AM Transmitter using Ring Diode Modulator

Here AM transmitter using ring diode modulator is demonstrated. The AM transmitter is build on breadboard, it's working principle is explained and test result are shown. This AM transmitter can be easily build at home in few hours.

Wireless communication is everywhere such as WiFi, GSM telecommunication, 433MHz RF communication, 2.5GHz communication and others. Historically, wireless communication started with amplitude modulation(AM) signal transmission. So AM signal transmission is the simplest form of wireless communication. AM signal transmission is possible with AM modulators. RF modulators are circuits that impresses signal to be transmitted, most often audio, also called modulating signal onto much higher frequency carrier signal, often called local oscillator(LO) signal or RF signal. There are couple of AM modulator circuits such as single diode(see AM Transmitter using Single Diode Modulator), double diode, ring diode modulators, BJT and FET transistor based AM modulator, Gilbert Cell mixers etc. 

Here AM transmitter is shown which is build using ring diode modulator. Usually, ring diode modulator uses transformers but here it is shown how a simple ring modulator can be build without the transformer.

The following picture shows AM diode ring modulator on breadboard.

The following pictures shows whole the AM transmitter(and testing probes).

The perfboard connected modules are RF crystal oscillator and audio pre-amplifier. The circuit is explained down below and the following video demonstrates how the AM transmitter with ring diode modulator works.

The following shows the circuit schematic of the AM transmitter with Ring Diode Modulator.

In the circuit diagram above, we have a crystal oscillator circuit which generates the carrier RF signal at 4MHz. Then there is audio pre-amplifier circuit which is biased using BJT voltage divider biasing technique. This amplifier takes the audio signal input from the audio source and amplifies it. The audio source used here is from PC sound speaker. Any speaker output from mobile or PC can be connected. And finally we have the ring diode modulator circuit. It takes as input the audio signal from the amplifier circuit and the the RF oscillator signal at the opposite side of the ring. The two signals are mixed such that the audio signal modulates the RF signal and at the output AM signal is produced. 

The ring diode modulator circuit diagram is shown below.

Here is 1N4007 but switching diodes such as 1N4148 or 1N914 should be used. However even with 1N4007 diode the modulator worked well. The RF signal magnitude should be 5 to 6 times the magnitude of modulating signal(audio signal). The high frequency RF carrier signal switches the diode on and off. During the positive half cycle of the carrier signal the diodes D1 and D3 conducts while the diodes D2 and D4 are switched off. When passing through the diodes D1 and D3, the mixing and modulation happens. When mixed signal is passed through a diode which is an active device, modulation happens and half wave rectified AM signal appears at resistor R4. During the negative half cycle, the diode D2 and D4 conducts and the diode D1 and D3 are switched off. During this cycle, again the audio signal and the RF signal gets mixed and modulated due to the diodes. The half cycle rectified AM signal appears at the resistor R5. The theory behind operation mechanism of such modulator is explained in greater details in how does Double Balanced Diode Ring Mixer Works with mathematical equations.