In our day-to-day life we frequently got used to listen the news, music and other programs through FM radio station by tuning to our favorite bands with wireless transmission and reception of data. Edwin Armstrong developed super-regenerative circuits and super-heterodyne receiver, which made radio broadcasting successful. In 1933, a revolutionary change was made by Armstrong in the broadcasting business with FM radio. The entire system depends on the FM radio transmitter to transmit the data from the transmitter end to the receiver end. But, are you aware of working of these FM transmitter and receiver circuits?
Introduction to Wireless Radio Transmitter
First and foremost, we must know how the data is transmitted from the transmitter end such as a radio station to receiver at receiver end. For transmitting data, primarily data have to be modulated using modulation techniques such as amplitude modulation or frequency modulation. The FM radio transmitter transmits data (mostly sound or audio signals in case of radio) using FM wave over a carrier wave whose frequency is corresponding to the audio signal amplitude. These FM radio transmitters consist of different blocks and can be designed by ourselves with simple electrical and electronics projects. The below figure shows a basic block diagram of an FM transmitter.
FM Radio Receiver
There are different types of radio receivers: PC controlled radio receivers, analog radio receivers, digital radio receivers, and so on. The receiver must be able to tune and amplify to work with FM signals by filtering out all other undesired stations. The transmitter consists of modulator, and different signals are transmitted from various stations and sources. Similarly, best FM radio receiver consists of a demodulator that works with all radio stations irrespective of carrier frequency.
The radio systems must be sufficient to receive all types of audio sources simultaneously because at a time several users or listeners may tune to multiple radio stations. But, at the same time, we must choose economical radio receiver. The AM/FM two band receivers are frequently used because of their advantages compared to single band receivers.
Requirements of FM Radio Receiver
- In general, radio receiver must work with AM signals and FM signals. But, in this system, receiver is chosen which will work with FM signals.
- Must be able to tune and amplify the desired the radio station.
- Should filter out all other radio stations except the desired one.
- Irrespective of carrier frequency, the demodulator of the radio receiver has to work with all radio stations.
FM Radio Receiver Circuit
The simple and best FM radio receiver circuit consists of RF tuner, RF to IF converter, IF filter, demodulator and audio amplifier.
It is a subsystem for receiving radio frequency transmissions and to convert selected carrier wave frequency along with associated bandwidth into a fixed frequency, which is suitable for further processing as frequently low frequency is used on the output.
RF to IF Converter
The actual radio frequency is converted into an intermediate frequency. In transmission or reception process, carrier frequency is shifted to a frequency called as intermediate frequency which is created using a local oscillator and mixing the carrier signal through the heterodyning process. The main reason for converting the RF to IF is due to poor performance of signal processing circuitry and failure of active devices, such as transistors to deliver much amplification.
The main function of the IF filter or intermediate frequency filter is to eliminate frequencies, other than intermediate frequency by filtering.
The information content can be recovered from the modulated carrier wave using an electronic circuit in the FM radio receiver called as a demodulator. This extraction of original information from carrier wave digital data using demodulator circuit consisting of staircase maker and the delay unit for creating an analog signal is called as demodulation process.
The output power of FM transmitters is much smaller than that of the AM transmitters. So, an audio amplifier is used for amplifying the output to generate a clear loud audible output.
FM Radio Receiver Circuit Working
The first stage TR1/VC1 of FM radio receiver circuit, to keep wirings as short as possible, is a critical part. L1 is stretched to a length of 13mm evenly, which is formed by the 1mm enamelled copper wire winding of 8 turns. The receiver circuit consists of a few electrical components, such as four transistors TR1 is BF199, TR2, TR3 and TR4 are BC547. VC1 is a tuning capacitor consisting of a built in trimmer VC2, and is said to be one section of miniature FM transistor radio. The moving vanes and spindle is called as earthy end connected to the capacitor C1 of 22pF. The value of RF choke L2 is in the range of 1 micro Henry to 10 micro Henry. The ordinary earphones connected in series are suitable for output providing impedance of 64ohm.
FM Radio Receiver Tuning-in
If we advance the potentiometer VR1 slowly towards the end of track connected to battery positive, then suddenly a slight increase in background noise will be heard at about half way point, which indicates onset of oscillation. After that, potentiometer VR1 must be backed off slowly until the oscillation just stops for tuning in some stations.
Then by adjusting the VC2 at high frequency around 108 MHz and, by stretching or squeezing the turns of coil L1 slightly at the end at around 87 MHz, we can obtain correct frequency range of 87 MHz to 108 MHz. Frequency spectrum over air can be shared by different radio stations through FM modulation.
Digital Radio Receiver
These are advanced radio receivers with digital radio technology with which listener can tune easily to favorite stations by searching stations with names and other searching options such as track, artist information. As technology is improving rapidly, these digital radios provide additional features such as slideshow, EPG functionality and other high quality listening experience to listeners compared to analogue FM radios. These digital radio receivers come in different shapes and sizes based on cost, silicon integration and power consumption capabilities.
The above discussed FM radio receiver circuit can be extended using an antenna at the junction between power electronics components C1, L1 and C2. Interesting fact is that, even a 1 meter long telephone wire can be used as an antenna for this circuit. Do you have innovative ideas to implement using a practical guide to build electronics projects on your own way? Then please post your suggestions, comments and ideas in the comment section below. We assure our technical help based on your requirement for designing free electronics project kits as given for students.