AM Transmitter using Single Diode Modulator

 Learning of wireless communication works is fun. Wireless communication is everywhere from cell phone, to wireless control in manufacturing factories, WiFi and so on. The modern wireless communication system uses complex RF circuit at high frequencies. They uses faster modulation techniques such as QAM(Quadrature Amplitude Modulation) and digital modulation such as BPSK(Binary Phase Shift Keying), ASK(Amplitude Shift Keying), FSK(Frequency Shift Keying) and top of it they use different types of multiplexing technologies such as FDMA(Frequency Division Multiple Access), TDMA(Time Division Multiple Access), OFDMA(Orthogonal Frequency Division Multiple Access) and so on. But the wireless communication began with simple amplitude modulation(AM) technique. Here it is shown how the simplest form of Amplitude Modulation works and how information is transmitted using AM transmitter. There are many ways of doing amplitude modulation such as using BJT transistor which was explained in the previous tutorial AM Transmitter with Crystal Oscillator. But here even simpler amplitude modulation technique is shown. This simple AM modulator uses diode and hence such modulator is called AM diode modulator. It is so simple that just one diode is required. This is called single diode AM modulator. Multiple diodes are also used as modulator and are called double diode AM modulator and modulator using four diodes are called ring diode modulator.

The following picture shows AM transmitter using single diode modulator build on a breadboard. It is driven by pre-amplified audio signal and 4MHz carrier signal. The audio pre-amplifier is on one plugin perfboard module and the 4MHz oscillator on the other perfboard module. 

AM transmitter with Single Diode Modulator

The AM transmitter was tested and the AM signal observed on oscilloscope.

testing AM transmitter

AM signal waveform on oscilloscope

 Now the following shows the AM transmitter using diode modulator circuit diagram.

AM Transmitter using Single Diode Modulator Circuit Diagram
In the above circuit diagram, the circuit around 2N3904 transistor Q3 is simple crystal oscillator. The circuit around 2N3904 transistor Q2 is the audio pre-amplifier circuit. The diode D1 along with resistor R11, RV1 and the LC tank circuit composes the diode modular. The diode used was simple 1N4007 diode.

The diode modular operation is as follows. At the input of the diode, the RF signal at 4MHz is mixed with the audio signal. The mixed signal enters into the diode which non-linearly mixes the signals and produces half rectified AM signal at the cathode. This half wave AM signal signal then enters the LC circuit which performs filtering and reproduces the other half wave signal to produce the full AM signal at its output. Filtering means that the signal entering into the LC circuit consist of signal composed of  desired AM signal and its harmonics but because the LC circuit to tuned to the AM signal frequency which is 4MHz, the AM signal passes through it and other unwanted signals with frequencies other than the AM signal frequency gets blocked. The value of the inductor and capacitor can be calculated using online LC circuit calculator as was done here.

Watch the following video that shows how this AM transmitter build using single diode modulator works. 

 In the test, audio was recorded with audacity free software and then played. The play enters the pre-amplifier circuit on breadboard via the PC speaker output. An electret microphone can also be easily adopted to fed in live audio into the AM transmitter. 

In the video, for sake of video recording the distance between the AM transmitter and the radio receiver was short and due to this there was bit of high noise as you can hear in the video. But in just some distance the received signal is clean. As a side note, AM signal are inherently noisy than FM signal from FM transmitter.

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