Circuit Envelope and Modulated Sources - Keysightrfmw.em.keysight.com/flash/eesof/ADS_2012_Ckt_Sim_Techs_v1/data/...amplifier with GSM and CDMA modulated sources ... Be able to understand

ADS 2012 Circuit Simulation Techniques (v.1 April 2013)

© Copyright Agilent Technologies 2013

LAB EXERCISE 4

Circuit Envelope and Modulated Sources

Topics: Circuit Envelope simulation using system components and a FET amplifier with GSM and CDMA modulated sources and results.

Audience: Engineers who have a basic working knowledge of ADS or have completed the prerequisite course.

Prerequisites: Completion of lab exercises 1, 2, and 3 of this course. Also, completion of Workspaces and Simulation Tools or equivalent experience, including basic circuit design concepts.

Objectives: Be able to understand and use Circuit Envelope and modulated sources to test circuits and systems.

Lab 4: Circuit Envelope & Modulated Sources

Copyright 2013 Agilent Technologies

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Table of Contents: Lab 4

1. Create a new Workspace 3

2. System Amp and PtRF source design 3

3. Envelope simulation controller setup 4

4. Simulate and plot the time domain response. 5

5. Simulate with Amp distortion 7

6. Amp with Demodulators for GSM 9

7. GSM Envelope Simulation with VAR setup 10

8. Copy FET AMP into the current workspace 12

9. CDMA setup for FET Amp 13

10. Example Data Display: ModSources_wrk 15

11. CDMA simulation and data equation 16

12. OPTIONAL – Distortion Effects using the Tuner 17



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Lab 4: Circuit Envelope and Modulated Sources

1. Create a new Workspace

a. Create a new workspace and library: My_Envelope. Select the Analog/RF and DemoKit_Non_Linear libraries as shown here.

b. In the new workspace, create a new cell / schematic named: Env_Basic.

2. System Amp and PtRF source design

a. In the new schematic, insert a behavioral amplifier from the System-Amps & Mixers palette: Amplifier2.

Notice that the default S parameters show different combinations of the coefficients. However you will simplify 3 of them by using the dbpolar function as shown here.

b. Set the S-parameters, as shown here, using the dbpolar function where S21 = 10dB of gain with 0 phase: S21 = dbpolar (10, 0).

Set S11 and S22 = dbpolar (-60,0).

Leave S12 as is: 0 (no leakage).



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c. Insert a PtRF_Pulse source (Sources-Modulated). Keep the default settings, except set the Rise = 5 ns, Fall = 10 ns, Width = 30 ns, and the Period = 100 ns. Also, P=dbmtow(0) and Freq = 1 GHz.

d. Insert a 50 Ohm resistor load, node names Vin and Vout, grounds, and wire as shown here, to complete this basic system test circuit.

3. Envelope simulation controller setup

a. Insert the Envelope controller and set the calculation frequency to1 GHz and Order=1. Later on, you will add distortion and increase the order.

b. Set stop = 50 nsec. This is enough time to see the entire pulse width, including the rise, fall, and delay.

c. Set the step = 1 nsec. This means the signal will be sampled every 1 ns resulting in 51 points of time sampled data.



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4. Simulate and plot the time domain response.

a. Run the simulation.

b. After the data display opens, plot Vin and Vout in a rectangular plot as the Magnitude of the Carrier in the time domain. Also, add a third trace using the Enter any Equation field (shown here) and type in the expression: ts (Vout) and Add it - this gives the composite waveform. The index [1] in the other traces gives the magnitude of the 1GHz carrier.

c. Put a marker on Vout to verify the rise time of 5 ns.

By sampling faster than the rise time, you get an accurate envelope.



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d. Insert another rectangular plot of Vout (Magnitude of carrier time domain). Then select the trace, and use Trace Options > Trace Expression to remove the index [1] so that the expression is: mag (Vout) and click OK. Then use Plot Options and turn off the X-axis Auto Scale: set X-axis from 5e8 to 1.5 e9 (step 1e8) and click OK to center the trace. By removing the index value [1], you get the magnitude of the fundamental (1 GHz) in the frequency domain. The increasing arrows represent the increasing magnitude of the pulse carrier as it rises during the time (5 ns). Use makers to see this – similar to the plot shown here.

e. Next, insert a List. When the dialog box appears, use the Enter any Equation field and type in the expression: what (Vout) and Add it. Click OK and you will see what dependencies there are for Vout.

The purpose of this is to show that both time and frequency exist in the circuit envelope data. There are 51 time points of the two frequencies: 0 (dc) and 1.0 GHz. The Matrix Size refers to the 1x1 matrix (ADS calls it scalar) and the data is complex (mag and phase of 1.0 GHz).

f. Insert a list of Mix (mix table) and use Plot Options to suppress the table format. This also shows how Envelope data is indexed. If you scroll through, you will see all 51 time points and all the index values for frequency.



