cadence pa design using spectrerf

________________________________________________________________________

August 2010 Product Version 10.1

SpectreRF Workshop

Power Amplifier Design Using SpectreRF

MMSIM 10.1

August 2010

PA Design Using SpectreRF ________________________________________________________________________

August 2010 Product Version 10.1 2

Contents

Power Amplifier Design Using SpectreRF....................................................................... 3

Purpose ....................................................................................................................... 3

Audience ..................................................................................................................... 3

Overview ..................................................................................................................... 3

Introduction to Power Amplifiers .................................................................................... 3

The Design Example ....................................................................................................... 4

Three Testbenches for PA Measurements .................................................................... 4

Example Measurements Using SpectreRF ....................................................................... 6

Lab 1: Power Related Measurement (Swept hb) ........................................................... 6

Lab 2: Linearity Measurement (Swept hb with hbac) ................................................. 19

Lab 3: Stability and S-Parameter Measurements (PSS and PSP) ................................ 30

Lab 4: Large Signal S-Parameter Measurement (LSSP Wizard) ................................. 44

Lab 5: Load-Pull Measurements (Swept hb) .............................................................. 52

Lab 6: Envelope Analysis (ENVLP and ACPR Wizard) ............................................ 73

Using the ACPR Wizard .................................................................................... 92

Lab 7: Envelope Analysis (fast-mode) ....................................................................... 98

Lab 8 EVM functions for OFDM signals ................................................................. 105

Conclusion .................................................................................................................. 114

Reference .................................................................................................................... 114



Power Amplifier Design Using SpectreRF

Purpose

This workshop describes how to use SpectreRF, which includes the new hb analysis, in

the Virtuoso Analog Design Environment to measure parameters that are important in

design verification of Power Amplifiers (PAs).

The hb analysis is one new GUI for the harmonic balance analysis, and has the similar

features with the harmonic balance engine in PSS and QPSS analyses.

Note: The procedures described in this workshop have been kept broad and generic. Your

specific design might require procedures that are slightly different from those described here.

Audience

Users of SpectreRF in the Virtuoso Analog Design Environment.

Overview

This workshop describes a basic set of the most useful measurements for PAs.

Introduction to Power Amplifiers

Power amplifiers are a part of the transmitter front-end used to amplify the transmitted

signal so the signal can be received and decoded within a fixed geographical area. The

main PA performance parameter is the output power level the PA can achieve, depending

on the targeted application, linearity, and efficiency.

Power amplifiers can be categorized several ways depending on whether they are

broadband or narrowband, and whether they are intended for linear operation (Class A, B,

AB and C) or constant-envelope operation (Class D, E and F). This workshop focuses on

the design of narrowband and linear PAs.



The Design Example

The PA measurements described in this workshop are calculated using SpectreRF in the

Virtuoso Analog Design Environment. The design example used to conduct the

measurements described in this workshop is the two-stage power amplifier, EF_PA_istg

and EF_PA_ostg, shown below:

The supply voltage is 5 V. There is a simple output matching network in the sub circuit

EF_PA_ostg. The power amplifier is designed to be driven by CDMA I/Q channel

baseband signals, modulated using QPSK schemes with a carrier frequency of 1 GHz.

Typical PA performance metrics are listed in the following table:

Measurement Acceptable Value

Output Power +20 to +30 dBm

Efficiency 30% to 60%

Supply Voltage 2.8 to 5.8 V

Gain 20 to 30 dB

Harmonic Output (2f, 3f ,4f) -30 to -50 dBc

Stability Factor >1

Three Testbenches for PA Measurements

Testbench One

The first testbench drives the PA by sinusoidal sources. In this workshop, you use this

testbench to make general measurements, including:

■ Power related measurements (input power, output power, supply voltage, supply

current, power gain, and power dissipation)



■ Efficiency measurements (drain efficiency and power added efficiency)

■ Linearity measurements (1 dB compression point, IIP3, and OIP3)

■ Noise measurements (NF or F)

■ Stability measurements (K-factors, B1f, and S-parameter)

■ Large signal S-Parameter measurements

You use a Periodic Steady State (PSS) or hb analysis followed by a Periodic Small Signal

(PAC/PSP/PNOISE, or hbac/hbnoise) analyses to make these measurements. (For details,

see Labs 1 to 4 on pages 6-44).

Testbench Two

The second testbench drives the PA by a sinusoidal source with a port adapter added at

the output power amplifier. You use this testbench to generate

■ Load-pull contours

■ Reflection contours

You use the swept hb analysis combined with the parametric analysis tools to measure

load pull. (For details, see Lab 5 on page 52).

Testbench Three

The third testbench drives the PA by modulation signals. You use this testbench to

generate

■ ACPR plots

■ Input and output trajectory plots

You use the Envelope (envlp) analysis to make these measurements. (For details, see

Lab 6 on page 73).



Example Measurements Using SpectreRF

To achieve optimal circuit performance, you should measure and evaluate several PA

characteristics or parameters under varying conditions. The most important trade-off in

PA design is between efficiency and linearity.

Begin the examination of the flow by bringing up the Cadence Design Framework II

environment to look at a full view of the reference design:

Action P-1: cd to ./rfworkshop directory.

Action P-2: Run tool virtuoso&.

Action P-3: In the CIW window, choose Tools — Library Manager….

