Signal Integrity Versus Headroom: Agilent DSO/DSA91304A Versus Tektronix DPO/DSA72004A Application Note When you choose an oscilloscope, you always need to make tradeoffs. But you also need to make sure you understand the claims made by competing oscilloscope vendors, so you do not unintentionally make a tradeoff that will adversely affect your measurement accuracy without buying you much advantage elsewhere. In February 2007, Tektronix introduced the DPO/DSA72004A oscilloscope, which Tektronix advertises as the world s first 20 GHz real time oscilloscope. Tektronix argues that some measurements of today and tomorrow require at least 20 GHz of bandwidth and they have the only solution. This paper discusses bandwidth requirements of today and tomorrow, and explores tradeoffs that you make when purchasing the Tektronix oscilloscope over the Agilent 90000 Series oscilloscope.
Bandwidth Requirements of Today and Tomorrow A number of existing documents explain how much bandwidth you need for accurate measurements, based on the third or fifth harmonic of a signal. The most important specification to determine needed bandwidth is rise time, since this determines what harmonic content is in a signal, not its bit rate. Accurate rise time measurements require oscilloscopes with low noise floor and fast response. When you look into future rise times of applications and what the DSO91304A is able to accurately measure, you will quickly see that the DSO91304A has more than enough bandwidth for your needs for the future because of its low noise floor and fast response. Do not be misled. Rise times over the next few years will not significantly decrease, as board material cost, connectors, and test fixtures continue to limit rise time speeds. Further solidifying this point, recently it was announced that the DSO91304A was qualified for SATA revision 3.0 by SATA-IO. The DSO91304A (or lower bandwidths) is the best tool for every digital application of today and tomorrow. Despite Tektronix claims that when you purchase its DPO72004A oscilloscope you are buying extra headroom, you will find very few applications that DPO72004A will measure that the DSO91304A won t (this is shown in the red box above). In fact when the headroom is truly needed, new real time oscilloscope technologies will be needed with lower noise floors and much flatter responses at 20 GHz and more. If you are measuring signals with rise times faster than 32 ps, a sampling oscilloscope such as the Agilent 86100C DCA is an excellent choice. 60 55 50 45 40 35 30 25 20 15 Green shading indicates applications for which the 90000 Series scopes are the best tools. Future Rise Times SATA SAS PCIe Fibre C HDMI Display Port USB 2008 Rise Time 2009 Rise Time 2010 Rise Time 2011 Rise Time Figure 1: Estimated future rise times of today s most popular applications 2
Noise Floor Vertical noise of an oscilloscope must be a very small portion of your measurement, as you want to focus on the measurement that you are making and not what limitations your measurement equipment has. The noise floor is what dominates the accuracy and repeatability of all measurements, especially jitter. The DSO91304A has the lowest noise floor in the industry. When using its 20GHz settings, the DPO720004A has a noise floor over 20mVrms on the 200 mv/div range! This noise may impact your measurement. You will notice that the Agilent noise floor is significantly less at every bandwidth setting, resulting in more accurate measurements. Agilent doesn t try to explain away a high noise floor by arguing noise floor as % of screen; Agilent s noise floor is significantly lower in direct comparison. At 20 GHz, the Tektronix oscilloscope will not permit measurements between 200mV/div and 100mV/div. Instead the scope will automatically default you back to the 16 GHz setting. Note: it does this without a warning that your bandwidth has been reduced! The lack of a 100 mv/div range has a double negative impact. Most signals of interest such as PCIe and SATA have typical amplitudes in the range of 