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g. Go back and set the Envelope simulation controller time step to: Step = 10 ns and Simulate. Now, watch what happens to your plot when you under-sample the envelope. With the time step greater than the rise-time, you still get the carrier but not the correct envelope. The first time point is on10 nsec but the actual rise time is 5 ns.

5. Simulate with Amp distortion

a. Edit the Amplifier by setting: Gain Compression Power = 5 (dBm is the default) and Gain Compression = 1 dB. Be sure to display these settings.

b. Set the Envelope controller Order = 5 and keep the time step at 10 ns. Also, set the source input power to 10 dBm: dbmtow (10). This will drive it into compression.

c. Simulate and view the data. On the frequency domain plot, use Plot Options to set the X-axis back to Auto Scale and place the markers as shown, where strong odd harmonics result from the amplifier distortion (summing out-of-phase). Notice that the envelope amplitude is now smaller than the magnitude of Vin or Vout. Also, the envelope shape is not accurate because the sampling rate is too slow.



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d. In the Envelope controller, speed up the sampling rate: Step = 1 ns and Simulate again. After updating, the plot shows the correct envelope. But Vin and Vout are still greater than the envelope magnitude – this is due to the compression.

e. To prove this, insert a List of Vout. Edit the list and, in Plot Options, use Suppress Table Format and click OK. Then scroll down to a time point in the middle of the data. Now, you can see that the large third harmonic is 180 degrees out-of-phase, making the envelope smaller than the magnitude of the fundamental.

Now you should have a good basic understanding of how Circuit Envelope works, how the time and frequency are set and what data results. The next step will be use a modulated source and look at the baseband data using different demodulation techniques.

f. Save all your work.

g. In the schematic, use Ctrl-A (selects all components) and then Ctrl-C to copy all the components. You will past them into the next cell. Then close the schematic.



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6. Amp with Demodulators for GSM

The GSM source is a phase modulation of the carrier (typically 900 MHz) where the phase variation can represent 1 or 0 in many applications.

a. Create a new cell /schematic: Env_GSM as shown here. Then paste (Ctrl_V) the copied components into the new schematic and delete the PtRF source.

b. From the Sources-Modulated palette, insert the GSM source and put a pin label (node name) at the “B” output as shown: bits_out. It looks like a non-connected pin but it is OK. Also, set the source FO=1 GHz and Power = dbmtow (10). Also, remove the amplifier compression (previously set to 5) so that: GainCompPower = (blank) – do the same with GainComp as shown. Check your schematic – it should look like this:

c. Go to the System-Mod/Demod palette and insert two demodulators: FM_DemodTuned as shown. Set the value of Fnom on the two demodulators as shown:1 GHz. Also, insert label names at each output: demod_in and demod_out as shown. These will be used to look at the demodulated GSM signal (baseband).



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7. GSM Envelope Simulation with VAR setup

a. Insert a variable equation VAR and set up the stop and step times for approximately 270 kHz modulation BW as shown. The variable: t_stop is set to cover approximately 100 us. It is convenient to use the BW value as the denominator. The sample rate t_step is set to: 1 / 10 times the BW. Also, the default time units (seconds) does not have to be specified.

b. Set up the Envelope controller as shown to use the variables for Stop and Start as shown here and run the simulation.

c. When the simulation completes, insert a rectangular plot of Vout: Spectrum of the carrier in dBm with Kaiser windowing. Then insert two markers across the GSM bandwidth (about 270 kHz apart).

This is the output spectrum around the fundamental frequency (0 Hz on the plot). The Kaiser window helps ensure that the first and last time data points equal zero - this improves the dynamic range of the computed spectrum. Also, with windowing, the noise floor is lowered

d. In the plot, the BW was not exactly ½ of 270 KHz or 135 KHz. For greater accuracy, sample with more resolution by changing the t_step to:

1 / (15 *270e3) and Simulate.

Afterward, you can now move the markers to the desired 135 KHz points.



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Now that you know how to plot the spectrum around the carrier, the next step is to plot the demodulated bits or baseband.

e. Next, in the Data Display, plot the two FM demod nodes as the Baseband signal in the time domain. These traces will be the real part, indexed to [0]. The demodulator only outputs a signal at baseband (similar to the dc component). Notice they are the same because there is no distortion .

f. Edit the plot add bits_out (baseband signal time domain). Then select it and use Trace Options > Trace Expression and change it to: plot vs (real(bits_out[0]),time+10u). Then select Plot Axes and select Right Y Axis and click OK. Finally change the right side units as shown.

This expression adds the source delay of 10 us so that you can see the ideal bits compared to the demodulated bits - you should see the source 001101010010 pattern.

Note on Demodulators – You could use phase-demodulators but the FM demodulators are easier to use for this example. Also, an equation can demodulate the data. In the baseband equation shown here: the unwrap function removes the +/- 180 degree transition format from the absolute phase. The diff function will differentiate the unwrapped slope. Dividing by 360 will give the value in Hz.

