Simple FM transmitter production experiment

At present, the most suitable transmitting experiments for DIY are FM equipment such as wireless microphones. Although there are many articles of this kind in various newspapers and magazines, newspapers and magazines lack interactive means, and it is inconvenient for enthusiasts to communicate with authors and other readers when they encounter problems during the production process. Moreover, the production articles selected by newspaper and magazine editors may not be verified, so some of the productions conceived by authors bring difficulties to beginners.

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

The coil in the picture is wound 6-8 times with 1.0mm enameled wire on a 3.2mm drill bit, which can cover 88-108MHz. 7 times is around 100MHz. After connecting as shown above, it is actually a simple transmitter. It can transmit when powered on, but it transmits unmodulated equal amplitude signals. When the nearby FM radio receives the signal, it will only be silent. Add a modulated signal as shown in the figure below, and it can enter the practical state.

At this time, if you connect the audio source signal output by a walkman, DVD player, etc. to the audio input terminal of the transmitter in the figure above, you can use the radio to listen nearby. Although the above figure can transmit, it is not practical. First, the transmission energy is very small and can only be used indoors. It is only a few dozen meters in an open area outdoors. Second, the frequency is unstable. Since the antenna is just a piece of wire connected to the oscillation circuit through a 100P capacitor, the environment around the antenna will affect the transmission frequency. If you want to make it usable, you should add two-pole amplification after it, as shown in the figure below.

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

The transmitter in the picture is easy to make successfully. Success here means that it can enter the working state after plugging in the antenna and turning on the power. If you want the transmitter to enter the best working state, you still need to do some debugging. In fact, the greatest joy of doing experiments for enthusiasts is

The fun is to perfect the work by debugging it yourself and get the best performance.

First of all, this output circuit works in a non-class A state, and has stricter requirements on load impedance. Usually, most transmitters have 50Ω output. It is not easy for amateurs to find 50Ω RF cables for experiments, but 75Ω RF cables can be bought everywhere. Moreover, 75Ω antennas are also easy to make. Therefore, this machine uses 75Ω output. Before powering on, a 75Ω resistor should be connected to the output end, and a 75Ω antenna should be connected after debugging. The maximum output of this machine is more than 1W. Do not use a small resistor of 1/8W, which will burn if connected. It is okay to burn a resistor, but once the resistor burns, the amplifier is equivalent to no load, and the tube is dangerous.

The most important thing that needs to be debugged after power is turned on is the operating point of the transmitting tube. The harmonic components of the output are greatly different at different operating points.

The transistor operating point selected for the local oscillator stage gives it greater dynamics. The measured second harmonic is more than 20 decibels lower than the fundamental wave, and the third and fourth harmonics are even lower, which can be completely ignored. The driver stage works in a Class A and Class B state close to Class A, and will not generate high-energy harmonics. The key lies in the output stage.

The selection of the output stage working point is more complicated. The voltage divided by the upper bias resistor and the lower bias 51Ω resistor should be lower than the emitter junction voltage of the transmitter, so that it works between Class B and Class C. When 1KΩ is selected, it is close to Class B. At this time, the operating current of the transmitter is 260mA, the output power is 700mW, the second harmonic is about 10dB lower than the main wave, and the third and fourth harmonics are even lower. The output main frequency energy is above 500mW.

As the upper bias resistor increases, the working state of the transmitting tube will be close to Class C, and the power meter indication will gradually increase, reaching 1300mW at the maximum, but at this time, the harmonics will gradually increase, and the main wave power will decrease. In professional manufacturing, this contradiction is not difficult to solve, but this circuit is designed for beginners, so the power efficiency index has to be abandoned. It is recommended to use 1KΩ for the upper bias resistor.

During the experiment, 2SC2053 generated a lot of heat. 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 two 2SC2053 in parallel. When using them in parallel, take current balancing measures.

If you just want to have some fun doing it yourself, you don't necessarily need such a large output power, nor do you need to transmit very far. In this case, you can make some changes to the circuit parameters. The circuit structure remains unchanged. Using a 6V power supply, the output power is also above 100mW, as shown in the figure below.

The circuit structure is exactly the same as the one in the figure above, except that the values ​​of several components are changed and the transistors are also adjusted. Since the output power is small, there is no need to consider the safety of the output tube when using 3355 as the output, and it can work stably for a long time.