How to use an oscilloscope to measure differential signals-oscilloscope Basics 100 Q (top)

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23. How do I use an oscilloscope to measure differential signals?

A: The best way is to choose a differential probe, the measured signal is the most realistic and objective; if there is no differential probe, you can use two differential probe to the oscilloscope two channels (such as CH1, CH2), and then use mathematical operations to obtain the CH1-CH2 waveform and analysis, at this time try to keep two probe exactly the same, Oscilloscope two channels of the Vertical scale (how many volts per cell) settings, otherwise, the error will be larger.

24. How to use the oscilloscope to measure the differential signal on the USB bus?

A: The USB signal test is divided into 2 cases: the first is the need to comply with the USB organization definition usb1.1/2.0 Bus physical Layer test specification, only through the USB conformance test can be played on the USB logo. The USB physical layer conformance test is divided into a number of test items, mainly to investigate the signal quality of the USB signal, such as Signal quality test, Droop & Drop test, inrush current test, HS specific T ESTs, CHIRP test, monotonic test, Receiver sensitivity test, impedance test (TDR), and so on.

The second case is to observe only the signals on the USB bus, you can select the appropriate differential probe to connect to the d+, D, directly to the USB signal observation. USB2.0 signal speed is faster, rise time is hundreds of picosecond, in order to ensure the signal packet true test, need to choose more than 2GHz oscilloscope and differential probe to test.

25. High-speed signal characteristics on the PCB board: XAUI interface 3.125GBd serial differential signal: 60PS, how much bandwidth is required to accurately measure the oscilloscope? How much measurement error can be reached?

A: The XAUI interface 3.125GBd serial differential signal, it sounds like a InfiniBand signal, with a sinusoidal interpolation, or similar to the equivalent sampling method to collect, but due to its own bandwidth and trigger jitter and other factors, in its measurement of the rise time in the 100PS to 130PS range, the use of 7 The GHZ differential probe guarantees error "3%, the error will be greater than 10% for the rise time measurement of the 80PS, although this is already the best solution in a real-time oscilloscope, the most accurate solution for rise time measurement is Agilent's network analyzer (which needs to be equipped with physical layer analysis software) because its bandwidth can be as high as 50GH Z.

26. What are the key issues that need to be considered to reduce phase noise in the design of clock phase noise parameters?

A: There are many metrics to measure performance in ADC,DAC devices: image bits, conversion speed, DC accuracy, switching performance, dynamic performance (SNR, SINAD,IMD), and more.

27. How to measure phase noise for the high-demand design of clock phase noise parameters?

A: From the perspective of the oscilloscope, you can test ADC,DAC analog and digital signal amplitude, time, converted signal quality, conversion speed, clock and data set/hold time parameters, but also through the TDS oscilloscope in the Advanced Computing function (spectrum analysis function) to qualitative measurement snr,sinad and other parameters.

28. Since it may be necessary to introduce an external clock so that the clock has a 2-choice 1 problem, what can be done to minimize the deterioration of phase noise?

A: First of all to analyze the source of jitter generation, oscilloscope to analyze jitter is a good tool, can now use the TDS5000B/6000B/7000B series oscilloscope with jitter analysis software for thorough jitter analysis, such as the determination of jitter (DJ), Random jitter (RJ), RJ and Dj separation, Finally, the jitter is eliminated by analyzing the cause of the jitter.

29. What is the difference between an external trigger and a self-trigger when looking at a waveform on an oscilloscope?

A: The usual trigger of the oscilloscope is the edge trigger, which has 2 trigger conditions, triggering the level and trigger edge, i.e., the oscilloscope triggers when the rising edge (or falling edge) of the signal reaches a certain levels (touch-generating level). The oscilloscope only uses external triggering when the signal has a problem, and there is no better problem. The problem is usually that the signal is more complex, there are many points to meet the trigger conditions, can not be triggered each time in the same position, so as to obtain a stable display. You need to use an external trigger. Examples are as follows:

　　

Observing the signal above, as the ABCD points will be triggered, oscilloscope display waveform will not be stable. The following signal can be used as the trigger signal, the oscilloscope will be able to display the full cycle.

30. The bandwidth of the tds3032b is 300MHz and the sampling frequency is 2.5g/s and the sampling frequency is 8 times times the bandwidth. What is the fixed relationship between bandwidth and sampling frequency? We also have an oscilloscope of other manufacturers, bandwidth 100MHz, sampling frequency only 200MHz. Why is the bandwidth sampling frequency of the two oscilloscopes so different?

A: Bandwidth is the most important indicator of the oscilloscope, because there is an ADC in the digital oscilloscope, and its sampling rate theoretically needs to meet the Nyquist sampling law, that is, the highest frequency signal of the measured signal each cycle theoretically need to pick at least 2 points, otherwise it will cause aliasing. But in fact it also depends on many other factors, such as waveform reconstruction algorithm. Tektronix oscilloscopes Use advanced waveform reconstruction algorithms, which require only 2.5 points per cycle of the measured signal to reconstruct the waveform. Some oscilloscopes use a linear interpolation algorithm, which may require 10 points. The general sampling rate is 4-5 times the bandwidth to reproduce the waveform more accurately.

Tektronix tds3000b Series is a "real-time sampling" oscilloscope, that is, its one-time bandwidth (the ability to capture a single signal) = Repeat bandwidth, you say that the other oscilloscope's single bandwidth is obviously less than 100MHz, you can look at its indicators.

31. How is the bandwidth in the oscilloscope indicator understood?

A: Bandwidth is the basic indicator of the oscilloscope, and the definition of the amplifier bandwidth is the so-called -3db Point, that is, in the oscilloscope input plus sine wave, the amplitude attenuation of the actual amplitude of 70.7% when the frequency point is called the bandwidth. That is, using an oscilloscope with 100MHz bandwidth to measure 1v,100mhz sine waves, the amplitude is only 0.707V. This is just a sine wave situation. Therefore, when we select the oscilloscope, in order to achieve a certain measurement accuracy, we should choose the maximum frequency of the signal 5 times times the bandwidth.

32. How is the total bandwidth of the measurement system obtained?

A: The total bandwidth of the measurement system =0.35/rise time (1GHz below oscilloscope).

How to use an oscilloscope to measure differential signals-oscilloscope Basics 100 Q (top)