Using an Oscilloscope for Verification and Debugging

Verification is to test whether the design complies with the corresponding standards (industry standards or custom standards) and how much redundancy there is. Verification and debugging are the main uses of oscilloscopes.

When using an oscilloscope for debugging, the main indicators we care about are:

Waveform capture rate - determines how quickly the instrument can find faults

Trigger system - determines how accurately the instrument can locate faults

Analysis capability – determines how much useful information the instrument can extract from the waveform.

When using an oscilloscope for verification, we are more concerned about the following indicators: signal fidelity - determines whether the collected samples can truly reflect the signal characteristics; sampling rate and memory depth - determines how fast a single capture can be and how many samples can be captured for verification testing; analysis tools - determines the degree and accuracy of in-depth analysis.

Signal fidelity is a relatively complex issue, covering the oscilloscope's bandwidth, sampling rate, interpolation, jitter noise floor, noise floor, time measurement accuracy, probe system, etc. There are many discussions and articles on this in the industry, so this article will not make a detailed analysis, but only emphasize the impact of the oscilloscope's frequency response on verification.

Frequency response is reflected in the oscilloscope indicators as bandwidth and rise time. Bandwidth represents the steady-state response capability of the oscilloscope, while rise time is the transient response. In experience, the product of bandwidth and rise time (10-90%) is a constant, which is related to the amplifier model of the oscilloscope. For example, for the amplifier model of Gaussian response, this constant is 0.35; while the amplifier model of high-performance oscilloscopes is more complex, the constant will be between 0.4 and 0.55. Of course, from the user's point of view, this constant should be as small as possible: the smaller the constant, the faster the rise time of the oscilloscope under the same bandwidth (steady-state response), that is, the better the transient response; and when the rise time is the same, the bandwidth required for an oscilloscope with a small product will be relatively lower - and for oscilloscopes, bandwidth and price are proportional, that is, oscilloscopes with a small product are more cost-effective.

Our verification test objects are generally pulse (non-sinusoidal) signals, such as communication signals, serial bus signals, high-speed pulse signals, modulated signals, etc., so the transient response of the oscilloscope is more important. Tektronix DPO oscilloscopes can provide the fastest rise time at the same bandwidth, which is very helpful for testing transient signals.

On the other hand, the different design structures of high-bandwidth oscilloscopes will also affect the correctness, accuracy and speed of verification tests:

In recent years, the bandwidth of oscilloscopes has been increasing rapidly. How to ensure the flatness of the amplitude response and the linearity of the phase response within the band while increasing the bandwidth has become an important issue. Experienced engineers know that it is impossible to obtain an ideal flat amplitude response and linear phase response if you start completely from hardware. Therefore, in the amplifier technology of high-performance oscilloscopes, major oscilloscope manufacturers are using software to increase bandwidth and optimize response DSP technology. The use of DSP technology can indeed obtain a relatively ideal amplitude and phase response, but it is not harmless. The figure below shows the response of the oscilloscope to a step signal. The blue is the complete analog response, and the red is the response after DSP processing.

Using an Oscilloscope for Verification and Debugging

After DSP enhances and corrects the amplitude and phase response, the oscilloscope can more accurately measure indicators such as rise time and eye diagram redundancy, which is conducive to the verification test of digital communication signals, computer bus signals, etc. However, from the red waveform, we can see that the dotted box part, which we call "pre-overshoot", is a "false waveform" that does not exist in the real signal. It is a distortion generated by DSP processing. For step signals, there is no reason for the waveform to oscillate when the rising energy has not yet been generated. Therefore, when using an oscilloscope to measure high-speed pulses, laser pulses or similar signals, DSP processing is no longer what the tester expects - the distorted waveform incorrectly indicates the physical behavior at each point in time.

Of course, DSP has other problems, such as incorrect display of overdriven signals, low data throughput, inability to export raw data before DSP, etc. Therefore, when users need to observe overdriven signals (such as overshoot details at the top of a pulse), need to use raw data collected by an oscilloscope for custom analysis (such as laser pulse measurement), or require higher processing speed, the oscilloscope is required not to use the DSP function.

Tektronix uses DSP frequency response correction and channel matching functions in all DPOs with bandwidths above 2.5GHz, and the DPO72004 also has DSP bandwidth enhancement function. However, Tektronix is ​​also well aware of the pros and cons of DSP functions, so when other companies "quietly" use DSP functions, Tektronix is ​​the only one that gives users the "right to know" and "right to control", that is, users can know whether the oscilloscope is using the DSP function, and can also turn on or off the DSP enhancement function as needed. In this way, if you are doing a pulse test that requires the original acquisition data of the oscilloscope, the user can choose to turn off the DSP function; and when doing serial signal consistency testing, Tektronix recommends turning on the DSP function.

