Frequency spectrum of the signal

[malli_1729]

Hi all,

I have a waveform whose characterstics are like this.

ON time = 15ms
OFF time = 285ms.

This will repeat for the amount of time we need.

In that ON time there will be pulses with frequency 5kHz. Each pulse has 5ns Rise time and 50ns width.

This is basic waveform for EFT(Electronic fast transients)

Can anybody help in finding the freq spectrum of this signal.

it will be very helpful to me.

thanking you.

 

[tjr]

Hai,
First u draw the signal and define limits on x-axis and y axis.then make an an equation x(t) and find out its fourier transofrom F(w). Refer simon haykin "signals and systems".

 

[malli_1729]

Thank You,

I don't need the exact frequency spectrum. I just want to know the frequency components which will have the most of the energy.

Waiting for reply.

 

[jiripolivka]

As you do not like to calculate the Fourier series, from your data I can estimate:

there will be a main spectrum line at 5 kHz, with a "rake" around of 3 Hz between the lines (300 ms repetition period).
Rise and fall times do not create other lines but they will determine the spectrum limits; 50 ns means the spectrum can extend up to 20 MHz, 5 ns, to 20 MHz.

Try to find a good spectrum analyzer to confirm it.

 

[Shawn Hermite]

The ON period contains 75 5 kHz pulses. For purposes of finding the dominant frequency contents, you can use the Fourier series analysis (which is strictly applicable to periodic signal running forever). I would use the on-line calculator **broken link removed** for trapezoidal wave by entering your rise/fall time values. The resultant coefficients shout tell you about the relative strengths of the harmonics. Remember that your fundamental frequency is 5 kHz.