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ScreenScope SSC-A531 . . . a 50MHz real-time standalone oscilloscope with a difference Review by MAURO GRASSI 16  Silicon Chip siliconchip.com.au Fig.1: this screen grab shows the two vertical channels displaying a square wave and a sinusoidal wave. In this case, the square wave is the 1.22kHz signal from channel 3. The sinusoidal wave is the output of an external signal generator at 400Hz. Triggering is set on a rising edge of the sinusoidal wave. Note that the trigger point is to the left of the main vertical axis. Fig.2: a pulse width modulated signal at a frequency of around 520Hz is shown in this screen shot. The duty cycle of the waveform is around 37% (average), as indicated by the text at the top of the display. Automatic measurements can be enabled on a per channel basis and you can display up to three automatic measurements simultaneously. The ScreenScope SSC-A531 is a digital dual-channel oscilloscope with a bandwidth of 50MHz and a wide range of features including FFT but it has no controls, no knobs and no screen. Instead, you connect your own LCD or CRT colour monitor to provide as big a screen as you want. Add a USB wheel mouse and you can control all scope functions as well as drag and move waveforms on the screen. A T SILICON CHIP, we are fortunate to have a number of highperformance digital sampling oscilloscopes and one of them, the Agilent MSO7034A, has a large screen which is great for easy viewing. It and other modern digital scopes also have a VGA or XGA output so you can have a much larger display if you want. Realistically though, many technicians simply cannot afford a modern digital scope and they certainly cannot afford one which has a big screen. But LCD monitors are now quite cheap. You can buy a high-performance LCD 22inch or 24-inch monitor for a few hundred dollars. What if you could get a cheap scope gadget which fed signals to a cheap large-screen monitor? Wouldn’t that be great? No squinting at a tiny screen, trying to glean signal details etc, etc. The people at Diamond Systems must have had a similar thought siliconchip.com.au process. They have developed and produced the ScreenScope SSC-A531, a 50MHz digital scope in a compact box with only three BNC sockets on the front panel but no knobs. On the back panel it has a 9-pin socket for connection to that nice big monitor. All the smarts are in that compact box – no laptop and scope software are required. What a great concept! XGA video signal The SSC-A531 outputs an XGA (1024 x 768 pixels) colour video signal The ScreenScope is built into a rugged aluminium case with three BNC sockets on the front panel. January 2010  17 Fig.3: the Fast Fourier Transform shows the frequency components of the square wave applied to channel 1. You can see the peaks corresponding to the odd harmonics of the fundamental frequency (1.22kHz). The square wave measures 3.2V peak-to-peak, as shown in the top area of the display. Note that the display refresh will slow down when the FFT option is selected. as shown in the accompanying screen grabs. Most LCD or CRT monitors would be suitable although the best display would normally be obtained with a native resolution which is precisely XGA. Monitors with higher resolution may possibly stretch the display and this pixel stretching could lead to slightly less than an optimum picture. We tested the ScreenScope with a widescreen 24-inch BENQ LCD monitor which has a native resolution of 1900 x 1200 pixels and in our case, the display was centred with black stripes on either side (ie, not stretched). The resulting screen display is bright and very easy to read. As already mentioned, the ScreenScope has no controls on the front panel, although it does have a membrane switch which is the On/Off button. Two of the BNC sockets are the channel 1 and channel 2 vertical scope inputs while the third BNC connector is reserved for an external Fig.5: in this screen grab, channel 2 shows a sinusoidal wave at around 400Hz while the blue trace is a previously stored waveform. Each of the four reference waveforms can store a trace in non-volatile memory. Each of the four waveforms can also be measured using the on screen markers or used as an input to the MATHs functions. In this case, the red trace shows the result of multiplying the two traces. 18  Silicon Chip Fig.4: channel 1 shows a square wave at 1.22kHz while channel 2 shows a sinewave at around 400Hz. The result of multiplying the two traces is shown as the MATH trace in red. The MATH trace can also be averaged to reduce noise and automatic measurements displayed at the same time. Both the measurement selected and its running average are superimposed on the display. trigger source or for other functions which we will mention later. Both 1x and 10x probes can be used and the ScreenScope is supplied with two 100MHz 10x passive probes. The rear panel has a DC power socket and two USB sockets, one for connecting a 2-button mouse (with click wheel) and the other for connecting a USB flash drive. And there is also the 9-pin port for connecting a video monitor. A USB flash drive can be used to Fig.6: measuring the period of a sinewave. The values of the two markers are shown in the top left corner of the window. The two markers are also shown as vertical red dashed lines and can be positioned using the mouse. Here we position them so that the delta value measures the period of the waveform. The delta value is shown as 2.4875ms, which agrees with the automatic measurement shown. siliconchip.com.au Fig.7: this screen grab shows a PAL video signal with the timebase set to 2µs/div. The line sync pulse occurs about 6µs from the start of the trace. store waveforms – more on this later. When the ScreenScope is turned on, a red LED glows next to the power button. It takes about seven seconds from initially being turned on to display a waveform, which is a shorter boot up time than many standalone oscilloscopes. From this point on, you control all functions via the mouse. For example, the timebase can be changed by moving the mouse pointer to the panel located in the upper right corner of the display, as seen in Fig.2 where it is shown set to 2ms/div. You can vary the setting using the mouse’s click wheel or the left and right buttons. Pressing the left button decreases the value, while pressing the right button increases the value. This works with most of the other controls too. The timebase can be varied from 3.3ns per division (3.3ns/div) down to an extremely low 1 hour per division. That is much slower than most conventional digital scopes but we should note that timebase settings from 100ms to 1hr/div use the so-called “chart recorder” mode that resembles a data logging mode rather than a standard oscilloscope sweep