3: Experiment Resonance in LRC Circuits Driven by Alternating
Current
Introduction
In last weeks laboratory you examined the LRC circuit when constant voltage was applied to it.
During this laboratory you will examine the same circuit but now you will apply alternating current
(AC). Our functions of interest from last week contained time as the independent variable. These
functions can generally be referred to as time-domain functions. In this laboratory the functions of
interest will be in the frequency-domain. That is, the independent variable is frequency.
1
Physics
1.1 Complex Impedance Revisited
This topic was previously introduced in Experiment 1, but a few of the impedances were simply
stated. The form of the impedance for a capacitor and for an inductor will be proven below.
The relationships of current to voltage for capacitors and inductors were explained in Experiment 2
section 1.2. These relationships are rewritten here.
Q1
=
Idt
C C
dI
VL = L
dt
VC =
(1)
(2)
Again the voltage/current relationship for the capacitor is best written in terms of derivatives.
Differentiating each side with respect to time equation 1 becomes:
dVC I
=
C
dt
(3)
Now assuming that alternating current is flowing through these types of elements, we will prove the
impedances which were only stated in Experiment 1. Recall complex impedance is only applicable
when one uses AC.
Complex impedance is a measure of the ratio of voltage to current and it also contains the phase
offset between the two. This relationship is summarized in the following equation.
~
V
Z=~
I
Now using the assumption that voltage is alternating:
~
~
V = V0 ei t = Z I
Experiment 3: Resonance in the LRC Circuit
(4)
(5)
1
Replacing the current in the inductor equation we have:
~
d VL
VL = L
dt Z L
~
i L
VL 0 ei t
ZL
i L ~
=
VL
ZL
=
(6)
Z L = i L
(7)
This implies:
In the same way, the voltage/current relationship for capacitors becomes:
~
d
I =C
VC 0 eiwt
dt
= i C VC 0 eiwt
~
= i C Z C I
(
(
)
)
(8)
This implies:
ZC =
1
i C
(9)
Mathematical Analysis
1.2 Properties of Complex Numbers
Complex numbers are of the form c = a + ib where a and b are real numbers and i = 1 . A
complex number can be visualized in a two-dimensional plane as it is in Figure 1. Figure 1a shows a
complex number in Cartesian coordinates. Figure 1b shows the same complex number in polar
coordinates which is of the form: c = rei . From well-known trigonometric relations we have the
follow relationships between Cartesian and polar coordinates.
r = a2 + b2
b
a
= arctan
Experiment 3: Resonance in the LRC Circuit
a = r cos ( )
b = r sin ( )
(10)
2
Imaginary
Axis
c = a + ib
= rei
Imaginary
Axis
c = a + ib
r
Im(c) = b
b
Re(c) = a
a
Real Axis
Figure 1a A complex vector, c, is shown in
Cartesian coordinates.
Real Axis
Figure 1b A complex vector, c, is shown in
polar coordinates
1.3 The LRC Series Circuit Driven by AC
Figure 2 is a schematic of the circuit you will construct during this laboratory. Using Kirchhoffs
Law on the single loop of this circuit we have:
VS + VL + VR + VC = 0
(11)
Notice that in Figure 2 there are three sources of resistance. The voltage drops of these three are all
included in VR . Also, in subsequent equations R represents the total resistance in the loop and
RR represents the resistance of only the resistor.
Figure 2 The complete LRC series circuit is drawn in (a) with all three resistors, including VS for the
Signal Generator, RL for the Inductor and RR for the resistor which you put into the circuit. An
equivalent circuit is drawn in (b) with the single resistor R = ( RS + RL + RR ) representing the total of all
of the circuit resistance. The internal voltage source, VS (t ) , is a sine wave, and the current through all
of the components is I.
