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

[Daniel Akeju]

Please find attached my work in progress. I made my current values default as you suggested and also tried to put an indicator for the sampling rate. It only worked when i made it a control. Please advise.

Thank you.

Daniel

[johnsold]

Daniel,

The sampling rate definitely needs to be a control.

In looking at the data, it appears that the input spectrum is flat up to 1000 and then drops off rapidly after that.  OK. You have a sampling rate of 16384 and stop frequency of 0.0625.  The highest frequency in the chirp pattern is the product fmax = 16384*0.0625 = 1024.

Anything above frequency 1024 is relatively meaningless because it was not excited.  The magnitude of the transfer function drops from near 1 to near 0.1. The magnitude of the load voltage also drops to about 0.1 times its low frequency value.  So I think that part is basically working. The Phase transfer function appears to be a linear change with frequency over that range (after allowing for the 2 pi wrap).

Lynn

[Daniel Akeju]

Lynn,

Thanks a lot. I have to do more work. The sampling frequency I had in mind was 32kHz. I wanted my Fmin and Fmax to be 0 and 2kHz respectively. Following your suggestion earlier, I divided 0 and 2k by 32k to get the 0 and 0.0625 that you see.

I wonder how to ensure this Sampling Frequency of 32kHz.

[johnsold]

Daniel,

It looks like it has to do with the number of samples.  The default values saved with your VI had a sample rate of 16384 and the number of samples = 8192.

When I changed the number of samples to 16384, I get the spectrum of the chirp flat out to 2048.  I do not have time to work out all the math, but the number of samples in the pattern has an effect on the range of frequencies.

Lynn