Frequency Modulator

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by Yusuf Ismail

Project Overview

The output of this circuit is an FM (Frequency Modulation) carrier signal between 0-8 khz. If no input signal is fed in, then it acts a simple voltage controlled oscillator.
The full circuit
Here's the link of a simulation of the project and the circuit as well

Note: the speaker part is not working in the simulation and should not be tested.

Tools and Materials

  • Prototyping Board: Chosen for simplicity.
  • LC358 Op-Am
  • Resistors:
  • 4x50k resistors.
  • 5x10k resistors.
  • 3x 1k resistor.
  • 2x 100k resistors.
  • 10k potentiometer
  • Capacitors:
  • 1 nano-farad capacitor.
  • 2x 100 micro-farad capacitors
  • OPA1013DN8 Single Supply Op-Amp.
  • 2N3904 Transistor: Acts as a switch or signal amplifier.
  • Two Switches: For controlling the circuit operations and signal output.
  • 10k potentiometer.
  • 2x 100 microfarad capacitors.
  • LM386 Audio Amplifier IC: Enhances audio signals.
  • An extra 0.1 microfarad capacitor to eliminate DC offset.

Project Files

Step-by-Step Instructions

Building the Voltage Controlled Oscillator (VCO) - Control voltage is the leftmost point in the circuit. - Testing: check the output signals on the scope to make sure the desired signals are of the right shape and that changing control voltage chan ges frequency. - Caution: Do not let control voltage exceed 1.5 * V+ where V+ is the positive rail of the Op-Amp - To get higher frequencies, you can replace the capacitor with a smaller capacitor, but there as a limit on how small you can go before the capacitance of the breadboard starts affecting your work.

Building a voltage adder - Resistor Configuration: Ensure 10 R2 = R1 when using digital signal 0-5 volts. However, if signal is 0-0.5 V make sure V1 = V2. Connect R2 to the input signal and R1 to the carrier voltage. - Voltage Adder Testing: To test the voltage adder, add a certain value of V1 to V2. Verify that vout approximates V1 + 1/10 V2 if using the shown configuration and V1+V2 if using R1=R2. - VCO Control Voltage Connection: Connect this vout to the control voltage of the VCO. Initially, set V2 to 0 and check if the carrier frequency remains stable. - Signal Modulation Test: Finally, use a function generator to produce a fixed frequency 5V wave of any shape. Ensure the output signal appears appropriately modulated as per the illustration provided.

RC integrator Make an RC integrator for simplicity. For our range, the following RC integrator works fine, but if you change the range of frequencies, you might want to consider revisiting the resistance and capacitance. R = 50k C = 1 nano farad. Connect the triangular wave of the VCO to Vin and measure Vout. Make sure Vout is a sinusoidal wave. If it is not, you might want to change the parameters of the circuit.

Speaker circuit - If the noise is still noise, add a bandpass to the input to filter DC signal and frequencies higher than 20k. - Test the speaker with a tone from your laptop or phone. An AUX cable should have a ground and at least one +ve terminal. Look up the pinout of AUX online. - If the speaker circuit works, congratulations! connect the frequency modulated signal to hear it.