The document discusses the AM diode envelope detector. It describes how the diode detector detects the envelope of an AM signal using a diode and low-pass filter. The diode rectifies the signal, leaving only the positive or negative half. The filter then removes the high frequencies, leaving an audio signal. While simple and low-cost, the diode detector introduces distortion and has limited sensitivity. However, it remained widely used in radios due to its convenience and low price.

1. AM Diode Envelope
Demodulator
UNIT 1
Presented By
Mrs. M.P.Sasirekha

2. Diode Detector
• 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.

3. Diode Detector
• 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.

4. 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.

5. Diode Detector
• 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).

6. • 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.

8. Diode Detector
• 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.

9. 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.

11. AM diode detection process
• 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.

12. 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.

13. 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.

14. Envelope detector disadvantages
• 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.

15. Thank you