KeysightOscilloscopes

Modular Oscilloscope or Digitizer for Wideband Measurements?

Blog Post created by KeysightOscilloscopes Employee on Aug 1, 2017

Written by Sheri Detomasi

 

There are many similarities and differences between oscilloscopes and wideband digitizers.  How do you know which is the right tool for your measurement need? 

 

Oscilloscopes use wideband data converters and typically provide a broad range of functionality.  They provide probing and visualization of time variant waveforms.  When debugging or troubleshooting a project, it’s important to see as much signal detail as possible.  Oscilloscopes typically provide waveform reconstruction filters for improved signal visualization. If the ADC waveform data is displayed with no waveform reconstruction, you would see a confusing cluster of points as shown in (a) below.  Whereas (b) shows with the waveform reconstructions.  Same with fast rise-times in (c) and (d)

 Waveform reconstructions

 

For visualization purposes, an oscilloscope also has continuous waveform acquisitions in display memory.  An oscilloscope can produce an extremely high waveform update rate > 1,000,000 waveforms per second.   Shown below, with an oscilloscope’s high speed waveform update rate and its ability to pick up glitches or unexpected events.   

 

Different measurement capabilities

 

 

 

Many oscilloscopes have a wide range of automatic measurement capabilities like rise/fall time, delay, peak to peak, zone triggering, etc. In addition, with the wideband acquisition, oscilloscopes are also ideal for high speed digital test, emerging serial protocols, and advanced communications.  With the wide bandwidth, vast measurement capabilities and robust user friendly interface, weather in bench or modular form factor, an oscilloscope is a general purpose tool that can be used for many applications. Keysight’s modular oscilloscopes, the M924xA series, range from 200 MHz to 1 GHz and feature the same functionality you would find on a benchtop oscilloscope.

 Digitizers

 

Digitizers are more purpose built.  Their main goal is to capture many channels of data with high resolution to achieve the best measurement fidelity.  While oscilloscopes typically have 8- to 10-bit ADCs, a digitizer is usually 10- to 16-bits.  This doesn’t tell the whole story though.  There are other noise factors to consider such as ADC differential and integral nonlinearities, thermal and shot noise, input signal distortion, as well as sample aperture jitter and ADC sample clock noise.  Therefore, a better measure of the resolution is the ENOB, or Effective Number of Bits.  One technique digitizers use to get even more ENOBs is digital down conversion (DDC).  DDC is extremely valuable when analyzing a small slice of spectrum within a wideband acquisition, allowing the user to reduce the bandwidth and ‘tune and zoom’ into a specific part of the signal.  Here is the digitizer DDC block diagram.

 ACD memory

 

 

It’s common for oscilloscopes to provide extremely wide bandwidth while digitizers provide higher ENOBs over smaller bandwidths. 

 

In normal use a digitizer will acquire many channels of data over longer time periods, producing lots and lots of data.  The data is either analyzed onboard or sent to a PC or storage device for post processing. For this reason, digitizers typically have deep memory buffers behind each ADC and very high data transfer rates.   For on-board processing, it’s useful to access the digitizers internal FPGA to do some real-time signal processing.  This allows the data processing and manipulation routines to reside in the hardware at GS/s processing rates and is useful for embedding algorithms to implement onboard custom filtering, correction routines, data reduction schemes as well as application specific routines.  This provides very specific application needs at very high speeds.  Here you can see a process for acquiring the data, processing, extraction, analysis and playback.

 

 

A digitizer is always connected to a PC and is controlled through a computer soft front panel or an automated program.  With this, there is less of a need for high-speed waveform update rate to the computer display.  The purpose-built nature of a digitizer makes it more of a dedicated tool for specific applications.

 

Characteristic

Oscilloscope

Digitizer

Ideal use

Interactive test and analysis with high performance user interface

Data capture with deep dive software analysis

Resolution and dynamic range

Good

Better

Measurement and analysis

Better automatic measures

Better data collection for post processing

Bandwidth

Better

Good

Acquisition memory (record time)

Limited

Good, extendable to external storage

Number of channels

Limited

Good, expandable

Waveform update rate

Better visual (display) update rate

Limited

Data streaming

Limited

Better, high data throughput

Triggering

Better

Good

FPGA access

Not typically

Possible

Probing

Better

Limited

 

 

 

A word of caution: some test equipment vendors will promote a digitizer as an oscilloscope or an oscilloscope as  a digitizer.  This may cause some confusion and the wrong choice can cause headaches down the road. Ensure you understand your needs and select the right instrument for your test application.  To find out more, check out these resources as well as a video of Keysight experts talking about the blog:

 

 

App Note: Understanding the Differences Between Oscilloscopes and Digitizers for Wideband Signal Acquisitions  

Webcast: "Oscilloscope or Digitizer for Wideband analysis - Why care?" .

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