**Signals and Systems Interview Questions for freshers experienced :-**

**1. What is meant by step response of the DT system?**

The output of the system y(n) is obtained for the unit step input u(n) then it is said to be step response of the system.

**2. Define Transfer function of the DT system.**

The Transfer function of DT system is defined as the ratio of Z transform of the system output to the input. That is , H(z)=Y(z)/X(z).

**3. Define impulse response of a DT system.**

The impulse response is the output produced by DT system when unit impulse is applied at the input.The impulse response is denoted by h(n). The impulse response h(n) is obtained by taking inverse Z transform from the transfer function H(z).

**4. State the significance of difference equations.**

The input and output behaviour of the DT system can be characterized with the help of linear constant coefficient difference equations.

**5. Write the difference equation for Discrete time system.**

The general form of constant coefficient difference equation is, Here n is the order of difference equation. x(n) is the input and y(n) is the output.

**6. Define frequency response (or) transfer function of the DT system.**

The frequency response of the system is obtained from the Transfer function by replacing z = e jω

ie, H(z)=Y(z)/X(z), Where z = e jω

**7. Give the state equations for LTI DT systems.**

State equations: q (n+1) = A q(n) + b x(n) and y(n) = c q(n) + D x(n)

Where, A=[-a1 –a2] b=[1] c=[(b1-a1) (b2-a2)] and D=[1]

[1 0 ] [0]

**8. What are the blocks used for block diagram representation?**

The block diagrams are implemented with the help of signal multipliers, adders, delay elements, time advance elements and constant multipliers.

**9. State the significance of block diagram representation.**

The LTI systems are represented with the help of block diagrams. The block diagrams are more effective way of system description. Block Diagrams indicate how individual calculations are performed. Various blocks are used for block diagram representation.

**10.What are the properties of convolution?**

- Commutative
- Assosiative.
- Distributive

**11.State the Commutative properties of convolution?**

Commutative property of Convolution is, x(n)*h(n) = h(n)*x(n)

**12.State the Associative properties of convolution**

Associative Property of convolution is, [x(n)*h1n)]*h2(n)=x(n)*[h1(n)*h2(n)]

**13. State Distributive properties of convolution**

The Distributive Property of convolution is,

{x(n)*[h1(n)+ h2(n)]}= [x(n)*h1(n) + x(n)*h2(n)]

**14. Define causal LTI DT system.**

For a LTI system to be causal if h(n)=0, for n<0.

**15. How the discrete time system is represented?**

The DT system is represented either Block diagram representation or difference equation representation.

**16. What are the classification of the system based on unit sample response?**

- FIR (Finite impulse Response) system.
- IIR ( Infinite Impulse Response) system.

**17. What is meant by FIR system?**

If the system have finite duration impulse response then the system is said to be FIR system.

**18. What is meant by IIR system?**

If the system have infinite duration impulse response then the system is said to be FIR system.

**19. What is recursive system?**

If the present output is dependent upon the present and past value of input then the system is said to be recursive system.

**20. What is Non recursive system?**

If the present output is dependent upon the present and past value of input and past value of output then the system is said to be non-recursive system.

**21.What is the difference between recursive and non recursive system**

A recursive system have the feed back and the non recursive system have no feed back.And also the need of memory requirement for the recursive system is less than non recursive system.

**22. Define realization structure.**

The block diagram representation of a difference equation is called realization structure.These diagram indicate the manner in which the computations are performed.

**23.What are the different types of structure realization.**

- Direct form I
- Direcct form II
- Cascade form
- Parallel Form.

**24. What is natural response?**

This is output produced by the system only due to initial conditions . Input is zero for natural response. Hence it is also called zero input Response.

**25. What is zero input Response?**

This is output produced by the system only due to initial conditions. Input is zero for zero input response.

**26. What is forced response?**

This is the output produced by the system only due to input. Initial conditions are considered zero for forced response.It is denoted by y (f ) (n).

