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2016

Oscilloscopes Blog

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Do you want to make sure your oscilloscope measurements are the best they can possibly be?  Don’t settle for just an average measurement; simply scaling your signal properly can dramatically improve measurement quality.  Why? Because both sample rate and the bits of resolution of your oscilloscope play a part in your scope’s measurements.

Sample rate is affected by the oscilloscope’s horizontal scaling.  The equation to remember is:

Sample Rate = Memory Depth/Acquisition Length

Memory depth is a constant value, and the acquisition length (or trace length) is a variable dependent on your time-per-division settings. As the time/division setting increases, the acquisition length increases.  Since all of this must fit into the scope’s memory, at a certain point the oscilloscope’s ADC will have to decrease its sample rate.  What does this mean practically?  Let’s look at a frequency measurement on a 100 kHz square wave. We know the frequency is precisely 100 kHz and is very stable, so we can use the standard deviation of our measurement to judge the quality of the measurement.  Figure 1 has our 100 kHz square wave scaled to be viewed over 20 ms of time.  And, the scope’s sample rate has been automatically decreased from 5 GSa/s down to 100 MSa/s in order to fit the entire trace into the oscilloscope’s memory.  And, the standard deviation of our measurement is 1.49 kHz (about 1.5%) after around 1,500 measurements.

frequency measurement on a 100 kHz square wavefrequency measurement on a 100 kHz square wave

But, look at what happens when we choose a much smaller time/div setting, effectively shortening the acquisition length and increasing the sample rate.  Figure 2 has the same signal, but horizontally scaled to 1.2 us/div. The standard deviation is now 1.5 Hz, which is one thousand times smaller than our previous measurement.

signal horizontally scaled to 1.2 us/div

All that changed was the horizontal scaling of the signal, and in turn the sample rate of the oscilloscope. So, proper horizontal scaling of your oscilloscope can have a dramatic effect on the quality of your time dependent measurement.

Just as horizontal scaling effects your time dependent measurements, vertical scaling effects your vertically dependent measurements (peak to peak voltage, RMS, etc.).  Again, let’s take the same 100 kHz square wave, but instead look at peak to peak voltage.  Figure 3 has the signal scaled to 770 mV/div.  And, the standard deviation of the peak to peak measurement is 18 mV.  By decreasing the V/div settings on the scope to 66 mV/div the measurement’s standard deviation becomes 1.22 mV.  This is almost a 15x improvement!

vertical scaling effects your vertically dependent measurements

By decreasing the V/div settings on the scope, the measurement’s standard deviation becomes 1.22 mV, almost a 15x improvement

Why does the vertical scaling make a difference?  By scaling the signal to fill as much of the screen as possible, we are able to take advantage of the oscilloscope’s full bits of resolution.  Bits of resolution is essentially a signifier of how precise an ADC is capable of being.  The higher the bits of resolution, the more vertical levels the ADC is able to detect.  For example, the image below shows a two bit ADC.  The red sine wave is the analog input to the ADC, and the blue waveform is the digitized version.  As you can see, there are four different quantization levels possible.

This image shows a three bit ADC digitizing the analog waveform.

This image shows a three bit ADC digitizing the same analog waveform.  By having more quantization levels, the ADC’s digital output is able to more closely approximate the analog input.

properly scaling signals on your oscilloscope can make a dramatic difference in the quality of your measurements

When vertically scaling a signal to fill only a portion of the oscilloscope screen, you are not utilizing the ADC’s bits to the full potential.  For example, if you scaled a signal to half of the 3 bit ADC’s screen, you would leave two quantization levels unused above your signal and two levels below.  This would mean that your three bit ADC would only be able to use four quantization levels, rendering it just as precise as a fully utilized two bit ADC.

Knowing how to properly scale signals on your oscilloscope can make a dramatic difference in the quality of your measurements.  Proper horizontal scaling significantly effects your time dependent measurement, and proper vertical scaling effects your vertically dependent measurements.  Next time you are in front of your scope, remember: good signal scaling makes great measurements!

Meet the Team!

Welcome to the new Keysight Oscilloscopes blog! We will be posting regularly on a wide variety of topics involving test and measurement: everything from industry updates to application news to oscilloscope tips and tricks.

To begin, we wanted to introduce the blogging team so you know their areas of expertise and where they will focus their posts and communications.

 

Daniel Bogdanoff, Keysight Technologies

Daniel Bogdanoff – Daniel has a degree in Electrical Engineering from Texas A&M University (whoop!) and works closely with oscilloscopes.  He will focus on helpful tips and techniques you should consider when working with benchtop equipment. In addition to working at Keysight, Daniel is also a Contributing Technical Expert for Electronic Design.

 

 

 

 

 

Takuya Furuta, Keysight Technologies

Takuya Furuta – Taku has a wide range of experience and in-depth oscilloscope product knowledge, including oscilloscopes from other manufacturers. His posts will cover the history of oscilloscopes, internal scope architecture and front end filters, and perhaps even some high speed digital applications.

 

 

 

 

 

Johnnie Hancock, Keysight Technologies

Johnnie Hancock – Johnnie began his career as an analog hardware designer and has been around as long as dirt. He has tremendous knowledge across a wide variety of applications with his most recent areas of focus being on oscilloscope-based power supply and automotive serial bus measurements. In his spare time Johnnie enjoys spending time with his four grandchildren and beautiful wife of 40 years.

 

 

 

Mike Hoffman, Keysight Technologies

Mike Hoffman – Mike has a great deal of experience around oscilloscope tips and tricks as well as deep technical knowledge. He will focus most of his topics around these areas and will also cover industry news.

 

 

 

 

 

Kenny Johnson, Keysight Technologies

Kenny Johnson – Kenny is an R&D engineer currently disguised as a marketing engineer. He spent a significant amount of his R&D career working on oscilloscope probes (design and project management) and has 17 patents related to this work. When not earning a paycheck he enjoys running, biking and canyoneering (just like 127 Hours but without the arm thing).

 

 

 

 

Don Schoenecker

Don Schoenecker – Don is a very technical member of the team who will cover a broad range of topics – everything from modular trends to application-specific posts. With over 30 years of experience in the use of test tools to improve product development, Don brings stories and a message of encouragement to help you improve your designs. As a Texas A&M Aggie from Colorado (whoop!), he is happy to be back in the mountains.

 

 

 

 


Robert Lashlee, Keysight Technologies

Robert Lashlee – Robert will write on a wide variety of topics and cover industry trends as well as new oscilloscope measurements and applications.