calculate amplitude Of a signal after application fft

Using Arduino Programming Questions
1

Hi, I'm using arduino mega2560 and sensor microwave Based on doppler effect and i applicated librairy fft To extract frequency and it's working perfectly, but i don't know how to calcluate the amplitude ?

2

Post your code (in code tags - use the </> icon).

Pete

3

Hi,Hi,thank you for replied
I'm using librairy fft posted by arduino
this is my code

#define LOG_OUT 1 
#define FFT_N 256 
#include <FFT.h> 

int bin = 0;
int frequency = 0;
int resolution = 9615 / 256;
int maxLogOut = 0;

void setup() {
   
  Serial.begin(9600);
     
  ADCSRA |= (1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0); //set adc prescaler=128
  ADMUX = 0x40; // use adc0
  ADCSRA |= (1<<ADATE);  //enable auto trigger
  ADCSRA |= (1<<ADEN);  //enable ADC
  ADCSRA |= (1<<ADSC);  //start ADC measurements

}

void loop()
 {
  
  cli();
    for (int i = 0 ; i < 512 ; i += 2) 
             { 
                 while(!(ADCSRA & 0x10)); 
                 ADCSRA |=(1<<ADSC); 
                 byte m = ADCL; 
                 byte j = ADCH;
                 int k = (j << 8) | m; 
                 k -= 0x0200; 
                 k <<= 6; 
                 fft_input[i] = k; 
                 fft_input[i+1] = 0; 
              }
    fft_window(); // window the data for better frequency response
    fft_reorder(); // reorder the data before doing the fft
    fft_run(); // process the data in the fft
    fft_mag_log(); // take the output of the fft
    sei();
    bin = 0;
    int k=0;
    int u =0;
    int w =0;
    int f =2;
    for( f =2;f<128;f++)
              {                               
                  if ( fft_log_out[f]>k)
                         {
                            k=fft_log_out[f];
                            w=f;
                           
                          }
              }   
              u = w*resolution;     
              Serial.print(" frequency is  : ");   
              Serial.println(u);       
              delay(250);

for amplitude i don't know how i can calculated

4

According to a comment in FFT.h, fft_mag_log returns 16log2((img^2 + real^2)^0.5) where (img^2 + real^2)^0.5 is the amplitude. So we have:
k = 16log2((img^2 + real^2)^0.5)
k = 16*log2(amplitude)
k/16 = log2(amplitude)
amplitude = 2^(k/16)

k is an integer (uint8) so in C this would be
amplitude = 2 << (k/16);

Pete

5

Or use the LIN_OUT keyword and get the magnitude directly via the fft_mag_lin array.

I suggest to read the documentation of any code you wish to use. It will save everyone some time.

6

i'm grateful
i add just this instruction " amplitude = 2<<(x/16);" in my programme

 fft_window(); // window the data for better frequency response
    fft_reorder(); // reorder the data before doing the fft
    fft_run(); // process the data in the fft
    fft_mag_log(); // take the output of the fft
    sei();
    bin = 0;
    uint8_t x=0;
    int u =0;
    int w =0;
    int f =2;
    for( f =2;f<128;f++)
              {                               
                        if ( fft_log_out[f]>x)
                         {
                            x=fft_log_out[f];
                            w=f; 
                          }
              }   
              u = w*resolution;  
              amplitude = 2<<(x/16);
              Serial.println(amplitude);

thank you all of you