4.8 Linear time invariant systems

When we apply a periodic input to a linear, time-invariant system, the output is periodic and has Fourierseries coefficients equal to the product of the system's frequency response and the input's Fourier coefficients ( Filtering Periodic Signals ). The way we derived the spectrum of non-periodic signal from periodic ones makes it clear that thesame kind of result works when the input is not periodic: If $x(t)$ serves as the input to a linear, time-invariant system having frequency response $H(f)$ , the spectrum of the output is $X(f)H(f)$ .

Let's use this frequency-domain input-output relationship forlinear, time-invariant systems to find a formula for the $RC$ -circuit's response to a pulse input. We have expressions for the input's spectrum and the system'sfrequency response.

• Multiplication by $\frac{1}{i\times 2\pi f}$ means integration.
• Multiplication by the complex exponential $e^{-(i\times 2\pi f\Delta )}$ means delay by $\Delta$ seconds in the time domain.
• The term $1-e^{-(i\times 2\pi f\Delta )}$ means, in the time domain, subtract the time-delayed signal from its original.
• The inverse transform of the frequency response is $\frac{1}{RC}e^{-\left(\frac{t}{RC}\right)}u(t)$ .

In this example, we used the table extensively to find the inverse Fourier transform, relying mostly on what multiplicationby certain factors, like $\frac{1}{i\times 2\pi f}$ and $e^{-(i\times 2\pi f\Delta )}$ , meant. We essentially treated multiplication by these factorsas if they were transfer functions of some fictitious circuit. The transfer function $\frac{1}{i\times 2\pi f}$ corresponded to a circuit that integrated, and $e^{-(i\times 2\pi f\Delta )}$ to one that delayed. We even implicitly interpreted the circuit's transfer function as the input's spectrum! Thisapproach to finding inverse transforms -- breaking down a complicated expression into products and sums of simplecomponents -- is the engineer's way of breaking down the problem into several subproblems that are much easier to solve and then gluing the results together. Along the way we may make thesystem serve as the input, but in the rule $Y(f)=X(f)H(f)$ , which term is the input and which is the transfer function ismerely a notational matter (we labeled one factor with an $X$ and the other with an $H$ ).

Transfer functions

The notion of a transfer function applies well beyond linear circuits. Although we don't have all we need to demonstratethe result as yet, all linear, time-invariant systems have a frequency-domain input-outputrelation given by the product of the input's Fourier transform and the system's transfer function. Thus, linear circuits area special case of linear, time-invariant systems. As we tackle more sophisticated problems in transmitting, manipulating, andreceiving information, we will assume linear systems having certain properties (transfer functions) without worrying about what circuit has the desired property. At this point, you may be concerned thatthis approach is glib, and rightly so. Later we'll show that by involving software that we really don't need to beconcerned about constructing a transfer function from circuit elements and op-amps.

Commutative transfer functions

Another interesting notion arises from the commutative property of multiplication (exploited in an example above ): We can rather arbitrarily choose an order in which to applyeach product. Consider a cascade of two linear, time-invariant systems. Because the Fourier transform of thefirst system's output is $X(f)H_1(f)$ and it serves as the second system's input, the cascade's output spectrum is $X(f)H_1(f)H_2(f)$ . Because this product also equals $X(f)H_2(f)H_1(f)$ , the cascade having the linear systems in the opposite order yields the sameresult . Furthermore, the cascade acts like a single linear system, having transfer function $H_1(f)H_2(f)$ . This result applies to other configurations of linear, time-invariant systems as well; see this Frequency Domain Problem . Engineers exploit this property by determining what transfer function they want, thenbreaking it down into components arranged according to standard configurations. Using the fact that op-amp circuitscan be connected in cascade with the transfer function equaling the product of its component's transfer function( see this analog signal processing problem ), we find a ready way of realizing designs. We now understand why op-ampimplementations of transfer functions are so important.