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A digital scope with a spectrum analyser! Review by Nicholas Vinen Tektronix MDO4104-3 Mixed Domain Oscilloscope This first-of-its-kind product combines a mixed signal oscilloscope (with four analog and 16 digital channels) and a 3 or 6GHz spectrum analyser (with an impressive capture bandwidth, over 1GHz). But it’s far more than just three instruments in one package. Tektronix must think that they’ve come up with a new type of test instrument since they’ve invented a name for it: “mixed domain oscilloscope”. We reckon they’re probably right. Digital storage oscilloscopes (DSOs) and spectrum analysers have both been around for yonks but while they’re individually useful, this unit can do some things that they can’t do by themselves. So what do they mean by “mixed domain”? It may help to think back to high-school mathematics. If you weren’t too busy making paper planes or programming games into 74  Silicon Chip your graphing calculator, you may remember that when a function is plotted on a graph, the x-axis is called the “domain” and the y-axis the “range”. For the classic oscilloscope display, the x-axis is time and the y-axis is voltage (or current). Hence these scopes operate within the “time domain”. Similarly, a spectrum analyser plots frequency on the x-axis and power on the y-axis. So we can say that a spectrum analyser operates with a “frequency domain”. So a mixed domain oscilloscope can display data in siliconchip.com.au either or both forms. We should point out that you can view the same signal either way, eg, as a plot of voltageversus-time or power-versus-frequency. Each view is useful for different purposes; a spectrum is invaluable for analysing a radio frequency (RF) signal but is not so useful for debugging a serial bus! It would have been tempting for Tektronix to just shoehorn two instruments into one box and call it something new. That is definitely not what they have done though. Clearly, a lot of effort has gone into integrating the two and the result is a device which allows you to capture and analyse data in ways that were not possible before. The power of mixed domains A digital spectrum analyser typically samples the signal at a high rate for some period (say 1ms), then converts the captured data to the frequency domain using a mathematical transform (eg, a fast Fourier transform or FFT). The display then shows the signal frequencies present during the capture period. For better frequency resolution (“resolution bandwidth”), a longer sampling period is necessary, to acquire more data for analysis. If data is captured over a longer period than merely necessary for the analysis, it is possible to “slide the window” (ie, the portion of data being analysed) within this period. This results in a series of spectrum plots, showing how the frequencies present in the signal shift over time. This can then be correlated with the time domain data captured by the oscilloscope portion of the instrument, so that the operation of the RF control circuitry can be observed simultaneously with the RF output. By this point, you should be starting to get an idea of what this device is capable of. In practice, the data for time and frequency domain analyses are stored separately. For the regular scope functions (ie, time domain), a generous 20Mpoints of storage is available. The spectrum analyser can capture an astounding one gigapoint (ie, one billion points). That corresponds to 2.5 milliseconds of signal when the spectrum analysis window has maximum span (>2GHz) and longer for smaller spans, to a maximum of 79ms (span of <125MHz). As well as allowing for a large “sliding window”, this also gives you a lot of capture bandwidth. This is the difference between the lowest and highest frequencies which can be displayed simultaneously. So you can, for example, monitor the RF output of a circuit at 900MHz and 2.4GHz simultaneously (see Fig.1) or even 2.4GHz and 5.6GHz (with the 6GHz model). Since many digital wireless devices can operate on multiple frequencies, this can be handy. A demonstration The Tektronix demo board provides a number of examples to demonstrates the MDO’s utility. One of the most interesting is the frequency hopping demo, shown in Fig.2. The screen is split, with the time domain display at the top and frequency domain at the bottom. The horizontal orange bar shows which portion of the time domain display corresponds to the spectrum analysis below. For this demo we have “frozen time” by pressing the run/stop button so what is shown in Figs.2-4 is based only on data stored in the scope’s memory. The frequency analysis time span (ie, the width of the orange bar) depends on the current resolution bandwidth. siliconchip.com.au Fig.1: both analog and digital channels are enabled here, as well as the spectrum display. You can see the serial commands between the controller IC and the voltage controlled oscillator (VCO). This also demonstrates the incredible capture bandwidth available as we can observe the output shifting from 900MHz to 2.4GHz without having to re-sample the data (orange trace shows frequency). Fig.2: the MDO4104 operating in mixed domain mode, with