what means phase-coherent ?

[fishress]

In the page of "what is caller-ID", the parameter of transmistion scheme is the so called "phase-coherent Frequency shift keyed". What is the meaning of phase-coherent in this case, or where I could get a reference of the "phase-coherent"? Thank you very much.

[Will.Spencer]

Phase coherent just means that both signals are in the same phase, i.e. they are in a fixed phase relationship.

FSK uses two signals, one to represent a binary 0 and the second to represent a binary 1.

In Caller-ID's implementation of FSK, binary 0 is 2200Hz and binary 1 is 1200Hz. These two signals are in a fixed phase relationship, i.e. they are phase coherent.

[fishress]

Thanks for your reply.

But only when f0 is twice (or other integer times) of f1, they could have the same phase. Now f0=2200Hz, whereas f1=1200Hz. How could they have the same phase if the signal is continuouse.

Or the duration of the signal to represent "1" or "0" is longer than 1/1200s. (I supposed that 1 cycle of 1200Hz signal represents "1" and more than 1 but less than 2 cycles of 2200Hz signal represents "0". ) If it is in this case, what is the duration to represent one digital signal "1" or "0"?

Thank you very much.

[Will.Spencer]

Originally Posted by fishress

But only when f0 is twice (or other integer times) of f1, they could have the same phase.

The most common forms of phase coherent FSK do utilize frequencies which are exact multiples, but this is not the only way to implement phase coherent FSK.

Exactly how this is done is a bit over my head. ops:

This PDF document might shed more light on the subject: http://rfwireless.rell.com/pdfs/TN_WJfsk.pdf

Originally Posted by fishress

How could they have the same phase if the signal is continuous.

How do you mean "continuous"?

The signals are not continuous, as the mark signal is only sent for 130mS and then again when the device needs to send a 1. The space signal is only sent when the device needs to send a 0.

In the other hand, the signals are continuous, in that the device is constantly sending either mark or space until the transmission ends.

[fishress]

Thank you for your document. I shall read it.

How do you mean "continuous"?

I mean the signal should be "phase continuous" (glitch free). For example, to send a sequence of binary signals, say "1010...", if we assume, both mark and space have the same starting phase 0 degree, after first mark and space "10", the ending phase is no longer 0 degree. The next mark still starts from phase 0 degree. Then there is a voltage jumping from a non-zero value to 0v to keep the "phase-coherent". Such kind of voltage jumping causes the signal non continuous. Is this the case in the caller ID signal?