Very High Frequency Lab1

8/7/2019 Very High Frequency Lab1

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Lab 1: Simple Frequency and Time Analysis

1) Open PSpice: Start->All Programs->EE Programs->PSpice Student->Capture Student

2) Create a new project: File->New->Project

a) Choose file folder (Your directory on the network [h Drive])

b) Select Analog or Mixed Signalc) Select Create a blank project

V s o u r c e

C L

1 n

VV

0

R S

5 0V S1 V a c

0 V d cV R L

R L

5 0

0

T 1

T D = 1 n sZ 0 = 5 0

0

V l o a d

0

Figure 1. Circuit Schematic for Part 3.

3) Draw the above circuit usinga) Draw Components: Place->Part (you may have to add the analog and source libraries)

i) Transmission Line: T/Analog (analog library)

ii) Resistor: R/Analog (analog library)

iii) Capacitor: C/Analog (analog library)

iv) Voltage source that can sweep over frequencies: Vac/Source (source library)

b) Connect Components: Place->Wire

c) Ground the Circuit: Place->Ground

i) Ground: 0 (you may have to add the source library)

d) Label Nodes: Place->Net Alias

4) Enter desired values for each of the components. Example: R =100ohms. If that value is displayed, you

can double-click in the value to change it.

If the property values are not displayed for the transmission line, double click the transmission line. Thiswill open a dialog box which contains all the attributes associated with the transmission line. Here, you can

select the property you wish to display (TD and Z0). Then, by clicking the Display button, you canchoose to display the Name and Value of the attribute.

If you are unable to find TD and Z0 among the various attributes, try changing the Filter by drop down

menu to PSPice.

5) Notation

a) k -kilo

b) MEG-mega

c) G-gigad) m-mili

e) u-micro

f) n-nano

g) p-pico

6) Setup up a simulation Profile: Pspice->New simulation profile

a) Choose an AC sweep for the Analysis type

b) Enter Start Frequency: 100MEG

c) Enter Ending Frequency: 10G

d) Choose Log scalee) Enter number of data points to take.


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Figure 2. Simulation Settings for AC Sweep from 100 MHz to 10 GHz with 1000 points.

7) Simulate: PSpice->Run

8) Check Results: (from the simulation window) View->Output File. Review errors, if any. If nodes are

floating you probably have connecting problems or you are using the wrong ground. If there is an error inyour schematic, this will be the first place to start looking for errors. It will be useful to try and understand

the information contained in the Output File for debugging purposes. Please note that at this point there will

not be any output visible on the graph until you have completed the next step.

9) Graphically monitor output: Click on Trace->Add Trace in order to plot a parameter such as current or

voltage. Please provide graphs which show the following.a) Source Current and Voltage

b) Load Current and Voltagec) Perform all of the following operations on at least 2 of the 4 parameters in sections a and b.

i) R() - real

ii) imag() - imaginary

iii) P() - phase

iv) M()-magnitude

10) Now click on the Toggle Cursor button. It will allow you to study and mark points on the graphs. Use

cursors to monitor your waveforms. Mark at least 2 points of interest on each of your above graphs.

11) Submit a labeled Bode-type plot of VSOURCE, and VLOAD. Do this by plotting 20*LOG10(voltage). You

can type this in the Trace dialog. Submit the circuit schematic.

12) Repeat the simulation for frequencies 100kHz to 100MHz. Submit a labeled Bode plot of the same

outputs. In this passive network, how can the voltage at the load be higher than the voltage at the source?


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13) Replace the VAC source with a sinusoidal source VSIN. (Select appropriate values for amplitude and

frequency. Set offset to 0.)

V l o a d

V r e s

V

V s o u r c eT 1

T D = 1 n

Z 0 = 5 0

V

R L

5 0

0

0

0 0

V SF R E Q = 0 . 5 GV A M P L = 1 0

V O F F = 0C L

1 n

R S

5 0

Figure 3. Circuit Schematic for Part 13.

This time run a Time Domain Response (Transient) simulation. Use the transient analysis to obtain plots of

the transient voltage waveforms VSOURCE and VLOAD for 5 periods of the wave. Submit a labeled plot of the

waveforms. What is the phase delay (in degrees) at 0.5 GHz due to the transmission line for the values you

have selected? (Please show calculation.) Does this make sense, given the transmission line parameters?

Use the following equation:

= 360Period

Delay

DelayT

t

Basic Transmission Lines in the Frequency Domain

In this laboratory experiment, you will use SPICE to study sinusoidal waves on lossless

transmission lines. Our goal is for you to become familiar with the basic behavior of

waves reflecting from loads in transmission lines, and compare the simulations withnumeric calculations and the Smith Chart.

2.1Basic Transmission Line Model

There is a standard lossless transmission line model T, which is specified by several

parameters. We will need to specify two of the parameters:

Z0, the characteristic impedance

TD, the time delay, which is the length of the line in time units.

The length of the line L is related to the time delay through

DpTuL = (2.1)

where up

is the phase velocity of waves on the transmission line.

As we saw in lecture and in our text, the phase velocity and characteristic impedance may

be derived from the lumped element model of the transmission line. With Lthe

inductance per unit length, and Cthe capacitance per unit length, we have

''

1

CLu p = (2.2)


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'

'0

C

LZ = (2.3)

2.1.1 A standard coaxial cable

For common RG-58 coaxial cable, the characteristic impedance is Z0 = 50 and thephase velocity up = 2/3 c. (Note: c = speed of light = 3e8 m/s)

Question 1: For such a transmission line, what are the inductance and

capacitance per meter?

