# Linear Time Invariant Systems

 Question 1
Which of the following options is true for a linear time-invariant discrete time system that obeys the difference equation:

$y[n]-ay[n-1]=b_0x[n]-b_1x[n-1]$
 A y[n] is unaffected by the values of $x[n - k]; k \gt 2$ B The system is necessarily causal. C The system impulse response is non-zero at infinitely many instants. D When $x[n] = 0, n \lt 0$, the function $y[n]; n \gt 0$ is solely determined by the function x[n].
GATE EE 2020   Signals and Systems
Question 1 Explanation:
\begin{aligned} y(n)-ay(n-1)&=b_{0}x(n)-b_{1}x(n-2) \\ &\text{By applying ZT,} \\ Y(z)-az^{-1}Y(z)&=b_{0}X(z)-b_{1}z^{-1}X(z)\\ \Rightarrow \, \, H(z)&=\frac{Y(z)}{X(z)}=\frac{b_{0}-b_{1}z^{-1}}{1-az^{-1}} \end{aligned}
By taking right-sided inverse ZT,
$h(n)=b_{0}a^{n}u(n)-b_{1}a^{n-1}u(n-1)$
By taking left-sided inverse ZT,
$h(n)=-b_{0}a^{n}u(-n-1)+b_{1}a^{n-1}u(-n)$
Thus system is not necessarily causal.
The impulse response is non-zero at infinitely many instants.
 Question 2
A continuous-time input signal x(t) is an eigenfunction of an LTI system, if the output is
 A k x(t) , where k is an eigenvalue B k $e^{j\omega t}$ x(t), where k is an eigenvalue and $e^{j\omega t}$ is a complex exponential signal C x(t) $e^{j\omega t}$, where $e^{j\omega t}$ is a complex exponential signal D k H($\omega$) ,where k is an eigenvalue and H($\omega$) is a frequency response of the system
GATE EE 2018   Signals and Systems
Question 2 Explanation:
Eigen function is a type of input for which output is constant times of input.
i.e. Where,
$x(t)=$ System input = eigen function
$H(s)=$ transfer function of system
$y(t)=$ system output
Here,
$y(t)=H(s)|_{s=a}\; e^{at}=k \cdot x(t)$
where,
k= eigen-value =$H(s)|_{s=a}$
$x(t)=$ eigen-function input
 Question 3
Let z(t)=x(t) * y(t) , where "*" denotes convolution. Let c be a positive real-valued constant. Choose the correct expression for z(ct).
 A c x(ct)*y(ct) B x(ct)*y(ct) C c x(t)*y(ct) D c x(ct)*y(t)
GATE EE 2017-SET-1   Signals and Systems
Question 3 Explanation:
Time scaling property of convolution.
If, $x(t)*y(t)=z(t)$
Then, $x(ct)*y(ct)=\frac{1}{c} z(ct)$
$z(ct)=c \times x(ct) * y(ct)$
 Question 4
Consider a causal LTI system characterized by differential equation $\frac{dy(t)}{dt}+\frac{1}{6}y(t)=3x(t)$. The response of the system to the input $x(t)=3e^{-\frac{t}{3}}u(t)$. where u(t) denotes the unit step function, is
 A $9e^{-\frac{t}{3}}u(t)$ B $9e^{-\frac{t}{6}}u(t)$ C $9e^{-\frac{t}{3}}u(t)-6e^{-\frac{t}{6}}u(t)$ D $54e^{-\frac{t}{6}}u(t)-54e^{-\frac{t}{3}}u(t)$
GATE EE 2016-SET-2   Signals and Systems
Question 4 Explanation:
The differential equation
\begin{aligned} \frac{dy(t)}{dt} &+\frac{1}{6}y(t)=3x(t) \\ \text{So, }sY(s)&+\frac{1}{6}Y(s) =3X(s) \\ Y(s) &=\frac{3X(s)}{\left ( s+\frac{1}{6} \right )} \\ X(s) &=\frac{9}{\left ( s+\frac{1}{3} \right )} \\ \text{So, } Y(s)&=\frac{9}{\left ( s+\frac{1}{3} \right )\left ( s+\frac{1}{6} \right )} \\ &=\frac{54}{\left ( s+\frac{1}{6} \right )} -\frac{54}{\left ( s+\frac{1}{3} \right )}\\ \text{So, }y(t) &= (54e^{-1/6t}-54e^{-1/3t})u(t) \end{aligned}
 Question 5
The output of a continuous-time, linear time-invariant system is denoted by T{x(t)} where x(t) is the input signal. A signal z(t) is called eigen-signal of the system T, when $T\{z(t)\} = \gamma z(t)$, where $\gamma$ is a complex number, in general, and is called an eigenvalue of T. Suppose the impulse response of the system T is real and even. Which of the following statements is TRUE?
 A cos(t) is an eigen-signal but sin(t) is not B cos(t) and sin(t) are both eigen-signals but with different eigenvalues C sin(t) is an eigen-signal but cos(t) is not D cos(t) and sin(t) are both eigen-signals with identical eigenvalues
GATE EE 2016-SET-1   Signals and Systems
Question 5 Explanation:
Given that impulse response is real and even, Thus $H(j\omega )$ will also be real and even. Since, $H(j\omega )$ is real and even thus,
$H(j\omega )=H(-j\omega )$
Now, $\cos (t)$ is input i.e. $\frac{e^{jt}+e^{-jt}}{2}$ is input
Output will be
$\frac{H(j1)e^{jt}+H(-j1)e^{-jt}}{2}=H(j1)\left ( \frac{e^{jt}+e^{-jt}}{2} \right )= H(j1) \cos (t)$
If, $\sin (t)$ is input i.e. $\frac{e^{jt}+e^{-jt}}{2}$ is input
Output will be
$\frac{H(j1)e^{jt}+H(-j1)e^{-jt}}{2}=H(j1)\left ( \frac{e^{jt}-e^{-jt}}{2j} \right )= H(j1) \sin (t)$
So, $\sin (t)$ and $\cos (t)$ are eigen signal with same eigen values.
 Question 6
Consider the following state-space representation of a linear time-invariant system.
$\dot{x}(t)=\begin{bmatrix} 1 & 0\\ 0&2 \end{bmatrix}x(t),$ $y(t)=c^{T}x(t),$ $c=\begin{bmatrix} 1\\ 1 \end{bmatrix}$ and $x(0)=\begin{bmatrix} 1\\ 1 \end{bmatrix}$
The value of y(t) for $t= log_{e}2$ is______.
 A 4 B 5 C 6 D 7
GATE EE 2016-SET-1   Signals and Systems
 Question 7
Consider a continuous-time system with input x(t) and output y(t) given by

