Archive for the ‘Ask Scope Guru Q/A’ category

The always-amazing, ever-evolving oscilloscope tackles protocol analysis—Why not?

January 19, 2011

As we take the plunge into 2011, I thought it would be interesting to take a moment and reflect back on the oscilloscope. The oscilloscope has come a long way in its 70+ year life span. The test and measurement industry’s most popular instrument, the oscilloscope was originally designed to enable viewing and evaluation of varying signal voltages.

To read the rest of this article, please visit my Scope Guru on Signal Integrity Blog, on EDN’s site

Measurements for Laser Engineers

July 29, 2010

Question: Being a Laser Engineer there are many times where I need to catch a pulse train of several hundred pulses at a time.  I am using a pulsing solid state laser operating anywhere between 10 and 20 Hz.  The issue is that I need to catch every pulse and then be able to export the data to Lab View for processing and integration with a full test setup. 

I am using a nanosecond photo detector to look at the widths of each pulse.  I need a scope to be able to detect and record each pulse of the set of pulses and then be able to conduct some basic statistics on the data: min, max, average, standard deviation, # of pulses, etc.

Typical Characteristics of the lasers are:

  • Pulse Width: 8-12 nanoseconds
  • Frequency: 10-20 Hz
  • Amplitude: 5 Volts or less
  • # of Pulses: 100-500 pulses per data set

The pulse shape is a Gaussian shaped pulse, and we would have 500 pulses each with a width of 8-12ns occurring at a frequency anywhere between 10-20Hz.  If it was 500 pulses at 20 Hz total time would be 25 s, 500 pulses at 15 Hz would be 33.33 s, and 500 pulses at 10 Hz, would be 50 s.  So, the total length of time depends on the number of pulses in the data set and the frequency of the pulsed laser.

Which scope would be best for me and is there one that I can control through Lab View?

Answer: First the good news–any Tektronix scope will work with Lab View.  But, here are specifics about making the measurements you need to make.

There are two ways to go about making the measurements you want.   The first way would be to use a lower end bench scope, single shot each pulse, then pull that into data into Lab View. Then you would need to re-arm the scope trigger before the next pulse would occur.  That puts a lot of variables on the PC side to ensure that the scope would be re-armed fast enough to catch the next pulse. 

What we would recommend is using a scope that has FastFrame Acquisition mode.  This mode allows the Oscilloscope to handle (by setup) the number of pulses (trigger event) and the number of data points to store every trigger event.  This allows for high resolution capturing of each pulse, without using up record length saving time in-between each pulse.   The time of trigger is also stored very accurately.  This acquisition mode was designed specifically for the type of acquisition you need.

The recommended Tektronix instrument with this feature is one of our DPO7000 series of oscilloscopes.

Why Are Most Scopes Grounded?

April 23, 2010

Gina BoniniQuestion: I am curious why A/C powered oscilloscopes do not have the ground lead isolated from the A/C chassis ground. Digital multimeters that plug in the wall do. Aside from hand held or portable scopes, shouldn’t that be the norm now by now, or are they, and my scope is just old??

Comments?

Answer: Interesting observation.  There’s actually a few different reasons:

  1. Digital Multimeters (DMMs) don’t draw a lot of power, so it’s pretty easy to power them with an AC/DC power supply that has low-power isolation transformers.  These transformer isolate the product from ground. The power requirements of many scopes are beyond what can be served by this design. That said, Tektronix does offer a unique oscilloscope with isolated channels, the Tektronix TPS2000 series.  This basic oscilloscope is specially designed with isolation transformers.
  2. DMMs and their leads are carefully designed so it’s very hard for you to touch any metal on them, so it’s hard to get shocked if they’re not grounded. In general, that’s not at all true of oscilloscopes. There are a few exceptions, like the Tektronix TPS2000 series and the Fluke handheld scopes. In these cases, the individual inputs are floating.
  3. DMMs are only capable of measuring low frequencies, so the stray capacitance between the chassis and earth ground doesn’t affect the measurements much. In addition to the safety issue (in #2 above), the large capacitance of a floating scope would serious affect the quality of most oscilloscope measurements.
  4. The dielectric in the transformers in most power transformers are not designed to have large voltages applied across them. Over time, high voltage differences across the transformer’s isolation barriers will cause them to fail, creating a reliability and a safety issue.