scope traces at the top and the RF spectrum underneath. The RF signal is “frequency hopping” and the orange trace in the time domain display, at top, shows the changing centre frequency with time. If the frequency resolution is made finer, for better peak discrimination, the orange bar necessarily gets wider and vice versa. There are limits to how far the orange bar can be moved, which is based on the RF capture timespan and this is determined by the frequency span. Here we have a short enough timebase on the upper display that we can show a spectrum analysis at any point in time that’s visible on the upper portion of the screen. At the top of the spectrum display is an automatically generated “marker”. For those familiar with scopes but not spectrum analysers, a marker is a cursor which highlights a particular frequency. The automatic markers (if enabled) appear at the tallest peaks (ie, frequencies with the highest powers). This unit can show up to 11 markers at once, detected using adjustable thresholds. The marker shows us that in this case, the RF signal November 2011  75 Fig.3: the same setup as Fig.1; all we did is change which portion of the captured data is being analysed for the spectrum display at bottom. Compare the position of the orange bar with Fig.1; at this later time, the RF signal has hopped to 2.403GHz (ie, up 30MHz) and so the peak has shifted. With this timebase (200μs), we can examine the spectrum at any point in time visible on the screen. Fig.4: we are still analysing the same data captured for Fig.1 and Fig.2. This time the spectrum analyser frequency resolution has been changed from 10kHz to 20kHz and now we are observing the spectrum as the RF centre frequency is shifting. The lower frequency resolution allows us to analyse a smaller time period, to better observe the effects of the “hop”, such as the overshoot. peaks at exactly 2.4GHz during the selected timespan, with a power of -15.1dBm. If more markers are shown, corresponding to lower peaks, their powers can be shown either as an absolute level or relative to the tallest peak. Markers can also be manually placed and the difference in frequency and power level between them read out. In Fig.3 we have moved the spectrum analysis window forward in time, where the RF output has “hopped” up by 3MHz. As you can see, the orange trace in the time domain section is actually derived from the spectrum analysis and shows the frequency of the highest RF peak over time. This immediately shows how the demo board’s RF output tends to “overshoot” at each frequency hop, before settling down at the target frequency. A frequency analysis during the transition (Fig.4) shows the range of frequencies output during the “hop’, as the oscillator frequency shifts. We changed the resolution bandwidth from 10kHz to 20kHz, allowing us to view the spectrum over a shorter period (compare the width of the orange bar with Fig.3). As well as increasing the size of the window being analysed, finer frequency resolution settings also slow the spectrum display update. There is one major restriction to this mixed domain mode; besides the scope running somewhat more slowly (depending on just how much number crunching it has to do), enabling the spectrum analysis also limits the offset between the trigger point and the start of the display. Be- Specifications Inputs..............................................................4 analog, 16 logic, 1 RF Bandwidth (analog inputs)..............................500MHz<at>2.5GS/s or 1GHz<at>5GS/s (2.5GS/s for 3-4 channels) Bandwidth (RF input)......................................50kHz-3GHz or 50kHz-6GHz Analog memory depth....................................20Mpoints (10kpoints at maximum update rate) Waveform update rate....................................Up to 50,000/s Size & weight..................................................229 x 439 x 147mm (5RU tall), 5kg USB Ports.......................................................Four host ports, one device port Other ports.....................................................Gigabit ethernet, VGA output, trigger out, frequency reference in Spectrum Analyser Capture bandwidth.........................................>1GHz Resolution bandwidth.....................................20Hz-10MHz Displayed Average Noise................................-152dBm/Hz (5MHz-3GHz, typical); -143dBm/Hz (3GHz-6GHz, typical) Residual Response........................................<-78dBm Spurious Response........................................-60dBc typical, 2nd and 3rd harmonic Maximum Input...............................................