Using an Oscilloscope for Verification and Debugging

In addition to the above matters that need to be paid attention to in application, there are some requirements for DSP functions. From the above figure, we can see that DSP requires real-time sampling at the Nyquist sampling rate. Some manufacturers' oscilloscopes will have unpredictable waveform amplitude distortion when the sampling rate does not meet the Nyquist bandwidth, mostly due to this reason.

At the same time, high-performance oscilloscopes generally have 4 channels. However, to achieve the calibration bandwidth on four channels at the same time, the sampling rate needs to be supported. The industry generally recognizes that a sampling rate of 2.5 times the bandwidth is the minimum requirement to ensure bandwidth. In this way, if an oscilloscope above 8GHz is used for signal verification (generally single-shot acquisition), Tektronix's DPO can provide full bandwidth performance on 4 channels at the same time (a sampling rate of 50G per channel can effectively guarantee a bandwidth of up to 20GHz), while an oscilloscope using a shared amplifier and ADC structure can only achieve full bandwidth indicators on two channels at most, and some can even only guarantee the performance of one channel.

In terms of storage depth, many verification tests require sufficient data. For example, in the jitter and eye diagram test of high-speed serial signals, it is required to capture a large amount of data at a time to perform accurate jitter measurement and estimation while ensuring a low bit error rate. This is to avoid the randomness and uncertainty of the results of capturing a small amount of data for analysis. For example, the HDMI test specification (CTS1.2 a Page 15) requires the capture of 1 million bits of data for eye diagram analysis, which requires the oscilloscope to use a 16M storage depth at a sampling rate of 10Gs/S for two channels. FBD Sigtest (Release notes Page 6) recommends capturing 1 million bits of data for eye diagram analysis. The PCIE 2.0 specification (Page 239) stipulates that it is mandatory to capture 1Mlillion data for eye diagram jitter analysis. The oscilloscope needs to use a single channel with a storage depth of 8M at a sampling rate of 40Gs/S.

另一个例子：为了减少EMI的串扰和辐射，在大多数高速串行信号中均使用了加入了扩频时钟(spread spectrum clock),它可以使串行信号的速率在一个适当的范围内进行漂移，从而使其频谱在一个较宽的范围内扩散，尖峰值显著降低，可以有效减少EMI问题。例如 FBD规范(Page15)明确规定需要支持频率很低的30-33K的频率的扩频时钟，其他如PCIE,SATAI,SATAII同样要支持此功能。为了验证Motherboard上的诸如此类的串行信号是否支持扩频时钟，而且确认其调制频率是否在30-33K之间。就必须一次捕获足够长时间的信号进行频率抖动分析。一次抓取的采样点数可以用下面的公式计算：每个扩频周期约位1/33k=30uS，由于是捕获高速串行信号，采样率至少为40Gs/S,即采样间隔为25pS，则捕获单个周期的总采样点数为30uS/25ps=1.2M,为了实现准确的扩频时钟的测量，一般建议捕获10个以上的扩频时钟周期, 所以总的采样点数为1.2M*10=12M.需要强调的是，此12M的存储深度必须使用在40Gs/s或更高的采样率下才有意义。

Some oscilloscopes are designed to physically implement the high-speed acquisition front end (up to 80 ADCs) and high-speed memory with a single SOC chip. Since there are too many functions implemented inside a single chip, the capacity of the high-speed memory on the chip is limited (no more than 2M at 40GS/s), and the memory cannot be expanded or upgraded. In order to make up for the defects of this design structure, this type of oscilloscope will add low-speed memory outside the chip to make up for the limitation of the high-speed memory on the chip, but the external memory cannot work at a high sampling rate, and generally can only provide 2GS/s, with a sample interval of 500ps. It is impossible to collect enough samples at the edge of the signal, and even aliasing may occur, so it cannot provide high-precision time test results. Tektronix DPO can provide a storage depth of 200M per channel without any usage restrictions, which is the highest capability in the industry. This capability allows engineers who use DPO for verification testing to work with ease.

In terms of analysis tools, engineers generally consider the following three aspects in order of priority: the first is accuracy, that is, the analysis tool can accurately obtain results; the second is completeness, that is, the analysis tool can complete as many required test items as possible; the third is speed, that is, on the basis of ensuring "accuracy" and "completeness", the analysis tool can also work quickly and automatically, and it is best to be able to generate standard test reports.