Amplitude Modulation, AM Tutorial Includes:
Amplitude modulation, AM AM basic theory & formulas AM bandwidth & sidebands Modulation index & depth AM efficiency AM demodulation / detection Diode detector Synchronous detector AM modulators Single sideband, SSB SSB demodulation
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 signal 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. In the early days of radio, these signal detectors were made using discrete components, but modern radios will use integrated circuits with inbuilt detectors.
As a result of its cost and convenience, the AM diode envelope detector has been widely used for many years in transistor portable radios.
Although its simplicity has been the main reason for its widespread use, its performance is not as good as other types of AM detector / demodulator, particularly with respect to the distortion levels.
Not only is the basic AM diode signal detector used for AM envelope detection, but is also widely used n RF circuits in general for signal level detection.
AM diode detector basics
The AM diode detector is an envelope detector – it provides an output of the envelope of the signal. As such the diode detector or demodulator is able to provide an output proportional to the amplitude of the envelope of the amplitude modulated signal.
As the name implies, the main component within the AM diode detector is a semiconductor diode, although in the days of valve / tube technology, diodes using this form of technology were also used.
The signal diode detector consists of two main elements to the circuit:
- Diode / rectifier: The diode in the detector serves to that enhances one half of the received signal over the other. In many instances Schottky diodes are used for this form of detector, because signal levels may be low, and Schottky diodes have a much lower turn on voltage (typically around 0.2 V) than standard silicon diodes (typically around 0.7 or 0.7 V).
- 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. As the capacitor in the circuit stores the voltage, the output voltage reflects the peak of the waveform. Sometimes these circuits are used as peak detectors.
When selecting the value of the capacitor used int he circuit, it should be large enough to hold the peak of the RF waveform, but not so large that it attenuates any modulation on the signal, i.e. it should act as a filter for the RF carrier and not the audio modulation.
The circuit typically has a relatively high source impedance. When linking the circuit to a following stage of the circuit, care should be taken not to land the detector too much otherwise the operation will be impaired.
Normally a resistor is placed across the capacitor - this may either be the load of the next stage, a volume control, or resistor in the circuit. This level of this should be determined by calculating the time constant of the capacitor and the load. This should be between the RF signal and audio modulation so that the RF is satisfactorily removed, but the audio modulation is left untouched.
It is worth noting in this circuit that the secondary of the transformer provides a DC return to ground. Sometimes when the AM signal detector is used using a capacitor connection tot he previous stage, then a resistor or choke (inductor) to ground must be used at the input so that a DC return path is provided. If not the circuit will not operate correctly.
The value of the resistor on the input providing the DC return path is normally critical, but it can help provide the require match without absorbing too much signal.
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 ensure that the optimum match is achieved and the best overall circuit performance is achieved.
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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