ADS Fundamentals - 2001 LAB 5: S-parameter Simulation and Optimization Overview - This exercise continues the amp_1900 design. It teaches how to setup, run, optimize and plot the results of various S-parameter simulations, including determining matching network values. OBJECTIVES? Measure gain and impedance.? Set up and use sweep plans, parameter sweeps, and equation based impedances.? Calculate values for a matching network.? Tune a matching network.? Use optimization to meet design matching goals.? Use Noise and Gain circles.
Table of Contents 1. Set up the simulation and circuit with ideal components...3 2. Simulate and plot data with marker readout modifications...5 3. Write an equation to vary Term port impedance...5 4. Calculate L and C values in the data display....6 5. Replace ideal with calculated values, simulate, plot S-parameters...7 6. Add matching components L and C, simulate, plot the results...8 7. Tune the input matching values...9 8. Set up an Optimization controller and Goals...11 9. Enable the components to be optimized....14 10. Simulate the optimized circuit...17 11. Set up a final matched circuit...18 12. Simulate Stability with Gain and Noise Circles...19 13. OPTIONAL Read and Write S-parameter Data with an S2P file...22 5-2
PROCEDURE 1. Set up the simulation and circuit with ideal components. a. Save the last schematic (ac_sim) design as: s_params. b. Modify the design to match the schematic shown here:? Delete the AC source and controller. Also delete the measurement equations, parameters sweep, and any unwanted variables, etc.? Insert terminations (Term) from the S-parameter palette.? From the Lumped Components palette, insert two ideal inductors: DC_feed to keep the RF out of the DC path. These are perfect shorts to dc and opens to AC. They provide a first approximation prior to implementing real component values.? Insert two ideal DC block / RF coupling capacitors: DC_block.? Delete the node names. For S-parameter simulation they are not required - the port terminations (num1 and num2) provide nodes. 5-3
c. Insert an S-Parameter simulation controller and set: Start=100 MHz, Stop=4 GHz, and Step=100 MHz. d. Save the design. 5-4
2. Simulate and plot data with marker readout modifications. a. Be sure the name of the dataset is: s_params and then simulate. b. When the simulation is finished, insert a rectangular plot of S21 (db). Insert a marker on 1900 MHz and verify that the gain is about 20 db. c. Insert a Smith chart of S11 and place a marker on 1900 MHz. Edit the marker readout (double click). In the dialog, Smith tab, change Zo to 50 as shown. Click OK and the marker will now read the value in ohms - referenced to 50 ohms. 3. Write an equation to vary Term port impedance. a. In schematic, write an equation for port 2 Term Z to be 35 ohms above 400 MHz: Z = if freq < 400 MHz then 50 else 35 endif. Type directly onscreen. b. Simulate and then insert a list of PortZ(2). Verify that Z is 35 Ohms above 400 MHz. c. Reset the value of port 2 Term to 50 ohms: Z = 50 Ohm. 5-5
4. Calculate L and C values in the data display. a. In data display, write an equation XC for the capacitive reactance of 10 pf at 1900 MHz. Then list equation XC and title the list using Plot Options as shown here. Afterward, change the value of the capacitor in the equation and verify that XC is automatically updated. b. Create a table for a range of inductor values and reactances. L_val is a range of swept values from 1 nano to 200 nano in 10 nano steps. In ADS, the syntax of two colons is a wild card (all values) and can also be used to indicate a range as shown here. The square brackets are used to generate the sweep. After writing the equations and listing them as shown here, scroll through the list. NOTE: The XL equation will be colored red (invalid) until you write the L_val equation. NOTE on data displays - You can copy the equations and tables (Ctrl C / Ctrl V) to other data displays. Or use the command File > Save As Template to save the data display as a template that can be inserted in other projects. c. Save the current data display and the schematic. 5-6
5. Replace ideal with calculated values, simulate, plot S-parameters. Now it is time to begin creating a matching network with the calculated values. a. Save the schematic with a new name: s_match. b. Change the component name (DC_Block) of both blocking capacitors to C and they will automatically become lumped capacitors as shown here. Assign the value for each C = 10 pf as calculated. Highlight the component name, type in C, and press Enter: DC_Block will become C. c. Change the ideal inductors (DC_Feed) in the same manner and set L = 120 nh each. According to the XL and L_val table, the reactance at 1900 MHz is about 1.5K, which is reasonable at this point in the design. d. The schematic should now look like the one shown here. If so, Simulate (dataset name should be s_tune). 5-7
