UNC Charlotte Agilent ADS Project 2

(Approx. 4 weeks)

Overview

The objective of the tutorial is to become familiar with S-parameters, transmission lines, and Smith charts, and matching network design.

NOTE: Use the Project Report Template and  keep answers to questions on consecutive sheets of paper with all plots at the end.

IN NO CASE may code or files be exchanged between groups, and each group must answer the questions themselves and do their own plots, NO COPYING of any sort!

Only turn in requested plots ( Pxx ) and requested answers to questions ( Qxx ).

Number all questions Q1 -- Qxx. and plots P1 ... Pxx

Part 1

  • In this part, the behavior of a transmission line with mismatch on one end is investigated.
    Power conservation and issues of "where the power goes" are explored.

  • Download the following tar-file (you may need to hold down the shift key while you click on the link):
    tpwp2.tar

  • When ADS first runs, you should have a new directory apps/agilent/ads (or hpeesof/ads) created in your home directory. Move the tar-file into the directory, and de-tar using the command 
        tar -xvf tpwp2.tar
  • You should find a new directory project2 created in apps/agilent/ads (or hpeesof/ads)
  • Run ADS
  • Go down through the directory tree to p2smith1 and right-click open-project  on p2smith
  • Go down through the directory tree to p2smith1/networks/p2txline1.dsn, and double click that design file. You should see the schematics on the right pane.
  • Double-click the p2txline1 schematic in the right half of the window, and the following schematic should appear.

  • Double-click the "gear" icon (shown below) in the upper right of the window to simulate.

  • Click the "rectangular plot" icon in the pop-up data plotting tool.
  • Drop the plotting box in the visible area, and in the pop-up window:

    Select DataSet -> S(1,1) -> Add -> dB
    Select DataSet -> S(2,1) -> Add -> dB

  • Select the plot options as shown below to plot S11 and S21 from -10 to 0 dB in 1 dB steps. (the picture below may show -20 to zero)

    Print the S-parameter plot and turn it in. You should have a "flat-line" plot of both S11 and S21. ( P1 )

  • What is the line impedance? (from formula or RFMD tables)
    ( Q1 )
  • What is the effective source impedance to the left of the line (i.e., what is the total source impedance seen to the left of the line)? ( Q2 )
  • What is the effective load impedance to the right of the line (i.e., what is the total load impedance seen to the right of the line)? ( Q3 )
    And so ... you should conclude a mismatch on one end.

  • Note that the S-parameters are plotted only from the perspective of the TERM devices, so the power transmitted to the right TERM device is only the power delivered to the TERM resistor. (It doesnt "know" anything about what is going on with the other resistor.)

    Draw a schematic on a sheet of paper with a 50 ohm 2 volt rms source and a load comprised of two 50 ohm resistors in parallel. At extremely low frequencies, this is a good model of the circuit since the transmission is electrically short (i.e., much less than a quarter wave). Questions Q4 - Q7 relate to hand calculations using this hand-drawn schematic.

  • What is the voltage across the two 50 ohm resistors that are in parallel? ( Q4 )
  • What is the power delivered to one of the two parallel resistors.? ( Q5 )
  • What is the maximum power available available from the source (i.e., replace the two 50 ohm resistors with a single 50 ohm load)? ( Q6 )
  • How many dB down is the power in Q5 relative to Q6? (10 log10( Q5/Q6 ) ) ( Q7 )
  • How does your answer to Q7 relate to the plot P1? ( Q8 )
    ... i.e., is there somewhere/something in the plot that reflects this?
  • For Q9- Q13, assume that the maximum available power from the source is 1 watt:
    (The answers to the questions below should agree with the foregoing calculations in Q4-Q8 AND with plot P1 for the above ADS schematic at low frequency, otherwise you have an error somewhere.  The answers should scale in proportion to the power sources.) )
  • What is power in watts delivered to the right-hand TERM in the above ADS schematic? (This would correspond to the power delivered to one of the two 50 ohm load resistors in parallel in your hand-drawn schematic.) ( Q9 )
  • What is the power delivered to the 50 ohm resistor (not TERM) in the above ADS schematic? (This would correspond to the power delivered to the second one of the two 50 ohm loads in parallel in your hand-drawn schematic.) ( Q10 )
  • What is the power reflected back to the left TERM in the above ADS schematic? (The left hand term would correspond to the 50 ohm internal resistance of the source in your hand-drawn schematic.) ( Q11 )
  • What is power delivered from the 1 watt source? (The total power delivered to both load resistors in your hand-drawn schematic.) ( Q12 )
  • Does the sum of the power delivered to the 50 ohm resistor (not the TERMS) in the above ADS schematic plus the right-hand TERM plus the reflected power equal the maximum available power? ( Q13 )