600-800 mv p-p. With the Tek scope set to 20 GHz bandwidth, you are forced to select 200 mv/div, resulting in a vertical deflection of only 3 to 4 divisions. This results in a much worse signal-to-noise ratio or dynamic range. Combined with the high noise level of the Tek scope at 20 GHz, this results in a double whammy on accuracy and repeatability. Noise (mv) Noise (mv) Noise Floor Tek 20G vs. Agilent 13G 100mV Tek 100mV Agilent 25 20 15 10 5 0 20G 18G 16G 13G 12G 10G 8G 6G 4G 25 20 15 10 5 0 20G 18G 16G 13G 12G 10G 8G 6G 4G Noise Floor Tek 20G vs. Agilent 13G 200mV Tek 200mV Agilent Figure 2: Noise floor comparison of DSO91304A versus DPO72004A 3
Frequency Response While the noise floor is very important, an even more telling story of some of the tradeoffs that must be made when purchasing because of only bandwidth is the frequency response of the two oscilloscopes. Figure 3 and 4 show frequency plots of actual oscilloscopes from Agilent and Tektronix. Each oscilloscope had user calibrations run before measurements were collected. Notice, the fluctuations beginning at 15 GHz of the DPO72004A; impacts measurement accuracy. At 200mV/div you can see up to a 2dB fluctuation for the DPO72004A, this results in nonrepeatable measurements. This shows itself in the following chart, which was taken by applying a signal with a 32ps rise time on a 2.5 GHz square wave. You can see how closely the mean error is between the oscilloscopes and how closely the measurements between the DPO72004A and DSO91304A are. Despite Tektronix data sheet bandwidth advantage, the fluctuation in its response (see figure 4) and high noise floor cause its measurements to be very similar to the 90000 Series. In fact the 90000 Series measurements show greater repeatability. Notice how the range increases to almost 10 ps when the bandwidth is set to 20 GHz on the Tektronix scope, significantly more than even the Tektronix DPO71254A shows. 4 2 0-2 -4-6 -8 DSA91304A 1.00E+07 6.00E+08 1.10E+09 1.60E+09 2.10E+09 2.60E+09 3.10E+09 3.60E+09 4.10E+09 4.60E+09 5.10E+09 5.60E+09 6.10E+09 6.60E+09 7.10E+09 7.60E+09 8.10E+09 8.60E+09 9.10E+09 9.60E+09 1.01E+10 1.06E+10 1.11E+10 1.16E+10 1.21E+10 1.26E+10 1.31E+10 1.36E+10 5 mv per div 10 mv per div 20 mv per div 50 mv per div 100 mv per div 200 mv per div 500 mv per div Figure 3: Frequency Reponse of the DSO91304A Figure 4: Frequency Response of the DPO72004A 4
Frequency Response Using an 8 Gb/s PRBS signal, you will notice that the DPO72004 and the DSO91304A eyes closely resemble each other despite the extra bandwidth. Nominal rise time Scope and Bandwidth 32 ps Tek 20 GHz 34.76 ps Mean Error Range Standard DCA reference Deviation 9.83 ps 1.53 ps 31.79 ps Measurement accuracy increases very little with Tek s BW even at 32 ps rise time 9% Tek 13 GHz 36.18 ps 4.75 ps 0.788 ps 31.79 ps Agilent 13 GHz 36.04 ps 7.57 ps 1.21 ps 32.07 ps 12% Figure 5: Measurement Accuracy of the DPO72004A versus the DSO91304A DSO91304A at 12 GHz DSA72004 at 20 GHz Fundamental Freq = 4 GHz, 3rd Harmonics = 12 GHz, 5th Harmonics = 20 GHz 5
Conclusion Tektronix claims that you need their 16 GHz boosted oscilloscope for headroom for next generation applications. However, next generation applications do not require more than 13 GHz. Agilent s 90000 Series oscilloscope is an excellent oscilloscope for today s and tomorrow s measurement needs. With the Agilent 90000 Series, you get the bandwidth you need for today and tomorrow, plus you don t sacrifice signal integrity. If you really want true bandwidth, a sampling oscilloscope is the best option as you will not have to tradeoff measurement accuracy for bandwidth. The Tektronix DPO72004A has a banner specification of 20 GHz; however, to get this 20 GHz you may have to make tradeoffs that will affect your measurements. The DSO91304A offers the performance you need without sacrifice. Related Literature Publication title Publication type Publication number Agilent Infiniium 90000 Series Data sheet 5989-7819EN Oscilloscopes 6
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