e. In the data display, plot the transmission (S12 and S21) and reflection (S11 and S22) data with markers as shown here. Notice that the gain stays relatively flat, and the leakage is reasonable, but the impedance is not matched. In the next steps, you will tune the input to 50 ohms and then optimize the output to 50 ohms. 6. Add matching components L and C, simulate, plot the results. Referring to the S11 data, a parallel C should move the marker toward the 50 ohm circle of constant resistance. Also, a series inductor can be used to move along 50 ohm circle toward the center of the Smith chart toward 50 ohms. By starting with reasonable values, the tuner can be used to match the input at this stage of the design development. a. Add series L = 10 nh and parallel C = 1 pf as shown here at the input termination. Rename the instances L_match_in and C_match_in as shown. b. Set the simulation step size to 10 MHz, simulate and view the response. You should see the gain is still reasonable but the input impedance S-11 is not yet close enough to 50 ohms. The next step will be to use the tuner to achieve the match. 5-8
7. Tune the input matching values. a. Position the data display so that you can see the results of using the tuner. Then select both matching components: L_match_in and C_match_in and start the tuner (Simulate > Tuning) or icon. b. Put the tuner in the Details mode. Set Simulate to: After pressing Tune and set Trace History to 1. The Min/Max range for C: 0 to 2 and for L: 0 to 40. Set step size 0.01 for C and 0.1 for L. As you tune, you can make them more precise if Use 1 or 0 Click here to change the value in steps. Then press Tune. Step size. desired. c. Tune the values until S21 and S11 are similar to the results shown here. Then click Update on the tuner and check the schematic to see that the component values are updated to approximately: L_match_in = 14.3 nh and C_match_in = 0.4 pf and cancel the tuner. If your values are not close, set them on the schematic and simulate without the tuner. Or, repeat the 5-9
tuning process if you have time. NOTE on results: The results show the input (S-11) match is good with reasonable gain but the output is almost an open. Therefore, it is reasonable to put a similar topology on the output and simulate the response. d. Add series L and parallel C to the output, directly before the port 2 termination. Rename them as L_match_out and C_match_out and set L = 10 nh and C = 0.5 pf. e. Simulate and check the response with a marker added to the S22 trace. Your data should be similar to the results shown here where S11 has shifted and S22 is now closer to 50 ohms. At this point in the amplifier design, it will be more efficient to use the optimizer to complete the matching process to achieve both S11 and S22 goals (near 50 ohms) without compromising gain or leakage. f. Save the design but do not close the window. 5-10
8. Set up an Optimization controller and Goals. a. Save the s_match schematic design with a new name: s_opt. b. Go to the Optim/Stat/Yield palette and insert an optimization controller and one goals as shown here. c. Edit the S-11 optimization goal In the dialog box, type in the following settings and click Apply after each one and OK when done.? Expr: db (S(1,1)) SimInstanceName: SP1.? Max= -10 (S22 must be at least 10dB to achieve the goal)? RangeVar=freq RangeMin=1850 MHz RangeMax:1950 No need to type quotation marks when using dialog box. 5-11
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d. Copy the S11 goal - select it and use the copy icon. e. On screen, change the goal expression to db(s(2,2)) as shown here. Note on quotation marks - The range values do not need quotes because they are values and not strings (variables). f. Set up the OPTIM controller. For this lab exercise, most of the default settings can remain. However, set the OptimType to Genetic which combines Random and Gradient methods. Also, increase the number of iterations, because Random optimization often requires more than other types. Set MaxIter = 200. Note on Optim parameter settings - SetBestvalues means that the components on schematic can be updated with the best optimized values. SaveSolns means that all successful optimization solutions (S matrix values, etc) can be written to the dataset but this can use more memory. Also, the default is to use all goals on the schematic, however, you can edit the OPTIM controller and select which goals to use in the simulation. 5-13
9. Enable the components to be optimized. a. Edit (double click) the inductor L_match_in. When the dialog appears, click the Optimization/Statistics/DOE Setup button. In the Optimization tab, set the inductor to be Enabled as shown and type in the continuous range from 1nH to 40 nh as shown here. Click OK and you should see the component annotation on the screen ready to be optimized. b. Go ahead and Enable the other three matching components as shown here in the same manner. You can edit them using the dialog box or you can type directly on-screen using the opt function and curly braces for the range: 5-14
