How to produce a Logarithmic Sweep (Chirp) over a Frequency Band

[Daniel Akeju]

Hello, I am new to LabVIEW. I wonder if someone can point in the right direction as far as producing a Logarithmic Sweep (Chrip) over a frequency band. My plan is to obtain a Transfer Function measurement over a frequency band eventually leading to Impedance for a DUT from the Impulse Response.

Thank you very much,

Daniel

[johnsold]

Daniel,

Welcome to the Forum.  Many of the participants here volunteer our time to help others and to learn more ourselves.

To be helpful we need as much specific information as possible.  Your request will generate more questions than answers.

Are you trying to generate the chirp signal in LabVIEW (LV) or will you be commanding an instrument (in hardware) to generate it? What frequency range? What is the duration of the chirp? What instruments or data acquisition (DAQ) devices do you have (or are planning to obtain)? How do they communicate with the computer?

Have you tried anything yet? If so, what did not work as you expected?

Since you are planning to eventually generate a stimulus and measure the response, you should design your chirp VIs with the idea that they will later be part of a bigger ssytem.  What is the end use of the data? Is this for R&D or a production tester or something else? Will the data be displayed to an operator or saved to a file?

Lynn

[Daniel Akeju]

I am very grateful.

Yes, I am trying to generate the chirp signal in LV. At the moment, my interest is in the audible range as far frequency. I am unsure of the duration of the chirp; I must look into this (I am studying a reference paper to learn more). I have the BNC 2110 adaptor; this provides the communication with the computer. So far, my setup gave produced impedance (real and imaginary) for a set frequency with the help of some MATLAB code as part of a research project. And the data is to be displayed.

Thank you very much.

[johnsold]

Daniel,

The BNC 2110 is just a connector block.  What kind of DAQ card do you have in the computer?

Some things which will affect the sweep rate are delays or transients in the response of the DUT.  If you do a step change from no signal to a sine wave at a constant frequency within the DUT bandwidth, how long does it take for the output to reach steady state?  If your sweep is faster than that, it will be very difficult to separate transient effects from the signals you need to measure the transfer function.

Do you really need a chirp or would a set of steps of frequency work?  For example a series of measurements at 100, 200, 500, 1000, 2000, 5000, 10000, and 20000 Hz covers the audio range logarithmically and does not require a chirp.

Lynn

[Daniel Akeju]

Thank you very much.

I am unsure of the DAQ card; I must check tomorrow morning and get back to you. I apologize for not having all the answers; I am just getting involved with the project. I will talk to my advisor tomorrow. Nonetheless, he made it clear that a chirp would be best for the project. Please allow me to reply with more information.

Thank you for your help.

[johnsold]

Think about this: Any chirp generated via a DAQ card will by definition be a set of steps in frequency.  The signal is coming from a digital computer through a digital to analog computer.  Event if you have a continuous voltage controlled oscillator, controlling it via a D/A converter will still produce frequency steps.  It is not a question of steps but how fine the steps are and whether the DUT can tell the difference compared to a true analog sweep.

I am not asking you to criticize or to confront your advisor, but consider whether he is being realistic.

Sometimes the thing which works best in an analytic model is completely impractical for implementation.

Lynn

[Daniel Akeju]

Thank you,

I talked with my advisor. He advised me to try stuff out with MATLAB first before proceeding to LabView. I have attached my reference paper. If you look on page 11 (Section 1.4), the guys try to offer reasons for why a chirp would be preferred.

[johnsold]

I skimmed though the paper.  It will take a bit of time to digest.  I will try to get back to you later today or tomorrow.

Lynn

[Daniel Akeju]

Hello,

I wonder if you have read the paper. I have worked a little in MATLAB; now I want to return to LabVIEW. A library for generating chirp was posted in this forum and I downloaded it. 

[johnsold]

I have been busy with some other things and have not yet had a chance to read it.

Lynn