In the worlds of radio and electronics, AM stands for amplitude modulation. For AM transmission of a signal, a carrier wave at a known, constant frequency is modulated by signal wave (which itself is an analog electronic representation of a sound wave), changing the amplitude of the carrier wave. This modulated wave is then sent through an antenna, so the signal can get picked up by nearby AM radio receivers. This guide will allow you to construct your own low-power AM transmitter, following FCC guidelines for hobby radio transmission.
Begin by constructing the oscillator portion of the circuit, outlined in blue on the circuit diagram. Once it is built, you can monitor the output V_carrier on an oscilloscope. You will likely have to zoom in pretty far on the time-axis of the oscilloscope -- AM radios expect carrier waves with a frequency of approximately 600-1600 kHz. Confirm that as you turn your potentiometer, the oscillator outputs primarily frequencies within this range. If this is not the case, double check your resistor and capacitor values.
An op-amp is the perfect candidate for generating a carrier wave, and is commonly implemented for use as an oscillator. This circuit has both positive and negative feedback elements, so the op-amp will always be operating such that it hits the rails, meaning it will always be outputting either +12V or 0V. If the non-inverting input V+ is greater than the inverting input V-, it will output +12V. Otherwise, it will output 0V. Since the non-inverting input (V+) is biased at a fraction of +Vcc (where R1 and R2 form a voltage divider), Vout will begin positive, so current will flow from the output to ground, charging the capacitor. As the capacitor charges, the voltage drop over it will increase, meaning the voltage to the inverting input, which is in parallel with the capacitor, will also increase. Eventually, the capacitor’s voltage will exceed that of the biasing at the non-inverting input, causing the op-amp to switch to outputting at the lower rail, or 0V. The capacitor will then begin to discharge until the threshold voltage is hit again, creating an oscillating signal. C4 and R11 are included for robustness, specifically to help prevent back-current from signals picked up by the antenna.
Next, construct the modulator portion of the circuit, outlined in purple on the circuit diagram. For now, use a function generator to create a signal to act as “V_audio” and send it to the inverting input. Generate a signal within the audible frequency range, 20Hz-20kHz. Monitor the output of the frequency generator and of the modulator on the oscilloscope. The oscilloscope should look something like the attached image, where the bottom is the output of the function generator, and the top is the modulated signal. Notice how the modulated signal is a high frequency wave whose amplitudes over time resemble the shape of the frequency generator wave.
This circuit involves an op-amp with negative feedback, so we know that the op-amp will adjust to try to make the difference between its inputs be 0. For that to happen, V_modulated must have a form such that when it is combined with V_carrier at the inverting input, it recovers the audio signal. This occurs when V_modulated has the form of the inverse of V_carrier, modulated by V_audio.
Next, construct the signal processor portion of the circuit, outlined in red on the circuit diagram, and attach it to the non-inverting input of the modulator op-amp. The potentiometer allows the strength of the audio signal relative to the carrier signal to be adjusted. This in turn will allow the modulated signal to contain more or less of the audio signal which can lead to some interesting sounds. C2 and R8 are included for robustness, specifically to help prevent back-current from signals picked up by the antenna. Monitor the audio signal and the modulated signal to ensure that they continue to line up as expected. An example is attached where the top waveform is the audio, and the bottom is the modulated signal.
Next, a power amplifier can be attached to the output of the modulator before putting the signal through the antenna in order to increase the range of the transmitter. This requires a high-frequency NPN transistor, which was not available in the makerspace. If you want to attempt this, I recommend looking into Class A or Class AB power amplifiers since while those are not the most efficient, they have the least warping of the signal inherent to their design. However, the transmitter will function fine without this component, so it is alright to skip this step.
Connect the output of the modulator to your antenna through C3. The capacitor is there to help prevent back current from any signals picked up by the antenna. All together, your circuit should look something like the attached image, where the long yellow wire sticking out leads into the antenna. Bring a radio receiver close to the antenna and tune it to the carrier frequency (which you can measure using your oscilloscope). Plug the audio cable into your MP3 player of choice (I used my laptop) and play a sound through it. You may need to adjust your carrier wave frequency slightly using the oscillator potentiometer, but you should be able to hit a point where you hear the audio you are playing through the radio. Feel free to try playing around with the modulator potentiometer and see how that impacts the sound. Also note that high-frequency analog electronics can be very susceptible to environmental changes, so for a more robust design you may want to contain your circuit in a metal shielding box. Happy transmitting!