What is an amplitude modulation transmitter?

Answer 1

Amplitude Modulation, AM, is a technique where a high frequency carrier of constant amplitude and frequency is modulated by a lower frequency signal. This modulation varies the amplitude of the carrier, and the resultant modulated carrier is what is transmitted. A typical carrier in the standard AM broadcast band is 550 kilohertz to 1.6 megahertz, while the modulating signal is audio, in a range less than 20 kilohertz.

Demodulation involves tuning the centerpoint of the carrier and detecting the envelope of modulation, often with a peak follower such as a diode, capacitor and resistor. Very simple AM radios can be built with only a few components, typically with less than six.

While there are several demodulation techniques, by for the most common is the superhetrodyne technique. Since selectivity and sensitivity are opposing factors, the RF stage of such a receiver generally trades selectivity for sensitivity, simply boosting the gain with a middle of the road semi-broad band amplifier. Then this is mixed with a local oscillator that is tuned to be exactly 455 kilohertz away from the desired carrier frequency. The mixed signal then is shifted to a center point of 455 kilohertz.

This is followed by a very steep skirt band pass filter, the IF stage, which filters out everything but the desired shifted carrier. This is where the selectivity comes in, and since the IF stage is tuned to only one frequency, it can be highly optimized. Actually, the IF stage is three stages, each tuned to slightly different frequencies, so that the response curve, while having steep skirts, and centered at 455 kilohertz, still has the bandwidth necessary to cover the audio range needed.

The output of the IF stage is then demodulated using some kind of peak follower, and the output is the original signal.

If you analyze the frequency domain of the modulated carrier, you discover that there is a carrier, and two side bands. The carrier contains no information, and consumes about 67% of the total power, while each side band carries the exact same information, but each is backwards to the other.

This has led to improvements in design. Sometimes you can suppress the carrier prior to the final amplifier in the transmitter. This gives you the ability to put more power into the side bands. The down side is that the receiver is more complex, and has to regenerate the original carrier and mix it with the suppressed carrier signal before then mixing it for the downshift into the IF stage.

You can take this even further, by suppressing both the carrier and one of the sidebands, since all of the signal is contained in both sidebands. Of course, this makes the receiver even more complex.

Some CB and Amateur radios are single sideband suppressed carrier, simply known as SSB, in order to boost range. Most broadcast stations are double sideband non-suppressed carrier, simply known as AM, in order to simplify the receiver. Different areas of the world use slightly different terminology, such as SSB-SC, but the intent is the same.

Answer 2

Amplitude modulation is commonly known by the acronym AM. It means the signal is carried by changes in the power of the waves (typically radio waves)which are transmitted at a constant frequency. It is the simplest form to both broadcast and receive. Early radio was AM.

FM or frequency modulation became more common in the late 20th Century. With FM the power remains constant but small changes are made to the frequency. It is considered better for radio audio transmission because:

a/ It is less prone to interference

b/ More data can be carried leading to better sound quality

c/ A narrower bandwidth is required between different broadcasts allowing more stations.

FM signals tend to use higher frequencies which do not travel so far so AM is still common in applications where range is more common than high quality.

Answer 3

A transmitter which modulates only the amplitude of the useful signal with the carrier signal is known as amplitude modulation transmitter.

Answer 4

Themodulation index of a modulation scheme describes by how much the modulated variable of the carrier signal varies around its unmodulated level

Answer 5

by varying the voltage amplitude of the carrier frequency

Answer 6

Amplitude modulation is a special kind of radio wave of high frequency.