I am in an undergrad Comm Systems class and this book is fricking awful, and my prof isn’t a whole lot of help. I am trying to figure out a way to find the in-phase and quadrature components of a signal.
One of the exercises in the book is asking to find the in-phase and quadrature components of the following signal when fc=1000Hz: Vsb=1 when f is between or equal to 900-1300Hz and is =0 for all other conditions.
Where so I even begin here? I know I have to do something with the signum function in here somewhere. The book has very few worked examples of anything and is of no help to me.
I need to know the method to do this because I have an exam coming up and I know this is gonna be on it.
OK, based on what you’ve said, it sounds like you’re talking about Quadrature Amplitude Modulation (QAM) and Vestigial Sideband (VSB) signals.
Ideally, if you’re book is that bad, you should look for another book or some sort of resource that explains it better for you. I’ll try to help out since I know bad books and professors exist, but if you’ve got a test coming up, you should find something good you can study from. Try Googling for “amplitude modulation”, etc. and see if you can find a good comm. signal page. I can help with the basic concepts, but doubt I’ll be able to help with the example problem.
This sounds like a description of the VSB filter. It is partially supressing the lower side band of the modulated signal. That is why there is more passband above the carrier frequency than below it.
Quadrature is a way of transmitting two signals on one carrier frequency by shifting one of the modulated signals by 90[sup]o[/sup] and transmitting the sum. The “in-phase” component is the signal modulated by cos(2pi f[sub]c[/sub] t) and the “quadrature” component is the signal modulated by cos(2pi f[sub]c[/sub] t - pi/2) = sin(2pi f[sub]c[/sub] t). It’s used in broadcasting color TV, but I didn’t think you could use it with VSB.
Sorry for the vagueness; I can answer theory questions, but it’s been a while since I worked out example problems…
Hey that actually makes sense sort of.
What is the purpose of the 90 degree phase shift?
Does it matter which signal is shifted?
What is a Vestigal Sideband or more specifically is it differenr from a standard sideband?
Thanks for your help. You already have made the idea of Quadrature somewhat understandable. I am still at a loss as to how to extract the 2 components from my given info though.
>>What is the purpose of the 90 degree phase shift?
So you can separate the two signals later
>> Does it matter which signal is shifted?
This question is meaningless. You just have two carriers of the same frequency and a phase difference of 90 degrees.
>>What is a Vestigal Sideband or more specifically is it differenr from a standard sideband?
A carrier modulate in amplitude carries simmetrical sidebands. Take one out and you get Single SideBand. But if your filter lets all of one and part of the other sideband, then you have a vestigial sideband.
Are you using phasor (complex exponential) notation? If so, the in-phase part is the real part, and the quadrature part is the imaginary part, assuming e[sup]j omega t[/sup] carrier (i.e., the carrier is cos(omega t) = Real{e[sup]j omega t[/sup]}).
It’s been a long while since I worked with this kind of stuff, but I don’t think this is necessarily about Quadrature Amplitude Modulation. A single modulated signal will in general have an in-phase and a quadrature part.
Quadrature modulation allows two separate signals to be carried by two carriers of the same frequency with a phase separation of pi/2 but separating them at the receiving end requires a synchronous detector.