Samples and cylcles per buffer

Hi. I am trying to use Daqmx to generate a sine wave and the use displacement sensors to measure the oscillations of a piezo. The problem that I am having is that the output sine wave stops changing after somewhere around 2kHz. Below 2kHz everything looks good. I am using an oscilloscope to measure the output frequency. At higher frequencies the frequency remains the same for a long period and then changes instead of changing when the code tells it to change. When I measure the Q curve of the piezo I am getting a step ladder instead of a smooth curve. I believe this has something to do with my cycles per buffer and samples per buffer but I am not sure. I have included the vi and the waveform generation vi that I am using. Please let me know what I am doing wrong.
Attachments:
Waveform Generation21.vi ‏38 KB
Dual RLP Phase & Deflection (sweep)1.vi ‏807 KB

I can't look at this because your version is too new. However I have
tried to code something very similar. The problem I encountered was
this: You can only set the sampling frequency to certain values (an
integer divided into the clock frequency) and the number of samples per
cycle is obviously an integer. So you cannot sweep the frequency with
arbitrarily fine increments. indeed figuring out how to sweep it is not
trivial as it may involve changing both the number of points per cycle
AND the clock divisor.
As a start you should probably decrease the number of points per cycle.
(Note that if you set the sampling rate DAQmx will round to a legal
value if necessary. If you need to know the actual sampling rate use a
property node to read it.)

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Dear Sir,
Thank you for using NI Developer Zone.
National Instruments has a product line designed specifically for acquisition of dynamic signals, it is called DSA - Dynamic Signal Analyzers. In according to the description of your application the card that best fits your needs is the NI-4472 that is available in PCI and PXI format. This card is able to acquire 8 channels simultaneously, as opposed to the regular DAQ cards that have a single A/D converter and a multiplexer. The NI-4472 has 8 Delta-Sigma A/D converters one per channel, what results in a better performance than having a single A/D converter in combination with a sample and hold and a multiplexer. To Acquire 64 channels you would have to have 8 boards and synchronize all 64 channels. The Synchronization process is very simple and with a couple of software calls we can synchronize as many as 112 channels for simultaneous acquisition at 102.4K samples per second per channel.
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For a 1 KHz sine waveform, this represent a phase delay of 0.288 degrees between consecutive channels. For a channel list of 64 channels each one having the same 1 kHz sine waveform, the phase delay between the first and the last waveform will be 18.432 degrees.
I really recommend the NI-4472 as a very good solution for your application, besides that the combination of the NI-4472 with LabVIEW and the sound and vibration toolset will give you a tremendous tool to solve not only your immediate need but also provide you a system that will be easily modified and adapted for future needs.
If you need more info, feel free to contact me and I can give you more details about this product.
Call 1 800 IEEE488 and ask to speak with the Computer Based Instruments (CBI) support team.
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http://www.ni.com/support/

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Message was edited by: jdredge

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Samples and cylcles per buffer

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