JimB said:Your attachment appears to be something cut from a PowerPoint presentation.
I dont think anyone mentioned distance yet,
steveB said:... copper wire is more like an antenna than a transmission cable. As the wire gets longer and the frequency gets higher, the energy radiates away, rather than being guided along the desired path...
... we have fortunate physics that makes the silica glass have low loss and low dispersion ...
... Dispersion is another issue for bandwidth we haven't mentioned too much, but you can imagine that if the different frequency components of the signal each travel at different speeds, then over a long distance, the signal will be distorted and unrecognizable.
One point to consider is that the frequency of the carrier is a big part of the potential bandwidth. Optical fiber uses light with frequency in the THz range. Microwave is in the GHz range. Radio is in the kHz-MHz range. Audio is in the 20-20kHz range. Hence, speaking has lower bandwidth and less information than radio which is less than microwave which is then less than optical frequencies.
On your query above, we can say the human voice does not use the entire 20 Hz to 20 kHz range. Even a symphony orchestra does not use this entire range. It is well known from telephone communications research that the human voice is much more limited in bandwidth. And, even when speech is filtered more than this, it is still quite understandable.
Why is the carrier frequency important? Because we modulate the carrier with information and typically the information frequency (which is the baseband signal, by the way) is a fraction of the carrier frequency. For example, you can't speak a microwave frequencies without becoming a microwave transmitter. You can't modulate a microwave carrier at optical frequencies because then you would have an optical (light) source. So you can think of a carrier having a channel bandwidth used for the information.
Why does the information frequency and the carrier frequency need this relation? Well, think of your Fourier transform analysis theory. The act of modulation makes the frequency content of the carrier broader than the narrow carrier itself. A pure carrier has a Fourier transform with impulse spikes at the carrier frequency (the negative frequency too). But a modulated carrier takes up the full channel bandwidth from F-f to F+f, where F is the carrier frequency and f is the information frequency bandwidth.
Why does the information frequency and the carrier frequency need this relation? Well, think of your Fourier transform analysis theory. The act of modulation makes the frequency content of the carrier broader than the narrow carrier itself. A pure carrier has a Fourier transform with impulse spikes at the carrier frequency (the negative frequency too). But a modulated carrier takes up the full channel bandwidth from F-f to F+f, where F is the carrier frequency and f is the information frequency bandwidth.
Why is copper wire itself a low bandwidth medium? Because a copper wire is more like an antenna than a transmission cable. As the wire gets longer and the frequency gets higher, the energy radiates away, rather than being guided along the desired path.
No, you are totally confused and messed up in your head.Q1: Do I have the information correct up to this point?
steveB said:Think of your math for system theory. Multiplication in the time domain becomes convolution in the frequency domain. This is why the frequencies add to give the upper and lower side bands.
Now I turn to amplitude modulation. In amplitude modulation, the amplitude of the carrier is modified by the amplitude of an information signal and the frequency of the carrier remains constant. This means that in case of amplitude modulation, we don't need bandwidth because frequency of the carrier always remains constant, i.e. if it's 500kHz then it's always going to remain at this value and hence no upper and lower frequency limits and therefore no bandwidth. Q4: Where do I have it wrong?
One of the problems I see here is that you are not using enough math to sort out the issues. There are differences between types of modulation. AM is different than FM and there are several ways to achieve both AM and FM. To see exactly what happens, you need to do it out mathematically for the case at hand. There are similarities in all cases, and were trying to give you the overall information in preparation for your more detailed studies, but you are not going to make sense of all this without using the math and without being patient and going through the course step by step.
One thing I want to ask is why you keep bringing up the center frequency of the baseband signal. Why do we care about this and why do you try to use this to determine bandwidth after modulation? Why is 1800 Hz any more special than the other frequencies? Is there a basis for this you found in a reference?
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