Window function and spectrum

[ustinoff]

If i multiply window function with input signal, and make DFT of product - carrier freq. will change?

In my Simulink model i have a generator of Blackman's window function and use Short-Time FFT unit. Input signal is sinus with carrier freq. 11 kHz. In spectrum i see signal component's on 11 MHz. I try to understand - is it right result?

 

[KlausST]

Hi,

I assume there is an input problem of "sample frequency" or "sample period". Check your inputs.

The window function does not change the frequency. But the amplitude peak is not that sharp anymore.. and unwanted frequency components (caused by frequency mismatch) are attenuated.

Klaus

 

[ustinoff]

Hi, Klaus.

I thougth about it: as you can see in left picture i made connetions with the same sample freq. in red colour. Also, i check what is sample freq. using - it's sample freq. what i need.

 

[KlausST]

Hi,

* What is your actual sample frequency?
* And how much samples do you use for calculating FFT?
* Do you use (or set up) any "undersampling" or "mixing" techniques?

Klaus

 

[ustinoff]

1) 44100 Hz;
2) use 1024 samples;
3) don't use.

 

[KlausST]

Hi,

with 44100Hz sampling frequency the max. FFT output frequency is 22050 Hz.

Check if you accidentally set it to 44100 kHz. Mind the "k" of kHz.

Or there is a problem with "." and "," if you set it to "44,100 kHz".


Klaus

 

[ustinoff]

I check - all sample times are correct.

 

[KlausST]

Hi,

if you need more help you should show us the screenshots, maybe you overlooked something. Four eyes see more than two eyes ;-)

But maybe there is an error in the software tool you are using.


Klaus

 

[ustinoff]

Screenshots

 

[KlausST]

Hi,

I´m not familiar with Simulink. Therefore I can´t see where the problem is.
I´m sure there are some experts who know the answer.

In the web i found this:

What Is Sample Time?

The sample time of a block is a parameter that indicates when, during simulation, the block produces outputs and if appropriate, updates its internal state. The internal state includes but is not limited to continuous and discrete states that are logged.

Note: Do not confuse the Simulink® usage of the term sample time with the engineering sense of the term. In engineering, sample time refers to the rate at which a discrete system samples its inputs. Simulink allows you to model single-rate and multirate discrete systems and hybrid continuous-discrete systems through the appropriate setting of block sample times that control the rate of block execution (calculations).

Klaus