Uncovering the secrets of testing and measurement - Test every Monday [Week 18]

Question 18-1: You can analyze signals without an oscilloscope, and the analysis function and interface are the same as those of a real oscilloscope. The prerequisite is that you have the waveform data file in hand. The figure below shows 4 waveforms measured by the N2750A series three-state active differential probe. These four waveforms are obtained by using one probe and one connection. Can you guess what these four waveforms are? You can choose from four options: single-ended positive signal (B), single-ended negative signal (A), differential signal (BA), common mode signal: (A+B)/2.

Question 18-2: This question is about a 50MHz square wave clock signal provided by a customer. It is measured using two 2.5GHz oscilloscopes, one equipped with a 1.5GHz active probe and the other with a 500MHz probe. When measuring the rise time, why is one result 1ns and the other 5.5ns? Please give a possible answer.

 

 

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Question 18-1: You can analyze signals without an oscilloscope, and the analysis function and interface are the same as those of a real oscilloscope. The prerequisite is that you have the waveform data file in hand. The figure below shows 4 waveforms measured by the N2750A series three-state active differential probe. These four waveforms are obtained by using one probe and one connection. Can you guess what these four waveforms are? You can choose from four options: single-ended positive signal (B), single-ended negative signal (A), differential signal (BA), common mode signal: (A+B)/2.

Answer: If you have the original waveform data file, you can use Excel to verify which one is BA, which one is (B+A)/2, etc. A more professional approach is to download the professional oscilloscope software http://www.agilent.com/find/N8900A  for analysis, because it has the same function and interface as the oscilloscope. The lower right corner of the figure is the single-ended positive signal B, the lower left corner is the single-ended negative signal A, the upper left corner is the differential signal BA, and the upper right corner is the common mode signal (B+A)/2. Using one probe to measure these four waveforms at the same time is only possible with a special probe. At Agilent, this probe is called the InfiniiMode probe.

 

Question 18-2: This question is about a 50MHz square wave clock signal provided by a customer. It is measured using two 2.5GHz oscilloscopes, one equipped with a 1.5GHz active probe and the other with a 500MHz probe. When measuring the rise time, why is one result 1ns and the other 5.5ns? Please give a possible answer.

Answer: The frequency of the clock signal determines the frequencies of its fundamental wave and odd harmonics, but does not determine its highest frequency component. Its rise or fall time is the key factor that determines its highest frequency component. If the rise time measured by a 1.5GHz active probe is 1ns, it means that the rise time of the signal is at least 1ns, that is, the knee frequency of the signal is at least 500MHz. If you use an oscilloscope and probe system with a Gaussian frequency response, the system bandwidth is at least 1.9 times the knee frequency, that is, 950MHz. When using a 500MHz passive probe, most people use its 10cm long black ground wire. At this time, its bandwidth is between 175MHz and 200MHz. It is normal to measure a rise time of 5ns to 6ns with an oscilloscope system with a 175MHz system bandwidth. This is the rise time of the measurement system itself, and it is impossible to measure a faster signal than it; if the spring ground of the passive probe itself is used, the system bandwidth can reach 500MHz. At this time, the measured result of the object with a rise time of 1ns will be about 2ns.

It can also be analyzed from another angle. If the frequency response of the oscilloscope and the probe are both Gaussian, you can use the formula Tr (oscilloscope display value) = RMS {Tr2 (oscilloscope) + Tr2 (probe) + Tr2 (test object)}. If the test object is an extremely fast signal, the rise time displayed by the oscilloscope is the rise time of the oscilloscope and probe system itself. In other words, if the system rise time of the oscilloscope and probe is much slower than the test signal, the test result displayed on the oscilloscope is not the rise time of the test signal at all, but the rise time of the test system itself.

The actual bandwidth of a 500MHz probe when using a 10cm alligator clip ground lead is about 175MHz, and it is impossible to measure a signal with a 1ns rising edge.