ازالة التعديل ( استخلاص الموجات ) - كيف يعمل جهاز استقبال الراديو .. الإلكترونيات

Demodulation (wave extraction) -
How does a radio receiver work .. Electronics

Demodulation (wave extraction)

145 Demodulation process
146 - Using a rectifier as a demodulator.

The process of integrating audio or civil programs into radio waves is called modulation or "embedding". Recovering the sound or image from the signal is called "demodulation", "detection", or also "demodulation". The amplitude of the unmodulated radio signal is regular and constant (145). It is worth noting that some transmission methods use "amplitude modulation". The figure represents a radio wave modified with a single tone. The process becomes more complicated when sending speech or music waves. The modulated waves then reach a diode in the receiver, which allows the current to pass in one direction to the next stage and thus can remove half of the wave and keep only the other half. As shown in the graph.

This process is followed by the process of smoothing the peaks of the signals by circuits that follow the valve circuit and give an image of the audio signal used to modify the transmitted signal. It is noted in Figure (146) that the coil tuning is not done by the variable capacitor S M). Then the signal voltages become similar to the "modulated" signal itself. As for the diode, it extracts the wave. In addition, the capacitor S1 contributes to smoothing the peaks of the radio frequency signal in order to obtain the audible output that is connected to the loudspeaker to be heard. However, the rule is to connect it to an amplifier before connecting it to the loudspeaker. However, if the signals are strong, it is possible to obtain a strong enough output to connect it directly to the loudspeaker without the need to use an amplifier (147).

The valve receivers contain a thermoelectric diode used to extract the wave. In this diode, electrons are emitted from the hot cathode and only travel in the direction of the anode (148). The audio signal is generated through the resistor M, and is taken through the capacitor S? to the amplification stage. In addition, the coil L A forms the secondary coil of the intermediate frequency transformer, and the diode is generally part of a diode-triode or a double diode-triode. The triode section is dedicated to amplifying audio frequencies, as the capacitor S A passes the radio frequencies.

Most receivers are characterized by being superheterodyne receivers derived from "heterodyne-ultrasonic action". This system is summarized by using an oscillator to generate a signal with a different frequency used to multiply the tuning signals in it or to pair them. This produces a fixed frequency that passes to the amplifier accompanied by fixed tuning circuits as well. The frequencies are lower than the frequencies that can be heard.

147 - Receiver with a crystal rectifier.
148 - Thermic diode for demodulation.

In Figure (149), the desired station is tuned by coil L and variable capacitor S M1 0, while L2 is the oscillator coil that is tuned by Sm?. Then the two signals, the selected signal and the oscillator signal, are mixed, and we get a signal difference with a frequency equal to the difference between the frequencies of the two signals. If we assume that the antenna circuit L has been tuned to receive a signal with a frequency of 670 kHz, then the frequency of the signal resulting from the mixing process is 670 - 200 = 470 kHz.

Figure (150) shows a frequency converter circuit in which the transistor is used, where L2 contributes to transferring the signals to the next stage and connects the signal to the base of the mixing transistor (Q). The coil L3 forms the tuning coil of the oscillator circuit itself, and is connected to both the collector of the transistor (Mg) and the emitter of the transistor (B). As for the resistor M3, it is responsible for ensuring the emitter bias.

If the frequency range of the signal selected in the tuning circuit by coil L1 ranges between 500 kHz and 1500 kHz for medium waves, and if the subsequent stages are to operate at a fixed frequency of 470 kHz, then coil L3 must be able to produce signals with a frequency ranging between 970 kHz and 1970 kHz. Since the ratio of frequencies covered by coil L3 (970 to 1970) is less than the ratio of frequencies covered by coil L1 (500 to 1500), we find that the value of the capacitance of capacitor cm2 is less than the value of cm. Therefore, it is always preferable to add another capacitor S in series to cm2. In other words, the frequency we obtain from the frequency changer (mixing circuit) is the average frequency of the device, and its value often ranges between 455 kHz and 470 kHz for most devices. . As for high frequencies, it is much greater than this value. Therefore, other files are used that are selected to suit high-frequency waves (short waves) as in Figure (151). Each of these files has a smoothing capacitor and a core, by which its inductance can be adjusted and tuned to the required frequency.

Some receivers contain an amplifier for radio signals (high-frequency signals) before the frequency changer stage (152). In these devices, the antenna circuit is tuned to the frequency of the signal to be heard, and valves can be used in them instead of transistors. After converting the signals to the fixed frequency that was chosen, they pass to the intermediate frequency amplification stage where they are transformed into intermediate frequency signals. After that, they must be converted to lower frequencies: audio frequencies.

149 - How the frequency changer works in the receiver, the ultrasonic variable.
150 - Using transistors in the frequency changer.
151 - Selecting other wave ranges.
152 - Radio frequency amplifier.