Note on Phase Distortion - Also, you could insert a Butterworth filter (Filters-Bandpass) between the amplifier and the source and set it as shown.

This would create some phase distortion as only the narrower bandwidth passes to the amplifier and the full signal goes to the first demodulator.

g. Save all your work and close all the windows except the Main window.



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8. Copy FET AMP into the current workspace

For the next step, you will use the FET Amplifier from the previous lab – you will copy and paste it from one workspace to another. .

a. In the Main window, use the command File > Recent Workspaces and select My_FET_AMP. Then open the schematic: S_params_Zmatch. Select only the schematic components and the DEMO KIT TECH INCLUDE shown here. You will not need the other items or controllers in the schematic. Use Ctrl+C to copy the items and then close the window.

b. After you copy the design, save it and close the window. Then use the File > Recent Workspaces command to return to My_Envelope_wrk.

c. Create a new cell / schematic named: FET_amp and use Ctrl+V to paste the FET amp as shown here.



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9. CDMA setup for FET Amp

The following steps will use a CDMA source and Envelope simulator to produce data for ACPR, a trajectory plot, and more using the Data Display from an ADS example workspace.

a. Replace the input Term with a PtRF_CDMA_ESG_FWD source from the Sources-modulated palette – be sure to insert the same CDMA source shown here. This source is based on a real signal generator. Set FO = 7 GHz and Power = dbmtow (RF_pwr) as shown. You will set up the variables next.

b. Add a new VAR block for the Envelope time settings: t_step, t_stop, bit_rate, sam_per_bit, and num_sym. Type in the values shown here. These are based on the CDMA signal.

c. Add one more VAR block for the source power as shown here: RF_pwr=0.

d. Insert a MeasEqn from any simulation palette or type MeasEqn in th component history. Type: CDMA_OUT = VOUT[1]. This is the only data you will need in the dataset.

e. Put a wire label (node name) on the output as shown here: VOUT. It will be the node voltage will plot the data in the next steps.



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f. Insert an Envelope controller from the Simulation Envelope palette. Then type in the values shown here:

Freq = 7 GHz, Order =3, Stop = t_stop and Step = t_step.

g. Verify that your schematic has the same values as the one show here and then Simulate.

The next step is to copy a built-in ADS example data display from an Example. To do this, you will unarchive the example – this will create a new workspace – and then copy the .dds (Data Display) to your current workspace (My_envelope_wrk).

a. Save and close the schematic window – you will come back to it after you unarchive the example.



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10. Example Data Display: ModSources_wrk

a. Use the Main window command: Open > Example and then navigate to Tutorial and open the ModSources_wrk.7zap. Then use the Unarchive Wizard (use defaults) and Finish it

b. When the ModSources workspace opens, go ahead and open the Data Display IS95FwdLinkSrc.dds as shown here. Notice that it shows the data from the example simulation.

c. Use the Data Display command: File > Save As and save the .dds file as Env_CDMA to your My_Envelope_wrk workspace as shown here.

d. Go to the Main window and use the command: File > Recent Workspaces and open your My_Envelope workspace again. The copied Data Display is now ready for your simulation data.



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11. CDMA simulation and data equation

a. Open the FET_amp schematic and click Simulate > Simulation Settings. Then browse and select the copied Data Display: Env_CDMA.

b. Run the simulation.

When the simulation completes, notice that the Data Display has red equations and no data in the plots. This is OK because you need to change the equation from the example value to your measurement equation data.

c. Change the Vfund equation to: Vfund = CDMA_OUT as shown here;

d. ACPR and Trajectory:

Look through the data on this page and verify that your FET amplifier has similar results. Notice that the trajectory diagram appears show QAM results and ACPR is about -50 for both channels. By copying the example Data Display, you saved a lot of time and effort.

e. Save all your work.

This is the end of the lab exercise – you can close all your windows and exit ADS unless you have time to do the optional step.



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12. OPTIONAL – Distortion Effects using the Tuner

a. Look at the Power Calcs page – these are the expected results from previous simulations: about 15 dBm at the output before compression.

b. In schematic, start the Tuner and select the RF_pwr variable to be tuned.

As shown here, set the Max =10 and set Step = 1. Then slowly increase power while you watch the trajectory and spectrum change – do this slowly so that the Data Display can update to the change. This will show that there is some distortion in the amplifier as power increases.

c. After you finish using the tuner, close it – you do not need to update the schematic. Then save and close all your work.

a. When finished, save and close the workspace and all open windows.

END OF LAB EXERCISE

Documents

Circuit Envelope and Modulated Sources - Keysightrfmw.em.keysight.com/flash/eesof/ADS_2012_Ckt_Sim_Techs_v1/data/...amplifier with GSM and CDMA modulated sources ... Be able to understand