Why cosine wave for phase deviation in phase modulation?

Click For Summary

Discussion Overview

The discussion revolves around the use of cosine waves in the context of phase deviation in phase modulation, particularly when the modulating signal is a sine wave. Participants explore the relationship between the modulating signal and the resulting phase deviation, questioning the introduction of phase shifts and the representation of these concepts in literature.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that phase deviation in phase modulation is proportional to the instantaneous amplitude of the modulating voltage, which is a sine wave, yet the phase deviation is represented as a cosine wave in their reference material.
  • Another participant requests visual aids, such as scans of figures, to clarify the relationship between the sine wave modulating signal and the cosine wave phase deviation.
  • A participant expresses confusion regarding the introduction of a 90-degree phase shift, questioning whether the situation described might actually pertain to frequency modulation instead of phase modulation.
  • One participant suggests that analyzing modulation as vector rotation may explain the preference for cosine representation, as the real component corresponds to cosine while the imaginary component corresponds to sine.
  • Another participant speculates that if the book illustrates phase modulation using a voltage-controlled oscillator (VCO), the tuning voltage could be 90 degrees out of phase with the frequency, which might account for the observed discrepancy.

Areas of Agreement / Disagreement

Participants express differing views on the introduction of phase shifts and the representation of phase deviation, indicating that the discussion remains unresolved with multiple competing perspectives.

Contextual Notes

The discussion highlights potential ambiguities regarding the definitions and relationships between phase modulation and frequency modulation, as well as the mathematical representation of these concepts. Specific figures referenced by participants are not provided, which may limit clarity.

asitiaf
Messages
21
Reaction score
0
Phase modulation is a system in which the amplitude of the modulated carrier is kept constant, while its phase and rate of phase change are varied by the modulating signal.
By the definition of phase modulation, the amount by which the carrier phase is varied from its unmodulated value, called the phase deviation, is made proportional to the instantaneous amplitude of the modulating voltage.
The rate at which this phase variation changes is equal oy the modulating frequency.
But in the book, i see that modulating signal is a sine wave, but the curve for phase deviation is cosine wave. Why?
 
Engineering news on Phys.org
asitiaf said:
Phase modulation is a system in which the amplitude of the modulated carrier is kept constant, while its phase and rate of phase change are varied by the modulating signal.
By the definition of phase modulation, the amount by which the carrier phase is varied from its unmodulated value, called the phase deviation, is made proportional to the instantaneous amplitude of the modulating voltage.
The rate at which this phase variation changes is equal oy the modulating frequency.
But in the book, i see that modulating signal is a sine wave, but the curve for phase deviation is cosine wave. Why?

Could you upload scans of these figures? That would make it easier for us to explain what is going on...
 
I don't see how a 90 degree phase shift would be introduced. If the modulating voltage is zero, and it is really phase modulation, then the phase deviation would be zero unless there is some shift introduced that you are not mentioning. Maybe it is frequency modulation?
 
When modulation is analysed as a vector rotation, the real component is the cosine while the imaginary component is the sine of the modulation angle. This may be why it is more natural to employ the cosine than the sine in the analysis of the modulation.
 
If book is showing phase modulation using VCO, the tuning voltage (which is in phase with freq) will be 90 degree shifted from phase (integral of frequency).

This is speculation though since OP has not posted the figures.
 
Last edited:

Similar threads

Why does increasing the message bandwidth decrease the FM bandwidth?
  • · Replies 5 ·
Replies
5
Views
2K
AM, FM modulation, sidebands question
  • · Replies 15 ·
Replies
15
Views
5K
Demodulation of modulating square wave
  • · Replies 26 ·
Replies
26
Views
3K
Phase modulation and intermodulation anomaly *Plots inside*
  • · Replies 2 ·
Replies
2
Views
2K
Frequency Shift Key Modulation: how are the carrier frequencies chosen
  • · Replies 5 ·
Replies
5
Views
2K
Basic Radio Questions -- Oscillators, Mixers and Modulators
  • · Replies 26 ·
Replies
26
Views
4K
Help with waveform control: frequency, phase and amplitude
  • · Replies 46 ·
2
Replies
46
Views
6K
Am vs Fm variable capacitor receiver, what is the difference?
  • · Replies 2 ·
Replies
2
Views
3K
Frequency of Modulated Signal: Carrier vs Message
  • · Replies 3 ·
Replies
3
Views
4K
Understanding the Relationship Between Time and Frequency in Modulation?
  • · Replies 4 ·
Replies
4
Views
2K