# Spectrum from the RFFT

Started by February 7, 2006
Hello people! Ipm working with the RFFT function and Im testing my program with a sinusoidal signal with some number of samples and different FFT length. But I noticed that the frequency resulting of the DSP is different from the nominal frequency, from the signal generator (seeing on the osciloscope, I can affirm that the generator is not the problem). In some cases, the difference reaches 12%. Someone of you had reach some similar result?

Thank you

Guilherme
_____________________
Guilherme Martinhon
Engenheiro Eletricista
Mestrando em Engenharia Elrica
ea: Instrumentao Eletrica
Laboratio de Ultra-Som
UNESP - Campus de Ilha Solteira

```Guilherme, The relationship of your sampling rate and FFT size is one that
will determine the frequency resolution. I have used the TI RFFT at various
resolutions and sampling rates and have validated performance against spectrum
analyzer outputs. In general your frequency resolution will be
SAMPLING_RATE/FFT_SIZE .

If this is all not news to you, then also performing the sum of squares on the
rfft result vector, or just the sum to experiment.

From TI's 54x DSP Lib Document:
"
On output x will contain the FFT(x) = y in the following format:
y(0)Re y(nx/2)im ? DC and Nyquist
y(1)Re y(1)Im
y(2)Re y(2)Im
"

I think I did one of the following,
Ignoring the DC y(0) and y(1),
FFTout(0) = y(2)^2 + y(3)^2
FFTout(1) = y(4)^2 + y(5)^2
etc..

Also try,
FFTout(0) = y(1)^2 + y(2)^2
FFTout(1) = y(3)^2 + y(4)^2
etc..

Ten

Spectrum from the RFFT
Guilherme Martinhon - Feb 7 16:05:00 2006
Hello people! Ipm working with the RFFT function and Im testing my program
with a sinusoidal signal with some number of samples and different FFT length.
But I noticed that the frequency resulting of the DSP is different from the
nominal frequency, from the signal generator (seeing on the osciloscope, I can
affirm that the generator is not the problem). In some cases, the difference
reaches 12%. Someone of you had reach some similar result?

Thank you

Guilherme
_____________________
Guilherme Martinhon
Engenheiro Eletricista
Mestrando em Engenharia Elrica
ea: Instrumentao Eletrica
Laboratio de Ultra-Som
UNESP - Campus de Ilha Solteira
```