 For all of these practical and safety reasons, most oscilloscopes will continue to be grounded.

Analyzing Frequency Content

April 8, 2010

Question: When I look into a analog signal in frequency domain (FFT), I see harmonics with certain dB. What I also see (which I shouldn’t) is some noise (!!) in between the harmonics with certain dB. What is this noise ?

Answer: Thank you for the question on analyzing frequency content with an oscilloscope. Depending upon your signal, instrument setup and other factors, there are a number of possibilities as to why you are seeing noise between the signal. As a general rule of thumb, to minimize noise coupling into the scope I first try to ensure proper grounding with short lead lengths. Depending on the signal type, you can also perform filtering or post-processing through some averaging techniques to reduce noise. Again, without knowing anything about the signal or your setup, these are some general considerations that may or may not be relevant.

I would, however, recommend a very nice application note on using Tektronix oscilloscopes for FFT analysis. You can download the application note here.

I hope this helps and thanks again for the question.

What’s the Best Scope for My Application?

April 1, 2010

Question: I’m looking to do some experimenting with digital chips. The clock frequencies I’ll be working with is 128 Mhz. I was thinking about purchasing a TDS2024B would this scope be adequate for this.

I would like to know some of the important differences between the TDS2000 and TDS3000 series oscilloscopes

Thank you for your help.

Answer: When choosing an oscilloscope, you typically want the oscilloscope’s bandwidth to be 3X to 5X the frequency of your signal, depending on the application. 

All oscilloscopes have a low-pass frequency response that rolls off at higher frequencies.  Traditionally, oscilloscope bandwidth has been specified as being the frequency at which a sinusoidal input signal is attenuated to 70.7% of the signal’s true amplitude. This is known as the “-3 dB point”, which is a term based on a logarithmic scale.  It means that a sine wave measured at the oscilloscope’s bandwidth rating will have an amplitude error of -3 dB, or almost 30%. 

Now, as you might recall, any complex signal – such as a digital signal – can be re-created by overlaying a series of sine waves of different amplitudes and frequencies.  These different sine waves are also known as spectral components.  To accurately represent a digital signal – like you might find running on a data bus – the fundamental, third harmonic and often the fifth harmonic spectral components must be captured during measurement.  If bandwidth limitations on the measurement system limit the captured bandwidth to less than these harmonics, the signal will lose key features.

Having said that, it’s also important to look at the rise time of your digital signal.  The edge speed (rise time) of a digital signal can carry much higher frequency components than its repetition rate might imply. You want your oscilloscope’s rise time to be at least 5 times faster than that of your digital signal.

If your clock is working at 128 MHz, you’ll probably want a scope with a bandwidth greater than 640 MHz.  I’d recommend that you take a look at our MSO/DPO4000 Series.

Now, to answer your other question:  One of the key differences between the  TDS2000 and TDS3000 is available bandwidth and performance.  The TDS3000C Series offers up to 500 MHz bandwidth and 5 GS/s sample rate compared to 200 MHz and 2 GS/s on the TDS2000B.  The TDS3000C also has deeper record length and more bells and whistles – more triggers, more automated measurements, DPO technology display, Wave Alert, and optional analysis packages like advanced waveform math and analysis, limit testing, telecommunications mask testing, and video debug.  I’d suggest going to www.tektronix.com/oscilloscopes.  We have a comparison tool there that will allow you to compare the TDS2000B and TDS3000C side-by-side to really see how they compare.

Good Luck!

Acquisition Modes

March 29, 2010

Question: What acquisition modes are available on most Tektronix oscilloscopes?

Answer: Here’s an overview of 8 acquisition modes commonly available.

