# Forum: DSP Regarding FFT

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Sir
If I want to extract fundamental component(50 Hz) from a sine
waveform(multiple cycles i.e. continuous) then I want to know whether I
need(minimum)
1. One complete  cycle of sine wave
2. Half cycle of sine wave
3. One quarter cycle of sine wave
or any other answer.
Secondly, if it is possible to extract fundamental component from half
cycle or lesser one complete cycle, then :
Is fundamental component is same for(Assume one complete sine wave have
256 samples)
1. 1 to 128
2. 129 to 256 and so on.
Kindly reply as early as possible.

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varun maheshwari wrote:
> I want to know whether I
> need(minimum)
> 1. One complete  cycle of sine wave
> 2. Half cycle of sine wave
> 3. One quarter cycle of sine wave
> or any other answer.

If you know the frequency, dc-offset and magnitude of the sine wave
(only phase is unknown) and there is no noise or distortions on the
signal, then you need only one point (e.g. one zero crossing) and the
information, if the slope is positive or negative to reconstruct the
full sine wave.

If only frequency and dc-offset is known, then you need one point and
the exact value of the slope in this point.

If only the frequency is known, then you need two points and the exact
value in one of these two points

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Johannes wrote:
> varun maheshwari wrote:
>> I want to know whether I
>> need(minimum)
>> 1. One complete  cycle of sine wave
>> 2. Half cycle of sine wave
>> 3. One quarter cycle of sine wave
>> or any other answer.
>
> If you know the frequency, dc-offset and magnitude of the sine wave
> (only phase is unknown) and there is no noise or distortions on the
> signal, then you need only one point (e.g. one zero crossing) and the
> information, if the slope is positive or negative to reconstruct the
> full sine wave.
>
> If only frequency and dc-offset is known, then you need one point and
> the exact value of the slope in this point.
>
> If only the frequency is known, then you need two points and the exact
> value in one of these two points

Thanks sir for writing to me
Sir I am giving 50 Hz sine wave, peak to peak value and dc offset.I am
working on FPGA with ADC and giving continuous sine wave to ADC ,
collecting samples from sine wave. After this we r giving it to FFT and
then IFFT to extract fundamental component, so here we want
Can I need:
1. One complete  cycle of sine wave
2. Half cycle of sine wave
3. One quarter cycle of sine wave
or any other answer.
for extracting fundamental frequency component  can we take arbitrarily
samples or collected from zero crossing and minimum how much
samples(full cycle,half cycle or anything else)

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Hi,

'any other answer' is correct: You do not need a full wave for sine
reconstruction: find amplitude, frequency and phase. You do not need a
FFT, either. Post some sampled data (and sample frequency) and I will do
the example calculation.

Cheers
Detlef

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