# find frequency from a complex number

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#### scdoro

##### Member level 5 hi all,

suppose i have a complex number of a signal as = 2+3i.

i need to express it in terms of Asin(ωt)

Qn1) how do i calculate the frequency,f of this signal?

Qn2) how di i calculate the amplitude A of this signal?

thanks

regards
scdoro

#### purnapragna what do you mean by complex number of a signal? Does that mean that it is the amplitude of the signal at some instant 't' ?

thnx

purna!

#### dunn

##### Full Member level 3 Have a look at:

h**p://www.ping.be/~ping1339/complget.htm

#### svensl

##### Full Member level 1 The magnitude is sqrt(Re^2+Im^2). The angle is phi=atan(Im/Re). Then you can write the complex number as A*exp(j*phi)=A*(cos(phi)+jsin(phi)).

#### tiger_shark

##### Member level 1 Hi,

As mentioned in the previous reply, you have a phasor and your sine wave will be y(t) = A Cos( w x t +theta) where

A = sqrt[real(phasor)^2 + imag(phasor)^2]
theta = phase(signal) = atan(imag/real)

However, this phasor representation is used when the circuit is working at a known frequency W so you cannot extract your frequency (W) from the phasor.

Regards - TS

#### scdoro

##### Member level 5 Hi again,

see attachment for my actual question.

I have found the result of the FFT of x is found to be
[0,-0.83536j,0,0.1464j,0,-0.1464j,0,0.8536j].

From here we can see that it is made up of 2 sine wave components.
given by x(t) = Asin(2Πf1t) + Bsin(2Πf2t).

Q1)And now i need to determine A,B,f1 and f2. How do i make use of the result i got which is in complex numbers to find those values A,B,f1 and f2?

Q2) and explain how the negative freq is represented in this FFT?

thanks

regards
scdoro

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