Experiment 3: Resonance in the LRC Circuit
3
Replacing the voltage of the circuit elements in equation 11 we have:
VS I ( Z L + Z R + ZC ) = 0 VS = I i L + R + 1
(i C )
(12)
The polar expression for the total impedance will turn out to be the most useful. Using the equations in
block 10 we have:
1
1
1
L
2
2
R + i L C = R + L LC exp i arctan R RC
2
(13)
We now wish to substitute 0 and Q , which are the same parameters from last week, into the
expression for the magnitude of the total impedance and then simplify. First recall:
0 =
ZTotal
1
1L
1
, and Q =
=
LC
R C 0 RC
1
= R + L
LC
2
(14)
2
2
L2
2
= R 1 + 2 0
R
2
L2 0 2 0
= R 1+
R2 0
L 0
= R 1+ 2
R C 0
0
= R 1+ Q
0
2
(15)
2
2
2
We can also simplify the angle as well. The result is stated below.
1 L 0
1
L
arctan
= arctan
R RC
R C 0
0
= arctan Q
0
(16)
Now let us find the current based on the impedance and the assumption that the voltage is of the form,
~
V = V0 ei t .
Experiment 3: Resonance in the LRC Circuit
4
~~
0 ~
V = I Z V0 eiwt = ZTotal exp i arctan Q
I
0
~
0
V0
I=
exp i t arctan Q
ZTotal
0
(17)
1.4 Voltage Over the Resistor
The voltage over the resistor is the current times the impedance of the resistor (simply its own
resistance). Let us evaluate its magnitude:
VR = I Z R = V0
=
ZR
ZTotal
V0 RR
(19)
0
R 1 + Q2
0
2
Notice the impedance of the resistor, RR , is only the resistance of the resistor and not the total
resistance, R . Also, notice that the voltage drop over the resistor is greatest when = 0 . This
frequency is called the resonance frequency.
Below in figure 3 there is a graph of this function. The "half power points", 1 and 2 , are also
shown in figure 3. Since P V 2 , the half power points occur when the voltage decreases to 1 2 of its
peak value, or
1
=
2
2
0
Q
=1
2
0
0
1 + Q2
0
1
2
(20)
which happens twice, = 1 , below resonance, and = 2 , above resonance. If you solve the two
Experiment 3: Resonance in the LRC Circuit
5
quadratic equations, you find that:
Q=
0
2 1
(21)
This is the quick way to measure Q, and it is exact, having no approximations in its derivation.
During your experiment you will determine the resonance frequency visually by varying the
frequency until the amplitude is at its highest value. You will eye-ball the frequency and for that
reason the uncertainty of your measurement is half of the range over which you are unsure if your
maximum amplitude is changing.
Question 3.1
What is the propagated uncertainty for Q in Equation 21?
Question 3.2
When determining the linear resonance frequency, you determine that the amplitude is greatest
between 21.03 Hz and 22.73 Hz. What do you record for the linear resonance frequency (with its
uncertainty)? What is the angular resonance frequency (with its uncertainty)?
Experiment 3: Resonance in the LRC Circuit
6
Figure 3 The Response Function of the LRC series-resonant circuit. A constant amplitude signal ( V0 )
drives the circuit, and changes in frequency lead to changes in the current that flows, which you
measure as the voltage drop across the resistor. The half-power points, 1 and 2 , are shown along
with their relation to the quality factor Q and the resonant frequency, 0 , which corresponds to the
peak of the response function. As the Q increases, the width of the function decreases.
1.5 The Q Multiplier
At the resonance frequency there is a very easy way to determine Q. The ratio of the amplitude of
voltage of the capacitor to the amplitude of the driving voltage is simply Q. The easiest way to derive
this result is to find the magnitude of the voltage drop across the capacitor and divide by the driving
voltage amplitude.
Experiment 3: Resonance in the LRC Circuit
7
VCO VC ( 0 )
=
V0
ZTotal 0C
=
V0
0 RC
L
= V0Q
C
V
Q = C0
V0
=
V0
R
(22)
In this derivation we recall equation 15 and use the fact that when = 0 ZTotal = R .
