DSO Bandwidth vs sample rate - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 21st February 2004, 09:00 PM   #1
bzo is offline bzo  United States
diyAudio Member
 
Join Date: Jan 2004
Location: San Francisco, CA
Default DSO Bandwidth vs sample rate

Can someone help me understand the relationship between the bandwidth vs the sample rate in digital storage oscilloscopes? A typical DSO might be speced at 100mhz bandwidth with 30MS/s sample rate. It seems to me that due to Nyquist, the maximum theoretical signal resolvable is half the sample rate, which would be 15mhz. So where does the 100mhz bandwidth come in? Am I missing something here?
  Reply With Quote
Old 22nd February 2004, 12:36 PM   #2
claudio is offline claudio  Italy
diyAudio Member
 
claudio's Avatar
 
Join Date: Feb 2002
Location: Italy
Hello,

Bandwidth
The first feature to consider is bandwidth. This can be defined as the maximum frequency of signal that can pass through the front-end amplifiers. It therefore follows that the analogue bandwidth of your scope must be higher than the maximum frequency that you wish to measure (real time).

Bandwidth alone is not enough to ensure that a DSO can accurately capture a high frequency signal. The goal of scope manufacturers is to achieve a specific type of frequency response with their designs. This response is known as the Maximally Flat Envelope Delay (MFED). A frequency response of this type delivers excellent pulse fidelity with minimum overshoot, undershoot and ringing. However, since a DSO is composed of amplifiers, attenuators, ADCs, interconnects, and relays, MFED response is a goal that can only be approached, and never met completely.

It is worth noting that most scope manufacturers define the bandwidth as the frequency at which a sine wave input signal will be attenuated to 71% of its true amplitude (-3dB point). Or, to put it another way, they allow the displayed trace to be 29% in error of the input before calling it a day.

Remember also that, if your input signal is not a pure sine wave, it will contain higher frequency harmonics. For example, a 20MHz pure square wave viewed on a 20MHz bandwidth scope will be displayed as an attenuated and distorted waveform. As a rule of thumb, try to purchase a scope with a bandwidth five times higher than the maximum frequency signal you wish to measure. Unfortunately, high bandwidth scopes are expensive, so you may have to compromise here.

On some scopes, the quoted bandwidth is not available on all voltage ranges, so check the data sheet carefully.

Sample rate
With analog scopes life was simple: you just selected the bandwidth that you required. For digital scopes, sampling rate and memory depth are equally important. For DSO's, the sampling rate is usually specified in mega samples per second (MS/s) or giga samples per second (GS/s). The Nyquist criterion states that the sampling rate must be at least twice the maximum frequency that you want to measure: for a spectrum analyser this may be true, but for a scope you require at least 5 samples to accurately reconstruct a waveform.

Most scopes have two different sampling rates (modes) depending on the signal being measured: real time and equivalent time sampling (ETS) - often called repetitive sampling. However, ETS only works if the signal you are measuring is stable and repetitive, since this mode works by building up the waveform from successive acquisitions.


http://www.picotech.com/applications..._tutorial.html

Claudio
  Reply With Quote
Old 23rd February 2004, 06:06 AM   #3
bzo is offline bzo  United States
diyAudio Member
 
Join Date: Jan 2004
Location: San Francisco, CA
thanks for the information, that link was useful.

Sounds like the resolution of a DSO is limited by the lesser of the bandwidth or sample rate. So in my hypothetical example of a 100mhz DSO with 30MS/s rate, this is effectively equivalent to a analog scope with 15Mhz bandwidth? (ignoring A/D resolution)
  Reply With Quote
Old 23rd February 2004, 03:53 PM   #4
claudio is offline claudio  Italy
diyAudio Member
 
claudio's Avatar
 
Join Date: Feb 2002
Location: Italy
Some DSO manufacturer, just show the SR, while the analog Bandwidth is given by SR/2 (Nyquist). So when choosing a DSO, check the SR first, since the bandwidth can be calculated from it. However keep in mind this: if you have a 15MHz signal, measuring it with your sample DSO (30 MS/s) will came up from 2 point (30/15), that is not very readable!
So my suggestion is: divide the DSO SR by 10, so that you will have 10 points to draw the signal, and it will be your bandwidth: a 30 MS/s DSO will have a 3MHz bandwidth.
Also check if the SR doesn't divide itself in case you are using both the DSO channels.

Claudio
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Asynchronous Sample Rate Conversion werewolf Digital Source 122 18th December 2009 07:10 AM
PMD100 Max Sample Rate? Filburt Digital Source 7 23rd May 2008 04:30 AM
Sample rate into BB PCM1798? Asgard Digital Source 14 22nd July 2006 12:54 PM
New sample rate converters from TI hifiZen Digital Source 3 15th March 2003 11:42 PM


New To Site? Need Help?

All times are GMT. The time now is 02:54 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2