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