display. This means that the samples are displayed as soon as they are acquired rather than after a complete sweep. This is a considerable advantage on very slow timebase settings, as you do not need to wait for the entire sweep to see the waveform, which could otherwise be a long time indeed. At 1hr/div, it would take 10 hours for the trace to make one sweep! Vertical resolution is fixed at eight bits while the vertical input sensitivsiliconchip.com.au Fig.8: the line sync pulse (which is around 4.7µs long) is followed by the colour burst signal, shown here using a timebase of 1µs/div for greater detail. ity can be varied from 50mV/div up to 10V/div (on a 1x probe). Display modes For each of the two channels, the ScreenScope allows you to select whether the trace is shown in “full” or “half” mode. In full mode, the waveform is shown at the full vertical 8-bit resolution, spread over the entire 600 pixels of the display window. However, because the viewable display is so large, in some cases, depending on the vertical scale setting, this resolution may be too coarse to achieve a good display. In this case, you should use the “half” mode, which effectively doubles the resolution by using the full vertical resolution to occupy only half of the viewable resolution (that is, eight bits for 300 pixels). Trigger options The ScreenScope can store more samples than are displayed on the screen at any time. This allows the waveform to be panned and zoomed using the mouse. This is very useful for investigating a waveform around its trigger point. All the usual triggering options, except video, are available. You can select to trigger on a rising or falling edge or on a positive or negative pulse width from any of the three channels. There is a configurable filter that can be applied to the trigger source to reduce noise and avoid unwanted triggering. This can be configured both as a low-pass filter to reject high frequency noise or as a differentiating filter that computes the gradient of the signal before applying it to the trigger circuit. The latter is useful for triggering from sharply rising waveforms (which exhibit high gradients) while ignoring low-frequency components. As with most oscilloscopes, the sweep mode can be automatic, triggered or single shot. MATHs features While the ability to add or subtract the input channels is more or less Features At A Glance Bandwidth: 50MHz real-time sampling Channels: 2 analog + 1 digital Sample rate: 240 megasamples (MS) per second Memory Depth: 4 kilosamples (KS) per channel Vertical Resolution: 8-bit ADC Video Output: 1024 x 768 pixels (XGA), 256 colours Size: 160 (W) x 227 (L) x 42mm (H) Weight: 0.95kg January 2010  19 The rear panel of the ScreenScope carries the USB sockets, a power socket and the video output socket. standard on all scopes these days, we did not expect to find the FFT (Fast Fourier Transform) facility which can be applied to channel 1 or channel 2. The FFT resultant trace is shown in red (see Fig.3). The scale can be set to dbV (for an unterminated waveform), dBm 50R (for a 50-ohm termination) or dBm 75R (for a 75-ohm impedance) – note that other impedances are also accounted for. This is simply a timesaving feature with the most common impedance settings. You can also enable averaging on the FFT channel to smooth out noise in the signal. Other MATHs features allow you to multiply and divide the amplitudes of two traces (see Fig.4 and Fig.5). The two traces can be chosen from among the two analog channel inputs, as well as from any one of four previously stored reference waveforms (see below). Note that when you enable any of the MATHs features, the display update frequency will decrease. Saving screen grabs & waveforms ScreenScope allows you to save up to four waveforms in internal non-volatile memory – these are the so-called reference waveforms (see Fig.5). These can be acquired from any of the two analog channels or from the result of the FFT or the arithmetic operations. You can even load a reference waveform from an external USB flash drive. For extra storage, an external USB flash drive allows you to save many more samples where it can function 20  Silicon Chip as a data-logging tool. Note that the data logging to USB may lose samples at very high sampling rates – this is a limitation of the packet size implemented for the USB transfer. Measurements & markers ScreenScope can make measurements of the waveforms that are displayed in the upper area of the screen. Measurements include the peak-to-peak voltage, amplitude, RMS voltage, rise time, fall time, duty cycle, frequency, period and positive and negative pulse width. Up to three measurements from three different groups can be displayed at any one time for both input channels. When selected, the measurements are displayed superimposed on the waveform window in red and white. Both the current reading and its running average are displayed. The units are auto scaling, meaning they change between mV and V or between µs and ms, say, depending on the measurement. Two screen markers can be moved around the waveform window using the mouse (see Fig.6). Actually ScreenScope refer to them as markers but they are displayed as red vertical cursors. The X or Y coordinates corresponding to the markers are then shown in the top-left corner of the waveform window, as well as the delta value (the difference between the two markers). This allows you to measure details of a captured waveform. The markers can be applied to any trace, including both of the input channels and any of the four reference waveforms. Calibration & probe compensation The output on channel 3 can be used for probe compensation as well as for calibration. It provides a 1.22kHz square wave for probe compensation. Calibration is also performed using channel 3. You simply connect the output of channel 3 to the analog channel you wish to calibrate using a short BNC cable. The oscilloscope does the rest. Note that you should run the calibration procedure at least 20 minutes after a cold start to allow for temperature drift. The calibration procedure takes around 10 minutes per channel. Firmware upgrades to incorporate new features or fix bugs can be down­ loaded from the manufacturer’s website and copied to a USB flash drive. The flash drive is then inserted in the host USB socket in the back of the oscilloscope. Conclusion ScreenScope offers a good range of user features with a good bandwidth at low cost – much lower than a standalone scope of the same specifications. The ScreenScope SSC-A531 is available from Diamond Systems and costs $A539 (including GST). For further information, contact: Diamond Systems, PO Box 105, Hurstbridge, Vic 3099. Phone (03) 9714 8269 or visit their website at www. SC screenscopetraces.com siliconchip.com.au