frequency response

[kima12]

I have attached a file for this problem. Can someone please see what i am doing wrong here. I am trying to plot the bode of  frequency response of a filter but nothing showing up. The examples i find dont make much sense. The graph output does not look like respose of a low pass filter. How can i get it to plot the respose and also search a db value corresponding to a certain frequecy i.e if i want to know the attenuation at 1500 hz.

Thanks

[camerond]

<warning: Rant ahead>

kima12, do you have any idea what functions you put in this VI?

<Interruption: Actually good advice here>   While the BD window is active, hit ctrl-H on your keyboard (for context help) and move your cursor over:

1. the function your two signals are fed into, and

2. the graph you followed this function with.

<End Interruption>

Do you see ANY reaason to believe that 2 would show any meaningful graph from the point at which you attached it to 1?

Do you have any idea what 1 does and what it uses to do it?

How can you make a Bode plot (log of amplitude response of a filter [etc.] with respect to signal frequency) when you are looking at a single frequency input and a single-frequency low-pass filter?

I'm sorry, I have no inkling of how to help you because I do not know if you know anything about LabVIEW, or about your problem, because the question as stated and the steps you took to solve it are so unrelated.

Cameron

[kima12]

I was under the assumption that the frequency respose function filter will take the time signal  Y then divide it by time signal X to compute the transfer function of the fiter. After that it would generate a range of frequencies with its corresponding amplitude from the behaviour of the transfer function it just analyzed (from time signal x and time sigal y either by doing fft or whatever). I do not have much experience in labview. How can i do this ? My final goal will be to physically use NI 9201 to capture both X AND Y signals, with Y coming from a device under test then perform frequency response analysis in labview. Thank you

[camerond]

OK, please first read the LabVIEW (notice the spelling, with capitalization) help files on the functions you put into this VI. After you get the Context Help, click on the "Detailed Help" link inside.

Then, to learn something about LabVIEW, do the online LabVIEW tutorials
LabVIEW Introduction Course - Three Hours
LabVIEW Introduction Course - Six Hours

Notice that there is no connection from your Frequency Response Function (Mag-Phase) VI to the signal generator or the filter function. It is not a filter, but an analyzer. That means it won't make them do anything, it just takes the data they give it (time data for one frequency, one low-pass filter) and tries to do something with it. That something is not "produce a graph".

Your project is certainly doable, but you've got to learn something about LabVIEW in order to do it. As several others (and I) have said often, we will help you if you get stuck, but you have to learn at least some of the basics first.

Cameron

[kima12]

I actually do have some basic experience in labview. I have done basic applications like analog and digital data input measurements using in compact daq, graphing and all that. I have not  done much in signal analysis or extensive programs. I am having a hard time dealing with dynamic data from express vi

[johnsold]

Everyone has a hard time dealing with the Dynamic Data Type. Some of us think of it as the "evil" data type. It obscures all information about the data type.  This is one reason that many experienced LV users avoid Express VIs (with a very few exceptions).

If you use the DAQmx VIs (not DAQ Assistant) you can select versions which produce waveform or array data types as outputs. Then you know what your data is and can focus on the analysis.

Lynn