[AMRadio] Physical Reality of Sidebands


Gary Schafer garyschafer at comcast.net
Sun Jan 16 17:07:57 EST 2005



Donald Chester wrote:
> 
>> From the beginnings of radiotelephony there has been a question whether 
> 
> sidebands exist as physical reality or only in the mathematics of 
> modulation theory.  

Don,

That is a question that can keep you up nights trying to visualize!
The sidebands do exist in reality but not because of the way we may 
receive them or detect them. re narrow ringing filters in the receiver.

The carrier also exists constantly with modulation but the reasoning 
tends to get a little fuzzy as the modulation frequency gets lower as 
you note.

I don't believe that the carrier still exists because of tank circuit 
ringing when 100% negative modulation is reached either. The Q of the 
tank only allows for a couple of cycles at most when no carrier is 
present. It decays very quickly. With modulation, even at voice 
frequencies, the modulation frequencies are much lower and there are 
many more rf cycles for each audio cycle. So one audio cycle would be 
equivalent to many rf.

Modulation is a multiplication process not one of adding audio to the 
carrier. Even though it works out the same when you add and subtract the 
modulating frequency to the carrier to figure where the side bands are. 
They don't get there by the addition / subtraction process but by the 
multiplication process.

When looking at a signal with a spectrum analyzer you are looking in the 
frequency domain. When looking at a monitor scope you are looking in the 
time domain.

When the modulating frequency gets very low it is hard to detect the 
side bands as you have noted. With very low frequencies as you describe, 
the carrier being off for 6 months and on again for 6 months, gets into 
how you are referencing it. Your reference must also be very long in 
order to still be in the frequency domain. The frequency domain and time 
domain kind of merge as I gather it. That is how it was explained to me.
I am still kind of fuzzy on the issue though when it comes to low 
modulating frequencies as you describe.

Another thought on the subject is to look at the opposite of what you 
describe. When mixing frequencies against a local oscillator to create a 
new frequency, you have the same modulation process taking place. You 
can look at the carrier, as it is usually way far away in frequency from 
the mixing signal, and it will appear the same when the mixing signal is 
present or not. You can in no way distinguish the difference.

Good subject.

73
Gary  K4FMX





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