Review of Modulation CircuitsAM GenerationAM DetectionDSBSC GenerationFM GenerationFM Detection

AM Modulators

The function of an AM modulator is to modulate a carrier wave using an intelligence signal, whichresults 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 asshown in the figure below. In such an arrangement the two signals will remain independent of eachother.

The production of a typical AM wave requires the use of a non-linear device, such as a transistor ordiode. A non-linear device is one that produces an output, which is not proportional to the input. Anexample 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. Theresult is the output signal shown. It will be noticed that this signal is only the top half of the AMwave form. To produce the full AM wave will require the use of a tank circuit. A diagram of the tankcircuit is shown together with its output.

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

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

AM DetectionThe demodulation of an AM wave can be done using a non-linear or a linear detector. A typical non-linear detector is the square law detector. This is based on a diode in series with a resistive load. Itcan be used to detect weak signals especially as occurs in radar systems.

The linear detector is the most commonly used because of its simplicity. An example of a lineardetector is the envelope detector. Since the shape of the AM signal is similar to the shape of themodulating signal, a circuit, which can follow the envelope, is desirable. A simple circuit is shown.

Even though the diode is a non-linear device, for large input signals it may be regarded as linear. Theoutput signal will therefore be proportional to the input. If the time constant, RC is correct, the outputvoltage will follow the modulation envelope and will be made up of the audio signal together with aDC voltage. The dc can be remove by use of a blocking capacitor. The correct value of RC can beobtained from

sm

mRC

ω

21−≤

DSBSC Generation

Circuits used to obtain DSBSC are called balanced modulators. The figure below shows an exampleof 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 carrierfrequency cycle.During the positive half, D1 and D4 are on and the other two are off. The circuit will therefore lookas 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 inopposite directions. This leads to their cancellation as they produce magnetic fields, which are equalin magnitude but opposite in phase.

FM GenerationVaractor ModulatorA typical circuit will be based on a varactor diode. When a diode is reverse biased, it causes acapacitance to develop across it. The capacitance may be approximated using

VC

1∝

where V is the reverse voltage. Silicon diodes usually have a capacitance of between 150 to 200 pF if1 volt is applied. This decreases to about 50 pF with applied voltage of 10 volts. The varactorcapacitance in the circuit is varied by the modulating voltage which subsequently causes theoscillator frequency to vary as required for FM generation.

Example:An oscillator operating at 100MHz has a 75 pF capacitor in its tuning circuit. What total capacitanceswing must the varactor supply to allow the modulator to have a 80kHz peak deviation?

FM Detection

Discriminators are used to extract the modulating frequency from an FM signal. In recent times thephase locked loop has become quite popular as a means of demodulating an FM signal. Mainadvantage: no need for coil adjustments as found in other forms of discriminators. A block diagramis shown.