Implementation example using the ST10 DSP library

AN2182

14/18

7.2

Sampling frequency and FIR coefficients

Using the flowchart in the previous section, the sampling period is the time needed between

point A and point B.

To calculate the FIR’s coefficients, the sampling period should be known. We measure the time

between points A and B and the result is 17.5 kHz.

Note that to reconstitute the filtered signal, the input signal should not contain frequencies

greater than 8.75 kHz.

The FIR’s coefficients are then computed according to this sampling frequency

7.3

Results

To analyze the filter’s frequency and phase responses, a sinusoidal signal of a 2V amplitude is

used. The input voltage varies between 0 and 2 V.

7.3.1

Frequency response

To draw the filter’s frequency response, the amplitude of the output signal is measured.

The following figure gives the filter’s response.

Figure 10. Filter’s frequency response

h0

()

h1

()

h2

()

h3

()

h4

()

h5

()

h6

()

h7

()

h8

()

h9

()

h10

()

h11

()

h12

()

h13

()

h14

()

0.23

c 1.61

–

()

sin

0.23

c 1.37

–

()

sin

0.23

c 1.14

–

()

sin

0.23

c 0.914

–

()

sin

0.23

c 0.68

–

()

sin

0.23

c 0.46

–

()

sin

0.23

c 0.23

–

()

sin

0.23

c 0

()

sin

0.23

c 0.23

()

sin

0.23

c 0.46

()

sin

0.23

c 0.68

()

sin

0.23

c 0.914

()

sin

0.23

c 1.14

()

sin

0.23

c 1.37

()

sin

0.23

c 1.61

()

sin

0.0428

–

0.049

–

0.0273

–

0.0214

0.0902

0.1579

0.2105

0.23

0.2105

0.1579

0.0902

0.0214

0.0273

–

0.049

–

0.0428

–

1402

–

1605

–

895

–

701

2956

5174

6898

7537

6898

5174

2956

701

895

–

1605

–

1402

–

···

2

15

–

⋅

==

=

-30

-20

-10

0

10

1

10

100

1000

10000

Frequency