Valid Frequency Domain Analysis ?

There seems to be two issues with using the MyDAC for frequency domain analysis ( Bode Plots etc. )
1) The system does not employ Anti-Aliasing Filters . This can create major frequency fold-back problems.
2) The analog inputs are multiplexed which leads to a channel to channel phase error .. up to 180 degrees at the Nyquist frequency.
Is there any 'behind the scenes' processing done to mitigate these issues ??
Thanks in advance
Solved!
Go to Solution.

Hello ProfP,
Thanks for the post! Sorry to leave you hanging for so long. I have some answers for the 2 points you asked about above.
1. You are correct, the myDAQ system does not employ Anti-Aliasing Filters. The ELVISmx Bode Analyzer artificially limits the input frequency when using the myDAQ to 20 kHz. At that frequency, you
only get 10 points per period from both the AO and AI.
Anti-alias filters behind the multiplexer would slow down settling behavior, so they were not included. You can add filters in front of the multiplexers (at the AI0 and AI1 pins). I'd recommend they be buffered, though. Charge injection on the multiplexer makes high speed scanning very sensitive to source impedance. Note the effect in the Settling Time versus Different Source Impedance figures (21 and 22) of the myDAQ manual: http://www.ni.com/pdf/manuals/373060b.pdf
2. You are correct. Two-channel acquisitions are phase-skewed by the 1/(2*scan rate) when the scan speed is faster than 40 kSps/ch. For example, if you acquire at 200 kSps/ch, the interval between consecutive measurements on AI0 and AI1 is 1/(2 * 200 kHz) = 2.5 us. Below 40 kSps/ch, the phase skew is constant at 12.5 us. This is intended to help very slow acquisition rates appear more like simultaneous acquisitions rather than keeping 180 degree phase skew unnecessarily. These are default configurations. You can adjust the conversion interval parameter through the DAQmx API if you prefer to customize the behavior.
Let us know if this is the information that you needed and thanks for using NI myDAQ!
Cheers
Corby
Academic Product Support Engineer

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NR not very trivial. IMO anyway.
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>
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Attachments:
sample.vi ‏204 KB
mathematical.png ‏103 KB

Motorbreath,
Here are some examples on how to use LabVIEWs graphcial coding to get things into the Frequency domain:
https://decibel.ni.com/content/docs/DOC-9981
http://zone.ni.com/devzone/cda/tut/p/id/13042
http://zone.ni.com/devzone/cda/tut/p/id/7111
http://zone.ni.com/devzone/cda/epd/p/id/2904
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http://zone.ni.com/devzone/cda/tut/p/id/4278
http://zone.ni.com/devzone/cda/tut/p/id/4541
http://zone.ni.com/devzone/cda/tut/p/id/3342
https://decibel.ni.com/content/docs/DOC-6390
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This is regarding clarification regarding frequency responce analysis

This is regarding clarification regarding frequency responce analysis
using
NI
PCI 4551 add-on card, especially in the singal generation part for
stimulating
the unit under test.
1. Is it possible to generate sine, step, ramp and arbitrary waveforms
using
this particular card?
2. Ifso what is the range of frequencies that can be possible to
generate
using
PCI 4551 card?( we may require from 1mHz to 2kHZ)
3. And what is the maximum amplitude of the generated signal for full
scale
value?
4. Whether real-time octave analysis for NI DSA software can serve the
purpose
of following tests: Frequency response test, Step response test,
Velocity
test,
Strain Measurement, Linearity Test, Cyclic test.
Or any other software tool is required?
Regards,
Sreedhar T

Hi SreedharT,
In response to your questions, I've included my responses. I highly advise referencing the NI-4551 User Manual for more details.
1. Is it possible to generate sine, step, ramp and arbitrary waveforms
using this particular card?
NI-DSA includes functions to generate standard white noise, pink noise, band-limited noise, periodic white noise, single or dual-tone sine, and chirp signals. For step, ramp, and any other arbitrary waveforms you can use NI-DSA to define and generate an arbitrary analog output signal.
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2. If so, what is the range of frequencies that can be possible to
generate using PCI 4551 card?( we may require from 1mHz to 2kHZ)
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1/1-octave analysis
center frequency fs=25.6 kHz:
High: 8 kHz
Low: 16 Hz
fs=51.2 kHz
High: 16 kHz
Low: 31.5 Hz
1/3-octave analysis
fs=25.6 kHz
High: 10 kHz
Low 12.5 Hz
fs=51.2 kHz
High: 20 kHz
Low: 25 Hz
1/12-octave analysis
fs=25.6 kHz
High: 10.68 kHz
Low: 11.05 Hz
fs=51.2 kHz
High: 5.34 kHz
Low: 5.52 Hz
3. And what is the maximum amplitude of the generated signal for full
scale value?
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Velocity test, Strain Measurement, Linearity Test, Cyclic test.
Or any other software tool is required?
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For the Velocity test, Strain Measurement, Linearity Test, and Cyclic Test I should ask for more information about each test before attempting an answer. However, to answer your question, there are no methods I know of in NI-DSA that would directly perform these tests. In contrast with that answer, you may be surprised this functionality may be included with a LabVIEW development environment.
Let me know if you have further questions or this does not resolve your issue!
Chad AE
Applications Engineer

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http://forums.sun.com/thread.jspa?threadID=5433169&messageID=10958328#10958328
http://forums.sun.com/thread.jspa?threadID=5433180&messageID=10958329#10958329
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Can we make a frequency response analyser using a Frequency generator and frequency counter?
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The outputs are Frequency, magnitude, and the phase as like solartron FRA.
somebody have an Idea for this
awaiting for the solution
thank you
"Thanks with regards "
by
..........Gireesh..........

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http://zone.ni.com/devzone/cda/epd/p/id/5646
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Hi,
I'm fairly new to Labview but feel I know most of the fundamentals.
I've been trying to convert the following bit of code from Matlab to Labview (I am aware 8.20 has a math script function but I don't have access to it). Please could someone have a look at my VI and possibly tell me why I am getting the 1802 error. It suggests that "(Hex 0xFFFFF8F6) Waveforms have different dt values." but I'm not sure what this means or how to fix it.
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% routine for calculating Transfer Function
% signal is the array containing the measured signal
% sigs is the array containing the input sine wave
% sigc is the array containing the input cosine wave
clear;
freq=11.45;
sfreq=360*freq;
cycs=10;
a=0;
b=0;
i=1;
for t=0:1/sfreq:cycs/freq;
n=cycs*360+1;
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Frequency Response Analyser v7.vi ‏1045 KB

Hi Ian,
It seems that you are attempting to use different dt in the waveforms, but I founded in this link two ways to avoid having this problem. Hope you find it useful. Thanks for using the forums!
http://digital.ni.com/public.nsf/websearch/C41966093C3D030F86256FF8007F8BF5?OpenDocument
NorSa
AE LATAM
NorSa
NI Applications Engineer Latin America
Para Soporte entra aquí

Valid Frequency Domain Analysis ?

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