Sample mode: The most basic of all the modes, sample mode delivers the highest accuracy for timing interval measurements. It retains and displays the first point from each sample interval, discarding the others. In many DSOs, sample mode interleaves the digitizers of two or more channels to achieve the instrument’s maximum sample rate.

Peak Detect mode: Determines the highest and lowest values for each sample interval, then displays all the samples between the two values, inclusive. Especially useful at slower sampling rates, Peak Detect allows you to see any extremes that occurred during the sample interval.

High Resolution (Hi-Res) mode: A Tektronix-patented process that calculates and displays the average of all the values in each sample interval. It runs at the highest sampling rate of the digitizer, providing maximum detail in the acquired waveform. It does not interleave channels. Because it works with more data per sample interval, Hi-Res mode increases the effective vertical measurement resolution. Sample, Peak Detect, and Hi-Res modes operate in real time, using the acquired data from one trigger event. Therefore these modes are suitable for the most demanding, single-shot measurements at frequencies up to the oscilloscope’s upper bandwidth limit. The remaining modes require a repetitive signal.

Waveform Database mode: The waveform database is a three-dimensional accumulation of source waveform data over several acquisitions. In addition to amplitude and timing information, the database includes the number of times a specific waveform point (time and amplitude) has been acquired. Using waveform database technology, the real-time oscilloscope processes a much larger sample of data than when using the sample acquisition mode.

DPO FastAcq mode: FastAcq optimizes the oscilloscope for analysis of dynamic signals and capture of infrequent events.

Average mode: Using the data from two or more acquisitions, this mode averages the corresponding data points on a point-by-point basis. Average mode improves the signal-to-noise ratio, removes uncorrelated noise, and makes viewing of repetitive signals easier.

Envelope mode: Builds a waveform “envelope” from the highest maximum values and lowest minimum values among the corresponding samples from two or more trigger events (acquisitions). Envelope mode is similar to Peak Detect mode (and in fact uses some Peak Detect capabilities) but is best used on repetitive waveforms, where it minimizes aliasing effects.

Random Equivalent Time (ET) mode: Accumulates a waveform record from acquisitions over many trigger events. The samples from any one trigger occur randomly with respect to samples from any other trigger, so eventually, samples will fill all parts of the waveform record. ET mode may capture several samples during any one acquisition, but cannot be used on single-shot signals. ET mode requires a repetitive waveform that remains consistent from trigger to trigger. In a DSO, random ET sampling allows sampling at rates higher than the oscilloscope’s nominal Nyquist frequency limits.

New Advances in Oscilloscope Technology

March 25, 2010

Question: What are some new advances in oscilloscope technology?

Answer: Besides making scopes faster, wider, deeper, there are many new capabilities in scope performance and time-saving features. For example digital storage oscilloscopes have traditionally been used for physical layer testing of AC/DC parametric measurements such as rise time, voltage swing, high/low values, etc. However with the wide adoption of mainstream buses like I2C and USB many scope now have built-in decode capability. In fact the new Tektronix MSO70000 mixed signal scopes have 20 GHz bandwidth across 4 channels as well as an integrated 16 high speed digital acquisition system. This allows for isolation of faults that occur only during specific bus cycles like a read or write cycle (DDR3 memory) or for monitoring of a wideband RF system that incorporates digital subsystems and controls like SPI or I2C.

Isolating faults during Read or Write cycles on a Mixed Signal Oscilloscope

Triggering on a Read Burst

Often, while monitoring signals in the digital domain, it’s important to be able to view the same signals in the analog domain as well. Maybe there is a glitch or some crosstalk that is causing an error on the bus or other digital problem. The next step would be to get an analog probe and probe the same point (double probe). While most new high bandwidth probes have lower loading than before this is inefficient and can still add some non-negligble loading to the bus. Some recent advances in scope architecture now allow for simultaneous signal acquisition in both the digital and analog domain, with a single probe. This is possible through an internal multiplexer technology called iCapture. Below are some examples of how this is done. For an interesting read check out the article “Twelve Things You Didn’t Know a Scope Could Do”  (please be patient–the PDF might take a few moments to download).