Power amplifier 5 watts FM range 88 - 108 MHz
Bandwidth
Beam Bandwidth:
The bandwidth of the capacitive modulated signal from the frequency spectrum can be found according to the following relationship:
BW AM = fusf - flsf = (fc + fm) - (fc - fm) = 2 fm
Radio transmitter station with amplitude modulation:
How does the station work:
A- The audio signal is converted into an electrical signal through the transducer (microphone).
B- The function of the filter is to pass the frequencies of the information signal coming from the microphone and prevent the passage of any other unwanted frequencies.
C- The voltage amplifier amplifies the information signal voltage to the level necessary to operate the power amplifier.
D- The power amplifier provides the ability to send information to the required level.
E- The oscillator gives the frequency of the carrier wave which is called the station frequency and it can be adjusted by the frequency of the crystal.
F- Amplify the carrier signal to reach the required power level.
G- The amplitude rectifier circuit produces the amplitude modulated wave (sideband + carrier wave).
H - the ultimate power amplifier that amplifies the modulated wave to bring its power to the desired level
It suffices to process the transmission and then radiation to lubricate the transmitter antenna.
Notes:
The frequency range of each transmitting station has an amplitude modulation of 10 k. Hz.
The main tasks of any receiving station:
A- Selecting the received modulated signal (wanted station signal) from hundreds of other signals and determining the bandwidth of this wave.
B- Dismantling the modified amplitude wave and extracting the information signal from the modulated wave (detection).
T- Provide sufficient magnification for the information signal
D- Providing the appropriate power transformer to convert the electrical information signal into the image that was at the entrance to the transmitter.
Frequency modulation:
The basic concept of frequency modulation is to modify or change the carrier frequency according to changing the amplitude of the information signal while the carrier amplitude remains constant. This means that by changing the amplitude of the information signal (increase or decrease of the amplitude), the carrier frequency will change to the right or left of the center frequency, that is, the frequency of the carrier wave, and thus the modified form of the wave "is a wave with a constant amplitude equal to the amplitude of the carrier wave, while its frequency is equal."
FFM = fc + Δ f = fc + kf Em = Modulated wave frequency
Frequency deviation:
It is the maximum deviation of the frequency of the formed wave with respect to the frequency of the carrier wave, and the frequency modulation system should not exceed this deviation, and in practice, the maximum deviation of the value should not exceed 0.001 from the value of the carrier frequency or less, and in practice the following has been agreed:
A- The maximum frequency deviation for FM broadcast is ± 75k. Hz.
B- The maximum frequency deviation for television sound transmission is ± 25K. Hz.
C- The maximum deviation of the cellular transmission frequency using FM is ± 5k. Hz.
Frequency division multiplexing (FDM)
Frequency division multiplexing is based on the principle that a number of signals share the permissible range of the communication channel. Each of these signals is modulated with a carrier that has a frequency different from that of other signals, and the different frequencies of these waves are separated with a sufficient frequency (protection or protection frequency (FG)) to prevent interference between the different spectra of these modulated signals and these carriers are known. By sub-waves, this type of message is used in telephony, radio and television broadcasting, and satellite communications, and the figure shows the signal distribution on the frequency spectrum for the process of modulating different signals using the frequency division multiplicity. The figure shows the process of integrating these signals into the transmitter.
Adding all the spectra of the signal after modulation, we get a composite signal that can be considered as a primary band signal involved in modulating a high-frequency (radio-frequency) carrier for the purpose of transmission. In the receiver, the modulated signals are extracted by the high-frequency carrier to pass the filters. Only the required band passes to separate each signal separately, then each signal is extracted by its subcarrier.
The fm rf amplifier uses the 2SC1971 transistor to provide 5W of output. Output matching is adjusted through the two 40pF trimmer capacitors also for the input. Note that the emitter of this transistor is directly grounded to the heat sink and should have good heat transfer. The driving force of 100 to 200 milliwatts can be applied in order to provide 5 watts of output. Use a phantom load to tune this amplifier and remember that the transistor is biased in class C, adequate filtering must be followed after the output to reduce all harmonics. Use ground level building technology in the PCB layout to get the best result, the better the grounding is. If you have a hard time finding a 10uH rf choke, try winding 1/2 meter of 0.2mm enamel wire onto a 33K 1/2 watt resistor and solder the coil to the ends of the resistor.