Sine Wave Question

So the middle of the wave indicates the basic energy state and the up and down indicates energy levels above and below? If that's the case why wouldn't very high intensity energy levels move the centerline off the graph? I can probably draw this out if there's any confusion.

Zipper730 said:

So the middle of the wave indicates the basic energy state and the up and down indicates energy levels above and below? If that's the case why wouldn't very high intensity energy levels move the centerline off the graph? I can probably draw this out if there's any confusion.

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Because the graph can be centered at any energy level you choose. What matters is the relative energy levels in your wave, not the absolute levels. It's modulation (manipulation of the waveforms at whatever energy level is in effect) that carries the message. The energy level merely serves to influence the range it will travel.(in the case of a radio wave)

One of the challenges of Amplitude Modulated radio is that the audio amplifier that provides the modulation of the basic Continuous Wave signal has to be about as powerful as the radio frequency amplifier. That is why so many CB radios have a built-in public address system - they already have a 5 watt audio amp that is required to modulate the radio signal. Frequency Modulated radio has the advantage of only needing a small audio signal to modulate the frequency of the RF signal, using various techniques that can be very simple and inexpensive.

XBe02Drvr said:

Because the graph can be centered at any energy level you choose. What matters is the relative energy levels in your wave, not the absolute levels. It's modulation (manipulation of the waveforms at whatever energy level is in effect) that carries the message. The energy level merely serves to influence the range it will travel.(in the case of a radio wave)

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What is the ratio between transmitted amplitude and minimum detectable received amplitude?

pbehn said:

What is the ratio between transmitted amplitude and minimum detectable received amplitude?

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Well, that is a complex question. Note that radio receivers can receive signals without ANY modulation. What you hear is kind of a "hole" in the naturally occurring static. In order to receive Morse Code an AM radio receiver needs a device known as a "Beat Frequency Oscillator." A BFO creates an interference signal between its frequency and the received signal; 1000 HZ is usually employed. This makes it much easier for the operator to read the Morse Code because "Beep, beeeep" is much easier to hear than "holes" in the static. Unmodulated CW Morse Code signals can be heard much further than AM modulated signals. All the US aircraft in WWII that were equipped with HF (also called Short Wave) radios had BFOs, even the fighters equipped with SCR-274-N. Long range communication by B-17's and B-24's and the like generally was done by CW Morse Code. I doubt the fighters used CW much, if at all, but all US pilots were taught to copy Morse Code, and radio navigational stations broadcast Morse Code identifiers and radio directional homing signals, generally using modulated signals in the US and CW overseas.

As the modulation level goes down and the peaks of the envelope of the signal get lower, the quality of the received audio decreases and the bandwidth of the signal goes down. So AM radio broadcast stations have to use near 100% modulation and are at least 20 KHZ wide to make things sound good. They even have stereo AM stations. Audio-only communications can be much narrower, perhaps 3 KHZ wide.