An outline of AM Modulation and demodulation

AM radio is one of the most obvious examples of AM modulation. AM stands for analog amplitude modulation. AM modulation has a reputation for poor sound quality. However, this isn’t really an appropriate way to express it. Humans can hear sound in frequencies from about 20 Hz to 16 kHz, and the music source is generally within this frequency range. For example, if you play music through a medium that only carries a frequency of about 3 kHz such as a telephone, naturally it will sound dull. While FM radio can transmit music up to about 15 kHz, AM radio can only transmit up to about 7.5 kHz. This is to comply with the radio wave band widths stipulated in the Radio Act, and it doesn’t mean that the sound quality of AM modulation itself is poor. However, the amplitude of the AM carrier wave can carry information. But the amplitude component is readily affected by noise, and since the modulation and demodulation is analog, the variation is also converted into sound, making it unpleasant to listen to.

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However, AM modulation is also used for aircraft radio, an application on which human lives depend. This is because humans can understand the transmissions even if they’re affected by noise to some extent. The simplicity of AM modulation is its selling point. Technology related to AM modulation is also used in the latest radios, and it includes some very important areas relating to research into radio modulation and demodulation. With amplitude modulation, the amplitude of carrier wave C is shifted proportionally to the information data in the modulating signal m(t). Amplitude modulation has long been used for radio and television broadcasting.

There are many types of amplitude modulation including AM (DSB-WC), DSB-SC, SSB-SC, VSB and so on. They each have their benefits and drawbacks, and are used as appropriate for the frequency band, transmitted power, required transmission quality and so on. AM radio uses the DSB (DSB-WC) modulation method where the direct current (positive component) is multiplied by the carrier wave and the modulated wave has a carrier wave component. The receiving wave includes a carrier wave component, so synchronization in the demodulation circuit is simplified. In double-sideband suppressed-carrier transmission (DSB-SC), the amplitude of the carrier wave is changed in accordance with the information. In DSB-SC modulation, the wave carrier is not transmitted in the modulated wave. DSB-SC is closely related to frequency conversion and single sideband suppressed carrier (SSB-SC) modulation, and is used in many applications. There is some confusion surrounding the various systems of amplitude modulation. For example, some people refer to AM as DSW, while others refer to DSB-SC as DSB. AM modulation is used as a generic term for amplitude modulation, but it includes a number of types, so it’s necessary to pay careful attention when discussing them. The types can be summarized as follows. Double sideband with carrier: DSB-WC(AM), referred to simply as DSW
Single sideband with carrier: SSB-WC
Double sideband suppressed carrier: DSB-SC
Single sideband suppressed carrier: SSB-SC


WC (With Carrier) means that the modulated wave includes a carrier wave component, and SC (Suppressed Carrier) means that there is no carrier wave component.　

The theory of AM modulation and demodulation

Amplitude modulation: AM radio
Amplitude modulation is often associated with the modulation method of AM radio. When the carrier wave is C(t) and the modulating signal is m(t), the modulated wave Sam is as follows.












Kam: Constant
With AM radio modulation, the amplitude of carrier wave Ac is shifted proportionally to the modulating signal m(t).

The modulated wave Sam when the modulating signal m(t) is a single sine wave is as follows.





If the initial phase ΦC of the carrier wave is 0, and Kam*Am is modulation factor m, the result is as follows.







The first equation shows the carrier wave component while the second equation shows that a spectrum where only the modulation signal frequency FM is separate on either side of the carrier frequency. Amplitude modulation shifts the spectrum of the modulating signal directly to the carrier wave. The upper part of the carrier frequency is the upper sideband (USB) while the lower part is the lower sideband (LSB). The bandwidth required for this modulation method includes the double sideband, so it is twice that of the modulating signal bandwidth. With this modulation, a carrier wave is transmitted and the receiver is built so as to be able to use it with demodulation, but the carrier wave has nothing to do with information transmission so it is not efficient from an electrical point of view. However, the receiver performs envelope detection and so the modulation factor must be 1 or higher, therefore the carrier wave component is unavoidable. A modulation factor of 1 or more results in over-modulation, so that synchronous detection is necessary.






Actual AM modulation and demodulation

Modulation
Using a modulator, the amplitude of the carrier wave is shifted proportionally to the amplitude of the modulating signal. A transistor or diode can be used as a modulator. For example, if the carrier wave and modulating signal are input with the transistor operating at grade C, a signal combining the DSB wave and harmonics is output. If the DSB wave is extracted with a resonance circuit, it results in an amplitude modulated wave including the carrier wave. In addition, if the carrier wave and modulating signal are input to a DBM (double-balanced modulator) and the nonlinearity of the DBM is used for modulation, a DSB-SC (double sideband suppressed carrier) amplitude modulation wave without a carrier wave can be obtained. AM radio uses a DSB wave with a carrier wave and asynchronous detection at the receiver. If the modulation index m is less than 1, the envelope of the modulated wave indicates the form of the modulating signal (information). If it is bigger than 1, the envelope no longer indicates the form of the modulating signal due to over-modulation. Therefore, it is important to note that the modulated wave does not include a modulation signal frequency component, as you can see by looking at the spectrum. This is strongly relevant to the demodulation method of the receiver.

Demodulation
Demodulation methods for AM modulating signals include synchronous detection and asynchronous detection, and the applet shown below allows you to switch between the two to see the difference. The receivers for AM radio broadcasts use envelope detection, a type of asynchronous detection, to simplify their structure and reduce their cost. Envelope detection is a detection method that can only perform demodulation when the envelope of the modulated signal indicates a modulating signal. The applet below can be set to over modulation with a modulation factor higher than 1, but in this case, since the envelope of the modulated signal does not indicate a modulating signal, envelope detection is not possible and it’s necessary to use synchronous detection. The applet below allows you to switch between synchronous detection and asynchronous detection. You can confirm that demodulation can be carried out properly even with over modulation if you use synchronous detection. However, please confirm this using a single sine wave. AM modulation and demodulation applet　

An AM modulation and demodulation applet

An AM modulation and demodulation applet Click the “Explain” button at the bottom right of the applet for how to use it.

The detection method used by the applet is as follows.
Synchronous detection: The carrier wave is multiplied with exactly the same transmission carrier wave, frequency and phase as the modulated wave.
Asynchronous detection: The modulated wave is rectified and signals with a level higher than 0 are extracted.

*Note: The modulation index of this applet is applied with a frequency 4 single sine wave.






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