Re: Newbie surppressed carrier question
- From: NoSpam@xxxxxxxxxxx (Bob Masta)
- Date: Sun, 25 Nov 2007 14:49:39 GMT
On Sat, 24 Nov 2007 19:11:57 -0800 (PST), el_squid_2000@xxxxxxxxx
wrote:
Ok - so if the carrier is surppressed, just how does a receiver
distinguish between stations ?
The AM process is really a form of multiplication The result
of multiplying two sinusoids, if you used a plain old multiplier,
would be signals at the sum and difference frequencies, and
none at either of the original frequencies. The only difference
with AM is there is effectively a constant added to the signal input
so that it never goes through zero, which means the carrier is
always present. If you looked at a spectrum of the broadcast
signal when there is only a pure sine signal input to the modulator,
you'd see 3 peaks: The carrier frequency, the carrier plus the
signal, and the carrier minus the signal.
In a real broadcast, there are of course many input frequencies,
but the above still holds. The side peaks become bands whose
width is the bandwidth of the incoming signal. Let's say the
signal covers a 5 kHz bandwidth (I think this is typical for broadcast
AM). Then a station at 1 MHz would have sidebands extending
5 kHz to either side, down to 995 kHz and up to 1005 kHz. The next
station on the dial will be at least 10 kHz away (farther, in
practice). so the sidebands will not overlap. If the carrier is
removed (say by using a multiplier instread of an AM modulator)
the sidebands stay in the same positions and don't interfere.
You can see how AM sidebands and multipliers work with my
free Daqarta signal generator and your Windows sound card.
(You don't have to purchase anything: The signal generator
continues to work after the trial period, along with most everything
else except inputs... which you don't need for these experiments.)
You can see waveform, spectrum, and spectrogram of the signal,
and play around with modulation depth and type. The only difference
compared to a broadcast situation is that the carrier will be in the
audio range as well. So, for example, you can set the carrier to
10 kHz and try modulating signals between 0 and 5 kHz, giving
sidebands from 5 kHz to 15 kHz. This is actually an advantage over
RF, since things are proportionally more spread out and easier to see
on the spectrum display.
Note that Daqarta's AM modulator works a bit differently from
that in a transmitter (see the Help for full details): When you
set Depth to 200% you get pure multiplication, which is the
supressed-carrier condition.
If you have any trouble with this, I'd be glad to answer questions.
Best regards,
Bob Masta
DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!
.
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- From: el_squid_2000
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