Principle and design of wireless receiver
In the early days of radio, the superstitious people thought it was a magical place far from the signal sent. But soon people will find these signals are electromagnetic waves, rather like light and infrared, frequency and wavelength is different. Radio wave frequency is much lower than the light, it is also longer wavelength. In the microwave spectrum, the wavelength in the range of the radio signals from VLF 25 000m until millimeter wave.
Signal has three different forms: continuous wave (CW), amplitude modulation (AM) and Frequency Modulation (FM). Continuous wave signal in this chapter only a brief statement, as most readers of this book is it less relevant. Wave signal amplitude (Figure 1) is a transmission frequency sinusoidal oscillation thereof. For example, a vibration signals 500 kHz maritime 500000 times per second. And other special continuous wave signal amplitude kept constant lies when it works. If the electric signal to form Morse code dots and dashes through the switch, the information can with the continuous wave signal transmission. Early in the radio, this signal is sent by ship on the ocean, but the real marine radio reported CW is still in use (less and less now, the U.S. has stopped using 500 kHz maritime defense equipment).
Figure 1 equal amplitude signals
Modulation is a technique in unadjusted signal (called the carrier) of added information. Unadjusted continuous wave signal and the signal does not raise the same. Modulated audio signal is typically derived from the speech or music. Radio, the modulated signal is the technical term information. Even taking into account citizens’ band radio and other radio transmission is the content of some talk show. However, when the as intelligence, I still want to keep the signal invariably difficult. Figure 2 shows a modulation signal (Fig. 2 (a), the information carrier (Figure 2 (b)), and amplifying the synthesized modulation signal (Fig. 2 (c)).
Figure 2 modulated signal
Transmit modulator with a carrier at twice the audio signal to form the (c) characteristic signals in Figure 2. The signal can be received at a set frequency. With attention to the peak signal amplitude modulated audio signal order change. AM signal carrier frequency is kept constant, but its intensity or amplitude changes in the audio signal with. In this case, the carrier frequency or phase of the audio signal with the corresponding change in the amplitude remains unchanged.