This is a relatively simple practical production, this paper intends to in-depth step by step from simple to complex, if you are willing to experiment hands simultaneously, and soon you will be able to make suitable for formal occasions, the use of FM transmitters. Of course, the experiment is to start from the most simple, below is a simple oscillator, which is the basis for the FM transmitter.
Figure in the coil of enameled wire in 3.2mm with 1.0mm drill around 6-8 on the circle, can cover 88-108MHz, 7 lap around at 100MHz. Click the image to connect, in fact, is a simple transmitter, the power to launch, but the launch is not amplitude modulated signal, a nearby FM radio receiver to the signal will only be muted. Like the following figure, as with modulated signals, can enter into the functional state.
Then, if you Walkman, DVD players and other audio signal connected to the output of the transmitter audio input graph, in the vicinity can use to listen to the radio. Although the figure can be fired, but not practical, one is the emission energy is very small, only for indoor use, but also in outdoor open to tens of meters. The second is the frequency of instability, because the antenna is a wire, through 100P capacitor connected with the oscillation circuit, so the antenna will affect the environment around the transmitting frequency. If you want to reach the level can be combined with the subsequent polarization of amplification, as shown below.
This should be a friends request Dachu amplifier circuit, the output power is satisfactory, but there are many problems, will be detailed below. Oscillator and amplifier together, it becomes a complete FM transmitter, as shown below.
The transmitter is easy to chart the success of production, referred to here is the success of the antenna power supply can be plugged into the working state, if want to work into the best state of the transmitter need to do some debugging. In fact, the fans do the experiment greatest pleasure is to make debugging work through their own hands to perfect, the best performance.
First, the output circuit in non-Group A state, the load impedance has more stringent requirements, usually more than the transmitter output to 50Ω, 50Ω enthusiasts experiments are not necessarily easy to find the RF cable, RF cable 75Ω everywhere buy Moreover 75Ω antenna is easy to make, therefore machine 75Ω output, the output power should be preceded by a 75Ω termination resistor, 75Ω followed after the completion of commissioning of the antenna. 1W maximum output over the machine, do not use the kind of small 1/8W resistors, connected to the fire. Burn down a little resistance, but resistance of an amplifier is equivalent to burning load, the tube in danger.
After the commissioning of the power needed to launch the main contents of the tube is operating point, the working point of the harmonic components of different output significantly different. The local oscillation level selection transistor operating point to a larger dynamic, the measured second harmonic Bobijibo low over twenty db, four times lower, completely negligible. A near-level work to promote the B class status, it does not produce large energy harmonics. Lies in the output stage. Output stage operating point of choice is too much trouble, under the bias on the bias resistor and 51Ω resistor divider formed should be lower than the emitter voltage of the launch tube to work in between B and C states. Close to B when used 1KΩ state, then launch tube current 260mA, output power 700mW, the second harmonic is about 10dB lower than the main wave, four times less energy in the frequency of the output 500mW or more.
With the bias resistor on the increase, the working status of the launch tube near the C class, power meter's instructions but added increases, the maximum up to 1300mW, but this time all of this harmonic is gradually strengthened, but the main wave power reduced. In the professional manufacture, is not difficult to resolve this contradiction, but the circuit is designed for beginners made simple type, had to give up power efficiency indicators, the proposed use on the bias resistor 1KΩ. Experiment 2SC2053 severe fever, although the experiment did not damage, estimated long-term use will not do, it is recommended for use when making two 2SC2053 2SC1970 or parallel. Parallel to the attention of current measures are taken. If you just want a taste of hands-on production of fun, does not necessarily require such a large output power, how far does not need to launch, then you can make s
ome changes to the circuit parameters, the circuit structure unchanged, with the 6V power supply, the output power In more than 100mW, shown below.
And on the same circuit structure diagram, just change a few component values, transistor also made adjustments. Since the output power less to do with the 3355 output tube output do not consider the security problem, you can work long-term stability.
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