c. Check the circuit as shown here and then Simulate and watch the status window. d. The status window reports progress. If the goals are met, the EF (error function) = 0. An iteration is successful if the EF moves closer to zero. Because of the Genetic type, your values may differ slightly. Because the solutions are not sent to the dataset, the S-parameters are not available in the dataset, but this is good because the dataset does not contain all of the unwanted solutions. So, if EF = 0, the next step is to update 5-15
component values and simulate. 5-16
e. Click the command: Simulate > Update Optimization Values. The enabled components should now have the best values as the nominal values. For example, the input inductor may look like the one shown here but your values may vary a little because of the random mode. The only way to check is to simulate with the new component values (next step). NOTE on optimization EF that does not reach zero - If an optimization does not meet the goal, you can loosen the goals or EF. Also, look for components that are being driven to the ends of their opt range and widen them. Or, try another optimization method, increase the number of iterations, or try another topology. 10. Simulate the optimized circuit. a. Disable the components. Edit (double clicking) each one and click the Optimization/Statistics button and then select Disabled as shown here. Notice that the component function changes from opt to noopt as shown here. Or, you can insert the cursor and change opt to noopt directly on-screen. b. Deactivate the Optim controller and goals. c. Simulate with the updated values. d. In the data display, plot the S matrix in db to see all four S parameters very quickly. This way you can verify the results which should be similar to the values shown here. Your values may differ but overall, the goals from the optimization should be met. If desired, you can plot the impedance on a Smith chart. 5-17
e. Save the schematic and data display. NOTE on results - the S-parameter goals were met, and the gain looks good. But the impedance values are too capacitive and not close enough to 50 ohms at 1900 MHz. Therefore, some modifications to the match will be used (next step). 11. Set up a final matched circuit. a. Save the existing schematic with a new name: s_final_match. b. While you could use various values or different topology, the following C and L (with R added) will be used for the remainder of the lab exercises be sure to add resistance to the inductors to provide the correct matching results. Go ahead and change the values by typing directly on-screen as shown here: L_match_in = 18.3 nh & R=12 Ohm C_match_in = 0.35 pf L_match_out=27.1 nh & R=6 Ohm C_match_out = 0.22 pf c. With the new final component values, Simulate again and verify the results by comparing them to the values shown here. Plot the entire S matrix by selecting S in the dataset. As you can see here, the S-parameters are very good at the carrier frequency of 1900 MHz. Also, the impedance values are very close to 50 ohms at the carrier frequency. With these results, the next steps will be to simulate noise circles and stability. 5-18
d. Save the design and data display. Close the data display but keep the schematic window opened. 12. Simulate Stability with Gain and Noise Circles a. Save the final design with the name s_circles. b. Go to the S-parameter simulation palette and insert stability measurement equations on the schematic: Mu and MuPrime. These can be used with their default settings (S-matrix). c. Scroll down in the palette and insert two measurement equations: GaCircle and NsCircle as shown and also insert the Options controller. In Options, set Temp = 16.85 to avoid the warning message for noise. Change the db gain in the GaCircle to 30 as shown. No setting is required for the NsCircle it will use Nfmin (calculated minimum noise figure) from the simulation data. d. Change the simulation frequency = 1850 MHz to 1950 MHz so that fewer data points (circles) will be created. Be sure the noise calculation is turned ON in the controller and simulate. Gain: 30 db. 5-19
e. Plot the measurement equations: NsCircle1 and GaCircle1 on a Smith chart, plot Mu s on a rectangular plot. f. Insert a list of nf(2), Nfmin, and Sopt. Center of the noise circle is within the 30 db gain circle. NOTE on results - On the Smith chart, inside the Gain Circle indicates the load impedance will result in 30 db of gain. The Noise Circle is different because its center indicates the optimum value of source reflection coefficient that will result in NF min. As you can see, the center of the Noise Circle is within the Gain Circle so both Gain and NFmin can be achieved. In addition, the listed value of nf (2) is the noise figure at port 2 for the circuit as it exists, but nf(2) could be improved if the source reflection coefficient is equal to Sopt. 5-20 g. Save and close all the designs and data displays. At this point, the amplifier is ready to be tested with the non-linear simulator, Harmonic Balance. However, before doing so, you will return to the system project in the next lab and build the two filters for the RF system.
