CartBy Robert Penfold
Everyday Practical Electronics
January 2005
Robert puts Pico's dual-trace PC Oscilloscope through its paces and finds that, together with its bundled software, it's easy to use and offers good value for money
The idea of virtual test gear was still relatively new when we first reviewed a unit from the PicoScope range. For the benefit of those who are not familiar with this concept, the idea is to have a test instrument that is formed by a combination of an interface and software running on a computer. The interface has any necessary input and output sockets, and might have some controls. In most cases though, all or most of the controls are of the on-screen variety and are provided by the software.
Provided you already have a suitable computer, the virtual approach permits a sophisticated piece of test equipment to be obtained at a significantly lower cost than the real thing. The virtual approach is not just about saving money though. Having a test instrument based on a computer has its advantages, such as the ability to save results to disk, print out graphics produced by the program, and export results to a spreadsheet.
A virtual oscilloscope such as a PicoScope is basically just a high speed analogue to digital converter. There is a potential “bottle-neck” in the link between the interface and the computer, especially when using a very fast converter that can produce many millions of samples per second. Most real-world virtual oscilloscopes, including the unit reviewed here, therefore have some memory in the interface unit to provide buffering.
Seeing Double
The PicoScope 3205 reviewed here is a virtual dual-trace PC oscilloscope that is the middle unit in a range of three. The main difference between the three interfaces is their maximum effective sampling rates, which for the PicoScope 3204, 3205, and 3206 are respectively 2.5GS/s, 5GS/s, and 10GS/s.
Note that these rates are for repetitive sampling and involve some software trickery. The true sampling rates are used for single shot operation, and are much lower. In order to accurately display a waveform it is necessary to take a number of samples per input cycle. The quoted bandwidths are 50MHz, 100MHz, and 200MHz respectively.
There are no controls on the interface, but it does have three BNC sockets and an indicator light on the front panel. Two of the sockets are the main (Channel A and B) inputs, and the third is an external trigger input. The latter also acts as an output for the unit's built-in signal generator. The light switches on when the unit is active.
On the rear panel there is a USB socket, and this is connected to a spare USB socket on the host computer using the supplied 2-metre cable. This is a standard USB cable of the type used with printers and scanners.
Power for the interface is obtained from the computer via the USB port. The port on the PicoScope is a high-speed USB 2.0 type, which should help to minimise any data “bottle-neck” problems. The unit is also compatible with USB 1.1 ports. Obviously, operation might not be quite as slick when using this type of USB port due to its much slower maximum transfer rate.
The interface is a neat and well made unit that measures about 185mm by 132mm by 33mm. Apart from the USB cable the only other accessory supplied is a BNC to BNC cable. Most users will also require two or three test leads of the type normally used with oscilloscopes, frequency meters, etc., but these should not add greatly to the cost. A printed Installation Guide is included, and there is the usual online Help system that is installed with the programs. The installation CD includes PDF manuals for the programs, and these are available in several languages.
Scope Software
The main piece of software provided with the interface is the PicoScope program, which provides the virtual dual-trace oscilloscope function. Some programs of this type have on-screen controls that look much like those of a conventional test instrument, but the PicoScope software uses conventional menus and toolbars (see Fig.1).
Virtual controls make a program more intuitive for users who have experience with the real thing, but a standard Windows-style interface is more compact and leaves masses of space to display waveforms. Also, the PicoScope user-interface is not exactly difficult to learn.
Below the title bar there is a conventional Windows menu system, and this includes a File menu that, amongst other things, permits the display area to be saved in various file formats such as JPEG. The display area can also be printed using any printer that is installed in Windows.
The Settings menu is used to access dialogue boxes that provide access to the main controls of the oscilloscope. For example, the Channels dialogue box (Fig.2) enables the user to select single channel operation using channel A or B, plus alternate or chopped dual trace operation. Other settings such as the sensitivity of each channel can also be set here.