+30dBm (1W) average continuous power Acquisition Length..........................................2.5ms (>2GHz Span) to 79ms (<125MHz span) 76  Silicon Chip siliconchip.com.au cause the spectrum analyser memory can only store data representing a limited time period, you can’t go back any further than that (before the scope was triggered) or there just isn’t any data to analyse. With the spectrum display turned off though, you can go back hundreds of milliseconds before the trigger, to see the events leading up to it, depending on the memory depth and timebase selected. In the mixed domain mode, the maximum delay is generally in the range of 2.5-79ms, with the longer periods available with a smaller frequency span. If you need to view earlier signals, the trigger settings must be changed. Normally this is not a problem since usually the RF phenomenon being investigated occurs after a particular digital or analog signal condition. But it’s something the user must keep in mind. User interface As DSOs go, this one is particularly easy and pleasant to use. We especially like the dual general-purpose knobs. In situations where there are two or more settings to adjust, the two most-used settings are labelled “a” and “b”, corresponding with those knobs. You can then adjust both without having to select between them using the “soft buttons” (which are along the right and bottom of the display). Speaking of the screen, it is a 26cm (10.4-inch) 1024x768 TFT LCD and is particularly crisp, with good contrast. Fig.5: the measurement menu. Measurements can be shown for both time and frequency domain signals but the largest choice is for the time domain (ie, the traditional scope display). The detailed information for each measurement helps you understand exactly what is being measured. Analog inputs Let’s take a closer look at time domain operation, ie, the scope functions. These are lifted from a Tektronix MSO4000-series DSO. In fact there are really only two differences; with three or four analog channels active, the sampling rate is 2.5 megasamples/second for the MDO4000-series compared to 5 megasamples/second for the MSO4000. Also the “aux input” BNC connector on the front panel has been removed to make room for the RF input. We assume that the reduced sampling rate is due to the main processor’s bandwidth being divided up between the four analog scope inputs and the RF input. As mentioned earlier, both 500MHz and 1GHz bandwidth options are available for the analog channels. Four passive 500MHz/1GHz probes are provided. These have a low 3.9pF capacitance and a high 10MΩ input resistance. They come with a very good range of accessories, including spare tips, both hook and grabber tips, plenty of ground springs, ground clip, colour coding rings, IC lead probes and so on. The analog inputs have low noise and with the supplied probes, give a sensitivity range from 10mV/div up to 100V/ div. Each channel has its own vertical control (scale, offset knob and on/off/select button). The amount of storage available is excellent at 20 megapoints. The zoom and pan functions work very well, allowing you to examine the overall waveform captured as well as the details. While the update rate is very good (50,000 updates per second), this is not available when using the full memory (20Mpoints). The maximum update rate is available with a memory depth of 10kpoints and is reduced when a larger memory is used (this is configurable in a number of steps). One nice feature of the analog inputs is that they have two different bandwidth limiting options: 250MHz and siliconchip.com.au Fig.6: this is the spectrogram display, which is used to capture shifts in the frequency spectrum over time. The normal spectrum analysis is shown at bottom while the upper display changes to show amplitude as colour and constantly scrolls up, with the latest spectrum appearing at the bottom. Note how the peaks in the lower display (which were constantly shifting) correspond to the “hotter” colours above. 20MHz (in addition to 500MHz/1GHz, ie, no limiting). These are useful for eliminating noise and ringing when the signals being examined have a relatively low frequency. There are five available sampling modes: normal, averaging (with a selectable number of averages), high resolution (very useful!), peak detect and envelope (min/max). Logic analyser All models come standard with a 16-channel logic analyser with an excellent time resolution of 60 picoseconds. Two logic heads are provided, which handle eight channels each. The physical arrangement for the logic probes is especially nice. The ribbon cables are thin and flexible, the logic heads are small and the eight connectors on each November 2011  77 With these optional serial decoders, the triggering options become even more powerful. You can then trigger when a particular value appears on the bus and it’s even possible to compare some bits in the serial packet and ignore others. As well as displaying decoded data values in the trace display (up to four buses at a time), serial data can also be shown in a list format at the bottom of the screen. The search function(s) (described below) allow you to jump to a point in time where a particular value appears on a bus. In short, the logic analyser on this scope is very powerful and comprehensive. Measurements Fig.7: the range of operations available when building functions for the “advanced math” mode. We can think of a lot of useful things that you could do with such a powerful feature, such as displaying and calculating real power drawn from mains. There are many measurements that can be applied to each channel. The menus are particularly nice, with a large graphical display showing what each one represents as you scroll through the list (see Fig.5). All