# negative frequency

Discussion in 'General Electronics Chat' started by Revathy Nair, Oct 16, 2013.

1. ### Revathy Nair Thread Starter New Member

Sep 18, 2013
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Do we have any negative frequency?

2. ### Papabravo Expert

Feb 24, 2006
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Only as a mathematical abstraction.

3. ### MrChips Moderator

Oct 2, 2009
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Yes, it exists mathematically and in real life.
When you take the Fourier Transform you will observe negative frequencies.

4. ### tshuck Well-Known Member

Oct 18, 2012
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Can you have less than 0 cycles per second?

No.

5. ### Revathy Nair Thread Starter New Member

Sep 18, 2013
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so is it just a theortical concept only?

6. ### MrChips Moderator

Oct 2, 2009
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Do imaginary numbers exist?

7. ### Revathy Nair Thread Starter New Member

Sep 18, 2013
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yes,but whenever i ask this question most of the replies are:''its just a theoritical concept"

8. ### LvW Active Member

Jun 13, 2013
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Yes - that`s true.
In this context, it is important to anser the question:
What is the meaning of the term "frequency"?

My answer: "Frequencies" do not "exist" as a physical phenomenon.
A "frequency" is just a measure to describe a sinusoidal signal, that means: It is a property of such a wave (similar to the amplitude).

Therefore the correct question is:
Do sinusoidal signals exist that have a negative frequency - with other words: A negative number of periods pro second?
Of course, the answer must be: No.

On the other hand, some people use negative values for frequencies in their calculations. Why?
My answer: From the mathematical point of view it is comfortable to use Euler`s expression based on exponential functions for describing a sinusoidal signal. And this expression contains also negative values for the variable called frequency. That`s all.

Following this approach, you have two alternatives to draw the Fourier spectrum of a periodic signal: Using
* positive and negative frequencies (two-sided symmetrical spectrum) with amplitudes A/2, or
* only positive frequencies (one-sided spectrum) with amplitude A.

Both are fully equivalent.

bug13 and Revathy Nair like this.
9. ### MrChips Moderator

Oct 2, 2009
17,087
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It's like asking: "Can you have negative time?"

Sure you can!

If frequency = n/time

and time can be negative, then frequency can be negative.

10. ### BobTPH Senior Member

Jun 5, 2013
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I believe the flux capacitor operates at a negative frequency.

Bob

11. ### LvW Active Member

Jun 13, 2013
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As a property of a sinusoidal signal or any other periodical mechanical phenomenon (mechanical pedulum, for instance) the thing called "frequency" is NOT a theoretical definition but a parameter that can be measured in reality.
And as such - it is based on positive times only.
(Do negative times really exist? Where ? And formerly spoken - what happens if n also is negative for negative times?).

Have "negative frequencies" ever been measured?

No - the answer is, of course, that a sinusoidal signal that is measured and/or visualized has two describing parameters: A finite amplitude and a finite and positive number of periods per second.

12. ### THE_RB AAC Fanatic!

Feb 11, 2008
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Well said.

I often wonder if anything negative exists in the real world, that is properly negative, ie; goes less than zero.

Most of the examples people will cite are just positive things occurring in a different direction (like - voltage or - current etc).

13. ### WBahn Moderator

Mar 31, 2012
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How can you tell the difference between

f(t) = Va * sin((+100r/s)t)

and

f(t) = Vb * sin((-100r/s)t)

If Va = -Vb ?

Give me a value of time and I'll tell you the value of f(t) at that time.

If two equal sized gears are meshed and one is turning at 4rpm then how is that any more or less real than the other gear that is turning at -4rpm?

14. ### crutschow Expert

Mar 14, 2008
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I think you are really asking if anything can have a less than zero magnitude which, I suppose, is impossible. Obviously many things are defined as negative and positive. For example, how about an electron charge and a positron (or proton) charge. What other words could you use besides positive and negative for the charge of those?

15. ### LvW Active Member

Jun 13, 2013
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I like to restrict myself on time - and in this context, something comes into my mind which may serve as a counter example: Group delay.

As you know, group delay can be negative - and this "delay" can be measured.
However, this parameter is based on a - more or less - formal definition (slope of the phase function) and I think it is really not an example for a negative time in the technical world. What do you think?

16. ### WBahn Moderator

Mar 31, 2012
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But what does a positive time mean? It is merely a measure of some quantity relative to some reference. We assign it a positive value if the moment in question occurs after the reference. How is that any more or less "real" then assigning it a negative time if the moment in question occurs before the reference.

17. ### LvW Active Member

Jun 13, 2013
674
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Hmmm - good question.
Here is my opinion:
At first - what is the "reference point"? I think it is the start time for the measurements (t=0). Thus, all measurements are performed for positive times.
However, if the "moment in question" occurs before the reference point (t=0) we have absolutely no information (no measurements). Thus, it makes no sense to assign negative time values to this "moment in question".
Of course, we can say at time tx=-30 minutes I had breakfast and 30 minutes later I started measurements. But coming back to the original problem (frequeny) it is necessary to combine the time with measurement results (occurences per second) - and this, I think, is possible for positive times only.

Oct 2, 2009
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19. ### MrChips Moderator

Oct 2, 2009
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I think the point about "reference point" is an important one.

On my oscilloscope I can dial a negative time and be able to examine events that happen prior to the reference point.

In Ham radio we tune in to SSB (single side band) RF transmissions. We can select the positive frequencies, upper side band, or negative frequencies, lower side band.

Last edited: Oct 18, 2013
20. ### WBahn Moderator

Mar 31, 2012
23,093
6,946
Not necessarily. Have you ever wondered by the NASA guy says, "T minus 1 hour"? It's because the reference for the measurement, T, is at a point in the future, namely main engine ignition.

A reference time in the future is often used when planning or preparing for some event.

Even in a physical measurement, we may take measurements prior to the start of the event we are measuring -- for instance, at t=0s we discharge a powerful lamp and record the response of various detectors. We would often be taking measurements before this to establish a baseline and those measurements are occuring at negative time relative to the reference.

Why?