[Photo] Simple FM FM transmission experiment

This is a relatively simple and practical production. This article intends to go from simple to complex step by step. If you are willing to experiment with synchronization, you will soon be able to make an FM transmitter suitable for formal occasions. Of course, the experiment starts from the simplest. The following figure is a simplest oscillator, which is the basis of FM transmission.

The coil in the picture is wrapped with a 1.0mm enameled wire on a 3.2mm drill bit for 6-8 turns, which can cover 88-108MHz, and 7 turns around 100MHz. Connect it as shown in the picture above. In fact, it is already a simple transmitter. It can transmit when powered on. However, it transmits an unmodulated constant-amplitude signal. When the nearby FM radio receives the signal, it will only be silent. Add the modulation signal as shown in the figure below, and you can enter the practical state.

At this time, if you connect the audio source signal output from the Walkman, DVD player, etc. to the audio input terminal of the transmitter above, you can listen to it with a radio nearby. Although the above picture can be launched, it is not practical. The first is that the transmitted energy is very small and can only be used indoors. It is only a few tens of meters outdoors. The second is the frequency instability. Since the antenna is just a piece of wire, it is connected to the oscillation circuit through a 100P capacitor, so the environment around the antenna will affect the transmission frequency. If you want to make it usable, you should add two poles to enlarge it, see the figure below.

This is a power amplifier circuit built at the request of netizens. The output power is satisfactory, but there are many problems, which will be described in detail below. The oscillator and the power amplifier are connected together to become a complete FM transmitter, see the figure below.

The transmitter in the picture is easy to make successfully. The success mentioned here is that you can plug in the antenna and turn on the power to enter the working state. If you want the transmitter to enter the best working state, you need to do some debugging. In fact, the biggest fun for enthusiasts to do experiments is to make the works more perfect and get the best performance through hands-on debugging.

First of all, this output circuit works in a non-class A state, which has stricter requirements on the load impedance. Usually the transmitter is mostly 50Ω output. It is not easy to find 50Ω RF cables for enthusiast experiments, and 75Ω RF cables can be everywhere. You can buy it, and the 75Ω antenna is also easy to make, so this machine uses a 75Ω output. Before powering on, you should connect a 75Ω resistor to the output. After debugging, connect the 75Ω antenna. The maximum output of this machine is more than 1W, don't use that kind of small resistance of 1 / 8W, connect it and burn. It's fine to burn a resistor, but when the resistor is burned, the amplifier is equivalent to no load, and the tube is dangerous.

The main content that needs to be debugged after power-on is the working point of the launch tube, and the harmonic components of different outputs at the working point are greatly different. The operating point of the transistor selected by the oscillating stage of this machine makes it more dynamic. The measured second harmonic is more than 20 decibels lower than the fundamental wave, and the third and fourth times are lower, which is completely negligible. The driving level works in the state of Class A and Class B, which is close to Class A, and does not generate harmonics of large energy. The key is in the output stage. The selection of the working point of the output stage is more troublesome. The partial voltage formed by the upper bias resistor and the lower bias 51 Ω resistor should be lower than the launch junction voltage of the launch tube, making it work between Class B and Class C states. When 1KΩ is selected, it is close to Class B. At this time, the working current of the launch tube is 260mA, the output power is 700mW, the second harmonic is about 10dB lower than the main wave, the third and fourth times are lower, and the output frequency energy is above 500mW.

With the increase of the upper bias resistance, the working state of the launch tube will be close to Class C. The indicator of the power meter will gradually increase, up to 1300mW at the maximum, but each harmonic at this time also gradually strengthens, and the main wave power instead reduce. In professional manufacturing, this contradiction is not difficult to solve, but this circuit is a simple type designed for beginners, so it has to give up the power efficiency index, and it is recommended to use 1KΩ for the upper bias resistor. During the experiment, 2SC2053 was severely heated. Although it was not damaged during the experiment, it is estimated that it will not work for a long time. It is recommended to use 2SC1970 or use two 2SC2053 in parallel during production. Pay attention to the current sharing measures when connecting in parallel. If you just want to taste the fun of hands-on production, you do not necessarily need such a large output power, nor do you need to transmit far, then you can make some changes to the circuit parameters, the circuit structure remains unchanged, with 6V power supply, the output power is also Above 100mW, see the figure below.

The circuit structure is exactly the same as the above figure, except that the values ​​of several components have been changed, and the transistors have also been adjusted. Due to the small output power, the use of 3355 for output does not need to consider the safety of the output tube, and can work stably for a long time.