Comparison of amplitude, frequency and phase modulations. Amplitude modulation. Balanced amplitude modulation with carrier suppression (double side band DSB)
Continuous modulation methods
Signal modulation methods
Lecture No. 7
In some cases, during telemetry it is necessary to transmit information about a continuous process using continuous messages. And if it is necessary to obtain information about an infinitely large number of gradations, then the signals with which continuous messages are transmitted must be continuous.
Continuous signal is formed using continuous modulation methods.
Modulation is the formation of a signal by changing the parameters of the carrier under the influence of a message.
With continuous modulation methods, RF is used as a carrier - a sinusoidal oscillation, or a non-sinusoidal one. Since a sinusoidal oscillation is characterized by such basic parameters as amplitude, frequency and phase, there are three main types of modulation: amplitude (AM), frequency (FM) and phase (PM). There are also varieties of these modulations, which will be discussed below, as well as oscillations of the main types of modulation, the so-called double modulations.
It is possible to transmit a continuous message directly without using an HF carrier, i.e. without modulation. However, modulation expands the possibilities of transmitting messages over the following reasons:
a) the number of messages that can be transmitted over one communication line increases by using frequency division signals and subcarrier frequencies;
b) the reliability of transmitted signals increases when using noise-resistant types of modulation;
c) the efficiency of signal radiation when transmitted over a radio channel increases. This is explained by the fact that the size of the antenna must be at least 1/10 of the wavelength of the emitted signal. Thus, transmitting a message with a frequency of 10 kHz and a wavelength of 30 km would require an antenna 3 km long. If this message is transmitted to a 200 kHz carrier, it will reduce the antenna length by 20 times (150 m).
Amplitude modulation (AM) is the formation of a signal by changing the amplitude of a harmonic oscillation in proportion to the instantaneous value of the voltage or current of another electrical signal(messages).
We will consider the case of amplitude modulation in which the transmitted message is a simple harmonic oscillation U c = UΩ cos Ω t(rice. A) where Ω is the frequency, and UΩ – vibration amplitude, HF – carrier, or carrier, U n = U w 0 = cos ω 0 t(rice. b), ω 0 is the carrier frequency, and Uω 0 – amplitude.
Under the influence of the message on the amplitude of the carrier, a new oscillation is formed, in which the amplitude changes, but the frequency ω 0 remains constant.
The carrier amplitude will change linearly.
U a m = Uω 0 + ku c = Uω 0 + k UΩ cos Ω t = Uω0 (1+ m cos Ω t).
Where k is the proportionality coefficient, and
– (4-2)
– the relative change in the amplitude of the carrier, called the modulation ratio or depth. Sometimes the modulation coefficient is expressed as a percentage. If the amplitude of the modulated oscillation increases to twice the amplitude of the carrier, then the modulation depth is 100%.
Amplitude - modulation oscillation will have the form shown in Fig. c), and its instantaneous value will be determined by the equality
Uam =Uω 0(1 + m cos Ω t) cos ω 0 t(4-3)
Opening the brackets and taking advantage of the fact that
cos Ω t cosω 0 t=}