Fm Radio Transmitter
Frequency modulation (FM) is a technique for wireless transmission of information where the frequency of a high frequency carrier is changed in proportion to message signal which contains the information. FM was invented and developed by Edwin Armstrong in the 1920’s and 30’s. Frequency modulation was demonstrated to the Federal Communications Commission (FCC) for the first time in 1940, and the first commercial FM radio station began broadcasting in 1945. FM is not a new concept. However, the concept of FM is essential to a wide gamut of radio frequency wireless devices and is therefore worth studying in 2007. 1] The objective of this project is to explain the design and construction of a simple FM transmitter. The salient specifications for the transmitter are as follows: one 3Volt supply, DC power consumption less than 80mW, transmit frequency at a vacant spot in FM band (88MHz-108MHz), transmitted power not to exceed 10uW, and the only active device type allowed is the BC547 NPN BJT. In addition, a pre-emphasis filter is required with a 3dB corner frequency equal to 1 / 75us. A variable capacitor with value 4pF . 30pF is used for tuning. The design has been simulated and constructed.
The built transmitter can transmit music at a distance of 300 meters. This project will explain the design decisions that were made in the process of design and construction. It is surprisingly powerful despite its small component count and 3Volt operating voltage. It will easily penetrate over three floors of an apartment building and go over 300 meters in the open air. FM TRNSMITTER CIRCUIT DESCRIPTION The circuit is basically a radio frequency (RF) oscillator that operates around 100 MHz audio picked up and amplified by the electrets microphone is fed into the audio amplifier stage built around the first transistor.
Output from the collector is fed into the base of the second transistor where it modulates the resonant frequency of the tank circuit (the 5 turn coil and the trim cap) by varying the junction capacitance of the transistor. Junction capacitance is the function of the potential difference applied to the base of the transistor. The tank circuit is connected in a colpitt’s oscillator circuit. Figure 1: Fm Transmitter Circuit Design The electrets microphone: An electrets is a permanently charged dielectric. It is made by heating a ceramic material, placing it in a magnetic field then allowing it to cool while still in the magnetic field.
It is the electrostatic equivalent of a permanent magnet. In the electrets microphone a slice of this material is used as the part of the dielectric of a capacitor in which the diaphragm of the microphone forms one plate. Sound pressure moves one of its plates. The movement of the plate changes the capacitance. The electrets capacitor is connected to an FET amplifier. These microphones are small, have excellent sensitivity, a wide frequency response and a very low cost. First amplification stage: This is a standard self biasing common emitter amplifier.
The 22nF capacitor isolates the microphone from the base voltage of the transistor and only allows alternating current (AC) signals to pass. The tank (LC) circuit: Every FM transmitter needs an oscillator to generate the radio frequency carrier waves. The tank (LC) circuit, the BC547 and the feedback 5pF capacitor are the oscillator in the cadre. An input signal is not needed to sustain the oscillation. The feedback signal makes the base emitter current of the transistor vary at the resonant frequency. This causes the emitter-collector current to vary at the same frequency.
The signal fed to the aerial and radiated as radio waves. The 27pf coupling capacitor on the aerial is to minimize the effect of the aerial capacitance on the LC circuit. The name ‘tank’ circuit is comes from the ability of the LC circuit to store energy for oscillations. In a pure LC circuit (one with no resistance) energy cannot be lost. (In an AC network only there resistive elements will dissipate electrical energy. The purely reactive elements, the C and the L simply store energy to be returned to the system later. ) Note that the tank circuit does not oscillate just by having a DC potential put across it.
Positive feedback must be provided. (Look up Hartley and colpitts oscillator in are reference book for more details. ) Trim cap: The slots inside the trim cap are shaped like the head of an arrow. The maximum capacitance value is when the arrow is in pointed to the 12 o’clock position. An 1800 turn brings the trim cap value to its minimum rated Value. With experimentation you will be able to build up a Table of total capacitance value (remember to add in the 10pf) to fm frequency. You can also change the Frequency by altering the space between the coils of L1. Spread out the L1 coil wide apart.
ASSEMBLY INSTRUCTION Components may be added to the PCB in any order. Note that the electrets microphone should be inserted with the pin connected to the metal case connected to the negative rail (that is, to the ground or zero voltage side of the circuit). The coil should be about 3mm in diameter and 5 turns. The wire is tinned copper wire, 0. 61 mm in diameter. After the coil in soldered into place, spread the coils apart about 0. 5 to 1mm so that they are not touching. (The spacing in not critical since tuning of the Transmitter will be done by the trim capacitor.
It is quite possible, but not as convenient, to use a fixed value capacitor in place of the trim capacitor – say 47pF – and to vary the transmitter frequency by simply adjusting the spacing of the coils. That is by varying L of the LC circuit rather than C. ) Adding and removing the batteries acts as a switch. Connect a half or quarter wavelength antenna (length of wire) to the aerial point. At an FM frequency of 100 MHz these lengths are 150 cm and 75 cm respectively. CIRCUIT CALIBRATION Place the transmitter about 10 feet from a FM radio. Set the radio to somewhere about 89 – 90 mhz.
Walk back to the FM transmitter and turn it on. Spread the winding of the coil apart by approximately 1mm from each other. No coil winding should be touching another winding. Use a small screw driver to tune the trim cap. Remove the screw driver from the trim screw after every adjustment so the LC circuit is not affected by stray capacitance. Or use a plastic screw driver. If you have difficulty finding for the transmitting frequency then have a second person tune up and down the FM dial after every adjustment. One full turn of the trim cap will cover its full range of capacitance from 6pf to 45pf.
The normal FM band tunes in over about one tenth of the full range of the tuning cap. So it is best to adjust it in steps of 5 to 10 degrees at each turn. So tuning takes a little patience but is not difficult. The reason that there must be at least 10 ft. Separation between the radio and the FM transmitter is that the FM transmitter emits harmonics; it does not only emit on one frequency but on several different frequencies close to each other. You should have little difficulty in finding the Transmitter frequency when you follow this rocedure. LEARNING EXPERIENCE It should already be clear from the above circuit description that there is a surprising amount of electronics which may be learnt from this deceptively simple kit. Here is a list of some advanced topics in electronics which can be demonstrated or have their beginnings in this project: Class C amplifiers; FM transmission; VHF antennas; positive and negative feedback; stray capacitance; crystal-locked oscillators; signal attenuation The simple half wave antenna used in the project is not the most efficient.