Analog Communications : Practice Questions and Answers

Frequency multiplier multiplies the frequency of the input by an integer, such that the output is a multiple of the input. Frequency multipliers are generally class C amplifiers.

If a source with low impedance is connected with a load with high impedance then the power that can pass through the connection is limited by higher impedance. Generally the two networks, pi and T are used for impedance matching.

For low level AM, modulation is done at low power of carrier and modulating signal, so power amplifiers are used to boost the carrier and modulating signal. Therefore, output power is low. Thus, the amplifiers used are linear.

In high level AM, modulation is done at high power of carrier and modulating signal therefore output power is high. Therefore, power amplifiers are not used to boost the carrier and modulating signal. Thus, all RF amplifiers are non-linear.

The quality factor Q, of a resonant circuit is a measure of quality of resonant circuit, thus indicating the performance of the resonant circuit. Bandwidth can be seen as the ratio of the resonant frequency to quality factor. A higher value for Q means a more narrow bandwidth, which we want in many applications.

Aliasing causes different signals to become indistinguishable when sampled. It happens when the sampling rate is less than Nyquist rate. To prevent aliasing, sampling signals should be at a rate which is twice of Nyquist Rate.

The response of a receiver to weak signals is limited by the noise. This noise is generated in the receiver only. Noise is the unwanted signal that is present in the passband of the signal.

Distortion in a receiver can occur in either mixer or in the detector. It can also occur in IF amplifiers. Distortion is the change in the shape of the waveform.

Neutralization cancels unwanted feedback by adding feedback out of phase with the unwanted feedback. It is a method of filtering noise out of signals.

Impedance matching is the input impedance of an electrical load or the output impedance of corresponding electrical signal source to maximize power transfer or minimize signal reflection from the load. Transformer coupling is basically used for impedance matching in RF amplifiers. It is usually used with a small load for power amplification.

Angular separation between the two half-power (-3dB) points is known as beam width. It is usually expressed in degrees.

Delta modulation is an analog-to-digital and digital-to-analog signal conversion technique used for transmission of voice information where quality is not of primary importance. It is the simplest form of differential pulse-code modulation (DPCM) where the difference between successive samples are encoded into n-bit data streams. The two major drawbacks in delta modulation are slope overload and granular noise. When step size becomes too small to track the original waveform it generates an error known as slope overload and granularity occurs when step size is too large.

A frequency tripler is a frequency multiplier in which an electronic circuit generates an output signal whose output frequency is a harmonic (multiple) of its input frequency. When a FM signal is passed through a frequency tripler, it increases its modulation index 3 times. So the modulation index of output is 3mf.

Frequency of DC signals is zero. For zero frequency, capacitive reactance XC is infinite as XC = 1/(f*C), where f is the frequency and C is the capacitance.

Power density of quadrature component is

Hilbert transform of a signal results in a signal with same amplitude but a phase shift of -90 degrees. Thus, a signal and it’s Hilbert transform have same amplitude spectrum i.e. they have same energy density spectrum and also correlation spectrum.

In television system in India, each of odd and even fields are scanned 25 times but alternately.

In SSB-SC(Single Side Band Suppressed Carrier), the carrier is suppressed and either of the upper side-band and lower-sideband, are transmitted. This reduces it’s bandwidth to the frequency of the message signal.

Signal to Noise ratio compares signal power to noise power. It determines how much signal is present in a transmitted wave compared to presence of noise. It is generally expressed in decibels(dB). For a hi-fi system, it should be equal to 50dB.

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