Archive for the ‘Oscilloscope Fundamentals’ 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

The oldest Tektronix scope out there, and why soccer will soon be America’s number 1 sport

August 2, 2010

Like most who read this blog, you know that the scope is the quintessential tool for general-purpose debug and electrical analysis. And I’d venture that most of you have a ubiquitous old scope lying around the house and/or lab somewhere.

Some folks have even made it their passion to collect information about some of these older scopes, as evidenced by these Web sites:

Classic Tektronix Scopes
Tektronix Resource Site
The Museum of Tek Scopes
AnaLog’s Tektronix T922R Service Notes

These vintage products from a different era were unique from the perspective that Tektronix built most everything in house—from the PCB, knobs, and switches, to the cathode ray tube, to the mechanical chassis.

To read the rest of this article, see my take on soccer and to read the 20+ comments generated on both soccer and on a favorite old oscilloscope, please visit my Scope Guru on Signal Integrity Blog, on EDN’s site.

It’s better than a Ferrari—why everybody needs a scope

June 11, 2010

You have probably used a scope sometime in your career, but you probably didn’t know there are a host of other things scopes can do. Matter of fact, the scope replaces many of the most important things in your daily life. The spring weather here in Beaverton, Oregon, has finally decided to come out so here’s some light-hearted reading as you enter the weekend.

Home-entertainment system

The large hi-res display of your scope allows you to view analysis results to 18 decimal points plus doubles as a HDTV. In addition to that, it also has a DVD drive that allows you to view movies, and it has Ethernet connections to stream live video. Not only does the giant disk storage help with organizing waveforms and measurement results, you can also use it to store your video and music albums. Make sure you also get a portable, battery-operated scope that will phase out your iPod, iTouch, iPhone, and iLoveSoaringStockValues.

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

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.

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.

Oscilloscopes and Audio Testing, Plus How to Win a Scope!

March 16, 2010

Question: What is needed on the scope to show that a tube or valve preamp stage is used in a pedal direct into the sound board:

The tube sound is often subjectively described by uncritical listeners (that is, not audio professionals) as having a “warmth” and “richness”, but the source of this is by no means agreed on. It may be due to the non-linear clipping that occurs with tube amps, or due to the higher levels of second-order harmonic distortion, common in single-ended designs resulting from the characteristics of the tube interacting with the inductance of the output transformer.

http://en.wikipedia.org/wiki/Tube_sound

I wish to show that a digital audio signal run through a tube post gain stage produces this “warmth”.   I can detail the exact 2nd harmonic characteristics of this sound at least in part mathematically.    ‘With that information, how would I go about illustrating the difference as scope traces?  The tube post gain vs straight digital.

Question 2:  What do I need to do to win a scope?  Used, cosmetic defect, I  don’t care, I’ve used them since 2nd year engineering school and lately have been wanting to work with them again.    

3rd and last question:  In the procedure taking a line level stereo audio signal and modulating it for FM transmission, how can a scope be used to optimize design for the highest quality audio reproduction possible?

Answer: Thanks for the excellent questions. I’ve divided the answer into three parts.

1. Vacuum Tube versus Solid State Sound

The discussion here will be about consumer audio products, but there are many similarities to professional audio products, too.

As a class, not all vacuum tube amplifiers sound the same.  And as a class, not all solid state amplifiers sound the same.  However, it has been generally found that there are differences in ‘tube’ sound versus ‘solid state’ sound.  There are differences in amplifier topologies; and differences in interfaces between preamp, power amp, and loudspeakers (i.e. load impedance and resonances).

 The following amplifier characteristics are thought to be some of the causes of audible differences:

Clipping characteristics, the proportions of low order harmonic distortion products and high order harmonic distortion products, frequency response, and damping factor (output impedance).

Clipping: The effects of clipping can be eliminated, as long as the amplifier, loudspeaker, and playback sound levels are such that the amplifier never clips.  Any amplifier that is driven into hard clipping will cause most music to sound terrible (although the intent of some electric guitar playing intentionally makes use of this highly distorted sound). 

