Simple FM Radio Circuit

An FM radio circuit is the electronic heart of a device that can pick up FM radio waves and convert them into sound you can hear.

Below are the functions of Radio, FM radio and FM vs AM.

Radio:

• A radio is an electronic device that catches radio waves and turns them into something you can hear.
• It uses an antenna to pick up specific radio waves.
• It then separates the chosen signal from other signals it receives.
• It makes the signal stronger and changes it back into its original form like music or speech.

FM Radio:

• FM is a common type of radio wave used for broadcasting.
• It is also used in many other things like walkie talkies, radar and some medical equipment.
• FM radio waves are better at resisting interference than another type of radio wave called AM.
• FM radio stations broadcast between 88 and 108MHz on the radio dial.
• FM radios can pick up these stations and play the sound with higher quality than AM radios.

FM vs. AM:

• FM radio can transmit a wider range of sounds than AM radio15kHz vs 4.5kHz.
• This wider range allows FM radio to produce higher quality sound.

Circuit Working:

Parts List:

FM Receiver Circuit Explanation:

Presented below is a simple FM receiver circuit with minimal components for local FM reception.

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

The trimmer VC sets the resonance frequency of the oscillator to the desired stations frequency typically between 88 and 108MHz.

The information signal used in modulation is extracted on resistor R1 and directed to the audio amplifier via a 220nF coupling capacitor C1.

The capacitance of the variable capacitor can be adjusted within a range of a couple of picofarads to approximately 20pF, making a 22pF trimmer a suitable choice.

The self supporting coil L, constructed with four turns of 22 SWG enameled copper wire features an air core with a 4mm internal diameter.

Capacitors C3 100nF and C6 100µF 25V, along with R3 1k, create a band pass filter for very low frequencies, effectively separating the low frequency signal from the high frequency signal in the receiver.

The IC LM386, serving as an audio power amplifier for low voltage consumer applications produces 1 to 2 watts sufficient to drive a small size speaker.

A logarithmic potentiometer VR of 22k, connected to pin 3 controls the volume, and the amplified output is obtained at pin 5 of IC LM386.

The receiver can be powered by a 6V to 9V battery.

The performance of this compact receiver depends on factors such as the quality and turns of coil L, the type of antenna and the distance from the FM transmitter.

Formulas:

Below formula describes the gain of the IC LM386 audio amplifier IC, specifically focusing on the impact of a capacitor between pins 1 and 8:

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

• Gv: This stands for the LM386s voltage gain, it indicates the amount by which the input signal’s amplitude (voltage) is increased by the amplifier.
• Z1-5 and Z1-8: The LM386s impedances, or resistance to current flow, between certain pins are represented.
• Pin 1 (inverting input) is denoted by Z1.
• Pin 5 (the bypassed capacitor pin) is denoted as Z5.
• Pin 8 (gain control pin) is denoted as Z8.

Formula Description:

Gain without a Capacitor:

Without a capacitor, the formula for gain is: Gv = 2 x 15K / (150 + 1350).

• 2 x 15K: This is equivalent to multiplying a fixed gain of two by a resistor value of fifteen thousand ohms (15K), which is probably the LM386s internal resistance.
• (1350 + 150): This is an illustration of the total impedance between pin 1 and pins 5 and 8.

Since there is no capacitor, the internal resistance between pin 1 and pin 8 (1350 ohms) is increased by the impedance of pin 5, which is often quite high.

The value that results from dividing the gain by the overall impedance is around 20.

By applying the formula: Gain (dB) = 20 * log(Gv), this corresponds to a gain of 26 dB (decibels).

Gain from a Capacitor:

This is the revised formula: Gv = 2 x 15K / 150.

A capacitor is positioned in this instance between pins 1 and 8.

By acting as a short circuit at low frequencies, this capacitor effectively eliminates pin 5s high impedance from the calculation.

The internal resistance now predominates and the overall impedance drops significantly to about 150 ohms.

This results in a considerable boost of about 200 or 46 dB increase in gain.

Note:

A significantly greater gain is possible by avoiding the high impedance and connecting a capacitor between pins 1 and 8 of the IC LM386.

Applications where it is necessary to greatly enhance weak audio signals may find this helpful.

But it is crucial to weigh the trade off: more gain also raises the possibility of distortion and noise.

Generally, the IC LM386 datasheet will provide suitable values for the capacitors according on the various gain needs.

How to Build:

Building an FM receiver circuit involves assembling the components mentioned in the circuit.

Colpitts Oscillator:

• Connect the transistor BF495 T1 with a 22pF variable capacitor VC, a 10k resistor R1, and the self supporting coil L.
• Ensure that the coil has four turns of 22 SWG enameled copper wire with a 4mm internal diameter.
• Place a 22pF trimmer as VC in the circuit to allow tuning between 88 and 108MHz.
• Adjust the trimmer to set the resonance frequency of the oscillator to the desired station.

Signal and Modulation:

• Extract the information signal from resistor R1 and feed it to the audio amplifier through a 220nF coupling capacitor C1.
• Use a 22k logarithmic potentiometer VR connected to pin 3 of the LM386 IC to control the volume.
• The amplified output is obtained at pin 5 of IC LM386.

Band pass Filter:

• Create a band pass filter for very low frequencies using capacitors C3 100nF and C6 100µF 25V, along with resistor R3 1k.
• This filter separates the low frequency signal from the high frequency signal in the receiver.

Antenna Connection:

• Connect the antenna to the circuit.
• You can use a telescopic antenna or a piece of isolated copper wire about 60cm long.
• Experimentally determine the optimum length of the copper wire for better reception.

Power Supply:

• Power the circuit using a 6V to 9V battery.

Testing:

• Once the circuit is assembled, turn on the power supply.
• Tune the variable capacitor VC to the desired frequency within the FM band 88 to108MHz.
• Adjust the volume control VR to test the audio output.

Note:

• Ensure proper connections and soldering.
• Experiment with the circuit and make adjustments if necessary especially if using different components.
• Be cautious with the power supply voltage to avoid damaging the components.

This simple FM receiver circuit is designed for local FM reception and can serve as a starting point for experimenting with radio electronics.

Conclusion:

Modern FM radio circuits often use integrated circuits ICs that combine several of these functions into a single package for simplicity and efficiency.

Some circuits may also include features such as automatic frequency control AFC to help maintain a stable and clear reception especially in the presence of interference.

References:

Radio receiver