Can two signals of different frequencies be in phase?

Discussion in 'General Electronics Chat' started by SDSI, Sep 16, 2016.

1. SDSI Thread Starter New Member

Sep 16, 2016
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Can two signals of different frequencies be in phase?

Feb 19, 2010
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yes

3. crutschow Expert

Mar 14, 2008
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If one frequency is an even harmonics of the other ( a factor of 2, 4, etc.) then yes, there zero crossings can be in phase.

Manish Chowdhary likes this.
4. dannyf Well-Known Member

Sep 13, 2015
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can two cars traveling at different speed be always next to each other?

Nov 29, 2011
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No.

6. AnalogKid AAC Fanatic!

Aug 1, 2013
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Also, odd harmonics have a fixed, repeating, complimentary phase pattern. That counts for me.

ak

7. wayneh Expert

Sep 9, 2010
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Yup, it's called music.

8. MrAl AAC Fanatic!

Jun 17, 2014
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Hi there,

The actual answer to this depends HIGHLY on the application, in fact, very highly, because in some applications a particular set of two frequencies would be considered 'in phase' when in other applications those same two frequencies would be consider VERY out of phase.

You've already seen several examples of applications where two frequencies can be said to be 'in phase', like music, harmonics, etc. Harmonics are often given a phase relationship to the fundamental, and sometimes it happens to be zero degrees while other times it may be different but constant. This means a frequency 2 times the first frequency could be called in phase and be perfectly descriptive. i am not so sure that "in phase" is the best way to describe this though, i would say the "same phase" is a better way to put it, or just describe them as both being at 0 degrees or whatever. Saying "in phase" might imply that they are of the same frequency.

An application that does not follow this rule though is for example a phase locked loop, In a phase locked loop, if the reference frequency is half of the target feedback frequency, even if they have some zero crossings in sync, the second frequency would be called very out of phase. It's only when it is brought up (or down) to the correct frequency can it be really in phase and then the loop locks. If the reference frequency is half or two times the feedback frequency the loop never locks properly.

Of course another example where we need the same frequency is in a three phase power system, where if phase A is called zero degrees and phase B has every other zero crossing synced to phase A but is twice the frequency, you better have some fuses on hand

So to be accurate, you have to specify the application as well as the type of waves themselves.

Last edited: Sep 16, 2016
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9. SLK001 Senior Member

Nov 29, 2011
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That's not the definition of "in phase".

Two signals are "in phase" when:

ƒ(α) - ƒ(β) = k

where "k" is a constant - and also the phase difference.

10. wayneh Expert

Sep 9, 2010
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Fair enough. I can concede that "in tune" is not synonymous with "in phase".

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11. BR-549 AAC Fanatic!

Sep 22, 2013
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As MrAL said, it depends on how you qualify phase. Can I and Q signals be considered in phase?

One signal always starts when another is 15 degrees ahead of it. Are they in phase?

Two signals....same frequency and both start at zero. One has amplitude of 5 and the other 10, are they in phase?

Is the second harmonic in phase, or in double phase?

And all cars travel at the same speed. They just have different wiper speeds.

12. AnalogKid AAC Fanatic!

Aug 1, 2013
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In NTSC video, I and Q are the same frequency, 90 degrees apart. Q stands for quadrature.

ak

13. nsaspook AAC Fanatic!

Aug 27, 2009
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Two signals would be coherent if they have a constant relative phase but coherence means more than just phase for electromagnetic waves.

Sep 16, 2016
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15. SDSI Thread Starter New Member

Sep 16, 2016
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What about when the even harmonic passes the falling edge of the original wave?

Sep 16, 2016
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17. SDSI Thread Starter New Member

Sep 16, 2016
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The application is a PLL

18. MrAl AAC Fanatic!

Jun 17, 2014
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Hi,

Ok then the two waves have to be both the same frequency.

In a more specific case where we have a half wave rectified sine wave perfectly rectified with no filtering, we have the fundamental at 0 degrees of course, and we have the 2nd harmonic at -90 degrees. The 4th harmonic also has a -90 degree phase shift, as do others. That means we can say that the second and fourth harmonics have zero degree phase shift with respect to each other, but to say that they are "in phase" seems inappropriate. Not only do they never have the same zero crossing, they only have coincident phase at every other peak of the 2nd harmonic. Also interesting though is that we might also call them at zero degrees because they are multiplied by a cosine wave in the reconstruction while the fundamental is multiplied by a sine wave.
The reason for stating the phase shift is so we know the relation to the fundamental so if we wanted to reconstruct the time signal from the amplitude and phase information we can do it just knowing that the amplitude is a certain amount and the "phase" or "phase shift" is a certain amount. Knowing these quantities for each harmonic allows us to reconstruct the original signal. So we might see a listing like this:
1st, amplitude=1, phase=0
2nd, amplitude=2/3pi, phase=-90
4th, amplitude=2/15pi, phase=-90

It looks like the 2nd and 4th are in phase, but calling them in phase seems inappropriate.

PLL correct operation depends on both frequencies being the same. If they are not, it can not properly detect the phase in many typical phase locked loop circuits. This could cause the frequency to jump up and down repeatedly and wreck havoc on the rest of the system which is expecting a lock at some point.

19. SLK001 Senior Member

Nov 29, 2011
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To be "in phase", the angular velocity of the two signals must be the same. The only way to do that is to have both frequencies the same. Zero crossings have nothing to do with determining whether or not two signals are "in phase".

Yes... I and Q are always in phase.

If they are the same frequency, then yes.

Yes

Not in phase (what is "double phase"?).

??? Very much non sequitur.

Feb 19, 2010
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