Degree of modulation and AM bandwidth
In this article, I will explain Degree of modulation and AM bandwidth. In this I have explained the Amplitude modulation for different values of degree of modulation along with the frequency spectrum.
In the AM the degree of modulation m is defined as the ratio of the amplitude modulation signal to said carrier signal. The volume level of an audio signal corresponding to the signal amplitude. A faint signal affects the carrier amplitude thus only slightly, while a loud signal causes a large change in amplitude. At a modulation depth of m = 1, and carrier amplitude modulation of equal size, wherein the amplitude of the periodic modulation product becomes zero. For practical use, these 100% modulation is unsuitable, since the recovery of the signal demodulators at these zeros can not work correctly.
If the signal amplitude larger than that of the carrier, then m> 1, This condition is called over-modulation and produces a distorted modulation product from which the original signal can no longer be recovered. The following graph shows the comparison of time and frequency diagrams for different degrees of modulation and signal frequencies.
Source of Image:
Own created diagrams using CAD software
The modulation depth can be determined quite easily from the Fourier diagram. It is calculated as the ratio of the sum of the two sidelines amplitude to the carrier amplitude.Bandwidth of the AM
For a modulation signal or frequency corresponds to the distance between the side lines of the modulation bandwidth of each product, the AM signal. The bandwidth is therefore twice as great as the highest frequency of the transmitted signal. The range commercial AM stations is set to 9 kHz internationally. Adjacent AM stations must comply with this carrier spacing. Only under these conditions, the sidebands of adjacent channels do not overlap and the information contained therein can be recovered unaltered. In the radio transmission range, the high-order transmitted AF frequency is limited to <4.5 kHz, and thus includes only the voice frequency band. This method is not suitable for high-quality music programs.
For a better understanding and ease of presentation, the amplitude modulation has been shown with only a single modulation frequency. Voice and music information includes an entire frequency spectrum in the baseband. This spectrum will appear in the AM signal in the sidebands. The rightmost upper sideband from the carrier OSB is in normal position. The frequency corresponds to the sequence of the base bands and the carrier frequency is higher. The lower sideband USB is left on the vehicle and has a downside compared to the baseband layer. The modulated signal frequencies are deeper closer to the support than the higher. The recovery of the information passed in the demodulator, which receives the complete signal with both AM sidebands and the carrier.
Source of Image:
Own created diagrams using CAD softwareThe current account of the AM
In all spectra AM carrier signal with the support line is at its highest. The majority of the transmission energy is required for the normal amplitude modulation for carrier transmission. The carrier is an auxiliary signal and itself contains no information. Apart from heat loss to the transmitter distributes the AM transmitter power on the pure carrier power P T, and the power in the two sidebands P S.
Source of Image:
Own created diagrams using CAD software
The radiated from the transmitter when theoretical maximum modulation index m = 1, power is distributed to 2/3 on the carrier. The single sideband contains information transmission only 1/6 of the total transmission power. In practical transmitter operating at m <1, the power ratio P S / P T is significantly worse.Outlook
To radio communications amplitude modulated signals have a very wide reach. In the long and medium waveband they run as so-called ground waves and the curvature of the earth fit quite well. In the short wavelength range between earth and sky wave higher-lying atmospheric layers can be reflected several times. The range itself weak shortwave transmitter can thus include the entire globe.
The poor performance record is a major disadvantage of amplitude modulation. With special versions such as the single-sideband or AM with suppressed carrier do you manage improvements, but which then lead to a greater technical complexity in the transmitter and receiver range.
Electrical noise often affect the amplitude. They are recognized in the modulation. Interference during the transmission of the AM signal, such as interference in a cable network, transmitting and receiving fluctuations in spark gaps that appear as amplitude changes. In the demodulation signal recovery by short-term amplitude noise can not be filtered out and deteriorate the signal quality.