analog and digital channels can be used for measurements although the list of available measurement modes for digital channels is smaller. We like the fact that you can have as many measurements on screen as you want but unfortunately, they take up valuable screen real estate and so if you have lots active at once there’s less room for traces. Triggering Fig.8: the amplitude shift keying (ASK) demo shows how the peak RF power can also be displayed in the time domain. This also shows how useful channel labels are. With 20 channels plus additional generated traces it’s easy to get confused as to what each represents without the names shown. head are colour coded. A ground wire can be connected for each head or separately for each pin; ground pins are provided which, if fitted, allow the wires to be plugged into standard 0.1-inch pitch, 2-way pin headers. Also supplied are “probe tips” for plugging the wires into sockets/vias/test points and plenty of “IC grabbers”. You also get a couple of little plastic blocks which allow the eight wires and ground for each logic head to be ganged together to form an 8x2 pin header socket, to suit male pin headers or PCB-mount IDC connectors. There are eight serial decoding/triggering options available, at additional cost. These are: Embedded (I2C/ SPI), Audio (I2S, left-justified, right-justified, TDM), Automotive (LIN/CAN), Extended Automotive (LIN/ CAN/FlexRay), Computer (RS-23/422/485/UART), Ethernet (10BASE-T/100BASE-TX), USB (Low, Full and HiSpeed) and Aerospace (MIL-STD-1553). 78  Silicon Chip As you would expect, there are many trigger modes. As well as the usual ones, including the commonly-used Edge and Pulse Width triggering, there are also Timeout, Run, Setup & Hold and Rise/Fall Time which are all useful for debugging high-speed digital buses. Then there is Video triggering which includes optional support for HDTV up to 1080p, as well as custom video triggering. We expect anybody working with video/TV these days would be involved with HDTVs and so would opt for this add-on. One interesting trigger mode is “Logic” which is very powerful. You can select a mix of any of the analog or digital inputs and specify which combination of states is required to trigger the scope. One input can even be designated the “clock”, which determines when the other channel states are sampled for triggering. You can also specify a minimum or maximum duration for this state to be held before the trigger occurs. There is the usual setting for auto/normal triggering and when analog channels are used as a trigger input, there are other options: AC/DC coupling, high-frequency and low-frequency signal filtering, noise rejection and so on. “Math” modes The “math” mode of this scope is the best we’ve seen. As well as the usual modes (add, subtract, multiply, divide and FFT) there is also an “advanced math” mode which lets you enter a custom formula. This can include parameters representing the data from one or two analog channels. The function is computed over the time domain and the result displayed as a new trace. A large array of operators are available for use in this mode, including integration and differentiation, trigonometric functions, logarithms and exponentials, absolute values, maximums, minimums and differences, periods frequencies, duty cycles . . . the list goes on (see Fig.7). siliconchip.com.au With the flexibility this feature provides, it has many uses. One example would be when measuring AC voltage and current using two analog channels – you could use the integration, multiplication and absolute value functions to display the instantaneous real power delivered to the load. You could probably also calculate and display the power factor in real time. Other features This scope has a search feature as standard. This allows you to quickly move along the time domain, jumping to particular points of interest, based on the search criteria. These locations can also be flagged with markers which is quite handy when using the zoom mode. The search criteria are similar to the triggering modes, ie, markers can be placed at points based on edges, pulse widths, runts, setup & hold violations, specific logic bus data values and so on. You can then use the marker navigation buttons to skip between the matching events and examine them. The search settings can be copied to the scope’s trigger settings and vice versa. So once you have found a point of interest, you can easily set it up to trigger on that condition. The marker locations can also be saved to memory. As well as the “live” traces, up to four reference traces can be enabled at once. Each shows a saved waveform. There is also an optional limit/mask testing feature. Masks can be created from traces or can be taken from a USB drive. The live trace(s) are then compared against the mask and any violations flagged. The scope also has an optional suite of “power applications”. These are very useful for testing and diagnosing switchmode power supplies. With the correct probe set-up (voltage measured on channel 1 and current on channel 2), additional information is displayed: power quality, switching loss, switching harmonics, output ripple, control pulse