  • Click the "Smith plot" icon in the data plotting window.
  • Drop the plotting box in the visible area, and in the pop-up window:

    Select DataSet -> S(1,1) -> Add
    (only plot S11 on the Smith chart)

    Click on the plot options, click Coordinate->both, select Grid, and select admittance line type as long dash, and pick a green color. Click OK and the Smith Chart should appear. To figure out which end of the curve is at what frequency, Select Marker->New from the Dataplot menu bar and click on one end of the plotted curve. Then Edit->Undo to delete the marker. (see below for an example of how the Smith chart should look ... depending on the shade of green you choose)

    Print the Smith chart plot and turn it in. You should have a "circle". ( P2 )

  • Why do you get a circle on the Smith Chart? ( Q14 )
  • What resistance (unnormalized) corresponds to where the circle intersected the purely resistive axis on the right? ( Q15 )
  • What is the reflection coefficient (magnitude and angle) where the circle intersected the purely resistive axis on the right? ( Q16 )
  • What is the VSWR where the circle intersected the purely resistive axis on the right? ( Q17 )

    • Part 2

  • Note that this is the same experiment as the pulse experiment from project 1, except now we are using a frequency sweep instead of pulses. Things get a bit more complicated compared to Part 1, since we have mismatches on both ends of the line.

  • Go down through the directory tree to p2smith1/networks/p2txline2.dsn, and double click that design file. You should see the schematics on the right pane.
  • Double-click the p2txline2 schematic in the right half of the window, and the following schematic should appear.

  • Double-click the "gear" icon in the upper right of the window to simulate.
  • Click the "rectangular plot" icon in the pop-up data plotting tool.
  • Drop the plotting box in the visible area, and in the pop-up window:

    Select DataSet -> S(1,1) -> Add -> dB
    Select DataSet -> S(2,1) -> Add -> dB

  • Select the plot options to plot S11 and S21 from -20 to 0 dB in 1 dB steps.
  • Print the S-parameter plot and turn it in. You should have a "rippled" plot of both S11 and S21. S21 should ripple between -6 and -8 dB. ( P3 )

  • What is the line impedance? (from formula or RFMD tables) ( Q18 )
  • What is the effective source impedance to the left of the line (TERM and the resistor)? ( Q19 )
  • What is the effective load impedance to the right of the line (TERM and the resistor)? ( Q20 )
  • Note that the S-parameters are plotted only from the perspective of the TERM devices, so the power transmitted to the right TERM device is only the power delivered to the TERM resistor, and the reflected power is only that reflected back to the left TERM.

    Draw a schematic on a sheet of paper with a 50 ohm 2 volt rms source (2 volts rms open-circuit voltage) and a load comprised of three 50 ohm resistors in parallel. At extremely low frequencies, this is a good model of the circuit since the transmission is electrically short (i.e., much less than a quarter wave). This is a good model at low frequency since the line is not very long in wavelengths at 1 MHz! Questions Q21 - Q24 relate to hand calculations using this hand-drawn schematic.