y(t) = x(t) cos(t)

This system is
 A linear and time-invariant B non-linear and time-invariant C linear and time-varying D non-linear and time-varying
GATE EE 2016-SET-1   Signals and Systems
Question 7 Explanation:
\begin{aligned} y(t)&=x(t)\cos (t)\\ &\text{To check linearity,}\\ y_1(t)&=x_1(t)\cos (t)\\ &[y_1(t) \text{ is output for }x_1(t)]\\ y_2(t)&=x_2(t) \cos (t)\\ &[y_2(t) \text{ is output for }x_2(t)]\\ \text{so, the}& \text{ output for }(x_1(t)+ x_2(t)) \text{ will be}\\ y(t)&=[x_1(t)+ x_2(t)]\cos (t)\\ &=y_1(t)+y_2(t) \end{aligned}
So, the system is linear, to check time invariance.
The delayed output,
$y(t-t_0)=x(t-t_0)\cos (t-t_0)$
The output for delayed input,
$y(t, t_0)=x(t-t_0)\cos (t)$
Since, $y(t-t_0)\neq y(t,t_0)$
System is time varying.
 Question 8
The following discrete-time equations result from the numerical integration of the differential equations of an un-damped simple harmonic oscillator with state variables x and y. The integration time step is h.

$\frac{x_{k+1}-x_{k}}{h}=y_{k}$

$\frac{y_{k+1}-y_{k}}{h}=-x_{k}$

For this discrete-time system, which one of the following statements is TRUE?
 A The system is not stable for $h\gt 0$ B The system is stable for $h \gt \frac{1}{\pi }$ C The system is stable for $0 \lt h \lt \frac{1}{2\pi }$ D The system is stable for $\frac{1}{2\pi } \lt h \lt \frac{1}{\pi }$
GATE EE 2015-SET-2   Signals and Systems
 Question 9
For linear time invariant systems, that are Bounded Input Bounded Output stable, which one of the following statements is TRUE?
 A The impulse response will be integrable, but may not be absolutely integrable. B The unit impulse response will have finite support. C The unit step response will be absolutely integrable D The unit step response will be bounded.
GATE EE 2015-SET-2   Signals and Systems
 Question 10
The impulse response g(t) of a system, G , is as shown in Figure (a). What is the maximum value attained by the impulse response of two cascaded blocks of G as shown in Figure (b)? A $\frac{2}{3}$ B $\frac{3}{4}$ C $\frac{4}{5}$ D 1
GATE EE 2015-SET-1   Signals and Systems
Question 10 Explanation:
\begin{aligned} g(t)&=u(t)-u(t-1)\\ G(s)&=\frac{1}{s}-\frac{e^{-s}}{s}\\ G(s) \times G(s)&=g(t)* g(t) \end{aligned} Maximum value =1
There are 10 questions to complete. 