Amplitude Modulation, AM Tutorial Includes:
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Modulation formats: Modulation types & techniques Frequency modulation Phase modulation Quadrature amplitude modulation
The diode detector is the simplest and most basic form of amplitude modulation, AM detector and it detects the envelope of the AM signal.
The AM diode detector can be built from just a diode and a few other components and as a result it is a very low cost circuit block within an overall receiver.
As a result of its cost and convenience, the AM diode envelope detector has been widely used for many years in transistor portable radios.
AM diode detector basics
The AM diode detector is an envelope detector – it provides an output of the envelope of the signal.
As the name implies, the AM diode detector consists of a diode or other non-linear device.
The diode detector consists of two main elements:
- Diode / rectifier: The diode in the detector serves to that enhances one half of the received signal over the other.
- Low pass filter: The low pass filter is required to remove the high frequency elements that remain within the signal after detection / demodulation. The filter usually consists of a very simple RC network but in some cases It can be provided simply by relying on the limited frequency response of the circuitry following the rectifier.
AM diode detection process
In rectifying the RF signal, the AM diode detector provides an output equivalent to the envelope of one half of the signal, i.e. it is an envelope detector.
In view of the operation of the diode detector, it may sometimes be referred to as an envelope detector.
The incoming amplitude modulated RF signal consists of a waveform of both positive and negative going voltages as shown. Any audio transducer would not respond to this.
The diode envelope detector rectifies the waveform leaving only the positive or negative half of the waveform.
The high frequency element of this is then filtered out, typically using a capacitor which forms the low pass filter and effectively ‘fills in’ the high frequency elements, leaving a waveform to which a transducer like a pair of earphones or a loudspeaker could respond to and convert into sound waves.
It is often necessary for diode envelope detectors used in various circuits to be matched to an impedance of 50Ω.
The basic circuit consisting of the diode, load resistor and the smoothing capacitor will never present a good match to 50Ω
If the detector diode is in its on state, the circuit will appear to be less than 50Ω.
To overcome this issue it is normal practice to use an impedance transformer to step up its impedance.
Advantages and disadvantages of a diode envelope detector
The AM diode envelope detector has been successfully used for many years.
Envelope detector advantages:
- Low cost: The diode detector only requires the use of a few low cost components. This made it ideal for use in transistor (and valve / vacuum tube ) radios using discrete components.
- Simplicity: Using very few components, the Diode AM detector was easy to implement. It was reliable and did not require any setup.
Envelope detector disadvantages:
- Distortion: As the diode detector is non-linear it introduces distortion onto the detected audio signal.
- Selective fading: One of the issues often experienced on the short and medium wavebands where the AM transmissions are located is that of selective fading. The diode envelope detector is not able to combat the effects of this in the way that some other detectors are able, and as a result, distortion occurs when selective fading occurs.
- Sensitivity: The diode detector is not as sensitive as some other types. If silicon diodes are used, these have a turn on voltage of around 0.6 volts as a result, germanium or Schottky diodes are used which have a lower turn on voltage of around 0.2 to 0.3 volts. Even with the use of the Schottky diode, the diode envelope detector still suffers from a poor level of sensitivity
The AM diode envelope detector has been available for many years. It has been widely used. Although amplitude modulation is used less these days, and other forms of AM detector can be easily incorporated into integrated circuits, the simple diode detector still has some advantages.
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