Question 3.3
What is the propagated error for Q in equation 22?
1.6 Voltage Phase Offsets of Circuit Elements
In this lab you will be asked to the measure phase offset from the driving voltage of the circuit
elements at various important frequencies. Below the phase offsets as a function of angular frequency
for the resistor, capacitor and inductor are derived. Having already found the current of the circuit this
process is fairly simple although the simplification looks messy. The voltage drop over each element is
the product of the current, which is found in equation 17, and the impedance of that element. Thus, for a
resistor
~
~
~
V R = I Z R = I RR
=
0
V0 RR
exp i t arctan Q
ZTotal
0
Define:
0
0
R ( ) arctan Q
(23)
(24)
Notice that at the resonance frequency the phase offset of the resistor is zero.
Below in Figure 4 there is a graph of this phase offset. Recall that 1 and 2 are two solutions to
the following equation:
2
0
0
Q
= 1 Q
=1
0
0
2
Experiment 3: Resonance in the LRC Circuit
(25)
8
Q 1 0 = 1
0 1
(26)
Q 2 0 = 1
0 2
(27)
Placing equations 26 and 27 into equation 24 we determine
R ( 1 ) arctan (1) =
4
R ( 2 ) arctan (+1) =
4
(28)
Figure 4 Phase ( R ) as a function of angular frequency. Note that the phase shift is positive at low
frequencies and negative for high frequencies. Resonance is at zero phase shift, and the half-lower
points are at plus and minus 4 .
Following the same procedure the phase offsets for the inductor and capacitor are:
Experiment 3: Resonance in the LRC Circuit
9
~
~
~
V L = I Z L = I i L
=
0
V0 L
exp i t arctan Q
+
ZTotal
0 2
Define:
0
+
0 2
L ( ) arctan Q
(29)
(30)
~
~
~ i
V C = I ZC = I
C
0
=
exp i t arctan Q
ZTotal C
0 2
Define:
V0
0
0 2
C ( ) arctan Q
(31)
(32)
Notice that we have used the identity i = ei 2 . Also notice that the resonance frequency C ( 0 ) = 2
and L ( 0 ) = 2 .
The Experiment
1.7 Target Values for R, L and C
Experiment 3: Resonance in the LRC Circuit
10
In the same way you selected your circuit elements in experiment 2 you must do so for this
laboratory as well. For this laboratory Q should be between 4 and 10. 0 should be between 104 and
5 10 5 rad/s (or the linear frequency should be between 1.6 and 80 kHz). Below is a table with the listed
options for R, L and C.
Circuit Element
Resistor (R)
Inductor (L)
Capacitor (C)
Options
10, 51, 100, 200, 510, and 1000
There is also a variable resistor.
0.55, 2.5, 5 and 10mH (milli Henry)
0.001, 0.01, 0.1 and 1.0F (micro Farad)
When you arrive in the lab measure the resistance and capacitance of the elements you have
selected and determine your theoretical values of Q and 0 with your measured values. Also, before
you begin using the oscilloscope, CALIBRATE it. You must calibrate both voltage channel 1 and
channel 2 because you are going to be using both.
Question 3.4 Mandatory
What are your target values for R, L and C? What values of Q and 0 do you expect?
Hint: if your Q from experiment 2 was between 4 and 10, then you can re-use those values.
1.8 Determining Q and 0 Experimentally
Figure 5 shows generally how the LRC series current is to be assembled. However, you will also
be measuring the voltage of individual circuit elements (i.e. the capacitor, resistor and inductor) at
different times. Recall that one lead from the signal generator is ground. There will also be a ground
from the oscilloscope, which displays the voltage over that circuit element. If you do not change the
order of circuit elements you may short-circuit your loop. In Experiment 1, Section 1.4 Common
Grounding, this concept is explained. If you do not remember, please re-read this section.
