As I understand this:
A phase shift can be due to a delay during transmission, in reference to a particular frequency.
If we apply the same signal to two different systems with different delays, the output signals look the same, and when both are compared with each other or the input signal, a phase shift may be evident unless the delay is an exact multiple of one cycle, which is 2pi radians, 360 degrees, or the period between two identical points on a cycle.
Lets say the signal is 1MHz, and:
the delay through the first system is equivalent to 90 degrees (250ns).
the delay through the second system is equivalent to 360 + 90 degrees (1250ns).
Comparing the output signals they look the same phase (both 90 degrees shift) because the phase reference is a zero crossing or a peak in a particular direction. Comparing either output with the input would yield a similar result, because the reference is still some recognizable point on any single cycle.
Now add some phase modulation to the input signal that just reverses the polarity by 180 degrees when the input signal is at its zero. See the link for a diagram. This modulation might be every second cycle for the explanation. This reversal will be seen at the output, but in its true position related to time, not related to just any cycle. In this case it would appear exactly 1 cycle delayed on one system, so 2pi rotation, or 360 degrees, when the two outputs are compared. Talking of phase rotation I expect comes from the analogy with multiphase rotary generators.
Diagram of phase reversal modulation for a digital signal:
http://www.answers.com/topic/phase-shift...
This arises in digital modulation schemes like QAM, as the timing of a symbol is taken from some reference time, the beginning of a symbol for example, not a reference phase. This might be important in a system using 90 degree phase shifts and combining them to modulate or demodulate carriers. These systems often use the two phases called I and Q, separated by 90 degrees. They need to be in the same phase rotation (cycle) when recombined, considering they can be modulated with different signals. Otherwise different data would be combined at some instant, considering what the other end is using. Phase rotation direction might also be an issue. Consider when a rotation is + 90 or -90 degrees.
A bit more detail...
The I and Q channels are seen as functions of sine and cos and can carry independent modulation. Each can be demodulated independently with the appropriate phase, no need to consider phase rotation for that. However the two (or many) streams are combined/merged to form one data stream, so the parts must be synchronized so the bits fall at the right time for merging.
I think it is very common to speak of "in phase" without considering the overall delay, and in many systems the overall delay may not be an issue, because the signal passes through the same equipment, and suffers the same delays. With enough bandwidth and multi-path reception (variable delay and echos), as well as group delay effects though...
The phase locked loop? It can lock a locally generated carrier to an incoming signal for each of I and Q channels, so the carrier is synchronized and can provide the reference phases for demodulation of the I and Q channels. If the modulation is fast enough, something else is needed to bring the 2 channels into correct timing for the next stage of decoding, often not shown in QAM explanations, which merges the streams. This involves a symbol synchronizing sequence that is recognized by the decoder, and not contained in the message data. In block diagrams a time shifter (adjustable delay) is sometimes shown. There is also some sort of level control for QAM, which has amplitude modulation too. These functions are likely to be in a DSP (computer).
If you are a student, you may have to deal with a prof who is picky about such semantics. Give him what he wants and know that as soon as you are done with that course you can forget such nonsense. 2pi is one cycle.
Any reference to n pi phase difference is about radians. Engineers and mathematicians usually use phase in radians and frequency in angular velocity (radians per second), not degrees and Hertz because nature (math) works that way. ie no need to always multiply by 2pi or 360/2pi. (2pi radians = 360 degrees)
PLLs are uses anywhere you need to stay synchronized to an external signal, not just for communications. There are some variations like DLL (delay locked loops), but the idea is that the locally generated signal is kept in sync with a feedback that comes from a phase (difference) detector.
For example say you need two frequencies. If you had two oscillators they would drift in and out of phase unless you use a PLL to synchronize one to the other.
Any communications needs to stay synchronized because you need a way to tell where the words/symbols/bits start and end.
A PLL is often used to multiply a frequency reference by comparing some fraction of the local (voltage controlled) oscillator to the reference. You computer has a clock chip that does this because the memory, the CPU, the pci bus, etc, all need different frequencies but they have to stay synchronized. When they are not synchronized you get something called metastability, which means digital mistakes that happen when a signal is sampled at the wrong time, ie in the middle of a transition.
http://en.wikipedia.org/wiki/Frame_synch...
http://en.wikipedia.org/wiki/Metastabili...
You don't have to use PLLs in digital communications. If you do use a PLL you need a phase detector. If one signal lags another by 2*pi, the signals are actually in phase.
2 pi is equal to 1 rotation of a Phaser and that will place you at the start point
is it same as 2*pi phase shift? i am not asking with respect to electrical meters but its related to why we need phase locked loops for digital communication
