# Digital Communication Viva Questions

Hey there! Welcome to another informative post on geekstocode! Today we will be looking into Digital Communication Viva Questions. This will help you with your interviews and even Lab viva exams. Thus, going through this will boost your knowledge about the topic.

This post will cover questions on Digital Communications. Thus, it contains all the important concepts that you need to prepare for your interview or Lab viva.

Therefore, let us not waste time anymore  and get started!

## Digital Communication Viva Questions.

#### 1. What do you mean by Digital System?

Digital systems are the systems who store, process, and communicate information in digital form. Hence, we find them in a wide range of applications.

For example, the digital computer, more commonly called the computer. In other words, digital systems are the systems that process a discrete or digital signal.

#### 2. How do you define a bit?

We define bits as the binary digits that is 0 and 1.

#### 3. What is the best Example of Digital system?

Digital Computer or a computer is the best example of a digital system.

#### 4. What are the types of number systems?

We have total of four types of number systems. They are:

1. Decimal Number System.
2. Binary Number System.
3. Octal Number System.

#### 5. What is a Logic gate?

Logic gates are the basic gates that make up the digital system. Also, the electronic circuit is a circuit that can operate on many binary inputs to perform a particular logic function.

#### 6. What are the basic Logic gates?

The three basic logic gates are :-

1. AND gate.
2. OR gate.
3. NOT gate.

#### 7. What is the difference between Latch And Flip-flop?

Latches are level triggered whereas flip flops are edge triggered. In latches, level triggered means that the output of the latches changes as we change the input and edge triggered means that the control signal only changes its state when it goes from low to high or high to low.

Latches are fast as compared to flip flops.

#### 8. What is the binary number system?

The system which has a base 2 is known as the binary system and it consists of only two digits 0 and 1.

For Example: Let us take decimal number 125

125 = 100 + 20 + 5

Which implies

1×100 + 2×10 + 5

1 ×102 + 2×101 + 5×100

Here, 125 consists of three bits, we start writing the numbers from the rightmost bit power as 0 then the second bit as power 1 and the last as power 2. So, we can represent a decimal number as

∑digit × 10corresponding position or bit

Here 10 is the total number of digits from 0 to 9.

#### 9. Write down the different analog pulse modulation techniques?

We have three different analog pulse modulation techniques:

(a) Pulse Width Modulation (PWM)

(b) Pulse Amplitude Modulation (PAM) and

(c) Pulse Position Modulation (PPM)

#### 10. Define PAM, PWM and PPM?

Pulse amplitude modulation (PAM) : Here, the amplitude of the carrier (pulse height) is proportional to the amplitude of the message signal.

The Pulse width modulation (PWM) : Here, pulse width of the carrier signal is proportional to the message signal’s amplitude.

Lastly, the Pulse position modulation(PPM) : Here, the pulse position of the carrier signal is proportional to the message signal’s amplitude.

#### 11. Compare PAM, PWM and PPM?

We can compare PAM, PWM and PPM by the following parameters:

#### 12. How do you calculate the bandwidth of a BPSK signal?

We calculate the bandwidth of BPSK by the following formula : B= 2Fc, Fc is the carrier frequency of a BPSK signal.

Amplitude Shift Keying is a type of amplitude modulation when the variations in the amplitude of a carrier wave represents the digital data. Also, we use the binary symbol d-amplitude carrier wave and fixed frequency for a bit length of T seconds in an ASK method.

#### 14.  What do you know about Frequency Shift Keying (FSK)?

As per the changes of the digital signal, if the frequency of the carrier signal varies, then that type of frequency modulation is said to be as FSK. Thus, if the message frequency is greater than the carrier frequency, then it is represented by ‘1’ and  represents ‘0’ if the message frequency is less than the carrier frequency.

#### 15. What do you understand about Phase Shift Keying (PSK)?

PSK is when at a particular time by varying the sine and cosine inputs, the system changes the phase of the carrier signal. Also, the ‘1800‘ phase shift carrier wave represents ‘0’ and ‘0’ phase shift carrier wave represents ‘1’.

#### 16. Explain about Binary Phase Shift Keying (BPSK)?

The system changes the phase of the carrier signal to 0 or 180 degrees for each binary data which is 0 or 1.

#### 17. What you understand about the Quadrature Phase Shift Keying 9QPSK)?

By the phase shift keying 9QPSK we understand that the system has changed phase of the carrier wave to four different shifts (45, 135, -45 and -135) for each two bits of binary datas (00, 01, 10 and 11).

#### 18. Compare B-W, Power, Probability of error and S/N of ASK, FSK and PSK.

The comparison of the above parameters are :

Bandwidth: FSK > PSK > ASK

Power:    PSK = FSK > ASK

Probability of error: FSK< PSK < ASK

Lastly, Signal to Noise Ratio: FSK > PSK > ASK

#### 19. Which modulation technique is known as ON-OFF keying? Justify your answer.

The Amplitude Shift Keying (ASK) modulation is referred as ON-OFF keying.

In the ASK modulation scheme, if the input is 0, then the output is also 0. But if the input is ‘1’ , the output is the carrier signal. Thus, ASK is also termed as ON-OFF keying.

#### 20. Give some differences between bit rate and baud rate?

Bit rate is the number of bits transmitted per unit time. Whereas, the baud rate is the number of signal units transmitted per unit time.

#### 21. Explain the aliasing effect of sampling?

Aliasing effect is a phenomenon when the sampling frequency becomes less than the Nyquist rate. Then the system cannot recover the original signal from its samples. Therefore, if aliasing occurs , then we cannot retrieve the original message signal from its samples.

## Conclusion:

Today we have covered Digital Communication Viva Questions. Make sure to go through all these important questions. Therefore, in this way you can cover all the important topics in the subject. Hence, you have now successfully prepared for your Viva on Digital Communications by reading these questions thoroughly.

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