Negative frequencies in spectrum analysis

In summary, negative frequencies in spectrum analysis are just to the left of the carrier on the spectral plot. They contribute in energy to the signal, but you can just take the positive part and multiply it by 2 as the negative part and positive part are the same.
  • #1
sukh_ece
10
0
Negative frequencies in spectrum analysis...

Hello everyone...
When we do the frequency analysis of a signal using Fourier transform, we get spectrum at negative frequencies also. What are they and what is the physical significance of such frequencies? Its hard for me to imagine a negative frequency. Plz share ur knowldge on this...
 
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  • #2
If your signal is real, you can ignore them - they are the same as the positive frequencies.
If you have a complex signal, they are similar to the positive frequencies, but correspond to a different "rotation" of the signal in the complex plane.
 
  • #3
Negative frequency, like negative distance, seems non-physical. However, we can locate physical points by position relative to some arbitrary zero reference, like on the X-axis. Negative simply means "to the left of the zero reference".

It is the same with frequency. When we discuss modulation of an RF carrier, is it very convenient to consider frequencies relative to the carrier. Negative frequencies are simply to the left of the carrier on the spectral plot.

Consider measuring the angular frequency of a clockwise rotating wheel that is illuminated by a strobe light that flashes at 1Hz. My observed (measured) frequency will be relative to 1Hz. If the wheel's angular frequency is 1Hz, I will observe 0Hz, i.e. it will appear stationary. If it is rotating 1.5Hz I will measure +0.5Hz. If it is rotating at 0.5Hz I will observe -0.5Hz, negative because it appears to be rotating counterclockwise.

The strobelight here is analogous to the 2.45GHz carrier on a WiFi signal. We observe the modulation behavior relative to this carrier.
 
  • #4
Thanks @mfb and @the_emi_guy...

If we can ignore the negative frequencies for a signal...then there is one doubt...
The bandwidth of a signal ranging from -fm to fm is 2fm...then why we consider negative frequencies in finding the bandwidth...
 
  • #5
That is just a definition of "bandwidth" - you could use half the bandwidth as "bandwidth" as well, if you add this factor of 2 everywhere else.
 
  • #6
@mfb...thanks...
I don't understand ur answer...where we can add the factor 2?...different frequencies contribute to the signal formation...a signals energy is equal to integration of the square of spectrum over entire frequency range (Parsevaals theorem)...that means negative frequencies exist and contribute in the energy of the signal ...then how can we ignore their contribution in the bandwidth...
 
  • #7
where we can add the factor 2?
In the definition of "bandwidth". Don't do that, but it would be possible.
then how can we ignore their contribution in the bandwidth...
Maybe "ignore" is not the best word. If they are exactly the same as the positive frequencies, they don't add new information about the wave. You can just take the positive part, and multiply it by 2, as the negative part and positive part are the same.
 
  • #8
Thanks...
 

What are negative frequencies in spectrum analysis?

Negative frequencies in spectrum analysis refer to the representation of frequencies that are below the zero frequency point in a spectrum. This is because in spectrum analysis, frequencies are typically represented on a logarithmic scale, with the zero frequency point being at the center. Negative frequencies can also occur due to mathematical transformations used in spectral analysis.

Why do negative frequencies appear in spectrum analysis?

Negative frequencies can appear in spectrum analysis due to the use of mathematical transformations such as the Discrete Fourier Transform (DFT) or the Fast Fourier Transform (FFT). These transformations use complex numbers to represent the frequency domain, resulting in negative frequencies being represented.

Do negative frequencies have any physical meaning?

No, negative frequencies do not have any physical meaning. They are simply a mathematical representation of the frequency domain in spectrum analysis. In the physical world, frequencies cannot be negative, as they represent the number of cycles or oscillations per unit of time.

How do negative frequencies affect the interpretation of a spectrum?

Negative frequencies do not affect the interpretation of a spectrum, as they are simply a mathematical representation. In most cases, they can be ignored and do not provide any additional information about the signal being analyzed.

Can negative frequencies be eliminated in spectrum analysis?

Yes, negative frequencies can be eliminated in spectrum analysis by using a technique called spectral folding. This involves taking the positive frequencies and folding them over to the negative side, effectively removing the negative frequencies from the spectrum. However, this technique should only be used when necessary, as it can also introduce errors into the analysis.

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