For lossless coaxial cables, the following formulas relate the differential inductance L

and capacitance Cto the radius of the inner conductora and the outer conductorb:

=a

bL ln

2'

(2.4)

=

a

bC

ln

2'

(2.5)

Question 2: For a different coaxial cable, = 0 and = 30. What is b/a ifZ0= 50 ?

Question 3: Ifb = 3 mm in question 2.2, what is a?

2.2A SPICE model of a transmission line problem.

Using SPICE, create a (matched) Thevenin source VAC with 1 Volt amplitude and 50

source impedance, leading to a transmission line model T, terminated in a 100 load.

Edit the transmission line so that it has a characteristic impedance of 50 . Also, createlabels Input and Load at the ends of the transmission lines, so that you can measure the

voltages conveniently.

0 0

L o a d

0

Z G

5 0

I n p u t

P A R A M E T E R S :d e l a y = 5 n s

Z L

1 0 0

0

V G1 V a c

0 V d c

T 1

T D = { d e l a y }Z 0 = 5 0

Figure 1. Circuit Schematic for Part 2.2

What we would like to do is to adjust the length of the transmission line and examine the

standing wave pattern at Input over one full wavelength at a frequency of 200MHz.


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Question 4: At 200 MHz, and with up = 2/3 c, what is the wavelength in the

transmission line?

Question 5: What is the time delay associated with /16? (Hint: Remember

thatf

L

u

LT

p

D

==

)

Use SPICE to simulate the steady state AC response of this transmission line for length 0,

/16, 2/16, , 15/16, . Center your sweep on the frequency of interest and sweep

linearly.

Figure 2. Illustration of Transmission Line Length Change for Part 2.2

One way to make this easier is to use a parameter for TD. Place the special part

PARAM. Double click on it and then on New Column Call it delay and set it to 5ns.

Assign {delay} (with the curly braces) to TD on the transmission line. When you create

your simulation profile, select the parametric sweep as an option. Choose GlobalParameter with a parameter of delay. Set the sweep range and increment based on your

TD calculations from above. Under General Settings set the sweep Range from Start

Frequency: 200Meg to End Frequency: 200Meg and increment Total Points: 1.

Using Excel, make a table of the voltage magnitudes and current magnitudes at nodes

Input and Load for each length.

Question 6: Use PSPICE, Excel, or Matlab to plot the magnitude of the voltage

at Input as a function of length. From the Voltage Values on the plot and

the relationship:min

max

V

VVSWR = , determine the VSWR, and from the

VSWR calculate | |.


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Question 7: Use PSPICE, Excel, or Matlab to plot the magnitude of the current at

Input as a function of length. From the Current Values on the plot,

determine the VSWR, and from the VSWR calculate | |. Do the voltage

and current yield the same VSWR and | |?

Question 8: Plot the magnitude of the impedance at Input as a function oflength using the data you collected with PSPICE. Plot the Real and

Imaginary Parts of the Impedance using PSPICE and also plot impedance

using a Smith Chart.

Question 9: Compute and VSWR directly using equations (2.6) and (2.7)

below. Do these agree with your measurements from question 6, 7 & 8?

From class recall that:

+=

1

1VSWR (2.6)

0

0

ZZ

ZZ

L

L

+

= (2.7)

Question 10: Plot the voltage magnitude at Load as a function of length. Howdoes the voltage change with length? From this, how do you think the

power delivered to the load will change with length?


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2.3A shortcut, and more load impedances

SPICE has a nice mechanism for scanning in frequency, but does not directly scan thelength of the transmission line. The electrical length of a transmission line is l,

lufllp

22 == (2.8)

Thus, changing the length of a transmission line from lto 10lachieves the same effect asscanning the frequency from 10fto f. Or to put it differently, if a transmission line is 1

at f0, then it is 0.5 long at 0.5f0 and 2 long at 2f0.

Question 11: If you have 1 meter of the coaxial cable described in question 4, atwhat frequency does it have length /2? At what frequency does it

have length 2.5? (Note that we are NOT changing the physical

length of the line, only its electrical length as defined above.)

Using a 1-meter length of transmission line, adjust your SPICE simulation,

sweeping linearly in frequency from 0.5 to 2.5 wavelengths. In this simulation weare not adjusting the Length of the Line. We are adjusting the frequency of the

system so as to produce similar effects to adjusting the length of the line.

Z G

5 0

0 0

V G1 V a c

0 V d c

T 1

T D = 5 n sZ 0 = 5 0

Z L

1 0 0

0

L o a dI n p u t

0

Figure 3. Circuit Schematic for Question 13 (Fixed Length)

Question 12: Plot the magnitude of the voltage at Input for the different

lengths (remember that you are really just adjusting the

frequency) properly relabeling the horizontal axis. (You can dothis by hand or by using text boxes in Pspice.) Does this agree

with your plot in question 6? What is the VSWR?

Replace the 100 load with a 25 load.

Question 13: Plot the magnitude of the voltage at Input, and compare to theprevious case of 100 . From the plot, what is the VSWR?

Replace the load with a short circuit, namely 0.001 .

Question14: Plot the magnitude of the voltage at Input. From the plot, find the

VSWR. From equations (2.6) and (2.7) calculate the VSWR. Do

these two results agree?


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Replace the load with an open circuit, namely 1 M. (remember that in

PSPICE, MEG = mega, M = milli)

Question 15: Plot the magnitude of the voltage at Input. Find the VSWR. Also,

calculate the VSWR. Do these two results agree?

Question 16: How are the plots from Question 14 and Question 15 similar

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Very High Frequency Lab1