Help me solve a AM modulation theoretical problem

[Highlander-SP]

AM modulation problem

A periodic signal f(t) is showed in the picture. For the amplitud modulation (AM), what should be the minimum amplitud of the carrier?
. For that Carrier amplitud, sketch the AM wave when the Carrier frequency is 100 KHz.
. Find out the AM wave spectrum
. Sketch the DSB-SC correspondent wave and its spectrum.

Please, i need to know how to desenvolve this, mainly the last item. Thanks

 

[Highlander-SP]

Re: AM modulation problem

May I say that the minimal amplitud of the carrier be 1 V (for does`t have overmodulation)?
Since we consider the signal amplitud the peak signal (Vpp/2), what shoul be that amplitud?

 

[flatulent]

Re: AM modulation problem

The amplitude of the carrier should be 1V so that the negative going modulation does not clip.

This is a classical way to give higher power output and still keep within government regulations. The modulator supplies an extra DC to the modulated amplifier which increases the carrier power.

The government regulations usually state the limit on carrier power with no modulation. Once they got wise to this modulating method they made it illegal.

 

[Highlander-SP]

Re: AM modulation problem

Thanks, but this is just a simple exercise from Lathi's book.
So, the AM DSB modulated is:

[Eo + K*a(t)]*cos wot

where:
Eo: Carrier amplitud ( that's we consider 1!?)
K: constant modulator
a(t): information signal or modulator signal
wo: carrier frequency

How can I get a(t)? Should be by Fourier series? How can I do that? What's the kind of Fourier series that would be better to solve this?

tks

 

[Highlander-SP]

Re: AM modulation problem

Is that question too hard?

 

[pthoppay]

Re: AM modulation problem

Hi,

Yes a(t) should be converted into fourier series and by multipying the carrier you just shift the frequecies by the carrier frequency. That would be the spectrum of the resulting signal.

Regards,
Prakash.

 

[Borber]

Re: AM modulation problem

From your equation and attached waveform you can only find entire part of equation Eo+K*a(t) and not only a(t). Different parameter values for Eo, K and a(t) can give you the same waveform.