Envelope Detector• An envelope detector is an electronic circuit that

takes a high-frequency signal as input and provides an output which is the "envelope" of the original signal.

• The capacitor in the circuit stores up charge on the rising edge, and releases it slowly through the resistor when the signal falls.

• The diode in series rectifies the incoming signal, allowing current flow only when the positive input terminal is at a higher potential than the negative input terminal.

• Any AM or FM signal can be written in the following form

• In the case of AM, φ(t), the phase component of the signal, is constant and can be ignored, so all the information in the signal is in R(t). R(t) is called the envelope of the signal. Hence an AM signal is given by the equation

with m(t) representing the original audio frequency message, C the carrier amplitude and R(t) equal to C + m(t). So, if the envelope of the AM signal can be extracted, the original message can be recovered.

A simple envelope demodulator circuit

A signal and its envelope marked with red

Drawbacks• The envelope detector has several drawbacks:• The input to the detector must be band-pass filtered

around the desired signal, or else the detector will simultaneously demodulate several signals. The filtering can be done with a tunable filter or, more practically, a superheterodyne receiver

• It is more susceptible to noise than a product detector

• If the signal is overmodulated, distortion will occur

• Most of these drawbacks are relatively minor and are usually acceptable tradeoffs for the simplicity and low cost of using an envelope detector.

Review of Modulation Circuits

• AM Generation• DSBSC Generation

AM Generation

• The function of an AM modulator is to modulate a carrier wave using an intelligence signal, which results in sum and difference frequencies, together with the carrier.

A possible way to achieve this is to use an operational amplifier to electrically sum the two signals as shown in the figure below. In such an arrangement the two signals will remain independent of each other.

The production of a typical AM wave requires the use of a non-linear device, such as a transistor or diode. A non-linear device is one that produces an output, which is not proportional to the input. An example of a circuit, which produces an AM wave, is shown.

The result of using the diode and resistor is to clip the negative half of the composite signal. The result is the output signal shown. It will be noticed that this signal is only the top half of the AM wave form.

To produce the full AM wave will require the use of a tank circuit. A diagram of the tank circuit is shown together with its output

If this circuit is incorporated into the original non-linear modulator circuit, the full AM wave can be produced

A diode ha s no gain. To produce gain, a transistor could be used in place of a diode. The circuit is as shown below.

DSBSC Generation

Circuits used to obtain DSBSC are called balanced modulators. The figure below shows an example of a balanced modulator. It uses diodes as the non-linear devices.

Circuits used to obtain DSBSC are called balanced modulators. The figure below shows an example of a balanced modulator. It uses diodes as the non-linear devices.

The diodes are paired and are turned on and off during the positive and negative half of the carrier frequency cycle.

During the positive half, D1 and D4 are on and the other two are off. The circuit will therefore look as follows

During the negative half, D2 and D3 are on and the other two are off. The resulting circuit is shown.

The modulating signal undergoes a 180o phase shift.

The current produced by the carrier signal is split at the centre taps of the transformers and flow in opposite directions. This leads to their cancellation as they produce magnetic fields, which are equal in magnitude but opposite in phase.

Baseband spectra and spectra of outputs from suppressed-carrier modulators. Amplitudes and phases are shown as functions of frequency f. (a) Baseband amplitude spectrum. (b) Baseband phase spectrum. (c) Amplitude spectrum of output from double-

sideband (DSB) modulator. (d) Amplitude spectrum of output from upper single-sideband (SSB) modulator. (e) Amplitude spectrum of output from lower single-

sideband (SSB) modulator. (f) Amplitude spectrum of output from vestigial-sideband (VSB) modulator

Balanced Modulator

• A modulator in which the carrier and modulating signal are introduced in such a way that the output contains the two sidebands without the carrier.