how to make an exponential x-axis in a waveform-cluster?

[Benjie]

I have some data sampled with a signal analyzer. I have the data in 2
arrays, X and Y axis.
The X-axis is frequency, which is exponential, 10 Hz to 30kHz, and the
Y-axis is the actual measured signal level.

Then in Labview I want to check this waveform against limits. So I
have set up with the methods "limit specifications" my limits, and
plan to use "limit testing" for testing my waveform is within certain
limits.

However, now to the problem.
"limit testing" in the frequency domain requires a signal input of the
kind which is a cluser of f0, df and then the actual data, spectrum.

f0 is 10Hz ofcourse
spectrum is my Y-array. No problem so far

BUT, df? what is df if the X-axis is exponential?
Does anyone has any advice for me how to solve this problem or some
clever work around?

On the pic you can see my graphwindow. The data has about 880 points of data. But it should still end at 30 000 if it were to match my X-axis.

Regards, Ben

[GerdW]

Hi Benjie,

you just have to create more values for your limit array! One possibility could be to interpolate values between your frequency limit steps to "fill in" the needed values for all df points...

Or to put it in other words: there is no exponential x-axis, but you can create more points for your x-axis!

[Benjie]

hmm, yes. I guess that is the only alternative. But is there anyone who has made something similar in the past?

I am not sure how to interpolate this data to an array of 30 000 data points without loosing data in the beginning.

Since it is exponential, the first few dozen datapoints are at really low freq. All about 10.xx Hz and increasing.

Then in the end of the array, i have like one datapoint at 15kHz, one at 15,5kHz etc. Will be alot of gaps in my array and alot of lost data in the beginning I think.

It's strange though that labview doesn't allow for waveform, or atleast let me put an XY-array into these methods since i believe always you want an logarithmic freq-axis when having a frequency-response to handle.

Anyway, anyone who has a nice little VI that interpolates data like this? 🙂

[Benjie]

Ok, found myself an example on how to do this. Thanks