If the intent is to play music very loudly at very low distortion, then either a very efficient loudspeaker is required, or a very powerful amplifier is required. 

For clipping testing, a function generator, scope, and load resistor are required. The shape of the clipped signal will vary from amp to amp, and according to the amount of overdrive to the amp.

Harmonic Distortion at moderate levels: For most amplifiers that are played at levels well below clipping, the distortion is very low. 

For harmonic testing, a low distortion function generator, scope that has an FFT function, and a load resistor are required.  The distortion may be low enough so that a scope cannot see any harmonics other than the 2nd and 3rd (higher order harmonics may not be visible).

In order to see low amplitude high order harmonics of amplifiers, a spectrum analyzer may be required.

Check the level of the harmonics, the number of harmonics, and the rate that their amplitude is reduced as the harmonic number increases.

Frequency Response: Most music fundamentals and harmonics are within the frequency response of the amplifier, but the amplifier may attenuate the signal at the frequency extremes.

For frequency response testing, a function generator, scope, and load resistor are required. 

The gain of the amplifier should be tested over its rated frequency range, both at low power, and at rated power.

Damping Factor: Different damping factors react with loudspeakers to cause different frequency responses, and different transient responses.

A function generator, scope, and multiple load resistors are required to test damping factor.

The output voltage is tested, as the load resistance is changed.  The damping factor (output impedance) is calculated from the output voltage changes, versus the load resistance changes.

2. Winning a Free Scope

Winning a free scope can be difficult but the more entries you have the better the chance. Tektronix participates in many events and shows throughout the year where we give out scopes as well as other prizes. I’d encourage you to check the ‘events’ page on the Tektronix site regularly for update-to-date details (www.tektronix.com/events) . In fact we will be giving away 5 Tektronix MSO2024 Digital Oscilloscopes during Embedded Systems Conference (ESC) Silicon Valley next month. And, we give away a scope each month at our Scope Central online community (www.tektronix.com/scopecentral). You can enter daily–thus increasing your odds of winning!

3. FM Stereo Transmission Quality

The quality of sound from an FM transmission depends on many factors.

The music source material (i.e. CD), playback source (i.e. CD player), sound boards, compressor (presence or lack thereof), and FM Transmitter are all responsible for the quality of sound.  Also, the correct setting of signal amplitudes at each point along the way is paramount to ensure good sound quality.

If the desire is to retain the original dynamic range of the music as it is recorded on the CD, then there should be no compressor in the station audio chain.  Some classical and Jazz stations follow this model.

The maximum input of each stage, including the transmitter modulator input, should be rated by the manufacturer.  These ratings will be used in setting the proper levels in the audio chain. A test CD with full amplitude sine wave (0 dB) should be put in the CD player, and the output sent down the audio chain. 

Starting from the CD player output, to the first input stage, a scope should be used to measure the signal level.  Set the gain levels of the audio at each point along the path so that the signal amplitude is near to, but not at the maximum input rating of each stage along the way.  Too much signal amplitude will cause distortion (or if the signal is way to large it will clip); and too small of a signal amplitude will have a poor signal to noise ratio.

Music CDs have wide variations in their recorded level output.  And without pre-playing and testing each CD for the loudest portion of a recording, it will be an unknown. 

A station philosophy decision must be made as to whether or not to change the gain on the sound board for each CD to maximize the signal to noise ratio.  The danger of adjusting the level individually for each CD, is that a loud portion will come along and cause clipping.

If it is decided to do gain adjustments, then for low level CDs, the gain of the sound board will have to be raised.  Again, the scope can help to set the level.  Then, it will have to be set again according to the next CD recording level.

A compressor can assist in reducing the severity of incorrect level setting, but the tradeoff is a reduction of dynamic range of the music. If there is a compressor in the audio path, it will be a compromise between dynamic range, versus the level you set into the compressor.  If the level is set so that the full scale CD does not cause the compressor to compress at all (might as well not use a compressor).  If the level is set too high, the compressor will be working all the time (the dynamic range will be severely limited).  The setting between those two levels is a compromise, and the best setting is up to the taste and goal of the station.