Simple FM Radio Circuit

An FM radio circuit is the electrical mind behind a device that can detect FM radio waves and transform them into audible sound.

The functions of radio, FM radio and FM vs. AM are presented below.

Radio:

An electrical device that converts radio waves into audible sound is called a radio.

It picks up certain radio waves using an antenna.

The selected signal is then separated from the various signals it receives.

The signal becomes stronger and returns to its original form such as voice or music.

FM Radio:

One popular kind of radio wave used for broadcasting is FM radio.

It is also found in a variety of other devices including as medical equipment, radar and walkie-talkies.

When it comes to avoid interference FM radio waves beat AM radio waves.

On the radio dial FM radio stations transmit between 88 and 108 MHz.

Compared to AM radios FM radios are able to pick up these stations and deliver the sound with greater quality.

AM against FM:

A wide range of sounds are likely sent by FM radio than by AM radio (15 kHz vs. 4.5 kHz).

FM radio can generate higher quality sound because of its wider range.

Circuit Working:

Parts List:

Description of the FM Receiver Circuit:

A simple FM receiver circuit with few parts for local FM reception is shown below.

Transistor BF495 T2, a 10k resistor R1, coil L, a 22pF variable capacitor VC and internal capacitances of transistor BF495 T1 make up the Colpitts oscillator.

The trimmer VC adjusts the oscillators resonance frequency to the required station frequency which is usually in the range of 88 and 108 MHz.

A 220nF coupling capacitor C1 directs the information signal used in modulation to the audio amplifier after it has been removed from resistor R1.

A 22pF trimmer is a good choice since the variable capacitors capacitance is easily changed from a few picofarads to about 20pF.

The self supporting coil L has an air core with an internal diameter of 4 mm and is made up of four rounds of 22 SWG enameled copper wire.

A band pass filter for extremely low frequencies is produced by the capacitors C3 100nF and C6 100µF 25V together with R3 1k.

This smoothly separates the low frequency signal from the high frequency signal in the receiver.

As an audio power amplifier for low voltage consumer applications the IC LM386 generates 1 to 2 watts which is enough to power a tiny speaker.

The amplified output is produced at pin 5 of the IC LM386 and the volume is controlled by a logarithmic potentiometer VR of 22k connected to pin 3.

A 6V to 9V battery can power the receiver.

The quality and turns of coil L, the kind of antenna and the distance from the FM transmitter are some of the factors that affect this little receivers performance.

Formulas:

The following formula explains the IC LM386 audio amplifier ICs gain paying special attention to the effect of a capacitor between pins 1 and 8:

Gv = 2 x 15K / (150 + 1350)

• Gv stands for the LM386s voltage gain it indicates the amount by which the input signals amplitude voltage is increased by the amplifier.
• Z1-5 and Z1-8 is the LM386s impedances or resistance to current flow between certain pins are represented.
• Pin 1 is the inverting input is represented by Z1.
• Pin 5 the bypassed capacitor pin is indicated as Z5.
• Pin 8 is the gain control pin is represented as Z8.

Formula Description:

Gain in the absence of a capacitor:

The formula for gain in the absence of a capacitor is Gv = 2 x 15K / (150 + 1350).

Multiplying a fixed gain of two by a resistor value of fifteen thousand ohms (15K) which is most likely the internal resistance of the LM386 corresponds to 2 x 15K.

An example of the total impedance between pin 1 and pins 5 and 8 is (1350 + 150).

Because there is no capacitor the impedance of pin 5 which is usually quite high raises the internal resistance between pins 1 and 8 (1350 ohms).

When the gain is divided by the total impedance a value of around 20 is obtained.

Using the following formula: Gain (dB) = 20 * log(Gv) which is equivalent to a gain of 26 dB (decibels).

A Capacitors Gain:

The modified equation is Gv = 2 x 15K / 150.

Here a capacitor is placed between pins 1 and 8.

This capacitor successfully removes pin 5s high resistance from the calculation by functioning as a short circuit at low frequencies.

At this point the total impedance rapidly decreases to around 150 ohms with the internal resistance taking center stage.

This leads to a significant increase in gain of around 200 or 46 dB.

Take note:

Avoiding the high impedance and adding a capacitor between pins 1 and 8 of the IC LM386 will result in a considerably greater gain.

This could prove useful for applications that need to significantly improve poor audio signals.

However it is important to consider the trade off, greater gain also increases the possibility of noise and distortion.

For best result the IC LM386 datasheet will give the right values for the capacitors based on the different gain requirements.

How to Build:

To build a Simple FM Radio Circuit follow the below mentioned steps for connections:

Oscillator Colpitts:

• Connect the self supporting coil L, a 10k resistor R1 and a 22pF variable capacitor VC to the transistor BF495 T1.
• Ensure that the coil includes four rounds of enameled copper wire that are 22 SWG and have an internal diameter of 4 mm.
• Configure the circuit using a 22pF trimmer as VC to enable tuning between 88 and 108MHz.
• To tune the oscillators resonance frequency to the desired station adjust the trimmer.

Modulation and Signal:

• Using a 220nF coupling capacitor C1 collect the information signal from resistor R1 and send it to the audio amplifier.
• To adjust the volume connect pin 3 of the LM386 IC to a 22k logarithmic potentiometer VR.
• The IC LM386s pin 5 shows the enhanced output.

Band pass filter:

• Use capacitors C3 100nF and C6 100µF 25V along with resistor R3 1k to create a band pass filter for extremely low frequencies.
• The low frequency and high frequency signals are separated in the receiver by this filter.

Connection to the Antenna:

• Connect the circuit to the antenna.
• A 60 cm long length of isolated copper wire or a telescopic antenna can be used.
• Find out through experimentation how long the copper wire should be to improve transmission.

Source of Power:

• A 6V to 9V battery can be used to power the circuit.

Testing:

• After assembling the circuit switch on the power source.
• Within the FM range from 88 to 108 MHz adjust the variable capacitor (VC) to the right frequency.
• To test the audio output adjust the VRs volume control.

Take note:

• Make sure the soldering and connections are correct.
• Adjust the circuit as needed by experimenting with it and especially if you are using different components.
• Use the power supply voltage carefully to prevent damage to components.
• This simple FM receiver circuit is often used as a starting point for radio electronics experiments and is designed for local FM reception.

Conclusion:

Integrated circuits (ICs) which reduce and improve efficiency by putting several operations into a single package are often found in modern FM radio circuits.

To maintain help a steady and clear reception particularly when there is interference some circuits might include features like automatic frequency control (AFC).

References:

Radio receiver