balanced modulator.pptx breif introduction
- 1. -The modulating signal x(t) is applied equally with 180o phase reversal at the inputs of both the diodes through the input center tapped transformer . -The carrier is applied to the center tap of the secondary. The Input voltage to D1 is given by : ……..(1) The input voltage to D2 is given by : ……….(2) The parallel RLC circuits on the output side form the band pass filters .
- 3. =K x(t) Cosct Thus, the diode balanced modulator produces the DSB-SC signal at its output .
- 4. Envelope Demodulation The envelope demodulator is a simple and very efficient device which is suitable for the detection of a narrowband AM signal. A narrowband AM wave is the one in which the carrier frequency fc is much higher as compared to the bandwidth of the modulating signal. An envelope demodulator produces an output signal that follows the envelope of the input AM signal exactly . It is used in all the commercial AM radio receivers . The envelope demodulator consists of a diode and RC filter . Figure 1. Diode Detector
- 5. Working Operation The standard AM wave is applied at the input of the demodulator in figure 1. In every positive half cycle of the input, the demodulator diode is forward biased and charge the filter capacitor C connected across the load resistance R to almost the peak value of the input voltage. As soon as the capacitor charges to the peak value, the diode stop conducting . The capacitor will now discharge through R between the positive peaks as shown in fig. 2 The discharging process continues until the next positive half cycle . When the input signal becomes greater than the capacitor voltage, the diode conducts again and the process repeats itself .
- 6. Figure 2 waveforms of Envelope detector The input-output waveforms for the envelope demodulator is shown in fig. 2. It shows the charging discharging of the filter capacitor and the approximate output voltage. It may be observed from these waveforms that the envelope of the AM wave is being recovered successfully. Here we have assumed that the diode is ideal and the AM wave applied to the input of the demodulator is supplied by a source having internal resistance Rs .
- 7. Selection of the RC time Constants -The capacitor charges through D and Rs when the diode is on and it discharges through R when the diode is off. -The charging time constant RsC should be short compared to the carrier period 1/fc . Thus, RsC << 1/fc -On the other hand, the discharging time constant RC should be long enough so that the capacitor discharges slowly through the load resistance R . -But, this time constant should not be too long which will not allow the capacitor voltage to discharge at the maximum rate of change of the envelope . -Therefore, 1/fc << RC << 1/W where, W = Maximum modulating frequency
- 8. Distortions in the Envelope Demodulator Output There are two types of distortions which can occur in the detector output such as : 1.Diagonal clipping 2.Negative peak clipping Diagonal Clipping This type of distortion occurs when the RC time constant of the load circuit is too long. Due to this, the RC circuit cannot follow the fast changes in the modulating envelope . The diagonal clipping is shown in figure below.
- 9. Negative Peak Clipping This distortion occurs due to a fact that the modulation index on the output side of the detector is higher than that on its input side. Hence, at higher depth of modulation of the transmitted signal, the overmodulation may takes place at the output of the detector. The negative peak clipping will take place as a result of this overmodulation as shown in figure below.