AM Transmitter with Crystal Oscillator

Today most technology has wireless communication installed in them. Sensors based system, manufacturing control system, homo automation, IoT(Internet of Things) are examples of such wireless based system. Examples of popularly used wireless technology includes WiFi using ESP8266 and NodeMCU, 2.5GHz based RF circuits and 433MHz RF module based circuit. These wireless communication modules are inexpensive and wireless system prototype can be easily build with these modules in few hours with all the libraries which can be found on the internet. Within these wireless module RF circuit is fairly complicated though it can be reproduced at home. Wireless communication started with development from AM(Amplitude Modulation), FM(Frequency Modulation) to PM(Phase Modulation), then QAM and into frequency multiplexing leading to high frequency RF circuit in the 900MHz 1800MHz GSM(Gobal System for Mobile Communication), CDMA and WiFi. Thus the basic of wireless communication is AM wireless system and understanding how AM wireless system is works is essential step for understanding how wireless circuit works. Here an AM transmitter is presented which is made up of Crystal Oscillator and AM modulator.

In AM wireless system, the carrier signal is modulated with changing modulating signal(audio signal for audio communication). The carrier signal frequency range for AM communication is 525KHz to 1605KHz. But frequency of carrier signal has nothing to do with AM. That is nature of the modulation of carrier signal that is used that characteristic AM and not the frequency. For example FM frequency range is in 86MHz to 106MHz but AM can designed that can be transmitted in these FM frequency range, that is, AM system with carrier frequency of 100MHz can be build. Here 4MHz is used for carrier signal which lies in the SW1(Short Wave 1) frequency range.

In AM the modulating signal which is the audio signal signal(20Hz to 22KHz) is impressed onto the RF carrier signal. This is done using modulator circuit. There are many AM modulation circuit or mixer circuit which can be used. These includes single diode mixer or double diode modulator, transistor mixer etc. 

Another circuit which is required in AM system is the RF oscillator that generates the carrier signal. Carrier signal can be generated in many ways like Colpitts oscillator, Hartley oscillator, Crystal oscillator, PLL and frequency synthesizer. Here the AM transmitter is build using crystal oscillator.

Thus the AM transmitter build here is build using BJT modulator and crystal oscillator which operates at 4MHz. The circuit is build and tested on a breadboard. 

The following shows the AM transmitter build on breadboard.

AM transmitter

The following shows the signal waveform on oscilloscope.

am signal on oscilloscope

The circuit diagram of the AM transmitter is shown below.

AM transmitter using modulator

The AM transmitter circuit above consist of three sections: Crystal oscillator circuit, audio signal input with pre-amplifier circuit and the AM modulator circuit. 

Crystal Oscillator

The crystal oscillator circuit section is made up of 4MHz quartz crystal X1 and 2N3904 NPN transistor Q3. This crystal oscillator is configured as Pierce oscillator. This oscillator was explained in details in the tutorial how to make simple Crystal Oscillator. The advantage of using crystal oscillator is that it produces stable frequency signal and the disadvantage is that the frequency is fixed and therefore cannot be tuned to other frequencies.

Audio Pre-amplifier

The audio pre-amplifier circuit section is made up of the 2N3904 transistor Q2 in the above circuit schematic. The audio input to this circuit is fed into amplifier which is shown as sine wave signal input. The audio is then amplified by the BJT amplifier. This circuit was build as a BJT Amplifier module for breadboard which is shown below. 

BJT amplifier module

This audio pre-amplifier is biased using voltage divider biasing which is explained in details in the tutorial how to bias a BJT using voltage divider biasing. There are several other methods of biasing a BJT transistor such as base bias, self bias, emitter bias. If you want to build BJT amplifier you can use the BJT amplifier design online calculator to easily calculate the component values. The voltage divider biasing method provides a stable bias for BJT. If you don't have oscilloscope you can also test an audio amplifier using PC oscilloscope which is explained in the tutorial How to Build BJT amplifier and test with Soundcard based PC Oscilloscope.

In the demonstration video below, audacity software was used to record audio and then fed that audio into the amplifier circuit.

AM modulator

The third section is the AM modulator using BJT transistor Q1 build with 2N3904 transistor. The 4MHz RF carrier signal enters into the base of the modulator transistor Q1 via the coupling capacitor C1. The amplified audio signal enters the emitter of the AM modulator via the coupling capacitor C4. The audio signal then modulates the carrier signal which appears at the collector. The collector is connected to an LC resonant tank circuit which is configured to resonate at the carrier frequency of 4MHz. This LC circuit performs filtering such that signal at 4MHz is generated at the output. The value of the inductor that was used is 150uH and capacitor that was used is 10pF. The calculated value of the C is 10.56 pF and closest value of 10pF was chosen. The values of these inductor and capacitor was calculated using the online LC circuit calculator

Video demonstration

The following video demonstrates how the above AM transmitter using crystal oscillator works.


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