What is the significance of negative frequency?

[magnetra]

A course in signal analysis and communication would have introuduced the concept of a "negative frequency" to anyone. A negative frequency component pops up everywhere whenever we deal with modulations AM(DSB,SSB,VSB,ISB).
A spectral analysis of a modulated signal has harmonics in both positive and negative frequency scale. Now I wonder what is the significance of this negative frequency component in practical sense. Does it have any implication in design of transmitters. Do negative frequency components really exist?????Is it realizable??
Please keep the mathematical part to minimum as possible....

 

[echo47]

Negative Frequency

Sure! Spin a wheel at 600 rpm: that's +10 hertz. Now spin it the other way: that's -10 hertz.

If you do DSP with a quadrature signal (I and Q, or complex number, or magnitude and phase, or the X and Y position of the wheel's valve stem), then positive and negative frequencies cause the vector to rotate in opposite directions. You can do all sorts of fun things with that information.

If you have only a simple signal instead of a quadrature signal, then you can't have as much fun. It's like knowing only the Y position of the wheel's valve stem.

 

[IanP]

Re: Negative Frequency

Scientifically speaking there is no such a thing as -Hz.
You can add any .. explenations ..ideas .. whatever you want .. frequency is not a negative value .. so the (-) is only for some convinence in some examples ..
Regards,
IanP

 

[armsgroup]

Negative Frequency

the negative sine has no physicl meaning and it's a result from the mapping equation made by fourier that's all
i discuss this matter with large no of doctors in communication

 

[ganlubbq]

Re: Negative Frequency

i think the Negative Frequency maybe used for doppler frequency for the opposition speed!

 

[maxwellequ]

Re: Negative Frequency

Here are two things that most people confuse: the physical reality and the (mathematical) model used to represent it.

From a physical point of view there are no negative frequencies: if you want to measure a frequency you use an oscilloscope to measure the period (T) and then compute f=1/T. There are no "negative periods", i.e. you always measure a positive T (and, therefore, you get a positive f).

Another thing is the mathematical representation: in DSP it is usual to consider "negative frequencies", which is an convenient way to represent phase relations. For example while
sin(2.pi.10.t)
is a signal with a positive frequency of 10 Hz,
sin(-2.pi.10.t)
is a signal with a NEGATIVE frequency of -10 Hz.

Now note that sin(-2.pi.10.t)=sin(2.pi.10.t+Pi)...

Another thing, most times misunderstood, is that fact that the sine function can be expressed as the sum of two complex exponentials, one with a "positive" frequency and another with a "negative" frequency. This is, again, a mathematical abstraction - it only means that the (real) sine function results from the sum of two phasors that, as the time passes, rotate in opposite directions.

For an intuitive discussion about this you may want to check:
https://www.amazon.com/exec/obidos/...t_1/103-7997422-5221411?v=glance&s=books&st=*


Rgds

 

[ankush_jn2000]

Re: Negative Frequency

Hi,

The concept of negative frequency is used for mathematical calculations only. As such there is no negative freq.

Bye...

 

[magnetra]

Re: Negative Frequency

So, from now on when i see a spectrum with a plot in the negative freq axis, i'm supposed to treat it only as a result of the mathematics involved. Thats ok.
Now I was wondering, considering a DSB-SC signal, its spectrum plot has plot of the baseband signal shifted to carrier frequency, now if the carrier frequency were less than the bandwidth of the base band signal, then there would be overlapping of the positive freq and negative freq components. How do we explain that??

 

[power-twq]

Re: Negative Frequency

I think negative frequency is

a signal that phase is 180.

best regards

A course in signal analysis and communication would have introuduced the concept of a "negative frequency" to anyone. A negative frequency component pops up everywhere whenever we deal with modulations AM(DSB,SSB,VSB,ISB).
A spectral analysis of a modulated signal has harmonics in both positive and negative frequency scale. Now I wonder what is the significance of this negative frequency component in practical sense. Does it have any implication in design of transmitters. Do negative frequency components really exist?????Is it realizable??
Please keep the mathematical part to minimum as possible....

 

[xvibe]

Negative Frequency

From what maxwellequ says: sin(x+pi)=-sin(x) and that's correct.
Consider this the following freq. spectrum:

-----f1--------0--------f2-----> freq. (Hz) , where f2=-f1 (Hz)
If sin(x+pi)=-sin(x) then in a real system this "two" waves will annulated because they are in opposite phases? Like if you have a signal band from 1Hz to f2 and another band from -1Hz to f1 with equal amplitues of the 1 to f2 band signal then the resulting band will be 0 because they are in opposite phases?

 

[rednewguy]

Re: Negative Frequency

negative frequency is similar to negative current.
we represent holes flow as positive current conventionally.
similarly anti-clockwise rotating phasor is represented by positive frequency and clockwise as negative frequency.
in real time both do occur, mathematically and physically

 

[joinfaisal]

Negative Frequency

If you consider a sinosoidal signal with only pos frequencies then this will not be a real signal like exp(jwt) will be a complex signal composed of cosine and sine signal.Now if you add negative freq like...exp(jwt)+exp(-jwt)/2 then now this is a real signal and in practive all the signal to be transmitted should be real signals.I think this is the only significance of neg freq.Its just a mathematical modeling of a signal that for any real valued signal say ACos(wt) you will always have pos and neg freq by defination in mathametics so while drwing spectrum you are bound to draw its component in neg freq as well. But it has no practical existence .Actual signal will always have pos freq.