Fourier Transform) spectrum analyzer capabilities offer 50 dB of dynamic range. In use, you get Normal, Average, and Peak Detect modes, with both linear and logarithmic scales on both amplitude and frequency axes.
Thanks to the associated software, you can also invoke seven different windowing types, including rectangle, triangle, Gaussian, Hanning, and Hamming. Automated measurements can make frequency, total power, THD (total harmonic distortion), SFDR (spurious-free dynamic range), SNR (signal-to-noise ratio), and IMD (intermodulation distortion) tests, to name a few. These should come in handy when making spectral tests on communications systems.
Expected Displays
Speaking of software, Pico Technology's supplied software endows these instruments with the look of a conventional benchtop scope. You'll feel right at home measuring period and frequency, rise time and fall time, as well as pulse width, duty cycle, and amplitude. You can also measure a signal's crest factor, its voltage at the cursor, time at the cursor, frequency at cursor, or amplitude at the cursor. The 3204 model is a 50 MHz scope that samples with a repetitive sampling rate of 2.5-Gsamples/s, and 50-Gsamples/s in single-shot mode. Its timebase spans 50-seconds/division down to 5-ns/division. The 3205 version is a 100 MHz scope; its timebase goes down to 2-ns/division. The 3206 is the 200-MHz instrument; its timebase goes down to 1-ns/division. All of these have timebases that are accurate to 50-ppm.
The PicoScopes also have an XY mode to plot one parameter against another. Significantly, the PicoScope 3000s use one digitizer/channel, so all samples are taken simultaneously on both channels (some scopes use one digitizer for multiple channels, which can lead to phase errors if making XY measurements).
Stunt Box
Moreover, thanks to the power of the host PC, you can also perform some additional stunts. A number of display modes can be used to reduce noise, capture glitches, or highlight jitter. There's even a chart recorder option that can be used to display slowly changing signals. Trigger options also include a save-to-disk mode that can help find intermittents.
Your PC display can also simultaneously show meter displays. With that, one or more parameters can be displayed numerically, just like a DMM or digital panel meter. You can even put up a bargraph. A multimeter can display AC volts in true RMS, DC volts, dB, and frequency. Dedicated parameters, such as pressure or speed, can be displayed by adding custom ranges.
You can also automate measurements. Measurements can include statistical indicators such as a signal's current value, its average, or its standard deviation. Similarly, you can set Pass/Fail thresholds. You can also customize a PicoScope, and, by using menus, you can load reference waveforms or previous settings. You can even load documents and web pages.
Data Acquisition Capabilities
Lastly, Pico's PicoLog data-acq software can be used to convert these scopes into data acquisition systems. A PicoScope 3000 can collect up to one million samples per channel, with data viewable during and after data collection, in both spreadsheet or graphical format. Naturally, you can also export the collected data to other PC applications, such as Excel spreadsheets. Pico Technology also includes an Excel macro to permit data to be collected directly into a spreadsheet.
If you want to spin your own custom software, or use third party software, Pico supplies free drivers and example code. Drivers are included Windows 98SE, Windows ME, Windows XP, and Windows 2000, and programming examples are supplied for C, Delphi, and VisualBASIC. Support is also provided for National Instruments LabVIEW, and Agilent VEE environments.
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