Bode magnitude graph cursor not following plot

[LetThePeopleSing]

Hi,

I was wondering if anyone would be able to shed some light on a problem that I am having regarding an XY graph cursor.

I have developed a simple VI to determine the characteristics of a low pass filter using a sine wave ramped in frequency and subsequently monitoring the gain of the filter to create a Bode plot.

Through analysis of the Bode plot I would like to determine the cut-off frequency of the low pass filter under test. I have introduced a cursor into the graph and using a property node I hoped to drive the cursor to the -3dB point in the Y axis and output the cursors corresponding X position thereby determining the cut-off frequency of the filter.

However the cursor never seems to follow the plot beyond the first step of the loop. It feels as if there is a break in the plot between the 1st and 2nd iteration of the while loop used to ramp the signal generator.

I've tried to work out why the cursor won't follow the plot to -3dB but so far I am stumped. 

Any help or advice that you could provide would be greatly appreciated ! I have provided the attached VI for clarity.

[GiulianoFranchetto]

Hi,

you have a piece of answer there : forum ni

i'll take a look at your VI

[GiulianoFranchetto · Accepted solution]

I have corrected your VI.

First, you needed to put the property node AFTER the while loop, because you need the graph to be done first and then look at the frequency cut off.

Second, in LV, the property nodes are read from top to bottom. So you need to place the cursor at Y = -3 first, and then read the X position.

I attached the corrected vi

[LetThePeopleSing]

Hi Giuliano,

Yes, that was me that posted that original question also. I have two logins as I am not allowed to access my private e-mail account in work.

It is a development of the VI that I had posted on the original query. I implemented your suggestion and it works fine when I am using a non dynamic data source however when I try and use express VI blocks and their inherent dynamic data it doesn't seem to work. That is, the cursor won't track the plot. I can't see why that would be ?

[GiulianoFranchetto]

The answer is 2 posts above this one 🙂

Hope it will help

[GiulianoFranchetto]

Is the problem solved?

[LetThePeopleSing]

Hi Giuliano,

I have not been able to check to see if your solution works as I believe that you are using version 12. Would it be possible for you to save the VI as version 10 and repost ?

Many thanks for your help !

[GiulianoFranchetto]

here it is