Windmill Software Ltd
Data Acquisition Intelligence

May 2009

The Newsletter for PC-Based Data Acquisition and Control
Issue 130           May 2009
--------------------ISSN 1472-0221----------------------

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* Choosing how Often to Sample an Analogue Signal
* DAQ News Roundup

Choosing how Often to Sample an Analogue Signal

When monitoring analogue signals, the data acquisition 
equipment digitises the signal before transferring the 
reading to the computer.  It does this by "sampling" the 
signal at regular intervals.  How do you choose the 
sampling rate to get an accurate picture of an 
analogue waveform?

The first thing to decide is how fast the signal is 
changing. For faster signals this is determined from the 
maximum frequency component in the signal. For slowly 
changing signals, such as those from industrial plants, 
you can use the maximum expected rate of change. 

For frequency components the Nyquist theorem demands 
that the signal be sampled at least twice in each cycle, 
otherwise the amplitude of this frequency component will 
distort the signal at lower frequencies (see the picture 
at So, sample 
at least twice as fast as the highest significant 
component. This rate would not mimic the waveform very 
closely though. 

To get an accurate picture of the waveform you will 
need a sampling rate of 10 to 20 times the 
highest frequency.

For slowly changing, essentially DC, signals, then all 
that is necessary is to consider the minimum time 
for a significant change in the signal. 

Related topics:
Monitor Issue 26, Multiplexing

DAQ News Roundup

Welcome to our roundup of the data acquisition and 
control news.  If you would like to receive more 
timely DAQ news updates then grab our RSS newsfeed 
at  Read for notes
on how to display the news on your own web site, 
read it via e-mail, mobile phone or in your browser.

Ageing satellites to hit GPS accuracy
   GPS devices calculate their position by comparing time 
   signals from at least four satellites. To keep that 
   many within range at all times requires a fleet of at 
   least 24.  For now there are 31 operating, but 13 of 
   them are more than four years past their design 
   lifetime. Bad news for position-referenced 
   data acquistion.
   Source: New Scientist

New Way to Test Water Quality
   Researchers at RMIT University have developed a 
   low-cost, portable way to test water quality and help 
   authorities deal with pollution.  The prototype sensor 
   uses selectively adsorbing polymers designed to 
   quantitatively detect specific water-borne contaminants.
   Source: RMIT University

New acoustic monitoring system for sewers
  Engineers from from Bradford University are developing  
  an acoustic monitoring system that will allow engineers 
  to rapidly identify blockages and damage in sewer pipes. 
  The key elements of the acoustic system are a small 
  multi-sensor array and an advanced, real-time signal 
  processing algorithm which overcomes the effects of 
  ambient noise and reverberation in the sewer environment.
  Source: The Engineer Online

Industrial Controls Shipments plunge
  US association NEMA's Primary Industrial Controls Index 
  experienced its largest quarter-to-quarter decline on 
  record, contracting more than 23 percent in the first 
  quarter of 2009 versus the fourth quarter of 2008. 
  Source: NEMA

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Windmill Software Ltd, PO Box 58, North District Office,
Manchester, M8 8QR, UK
Telephone: +44 (0)161 833 2782
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