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13. OPTIONAL Read and Write S-parameter Data with an S2P file You can read or write data in Touchstone, MDIF, or Citifile formats. ADS can convert supported data into the ADS dataset format. Typically, these data files are put in the project directory but they can also be sent to the data directory. You can control where they reside. a. Open a new schematic window using Ctrl N where N means new (schematic). Next, save the new design with the name: s2p_data. b. Click on the HP-IB icon (Instrument Server). c. When the dialog box opens, click the box to WRITE and Write to File then select the Touchstone format. You are going to write (convert) an existing ADS dataset (s_params) into a Touchstone file. It will represent measurement data from a network analyzer. d. In the File Name field, type: my_file.s2p. This will be the name of the Touchstone format file that will be converted from ADS data. e. Select the Output Data Format as Mag/Angle. f. In the Datasets field, select the dataset: s_params. This was the dataset from the simulation using the ideal components. g. Check the dialog box and click Write to File. Check the Status Window. If successful, you will see a message. This means my_file.s2p is now a Touchstone file in the data directory of the amp_1900 project. You can check this if you want and you can use a text editor (ADS Main window: Options > Text Editor) to look at or to modify the file. 5-22
h. In the empty new schematic, insert an S2P component from the Data Items palette. Notice that the component variable (file=) is not yet assigned. h. To assign the data, edit the S2P component and another dialog box will appear. Next, browse for the file name. When the next dialog appears, select my_file.s2p and click the Open button and the file name will be assigned (shown here). i. In the schematic insert an S_Params template (Insert > Template) and wire the S2P component to the ports and insert a reference ground as shown here. i. From the Simulation- S_Param palette, insert a Sweep Plan and set it to Start= 100 MHz, Stop = 3 GHz and Step=100 MHz as shown. Sweep plans are normally used for frequency sweeps within sweeps but you can use it here to see how it replaces the Frequency settings in the S- parameter simulation controller. Simulation Controller: Frequency tab - j. To use the Sweep Plan, edit the simulation controller. In the Frequency tab, select the SwpPlan1 as shown here. Go to the Display tab, select SweepPlan and remove the start, stop and step as shown here. 5-23
k. Simulate and the results will automatically appear in the Data Display window. l. Zoom in on the S21 measurement and add the S21simulation data from the original s_params simulation to the plot to verify that the Touchstone file correctly represented the data. Also, you can try setting the trace thickness using Trace Options. As you can see here, the S21 data from the s_params dataset has been added to the simulation results. The two traces are identical except that the s2p simulation only goes to 3 GHz. Here, the trace thickness and types have been adjusted using Trace Options) to show both traces more clearly and markers also show the two traces. 5-24
EXTRA EXERCISES: 1. Z_PORTS - In a separate schematic, set up an S parameter simulation of an impedance that is described by an equation as shown here. Plot the response and try adjusting the values. 2. Set up the simulation using a sweep within a sweep using two or more Sweep Plans. 3. REFER TO THE OPTIONAL EXERCISE and use the text editor to edit the s2p file, change some values, and simulate to verify that you can do this. 5-25
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