Disscussion

The purpose of this simulation is to demonstrate the characteristics and operation of an envelope detector, and to provide a comprehension of the stages that a modulated signal is subjected to at the receiver, so that the original transmitted information is recovered.

An AM signal, once received by a receiver, is subjected to several stages in the demodulation process. Figure 12-1 illustrates the final detection and filter stage using a simple diode detector. Other more complex detectors that use the popular PLL (phase-lock-loop) circuitry allow, together with AGC (automatic gain control) circuitry, modulation indexes of close to one.

Because the circuitry involved in the detection process is fixed, a fundamental requirement for a signal at the detector's input is that the sidebands are situated on either side of a fixed frequency. This fixed frequency is called the IF or intermediate frequency and is produced by the mixing of a local oscillator frequency with the RF spectrum which has been filtered in the RF stage of the demodulation process. The fixed value of the intermediate frequency is 455 kHz. This IF signal is applied to the input of a highly selective IF amplifier.

The local oscillator (LO) frequency in the popular superheterodyne receiver is adjusted through the tuning control to 455 kHz above the RF carrier, fLO = fc + fIF. Why is the LO necessary? Remember that the detector requires the message signal to be frequency translated to either side of a fixed intermediate frequency. Injecting the RF spectrum and the local oscillator frequency through a mixer will produce the sum and difference of the frequencies involved. It is the difference frequencies that produce the IF spectrum required. Consider a carrier frequency of 1050 kHz carrying a 5 kHz message signal.

The IF filter features steep roll off characteristics which reject all frequencies other than the IF frequency translated spectrum. The output of the filter constitutes the input to the detector.

The envelope detector of Figure 12-1 is designed to subject the signal to a half wave rectification process. The RC time constant should be such that the charge time is fast and the discharge time is slow. This will ensure that the detector follows the amplitude variations of the envelope. The RC time constant of the envelope detector should be designed such that:

One of the main drawbacks of the envelope detector is the effect of the diode voltage drop Vd. This 0.7 V drop represents a delay between the point where the signal reaches the input and where the capacitor is able to allow the output to react to the input. This ultimately results in power lost because the modulation index is restricted from reaching its optimum level of one. The detector of Figure 12-2 will detect modulation signals over a range of frequencies with the particular low pass filter portion supporting a cutoff frequency of 2 kHz for purposes of demonstration.

2π mfm