  • Calculate the voltage across the three 50 ohm resistors that are in parallel at 1 MHz in your hand-drawn schematic? ( Q21 )
  • Calculate the power delivered to one of the three parallel resistors.? (This would correspond to the power delivered to one of the three 50 ohm load resistors in parallel in your hand-drawn schematic.) ( Q22 )
  • Calculate the maximum power available available from the source at 1 MHz (i.e., replace the three 50 ohm resistors with a single 50 ohm load)? ( Q23 )
  • Calculate how many dB down is the power in Q23 relative to Q22 at 1 MHz? (10 log10( Q22/Q23 ) ) ( Q24 )
  • How does your answer to Q24 relate to the plot P3 at 1 MHz? ( Q25 )
  • For Q26- Q28, assume that the maximum available power from the source is 1 watt:
    (The answers to the questions below should agree with the foregoing calculations in Q21-Q25 AND with plot P3 for the above ADS schematic at low frequency(1MHz), otherwise you have an error somewhere)
  • Calculate the power in watts delivered to the right-hand TERM at 1 MHz in the above ADS schematic? ( Q26 )
  • Calculate the power delivered to the right hand 50 ohm resistor at 1 MHz in the above ADS schematic? ( Q27 )
  • Calculate the power delivered from the source at 1 MHz in the above ADS schematic? ( Q28 )

  • What is the length of the line in wavelengths at the bottom of the S11 dip near 14 MHz?
    (you should be able to figure this out from the schematic and P3, but you can also calculate from the transmission line geometry to double-check your answer) ( Q29 )
  • Looking into the left side of the line at the bottom of the S11 dip near 14 MHz, use a Smith chart to figure out the impedance. What is the impedance looking into the left side of the line.? ( Q30 )

  • Note that the S-parameters are plotted only from the perspective of the TERM devices, so the reflected power is only that reflected back to the left TERM.

    Draw a schematic on a sheet of paper with a 50 ohm 2 volt rms source and a load comprised of the left-hand 50 ohm resistor in parallel with the answer to Q30. At the S11 dip near 14 MHz, this is a good model of the circuit since the transmission has transformed the impedance (we cant use the previous low frequency model anymore!). Questions Q31 - Q37 relate to hand calculations using this hand-drawn schematic.

  • Calculate the voltage across the 50 ohm resistor in parallel with the Q30 resistor near 14 MHz in your hand-drawn schematic? ( Q31 )
  • Calculate the power delivered to the Q30 resistor near 14 MHz in your hand-drawn schematic.? ( Q32 )
  • Calculate the maximum power available available from the source (i.e., replace the 50 ohm resistor and Q30 resistor with a single 50 ohm load)? ( Q33 )
  • Calculate how many dB down the power in Q32 is relative to Q33 near 14 MHz? (10 log10( Q32/Q33 ) ) ( Q34 )

  • Consider that all the power in the Q30 resistor near 14 MHz must be dissipated in the right-hand TERM and right-hand 50 ohm resistor (the line is lossless). And since the voltage across these two resistors is the same, and they have equal resistance, the total power must be equally split between them.

    Considering this, and the answer to Q34, calculate how many dB down is the power delivered to the right-hand term relative to the maximum power available from the source; and does your answer properly relate to the S21 plot P3 at 14 MHz? ( Q35 )

  • Using the Q30 resistance in parallel with the left-hand 50 ohm resistor, calculate the effective load resistance seen by the left-hand TERM at 14 MHz? ( Q36 )

  • From the foregoing answer, calculate the return loss be as seen by the left TERM at 14 MHz (this should correspond to S11 at 14 MHz)? ( Q37 )

  • Change the S-parameter box in the ADS schematic to a stop frequency of 20 MHz by double clicking the S parameter box on the schematic. Rerun the simulation.
  • Click the "smith plot" icon in the data plotting window.
  • Drop the plotting box in the visible area, and in the pop-up window:

    Select DataSet -> S(1,1) -> Add
    (only plot S11 on the Smith chart)

    Click on the plot options, click Coordinate->both, select Grid, and select admittance line type as long dash, and pick a green color. Click OK and the Smith Chart should appear. To figure out which end of the curve is at what frequency, Select Marker->New from the Dataplot menu bar and click on one end of the plotted curve. Then Edit->Undo to delete the marker.