Set up the circuit in Figure 5 and connect the oscilloscope across RR (i.e., measure VR ). This is
proportional to the current flowing in the circuit and should show a large amplitude when you change
the frequency of the signal generator (sinusoidal wave) so as to pass through resonance, 0 (= 2 f0 ) .
Now find 1 and 2 by changing frequency until the sinusoidal wave amplitude VR decreases to
0.707 of the voltage at resonance. Verify that equation 21 gives you a value of Q close to what you
calculated.
When using the variable resistor, measure your final resistance with the multimeter.
Experiment 3: Resonance in the LRC Circuit
11
Figure 5 Setup for LRC circuit
3.3 Graphing Frequency Response
Now measure several values of the amplitude of the resistors voltage, VR , at different frequencies,
recording them in a table. Make sure that at least 3 points are below 1 , 10 points between 1 and 2
and 3 points above 2 (include the 0 , 1 , and 2 points as well). Plot VR as a function of for your
data (with error bars). Fit your data to the functional form of equation 19:
y=
A
x C
1+ B
C x
2
2
and from the coefficients of the fit, determine values (with error) for 0 and Q.
Question 3.5
What are the coefficients A, B, and C in the equation above? What is the independent variable x?
3.4 Q Dependence on R
Restore your circuit to the original form (cf. Figure 5), but increase R to some new value, for the
ROld
1L
total resistance of the circuit. Since you expect that Q =
, the new value of Q should be R
of
RC
New
your original Q. Verify by finding the new 1 and 2 , and check that 0 does not change.
Question 3.6
What is the relationship between QNew and QOld if only the resistance is changed from RNew to ROld ?
Hint: L and C are not changed. Write out the equation for QNew and QOld and substitute.
Experiment 3: Resonance in the LRC Circuit
12
3.5 The Q Multiplier
Common ground leads on our electronic equipment (the oscilloscope and signal generator) are
typically the same or very close in voltage. Thus you cannot connect the scope across the capacitor in
Figure 5. If the oscilloscope ground is connected to the node between RR and C, then you will effective
remove the resistor from the circuit. Rearrange the circuit so that the capacitor is the last element. This
allows you to measure the voltage across the capacitor without shorting the resistor. For diagrams
and a more thorough explanation review Experiment 1, Section 1.4 (Common Grounding).
When viewing more than one signal, the oscilloscope may picture the two or at least one of the
signals as fuzzy or jumpy. This is certainly true if each signal has a slightly different ground voltage.
The oscilloscope can only make one steady at a time when the two grounds are different. You can make
the grounds exactly the same by connecting the TTY output on the signal generator (the middle BNC
port) to the lower right external trigger on the oscilloscope. Make sure to turn the two switches above
the external trigger to external. Keep this connected for section 3.6 as well.
Now measure VCO , the amplitude of the voltage across the capacitor at resonance. (Make sure that
you stay at the resonant frequency for this section of the experiment). Next, disconnect the entire circuit
from the generator and measure a proper value for VS (the internal voltage of the generator). Determine
the Q of the circuit by taking the ratio of the capacitor voltage amplitude at resonance to the drive
voltage (cf. Equation 22). Compare this value of Q to the expected value found earlier. Comment on
why the circuit was removed from the signal generator before measuring VS .
Hint: If the amplitude of the source signal is too great for you to read on the oscilloscope screen, then
you must turn the signal voltage knob at the lower right on the face of the signal generator. Afterward
you can take the proper measurement.
Analysis
Compute the following:
Theoretical value of Q;
Theoretical value of 0 ;
Theoretical bandwidth 1 2 ;
Experimental bandwidth 1 2 ;
Experimental Q using measured 1 2 and 0 ;
Ratio of QNew to QOld from Q multiplier.
Compute t-values for the following:
Experimental 0 with theoretical 0 ;
Q from bandwidth with theoretical Q;
Q from fit with theoretical Q.
Experiment 3: Resonance in the LRC Circuit
13