There are many ways to build an AM modulation circuit and therefore different types of AM modulation circuit. The simplest one was to add the carrier signal and message signal and pass this combined signal through a diode, then use a LC filter to resonant at the carrier frequency. This was illustrated in the guide AM modulation using diode. With transistors we can apply both the signals to the base of the transistor or apply the one signal to the base and the other into the emitter of the transistor(emitter AM modulation) with AM signal appearing at the collector for both cases. The AM circuit with inputs to the base is illustrated in AM modulator circuit with transistors. AM modulation circuits can also be built using transformers and diode rectifier in bridge configuration. Here a square law AM modulation circuit is tested and explained.
Below is the circuit diagram of square law modulator for AM signal generation.
The message signal, the mixed signal before the diode and the resultant AM signal waveforms are shown below.
The following shows the AM signal frequency spectrum which is of double sideband AM signal.
In the circuit, the message signal between 20Hz to 20Khz with amplitude around 500mV and carrier signal of 50Khz with amplitude around 2V is connected in series and applied to the 1N4148 switching diode. The output from the diode is the AM signal with lots of harmonics, to reduce them the AM signal is applied to the LC filter resonant at the carrier frequency of 20KHz. The online LC filter calculator was used to calculate the inductance given the capacitor value of 0.1uF and fr=50Khz. This gives L=101uH. There are many oscillator circuit that one can use.
Here are some of the tested oscillator you can use for AM carrier signal generation:
- Colpitts oscillator with LM358 and TL072 Op-Amps
- Drain Feedback biased E-MOSFET Colpitts Oscillator
- How Pierce crystal oscillator Works
The message signal comes from electret microphone(30 to 50mV amplitude level signal) and an electret microphone pre amplifier circuit amplifying it to at least 500mV before connecting it to the carrier signal.