    Print the smith chart plot (without the marker) and turn it in. Please write by hand on the plot the start and stop frequencies on the curve (mark the end-points). You should have a "semi-circle" in the left side of the chart. ( P4 )

  • From the Smith Chart (using a compass, compare the ADS Smith Chart plot to the Smith Charts used in class) what is the impedance (unnormalized) at 1 MHz? ( Q38 )
  • From the Smith Chart (using a compass, compare the ADS Smith Chart plot to the Smith Charts used in class) what is the impedance (unnormalized) at 14 MHz? ( Q39 )
  • From the Smith Chart (using a compass, compare the ADS Smith Chart plot to the Smith Charts used in class) what is the Susceptance (unnormalized) at 20 MHz? ( Q40 )
  • Again use the marker and drag it with the mouse to observe the read-out and find the impedance (unnormalized) at 20 MHz? ( Q41 )

  • ( NOTE: the above answers should agree with all of your hand calculations above. Remember, the impedance you observe consists of everything except the left-hand TERM. )

  • Change the S-parameter box in the ADS schematic to a stop frequency of 100 MHz by double clicking the S parameter box on the schematic. Rerun the simulation.
  • Replot the Smith chart from 1 to 100 MHz and plot as before,
  • Print the Smith chart plot (without markers) and turn it in. You should have a "circle" in the left side of the chart. ( P5 )

  • Why do you have a "circle" now? ( Q42 )

    • Part 3

  • In this part, we will experimentally determine the wavelength and impedance of a microstrip line of width 0.025 inches (25 mils) on a 0.05 inch (50 mils) thick epoxy-fiberglas FR4 board (apologies to the metric system ...). This is a good way to check your microstrip line impedance for future projects.

  • Go down through the directory tree to networks/p2txline3.dsn, and double click that design file. You should see the schematics on the right pane.
  • Double-click the p2txline3 schematic in the right half of the window, and the following schematic should appear.

  • Double-click the "gear" icon in the upper right of the window to simulate.
  • Click the "rectangular plot" icon in the pop-up data plotting tool.
  • Select the plot options to plot S11 and S21 from -30 to 0 dB in 3 dB steps.
  • Print the S-parameter plot and turn it in. You should have a plot of both S11 and S21. S11 have a null near 30 MHz. ( P6 )

  • What is the electrical length in wavelengths of the line at the null of S11 near 30 MHz??
    (you should be able to figure this out from the schematic and P6 alone, but you can also calculate from the transmission line geometry to double-check your answer) ( Q43 )
  • What is the peak value of S11 near 15 MHz? ( Q44 )
  • What is input impedance looking into the left end of the line at the peak value of S11 near 15 MHz?
    (Hint: is the impedance looking into the line at its largest or smallest value there?) ( Q45 )

  • Change the S-parameter box in the schematic to a stop frequency of 25 MHz by double clicking the S parameter box on the schematic. Rerun the simulation.
  • Click the "smith plot" icon in the data plotting window.

    Click on the plot options, click Coordinate->both, select Grid, and select admittance line type as long dash, and pick a green color. Click OK and the Smith Chart should appear. To figure out which end of the curve is at what frequency, Select Marker->New from the Dataplot menu bar and click on one end of the plotted curve. Then Edit->Undo to delete the marker.

  • Print the S11 smith chart plot (without the marker) and turn it in. Please write by hand on the plot the start and stop frequencies on the curve (mark the end-points). Also mark the frequency where the impedance crosses the purely resistive axis. You should have a "semi-circle" in the right side of the chart. ( P7 )

  • From the Smith Chart what is the impedance ("un-normalized") looking into the left end of the line at 15 MHz? ( Q46 )
  • What is the line impedance?
    (DONT USE A FORMULA to calculte this answer! figure it out from the experimental data.)?
    (Hint: experiment with quarter wave line sections on the smith chart, and see that an impedance of Zo/k is transformed into an impedance of kZo; a transformation ratio of k^2.)     ( Q47 )
  • What is the effective dielectric constant of the line?
    (DONT USE A FORMULA to calculte this answer! figure it out from the experimental data.)
    (Hint: lambda = lambda0 / sqrt(relative dielectric constant).     ( Q48 )

    • Part 4

  • In this part, we design matching networks.

  • Design a 2-element matching network to match a load of 250 ohms in series with a 2 nH inductor into 50 ohms at 4 GHz. Proceeding from the load, use a shunt inductor then a series capacitor for the matching network. (use file p2match1 as a starting point and find the proper values for the 1000 pF capacitor and 1000 nH inductor).

