preemphasis

We have performed amplitude modulation using multisim live software. • In the experiment we observed that for different values of modulation index different output waveforms were obtained. • For the value m=0.133(m<1), it is observed that in output signal the carrier amplitude changes by only 50% and therefore, it will be under-modulated. • For value of m=1, approximately 100% modulation is observed in its output signal. • For value of m=2.2(m>1), it is observed that our output signal obtained is overmodulated as modulation changes over by 100% which cause distortion in its output. • Therefore, we can say that as the if there is increase in the value of modulation index , phase reversal is experienced by carrier, due to which wave leads to distortion at higher value of m. -------- High frequency signals have less energy and are more sensitive to noise.If they are transmitted without increasing their energy noise may destroy the information in the signal. Increasing the amplitude of high frequency components of messaging signals before modulation is called as pre-emphasis. Reduction in level of amplitude of received high frequency signal by the same amount as that of increase in pre-emphasis is defined as de-emphasis. Here pre-emphasis circuit acts as high pass filter and de-emphasis circuit acts as low pass filter ------------- From the Experiment ,we can conclude that Mixer circuit creates new frequencies from two signals applied to it. In its most common application, two signals are applied to a mixer, and it produces new signals at the sum and difference of the original frequencies. Mixers are widely used to shift signals from one frequency range to another, a process known as heterodyning, for convenience in transmission or further signal processing. For example, a key component of a super heterodyne receiver is a mixer used to move received signals to a common intermediate frequency. Frequency mixers are also used to modulate a carrier signal in radio transmitters. Here mixer produce difference frequency------- ------------ If the sampling rate is equal to or less than the Nyquist frequency, raising it will improve the reconstructed signal by removing aliasing distortion. However, all other circumstances being equal, if it is already far above, no improvement is projected. A number of approaches exist for reconstructing a continuous time signal from a discrete time signal. It is crucial to remember, however, that reconstruction is not the inverse of sampling, and that it only generates one conceivable continuous time signal that samples to a discrete time signal ------------------ The experiment showed how a low pass filter is used to smooth out a PAM signal and reconstruct samples of the PAM signal to reproduce an original message signal. With regards to removing unwanted higher frequencies, a high order filter with a sharp roll off may be used. This is done in commercial components. Selection of a duty cycle of 10 % or less is useful in avoiding aperture distortion. Also, Nyquist theorem needs to be adhered to reproduce a signal with no distortion -------------------------- ASK is a modulation technique used in digital data communication for a large number of low-frequency RF applications. The source transmits a large amplitude carrier when it wants to send a '1', and it sends a small amplitude carrier when it wants to send a '0' in its simplest form.