RF System Design - Chapter 5

Chapter 5

RF System Design

PathWave Advanced Design System (ADS)

Keysight PathWave Advanced Design System (ADS) provides all the necessary capabilities to perform RF system design and simulations. RF system design is an important and critical step to validate the system performance for first pass success. RF system architecture can be implemented using RF System models available in the Analog/RF library.

Case Study 1: Receiver System Design

1. Create a new workspace Lab5_RF_System_Design_wrk. Create a new schematic cell and name it Lab5a_RFSystemDesign.

2. Place Amp and Mixer2 from the System-Amps & Mixers component library palette menu. Set their characteristics as shown in Figure 1.

3. Place the Chebyshev Bandpass Filter component at the Mixer output from FiltersBandpass component library palette menu. Set the characteristics as shown in Figure 1.

4. Copy and paste the amplifier twice after the BPF component. These amplifiers will be used as two-stage IF amplifiers. Change the following specifications:

TOI = 20

SOI = 30

NF = 3 dB

5. Once completed, the schematic will look similar to the one shown in Figure 1.

6. The next step is to connect the RF and LO sources and setup the simulation to observe the system response. Place P_1Tone and Osc source from the Sources-Freq Domain library. Connect the components and set their characteristics as shown in Figure 2. Make sure to change the PhaseNoise list in the Osc source.

7. Place the HB (Harmonic Balance) simulation controller from the Simulation-HB library and set its characteristics as show below.

8. Place a Term component at the output (after the second IF amplifier). Click Wire Label. Enter vout in the pop-up window, and click the “+” terminal of the Term component.

9. Once completed the receiver system diagram will look like the design shown in Figure 2.

10. Save the design and click the Simulate icon. Insert a rectangular plot in the data display window. Add vout from the measurement list and select Spectrum in dBm to observe the output spectrum.

Case Study 2: Phase Noise Simulation

Phase noise is an important simulation for receiver systems. The example below shows how to perform phase noise analysis using the Harmonic Balance simulator in Pathwave ADS.

1. Right-click Lab5a_RFSystemDesign and click Copy Cell.

2. In the pop-up window, give name the copied cell Lab5b_RFSystemDesign_PhaseNoise.

3. Open the schematic design for the copied cell. From Simulation-HB library place NoiseCon (Noise Controller) block onto schematic.

4. Double-click the NoiseCon block to open the properties. Set the following parameters:

Freq tab:

Sweep Type = Log

Start = 10 Hz

Stop = 100 kHz

Num. of pts. will automatically become 5. This indicates five noise analysis frequencies (i.e. 10 Hz, 100 Hz, 1 kHz, 10 kHz and 100 kHz). These are the same frequencies specified for the oscillator, which was used as an LO source in the system.

Nodes tab:

Select Pos Node = vout from the drop-down box, which is the output node where we provided a label in the earlier lab exercise

Click Add.

Phase Noise Tab:

Phase Noise Type = Phase Noise Spectrum

Under Specify Phase Noise Carrier, specify Frequency as 70 MHz. Alternatively, we can also specify carrier mixing indices such as {-1,1} etc.

5. Click OK. The Noise Controller is setup. We have one extra step in linking this Noise Controller to our HB simulation controller.

6. Double-click HB controller, and go to the Noise tab.

Check Noise Cons option.

From Edit drop-down box, select NC1 (name of noise controller).

Click Add.

Click OK. Close the HB simulation controller properties box.

7. Run the simulation. A new data display window will come up. Insert a new rectangular plot and select vout to be plotted in dBm to see the same spectrum as in the earlier lab.

8. Insert a new rectangular plot. In the window that pops up, select pnmx (i.e. Phase Noise). In the Plot Options, change the X-axis to Log. Click OK to see the Phase Noise plot at various offsets.

Case Study 3: Two-Tone Simulation of the Receiver System

Performing a two-tone simulation is important for system level analysis. The example here shows how to perform two-tone simulations on frequency converting based systems.

1. Right-click Lab5a_RFSystemDesign and click Copy Cell.

2. In the pop-up window, enter the name as Lab5c_RFSystemDesign_2Tone and click OK.

3. Open the schematic of the copied cell. For a two-tone simulation, we need to change the P_1Tone source, which is currently used for the RF source. Delete the one-tone source. From the Sources-Freq Domain library, place a P_nTone source on the schematic.

4. Double-click the P_nTone source and edit the properties as below:

Click Freq[1] and enter 2.399 GHz. Click Apply.

Click Add to insert a second tone frequency with the name Freq[2]. Enter the frequency as 2.401 GHz. Click Apply.

Click P[1], which is the power in the first tone. Enter the power as polar(dbmtow(-50),0). Click Apply.

Click Add to insert power for the second tone. Enter the same power as for the first tone. Please note that for a two-tone test, it is mandatory to have the same power in both tones. If this is not the case, the analysis will not be valid.

5. Since we have three source frequencies in the schematic, we need to modify the HB simulator to specify these frequencies for proper mixing product calculations. Double click the HB controller and specify the following frequencies, in order:

Freq[1] = 2.33 GHz

Freq[2] = 2.399 GHz

Freq[3] = 2.401 GHz