Clean Sweep
Many of the controls can also be accessed via the toolbar below the main menu, which has a mixture of buttons and dropdown menus. The drop-down menu shown in Fig.3 is the timebase control, and it goes from 50ns to 50s per division. There is another toolbar along the bottom of the screen, and this controls the trigger function. There is an on/off button plus menus that provide the standard options such as triggering on the rising or falling edge, auto, repetitive, or single-shot operation, and the trigger level.
There is also an ETS (Equivalent Time Sampling) option for use with fast repetitive waveforms. This mode samples a number of cycles in an attempt to build up a more accurate model of the waveform, and effectively extend the bandwidth of the system. The fastest sweep rate is an impressive 2ns per division when using this mode.
A 10MHz signal display using a sweep rate of 50ns per division is shown in Fig.4. The sweep rate can be boosted to an effective rate of 10ns per division using the timebase's X5 feature. In Fig.5 an effective sweep rate of 10ns per division has been obtained with the aid of the ETS feature, and this seems to provide a more detailed view of the signal's waveform.
Extras
A couple of extras are available from the PicoScope software, and one of these is a spectrum analyser facility. This can have various maximum frequencies from 95Hz to 50MHz. The result of feeding the input with stray noise is shown in Fig.6, and as one would probably expect, this shows strong components at 50Hz and its harmonics. This analysis covers a range of 0 to 381Hz. The other additional facility is a voltmeter that, like the spectrum analyser, appears in a new window. This can be used to read a.c. and d.c. voltages, frequency, and a.c. voltage with decibel scaling.
PicoLog
The software bundle includes a separate data logging program called PicoLog. This is inevitably less straightforward to use than the PicoScope software. Actually, PicoLog consists of two programs, which are the data recorder and the player. Since the recording program also has some playback facilities, it is possible to log and analyse data without resorting to the playback program. We will only consider the recording program here. The playback facilities of the two programs seem to be similar.
A certain amount of setting up is required before the recorder is ready to do anything useful, and the software includes a “guided tour” that helps to get you started. The first task is to select a filename for the new data to be saved under. Then the Settings menu plus some dialogue boxes are used to set parameters such as the rate at which data will be read, the maximum number of samples to be taken, the input voltage range, and the channel to be used.
With these preliminaries completed the data logging can commence, and there are tape recorder style control buttons just beneath the left end of the main menu. These provide re-record, record, pause/resume, and stop functions.
Three control buttons on the other side of the screen are used to view data. The first button launches a simple text editor that can be used for notes. The other two buttons enable the data to be viewed as a table of results or a graph. Various control buttons in the graph window enable part of the graph to be expanded for a detailed view, the graph to be printed or copied to the clipboard, etc.
The built-in signal generator is controlled via the PicoScope program. It is switched on via the Settings menu, and activating it produces a small pop-up window (Fig.7). This offers a sine, square, or triangular waveform, and output frequencies from 1Hz to 1MHz or more. Inevitably, the quality of the output signal reduces somewhat at higher frequencies, but good quality signals are produced over the audio range and beyond.
The output level is about one volt peak-to-peak and is not adjustable, but obviously an external attenuator can be used to provide lower signal levels. The oscilloscope and signal generator functions can be used simultaneously. However, the trigger input is used as the output of the signal generator, so external triggering is not available when using the signal generator.
The minimum hardware requirements needed to run the software are not very demanding. It should all work properly if you have a PC that has 10 megabytes of spare hard disk space and runs under Windows 98 SE, ME, 2000, or XP. Obviously a vacant USB port is required as well, and the unit needs to be connected direct to the computer or via a powered hub. A passive hub is unlikely to provide the unit with sufficient power.
Although the software will run on virtually any PC, at times it seemed to place quite high loading on the 2.4GHz Pentium 4 PC used during the review. There is probably some advantage in using a fairly modern PC.
At respective prices of £399, £599, and £799 the 3204, 3205, and 3206 PicoScopes appear to be aimed more at professional users and educational establishments. Although the prices are quite high, so are the specifications. Together with the bundled software, all three units offer good value for money. The software offers some advanced features but it is still reasonably easy to use. In a review such as this it is not possible to fully cover all the available features, and prospective buyers would be well advised to obtain the demonstration software so that they can check out the PicoScope system in detail.
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