This project consisted of building two separate devices to receive and transmit a signal over the FM spectrum of 88 to 108 MHz. Originally the idea was to make a discrete pirate radio station that can be easily transported inside a pocket. When the transmitter is turned on, any radio in its transmission radius can pick up whatever is played through the auxiliary input of the transmitter. The second unit simply acts as a FM receiver to pick up the signal being broadcast through the transmitter and play through an auxiliary port with headphones, this unit was made only if an FM radio is not available, but can also function as a spy bug to listen in on the transmitter if a microphone is added in place of the auxiliary input port of the transmitter. Both the pirate radio transmitter and receiver cover the whole FM spectrum but can easily go outside this spectrum if needed by adjusting the variable capacitor.
The FM transmitter used to broadcast a pirate radio station is powered off a 3V watch battery and a toggle switch to turn on the unit. The audio is fed through an auxiliary input connected to any device that can transmit an audio signal through an auxiliary jack. First the input signal is amplifier by a common emitter configuration to be strong enough to transmit through an antenna. Next it is modulated by the variable capacitor and inductor. This process gives the signal a carrier frequency between 88 and 108 MHz depending on the set value of the 1-30 picoFarad variable capacitor. The input signal data is now stored on this frequency and sent through the antenna. Without a variable capacitor the carrier frequency would be set to a constant value depending on the used capacitor and inductor. For the transmitter, a 28 gauge copper wire turned 5 times with about a 6 mm diameter was used. The antenna in for this unit is not completely necessary for transmission inside a room but to get a coupe hundred feet a 30” inch wire is attached to the emitter.
The FM Receiver operates to collect desired frequency waves and convert the information carried by them to a usable form. In this case the receiver will aim to catch the transmitted signal and play it back through a pair of headphones. To do this the receiver must catch the wanted signal in the sea of transmitted signals around it. The inductor and variable capacitor are used to filter out all of these unwanted frequencies being picked up by the antenna. After the capacitor is set to a value that picks up the desired frequency, it is then amplified by the transistors and a LM386 op-amp, specialized for low voltage audio amplification. This amplified signal is sent to the headphones to be heard with as little noise as possible.
Antenna: Modulated signals from Q1 are fed into the antenna that transmits out electromagnetic waves.
L1 and Variable_C (C3): The inductor and variable capacitor, used as a trimmer capacitor, sets the audio signal onto a carrier frequency in a process called frequency modulation. (Note in the LTSpice design C3 has a specific value for the A/C analysis seen below. In the physical design it is a variable capacitor allowing you to change the frequency.)
C1: coupling capacitor to block DC from interfering from the audio jack?
C2: coupling capacitor to block AC from interfering with the battery?
C3: Coupling capacitor assists in modulation by allowing current to flow to the emitter, the current flows through R4 and creates a voltage drop. Changing Vbe reducing Q1 conductivity, when this happens C3 charges again, reducing current through R4 as this happens Vbe increases and conductivity increases, this repeats and assists in the modulation.
R1 and R2: Provides a low base current to the transistor.
R3: Provides biasing for the transistor.
R4: (See C3) Part of C3 modulation circuit.
Q1, 2N3904: The key to the modulation through parasitic capacitance in the BJT, by varying the base current the capacitance changes between, emitter, base and collector to change the frequency.
Antenna: a shielded copper wire that picks up transmitted radio waves by receiving radio waves at the antenna which causes atoms inside the antenna to shift back and forth. These shifts create an electric current that goes into our circuit.
L1 and Variable_C (C1): The inductor and variable capacitor, used as a trimmer capacitor, allows unwanted frequencies to be filtered out of the circuit. These two pieces are tuned to have the same resonance frequency of our transmitting signal. When the antenna pulls this signal into the circuit the same resonance frequencies cause a strong vibrations in the antenna which allows the small changes in the electromagnetic force of the radio wave to become a current in the circuit. (Note: C1 has a specific value for testing in LTSpice while in the physical design it is a variable capacitor.)
Q1 and Q2: form the amplifiers and detect the frequency modulated signal that the transmitter puts out. When the antenna voltage increases Q2 turns on more while lowering the collector voltage. (180 phase shift). This decreases the current to the base of Q1 since they are connected and as a result increases the collector voltage, this acts as a positive feedback amplifier.
LM386: It is an op-amp chip designed for low voltage audio amplification of the audio signal.
R3: Supply voltage
Variable_R (R1): This variable resistor of 10 kohms allows the gain to be increased and decreased. The result is adjustable volume through the headphones.
C4: Coupling capacitor so only the AC signal is heard through the headphones.
C3: Bypass capacitor from opamp to reduce noise.
Design:
Reference:
https://archive.org/details/MakingATransistorRadio-LadybirdBook/page/n19
https://www.boondog.com/tutorials/rfTransmitter/rfTransmitter.htm