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Description

1 Objectives

The purpose of the lab is to investigate the frequency response of second order circuits and further practice circuit design and analysis techniques in the frequency domain.

2 Introduction

It is useful to format a transfer function as a multiplication of known functions so that its frequency response can be easily sketched without the need for complex tools. Since the y-axis of Bode plots are in dB-scale, Bode plot of the overall transfer function can be simply obtained by graphically adding Bode plots of individual first or second order sections on the same frequency axis.

Incasethe derivationofH(s)alreadyyields multiplicationofknownfunctions,then itisbest tokeep them in these formats.However,ifthederivationyields multiplicationoftransferfunctionswith highorder numeratoror denominatorpolynomials,thenyoumayneedtodecomposethemintosmaller knownsections. Table1shows first order functionsandtheirmagnitudeandphase responses.

Table1:FirstOrderFunctions

 H (s ) Bode Magnitude Bode Phase H (s ) Bode Magnitude Bode Phase

In case any section of the transfer function has a second order denominator with complex poles, then that section cannot be decomposed into first order functions. Table 2 shows the second order functions with complex poles and their magnitude and phase responses. Remember that second order polynomials with real poles can always be expressed as a multiplication of two first-order polynomials, so that a combination of first order functions in Table 1 can still be used.

Table2:SecondOrderFunctions

3 Calculations

1. Derive the transfer functions for the circuits shown in Figs. 1(a), 1(b) and 1(c):

HLP (s ) =

VLP

Vi

(s ) HHP (s ) =

VHP

Vi

(s) HBP (s ) =

VBP

Vi

(s )

R1 R2

Vi C1

VLP

C2 Vi

C3 C4

R3

VHP

R4 Vi

C5 R6

R5

VBP

C6

(a) (b) (c)

Figure 1: Second order (a) lowpass filter (b) highpass filter (c) bandpass filter

Express the transfer functions as

H (s ) =

P (s )

Q(s)

(1)

where P (s ) and Q (s ) are polynomials of s with coefficients in terms of resistors and capacitors. Remember that negative powers of s are not allowed in polynomials, so all the terms in P (s ) and Q (s ) must contain s n where n ≥ 0.

2. Find the resistor and capacitor values such that the transfer functions can be formatted as follows:

1

HLP (s ) = s

1 +

1

s , f1 = 4kHz , f2 = 8kHz (2)

1 +

2πf1

s

2πf2

s

3

HHP (s ) = s + 2πf

s

s+2πf4

1

, f3 = 4kHz , f4 = 8kHz (3)

5

HBP (s ) = s + 2πf

s , f5 = 4kHz , f6 = 8kHz (4)

1 +

2πf6

When calculating component values, you can make reasonable approximations. For example, 1 + x ≈ 1 if

x 1, which typically requires x < 0.1. Similarly, 1+x ≈ x if x 1, which requires x > 10.

3. Sketch the magnitude and phase Bode plots for HLP (s ), HHP (s ), and HBP (s ).

4. Calculate the output voltages VLP (t ), VHP (t ), and VHP (t ) for Vi (t ) = 0.5 sin(2π6000t ).

4 Simulations

For all simulations, provide screenshots showing the schematics and the plots with the simulated values prop- erly labeled.

Draw the schematics for the circuits in Fig. 1 with the calculated component values. Perform the following simula- tions for each circuit:

1. Obtain the magnitude and phase Bode plots of the transfer function using AC simulation, and measure the

3-dB frequencies and passband gains. Also measure the magnitude and phase of the transfer function at 6kHz.

2. Apply the input Vi (t ) = 0.5 sin(2π6000t ) and obtain the time-domain waveforms for the input and the output voltage using transient simulation. Measure the magnitudes of the input and the output voltages, and the phase difference between them.

5 Measurements

For all measurements, provide screenshots showing the plots with the measured values properly labeled.

Build the circuits in Fig. 1 with the simulated component values. Perform the following measurements for each circuit:

1. Obtain the magnitude and phase Bode plots of the transfer function using the network analyzer, and measure the 3-dB frequency and passband gain. Also measure the magnitude and phase of the transfer function at

6kHz.

2. Apply the input Vi (t ) = 0.5 sin(2π6000t ) and obtain the time-domain waveforms for the input and the out- put voltage using the scope. Measure the magnitudes of the input and the output voltages, and the phase difference between them.

6 Report

1. Include calculations, schematics, simulation plots, and measurement plots.

2. Prepare a table showing calculated, simulated and measured results.

3. Compare the results and comment on the differences.

4. The same transfer functions can be obtained using different combinations of resistors and capacitors. Explain your reasoning for your selection. What are the trade-offs if you change your selection of components to realize the same transfer functions?

7 Demonstration

1. Build the circuits in Fig. 1(a), (b) and (c) on your breadboard and bring it to your lab session.

4. For the lowpass filter in Fig. 1(a):

• Show the frequency response using the network analyzer.

• Measure and verify -40dB/dec slope at the stopband.

5. For the highpass filter in Fig. 1(b):

• Show the frequency response using the network analyzer.

• Measure and verify +40dB/dec slope at the stopband.

6. For the bandpass filter in Fig. 1(c):

• Show the frequency response using the network analyzer.

– Measure the low and high 3-dB frequencies.

– Measure the magnitude and phase at a passband frequency fx determined by your TA.

• Show the time-domain input and output waveforms using the scope at the frequency fx .

– Measure the gain.

– Measure phase difference between the input and the output.