  • Plot the the Immitance Smith chart for the load alone (no matching network) from 3 to 5 GHz. ( P8 )
  • What are the values for the matching network inductor (nH) and capacitor (pF)?. ( Q49 )
  • Printout the schematic of your final matching network along with the load. ( P9 )
  • Plot the the Immitance Smith chart for the matched load from 3 to 5 GHZ (label, by hand, start and stop frequency). ( P10 )
  • Plot S11 (-30 to 0 dB) for the matched load from 3 to 5 GHZ. ( P11 )
  • Over what frequency range is your return loss better than 10 dB?. ( Q50 )

  • Design a 2-element matching network to match a load of 20 ohms in series with a 5 nH inductor into 50 ohms at 4 GHz. (use file p2match2 as a starting point and add a 2-element input matching network).

    NOTE: you might not be able to use a shunt element as the first element of the 2-element matching network! So redraw the schematic as required.

  • Plot the the Immitance Smith chart for the load alone (no matching network) from 3 to 5 GHz. ( P12 )
  • What are the values for the matching network inductor (nH) and capacitor (pF)?. ( Q51 )
  • Printout the schematic of your final matching network along with the load. ( P13 )
  • Plot the the Immitance Smith chart for the matched load from 3 to 5 GHZ (label start and stop frequency). ( P14 )
  • Plot S11 (-30 to 0 dB) for the matched load from 3 to 5 GHZ. ( P15 )
  • Over what frequency range is your return loss better than 10 dB?. ( Q52 )

  • For the past 4 months, you have been designing the world's greatest 4 GHz amplifier chip with 300 ohm output impedance and only 0.05 pF stray capacitance ... a perfect impedance match to the 300 ohm antennas you bought last month for the prototype radios!

    To your horror, your boss comes into your office and says that the president of the company insists you package your chips into the packages he bought from a friend on a fishing trip. The packages are so not RF. After the obligatory bond-wire inductance of 2 nH connecting your chip to the package bond area, they have a stray capacitance of 1 pF to ground and another 1.5 nH series inductance because the pins are longer than a Weldon lecture ... And if that wasnt bad enough, the president traded all of your 300 ohm antennas for a bucket of real shiny 50 ohm antennas ... so you now need a 50 ohm output!

    To make matters worse, the boss says the program is over budget so you cant use any inductors or capacitors because they will cost too much, but you can use microstrip transmission line sections laid out as foil patterns on the system board.

    Based on the great stuff you learned in RF class, you save the day by matching the whole mess with a single little piece of microstrip ... and the boss gives you 10,000 Enron options as a bonus.

    A schematic of the whole situation is below, you need to make the impedance look like 50 ohms looking into the transmission line from the left. (also see the really handy file p2smith1/networks/p2match3)

  • Plot the the Immitance Smith chart for the chip alone (the 300 ohm and 0.05 pf) from 3 to 5 GHz. ( P16 )
  • Plot the the Immitance Smith chart for the chip in the package (the chip plus the two inductors and 1 pF, with no transmission line matching network) from 3 to 5 GHz. ( P16 )
  • Notice what the package did to the chip!
    Almost took it from an open circuit to a short circuit ...
  • What are the values for the microstrip matching network length (meters) and width (mils)?. ( Q53 )
  • Printout the schematic of your final circuit. (including matching network along with the load, ie., the chip plus the two inductors and 1 pF). ( P17 )
  • Plot the the Immitance Smith chart for the matched load from 3 to 5 GHZ (label start and stop frequency). ( P18 )
  • Plot S11 (-30 to 0 dB) for the matched load from 3 to 5 GHZ. ( P19 )
  • Over what frequency range is your return loss better than 10 dB?. ( Q54 )
  • Sketch your transmission line to 1:1 scale. ( Q55 )

    • Report

    Use the Project Report Template  (also shown as pdf file)

    Write any comments or observations you may have directly on the printouts. Type or clearly handwrite. Do not add extraneous pages or put explanations on separate pages unless specifically directed to do so. The instructor will not read extraneous pages!

    Only turn in requested plots ( Pxx ) and requested answers to questions ( Qxx ). All plots must be labeled P1, P2, etc. and all questions must be numbered Q1, Q2, etc.

    Turn in a separate sheet with answers to all of the specific questions above.

    Copyright © 2002 T. Weldon