modulation or transistor SOA (safe operating area) characteristics. It can also display “histograms” above the traces, for a given channel. The scope gathers statistical data on the data captured from this channel and shows the distribution in the histogram, based on the spread of rising/falling edges (ie, jitter) or other aspects of the signal. The display includes readouts of the mean, standard deviation and other properties of the signal. That’s a lot of features in one unit. In fact there are even more that we could list but we don’t really have the space. Let’s just say it’s feature-packed! Spectrum analyser Compared to the scope portion, the spectrum analyser is pretty easy to drive. There are five dedicated set-up buttons plus a numeric keypad for entering frequencies (which can also be altered with the two general purpose knobs). The five set-up buttons are labelled “RF”, “Freq/ Span”, “Ampl”, “BW” and “Markers”. As well as turning the spectrum analyser on and off, the “RF” button presents a menu where average/min/max readings can be turned on or off. The same menu also lets you enable and disable the RF traces in the time domain section (frequency, amplitude and/or phase), toggle the spectrogram mode (see Fig.7), configure automatic markers and change the RF trace labels, etc. siliconchip.com.au Save Up To 60% On Electronic Co Components New ET-Easy T A Arduino Stamp Includes ATMega168 with Installed Bootloader Direct USB Program Download Up to 22 I/O Points, 10-Bit ADC Included Compact, Easy to Use and Program Only $24.90 Ultrasonic Range ge F Finder ind der Only $14 $14.90 90 Ideal for use on Robots and Water Tanks Measures from 3cm to 3m High Accuracy Ready to Run, No Set-Up Required 10A 1 10 A So Solar ar Regulator for Lighting Only O Onl nly ly $$36.90 36.90 Hig Efficiency PWM Charging High Cha es Batteries During Daylight and Charges Switches Lights on at Night Sw S wii Suitable for 12V and 24V Systems S uitt LED D Status Indication for Charging, Low Battery etc We aare re e yyour ourr on ou oone-stop one ne-stop sh shop for Microcontroller Boards, PCB Manufacture aand Electronic Components www.futurlec.com.au November 2011  79 The rear panel carries three USB ports (there are two more on the front panel), plus an ethernet socket, a video output socket and BNC connectors for a 10MHz reference (input) and an auxiliary output. The “Freq/Span” button lets you select the displayed frequency range by setting either the centre frequency and span or the start and stop frequencies. “Ampl” controls the vertical axis, allowing you to select the display units, reference level and scale or turn automatic scaling on or off. The “BW” button sets the bandwidth resolution as either a ratio to the capture bandwidth or an absolute value in Hertz. The “Markers” button controls the automatic and manual markers, as mentioned earlier. There are also controls to select which FFT window type is used for the frequency analysis: Kaiser, Rectangular, Hamming, Hanning, Blackman-Harris and Flat-Top. Various measurements can be taken on the RF signal, including channel power, adjacent channel power ratio and occupied bandwidth. These are made through the same measurement interface as for the scope. There is also an option available to add triggering based on RF power level to the unit (MDO4TRIG). If installed, the overall RF power level can be used as an input for the Pulse Width, Runt, Timeout, Logic and Sequence trigger modes. Accessories In addition to the analog and digital probes and associated accessories, the unit comes with a protective front panel cover, a BNC-connector to N-connector adaptor for the RF input (for signals <5GHz), a printed user manual, documentation and software CDs, calibration certificate and power cord. The main unit has a VESA monitor mount on the back. 80  Silicon Chip This means that it can be attached to various stands, wall brackets, mounted in a rack, etc. It also has a Kensington lock for theft prevention. Conclusion The MDO4000-series is innovative and feature-packed. It will be an invaluable tool for engineers working with wireless communications. This performance comes at a price, though; both monetary and in terms of some performance compromises. While the main processor in this unit is no doubt quite powerful, it has quite a lot of tasks to perform when all the features are running at once. It can get a bit bogged down if you try to do too much at once. For that reason, it’s best to keep the resolution bandwidth at a lower setting (ie, larger frequency step) initially and then increase the resolution once the signals you want to examine are on-screen. The base model, MDO4054-X, with 500MHz analog inputs (2.5GS/s) and 3GHz spectrum analyser costs AUD $23,205+GST. The top-rated model (MDO4104-6) comes in at $33,180+GST. The price for the various options, including the serial decoding modules, varies but the latter are in the range of $1600-1800 each. For more information or to purchase an MDO4000-series scope, contact TekMark Australia on 1300 811 355 or email enquiries<at>tekmarkgroup.com Or for New Zealand, call Nichecom Limited on +64-4-232-3233 or e-mail TektroSC nix<at>nichecom.co.nz siliconchip.com.au