1. HOME
  2. HF-Technik
  3. Funktechnik
  4. Digital modulation: Amplitude shift keying(ASK)

技術情報

  • RF design guide
  • Calculation tools
  • Radio technology
  • Technical literature
  • Modem evaluation

Introduction

下線 トップへ戻る

This Java applet allows you to experience the basics of ASK/OOK modulation and demodulation visually. It’s interesting to look at the spectrogram and try changing the various parameters. You can understand the background to the characteristics of the signal. To run the applet, click the picture.

*Java Runtime Environment version 5.0 or higher is required to run the applet.
Download:http://www.java.com/en/download/index.jsp

An outline of digital amplitude modulation and demodulation

下線 トップへ戻る

Digital amplitude modulation and demodulation includes ASK and OOK. ASK stands for Amplitude Shift Keying, and OOK stands for OnOff Keying. The modulation and demodulation circuits for digital amplitude modulation are simple and relatively cheap, and so they are used in relatively low cost systems where long communication range is not necessary such as RFID, ETC, keyless entry and so on. Demodulation methods for ASK modulating signals include synchronous detection and asynchronous detection. However, in most cases, asynchronous detection is used because it is cheaper to implement.

The theory of digital amplitude modulation and demodulation

下線 トップへ戻る

◆Modulation
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.With ASK/OOK modulation, a modulator is used to shift the amplitude of the carrier wave proportionally to the amplitude of the modulating signal (digital signal). In the case of a digital signal, the modulating signal applied to the modulator is called digital modulation. The modulator is the same as for analog modulation. With an ASK modulating signal, the digital signal value is either -1 or +1, and when it is changed to 0 and +1 it is called OOK. With OOK, the amplitude direction of the modulated wave is indicated by the presence or absence of a carrier wave. In other words, when the signal is 0, there is no carrier wave and when it is 1, there is a carrier wave. The spectrum of the ASK modulated wave is centered on the carrier frequency and the square wave spectrum which is the modulating signal takes a spread-out form.

ASK_03.gif

Expressed as a formula, carrier wave C(t) takes the following form.
eq024

The ASK modulated wave is modulating signal m(t) multiplied by carrier wave C(t), and is expressed as a formula as follows.
eq025


◇ASK constellation
ASK can also be expressed as the constellation in the figure below, with the information at amplitude point 0 and 1 at phase 0 rad corresponding to 0 and 1. 0 rad means that even if the information signal changes, there is no phase shift.

ASK_06

◆Demodulation
The receivers 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 can be set 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 allows you to switch between synchronous detection and asynchronous detection. You can confirm that demodulation can be carried out properly even with a modulation index over 1 if you use synchronous detection.

The detection method used by the applet is as follows.

 Synchronous detection:
 The modulated wave is multiplied with the carrier frequency which is exactly the same frequency and phase as the transmission carrier wave.

Asynchronous detection:
 The received signal is multiplied by a squarer. Synchronous detection
When the ASK modulated wave is multiplied to become a regenerated carrier wave, it is as follows. The second term in the braces is an unwanted component, so only the LPF signal component is added. The determination device determines the level of the signal and the transmitting end information signal is obtained.

◆Synchronous detection

ASK_04

When the ASK modulated wave is multiplied to become a regenerated carrier wave, it is as follows.
eq026

The second term in the braces is an unwanted component, so only the LPF signal component is added.
eq027
The determination device determines the level of the signal and the transmitting end information signal is obtained.

◆Example of asynchronous detection

ASK_05

The ASK modulated wave is multiplied by a squarer as follows.
eq028

The second term in the braces is an unwanted component, so only the LPF signal component is added.
eq029
The determination device determines the level of the signal and the transmitting end information signal is obtained.

ASK/OOK modulation and demodulation applet

下線 トップへ戻る

An ASK modulation and demodulation applet Click the “Explain” button at the bottom right of the applet for how to use it.
Demodulation methods for ASK modulating signals include synchronous detection and asynchronous detection, and the applet shown below allows you to switch between the two to see the difference.

ASK_00

Click the image: ASK/OOK modulation and demodulation applet

back
このページのトップへ