Expert: Cezar L. Palconet - 9/1/2010

Question
QUESTION: what if we add AM with narrow band FM?
what type of signal we will get

ANSWER: Hi Devesh.

Are you just going to combine them as individual signals on a emitter or a radiator?

If that is the case, each one would be distinct from each other and you would end up with the same, an AM signal which is demodulated with an AM receiver and an FM signal with is demodulated by an FM receiver, however you may face problems with the amplifiers due to linearity issue, but if you already have gone around this problem then you have full mode modulation system.

But if you are into something else, keep in mind that they are already modulation and in order to bring them up into the next higher order, you would have to resolve the phase and amplitude linearity.
imagine each time you increase the amplitude you widen the spectra and vice versa, so each change in information would be incoherent with the other, then consider how to recover it.
you will get something like a circular pattern which loops itself each time the amplitude and frequency changes, and if the information are different, the amplitude would cancel out each impulse of frequency shift that will happen and vice versa.



Interesting but daunting…

good luck!

Regards.


---------- FOLLOW-UP ----------

QUESTION: are you trying to sy that we will get only carrier signal
assume it as a heroical situation not practical one
we are adding an AM Signal with narrowband FM.
I THINK WE WILL GET DSB Signal with carrier.
am i right.
please suggest.
and thanks for your quick reply

Answer
Hi, Devesh

That is you, assuming that your narrow band FM has a constant frequency and magnitude, then it becomes an oscillator, in which you mix your AM that is carrying information ( varying in magnitude as per intelligence content) YES you have a DSB, but if the intelligence content is the same for both, then it becomes incoherent(lisagous), or if the intelligence for both is different in term of time and magnitude then, it is also incoherent(non linear).

Your heroical situation is, as I have said "interesting" but you still need to put some form of predictability on the intelligence content.

As I have mentioned many times earlier, the carrier is no longer relevant with the advent of high speed computing, it is the predictability and cohesiveness of the information or intelligence contained therein that is most important.

regards,