**27. What is complete response?**

The complete response of the system is equal to the sum of natural response and forced response .Thus initial conditions as well as input both are considered for complete response.

**28. What are the steps involved in calculating convolution sum?**

The steps involved in calculating sum are

- Folding
- Shifting
- Multiplication
- Summation

**29. Give the transfer function using state variables.**

H(z) = Y(z) / X(z) = c ( zI – A ) -1 b + D

Where, A=[-a1 –a2] b=[1] c=[(b1-a1) (b2-a2)] and D=[1]

[1 0 ] [0]

**30. What we have to do, if the spectrum of a discrete signal is to be expanded?**

We have to decimate the discrete signal.

**31. Why for signal analysis we use only sinusoidal waves and not other signals?**

We use only sinusoidal waves and not other signals because:

- The response of sine wave to a LTI system is also sinusoidal.
- The sinusoidal analysis of electric network is more simple and convenient.

**32. What do you mean by sinusoidal fidelity?**

sinusoidal fidelity is an important characteristic of linear system. If input to a linear system is a sine wave, the output will also be a sine wave at exactly the same frequency. Only the amplitude and phase can be different.

**33. Define Aliasing effect.**

Aliasing is an effect that causes different signals to become indistinguishable (or aliases of one another) when sampled. It also refers to the distortion or artifact that results when the signal reconstructed from samples is different from the original continuous signal.

**34. What is the double curse effect of Aliasing?**

Due to Aliasing high frequency contents are recovered at low frequency so both high frequency and low frequency contents are lost.

**35. What are mutually orthogonal functions?**

Two vectors are said to be orthogonal if their product is zero. i.e, the two vectors have nothing in common. Example, Trignometric and exponential functions.

**36. Define linearity or linear system.**

A linear system is a system that possesses the property of superposition, i.e, additive property and scaling or homogeneity property.

**37. Define fundamental frequency.**

It is the smallest frequency with which a signal repeats itself.

**38. What are passive and active filters?**

A passive filter is a kind of electronic filter that is made only from passive elements – in contrast to an active filter, it does not require an external power source (beyond the signal). An active filter is a type of analog electronic filter, distinguished by the use of one or more active components and require an external power source.

**39. Is it possible to design a filter which can give good results both in time domain and frequency domain?**

No it is not possible to design such kind of filter.

**40. What are the advantages of digital filter over analog filters?**

Digital filters have the following advantages compared to analog filters:

- Digital filters are software programmable, which makes them esay to build and test.
- Digital filters require only the arithmetic operations of addition, subtraction and multiplication.
- Digital filters do not drift with temperature or humidity or require precision components.
- Digital filters have a superior performance to cost ratio.
- Digital filters do not suffer from manufacturing variations or aging.

**41. Which property of Fourier transform is used in Analog modulation?**

Time shifting property of Fourier transform.

**42. What are the classification of the system based on unit sample response?**

FIR (Finite impulse response) system and IIR (Infinite impulse response) system.

**43. Define FIR system.**

If the system has finite duration impulse response then the system is said to be finite impulse response (FIR) system.

**44. Define IIR system.**

If the system has infinite duration impulse response then the system is said to be infinite impulse response (IIR) system.

**45. Give one example of FIR and IIR filters?**

Windowed sinc filters are the examples of FIR filters and moving average filters are the example of IIR filters.

**46. Which filter (FIR or IIR filters) is always stable and why?**

FIR filters are always stable because they do not contain poles.

**47. Define Fourier series?**

Fourier series is the representation of a function f(t) by the linear combination of elements of a closed set of infinite mutually orthogonal functions.

**48. What is Hilbert Transform?**

Hilbert transform of a signal x(t) is defined as the transform in which phase angle of all components of the signal is shifted by ±90∘.

**49. What do you mean by linear phase filter?**

If the system is having even symmetry around some frequency other than zero then the filter is said to have linear phase.

**50. Can we make non linear phase of IIR filter as linear phase?**

Yes linear phase can be obtained by bidirectional filtering at the expense of double the execution time and program complexity.