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Message started by aaron_do on Feb 5^th, 2007, 12:01am

Title: unwanted? paths
Post by aaron_do on Feb 5^th, 2007, 12:01am

Hi all,

lets say you have an inverting amplifier at RF. Suppose there's an unwanted signal path which results in only 160 degrees phase shift as opposed to 180 degrees. At the output, these two signals combine.

How would this affect the throughput of a system. My initial thoughts are that it would not have much effect on an FM modulation system, but would affect a PM modulation system. Is this correct? I suppose it would also depend on the type of demodulation. I'm more interested in the theoretical/ideal world type answer.

thanks,
Aaron

Title: Re: unwanted? paths
Post by mg777 on Feb 8^th, 2007, 11:11pm

From trig, A₁.cos(θ) + A₂.cos(θ+δ) = A.cos(θ+φ)

where φ = tan^-1[A₂.sin(δ) /(A₁+A₂.cos(δ))], and A is the hypotenuse of the corresponding right triangle.

As long as A₁ and A₂ are constant, the phase shift φ is static and an FM demodulator can't tell it from propagation delay. The case of A₂ ≪ A₁ is obvious. But if you have some amplitude fluctuations prior to clipping, then there'll be a time varying φ due to AM-PM conversion.

The effect of a time varying φ on FM can be difficult to calculate - depends on whether the FM is for analog (voice) or digital (FSK/GMSK/CPSK). If you have CPSK with modulation memory and a trellis coding layer, it becomes a serious question for a communications theorist. In my understanding, FM and PM are the same beast - in one the message is integrated, in the other it is not. In an FM demodulator you need to count periods to tell what the frequency is, so you end up integrating somewhere.

M.G.Rajan
www.eecalc.com

Title: Re: unwanted? paths
Post by aaron_do on Feb 9^th, 2007, 6:04am

thanks for the reply,

I'll look into that further. I'm really interested in the different types of modulation and how you design around them, but every time i finish reading a section in my digital and analog communications systems book i forget it before i pick the book up again......

Aaron

Title: Re: unwanted? paths
Post by aaron_do on Mar 7^th, 2007, 5:44pm

Hi Mr. Rajan,

thanks for your help.

regarding the formula you derived, I have a slight correction...

A = √(A₁² + 2A₁A₂cosδ + A₂²)

cheers,
Aaron