Lab 1: Power Related Measurement (Swept hb)

Power related measurements include input power, output power, supply voltage, supply

current, power gain, and power dissipation. To make these measurements, you use a

swept PSS analysis to sweep the input power level.

Action 1-1: Open the schematic view of the design EF_example_simple in the library

RFworkshop.

Action 1-2: Select the PORT1 source. Choose Edit — Properties — Objects and

ensure that the port properties are set as described below:

Parameter Value

Resistance 50 ohm

Port Number 1

DC voltage (blank)



Source type sine

Frequency name 1 RF

Frequency 1 fin

Amplitude 1 (dBm) pin



Parameter Value

Resistance 50 ohm

Port Number 2

DC voltage (blank)

Source type dc

Action 1-4: Check and save the schematic.

Action 1-5: In the Virtuoso Schematic Editing window, choose Launch — ADE L.

Action 1-6: (Optional) Choose Session — Load State in the Virtuoso Analog Design

Environment window, select Cellview in Load State Option and load

state “Lab1_Power_hb”, then skip to Action 1-12.

Action 1-7: In the Virtuoso Analog Design Environment window, choose Analyses —

Choose….

Action 1-8: In the Choosing Analyses window, select hb in the Analysis

field of the window. Set up the form as follows:



Note: The hb analysis is one new GUI for the harmonic balance analysis, and has the

similar features with the harmonic balance engine in PSS and QPSS analyses.

For the hams/maxharms parameter, 5-8 is enough usually. More harms/maxharms will be

used for the strong non-linear signal.

By default, the tstab method is used for Harmonic Balance Homotopy Method.

Action 1-9: Make sure that Enabled is selected. Click OK in the Choosing Analyses

form.

Action 1-10: In the Virtuoso Analog Design Environment window, choose Outputs —

To be Saved — Select on Schematic.

Action 1-11: In the schematic, select the positive terminals of PORT2, and PORT1.

Press the ESC key to end the selection process.

The Virtuoso Analog Design Environment window looks like this:

Action 1-12: Choose Simulation — Netlist and Run to start the simulation or click the

netlist and Run icon in the Virtuoso Analog Design Environment

window. After the simulation finishes, use the next actions to plot the

simulation results.



Action 1-13: In the Virtuoso Analog Design Environment window, choose Results —

Direct Plot — Main Form.

The Direct Plot form appears.

Action 1-14: In the Direct Plot Form window, choose hb as the Analysis type. Choose

Power in the Function field. Choose 1G in the Output Harmonic list

box.



Action 1-15: Select Port2 on the schematic.

The waveform window shows the output power versus input power.



For the design example given, when the input power level is -5 dBm, the output power

level is close to 20 dBm. Thus, -5 dBm is assumed to be the normal operating condition.

All subsequent plots are based on this assumption.

Action 1-16: In the Direct Plot form, change the Plotting Mode to Replace and set up

the form as follows:



Action 1-17: Click Port2 to show the output Power Spectrum.



Action 1-18: In the Direct Plot form, click Power Gain in the Function field and set up

the form as follows:



Action 1-19: In the schematic, click the positive and negative terminals of Port2, and

then click the positive and negative terminals of VCC.

The following plot shows the drain efficiency of the PA.

Action 1-20: In the waveform window, click the Add Sub window icon.

Action 1-21: In the Direct Plot form, set the Plotting Mode to New SubWin. Select

Power Added Eff. in the Function field. Select the Output Harmonic as

1GHz.

To use the Power Added Efficiency (PAE) function, you only need to select the output

terminal, input terminal, and DC terminal in turn. The result is a plot of the power added

efficiency versus the input power level.



Action 1-22: In the schematic, select the positive terminals of PORT2, PORT1 and

VCC in turn.

The waveform window updates.



Notice that for a PA with high gain, the PAE is nearly equal to the drain efficiency. You

find that the efficiency of the PA around the nominal operating condition is only about

20%.

Action 1-23: Close the waveform window, the Direct Plot form, and the Virtuoso

Analog Design Environment window.



Lab 2: Linearity Measurement (Swept hb with hbac)

The 1 dB compression point is defined as the input signal level that causes the small

signal gain to drop by 1 dB. The suggested approach to measuring the 1 dB compression

point is to set up a swept PSS analysis that sweeps the input power level.

When the circuit is driven by two RF tones ( inf and 2inf ), the third order intercept point

is the intercept point of the first order fundamental power term ( inf , 2inf ) and the third

order inter-modulation power term ( 22 inin ff −× , inin ff −× 22 ), expressed in decibel

form.

There are at least four ways to measure IIP3 and OIP3 using SpectreRF:

1. PSS/hb analysis with two large tones

2. QPSS/hb analysis with one large tone and one moderate tone

3. Swept PSS/hb and PAC/hbac analyses

4. Rapid IP3 using AC or PAC/hbac analysis

The recommended approach is method 4, rapid IP3 using AC or PAC/hbac analysis,

because it is faster and more accurate than the other approaches.

Action 2-1: If it is not already open, open the schematic view of the design

EF_example_simple in the library RFworkshop.



Parameter Value

Resistance 50 ohm

Port Number 1

DC voltage (blank)

Source type sine

Frequency name 1 RF

Frequency 1 fin


PAC magnitude (dBm) pin




Action 2-4: From the EF_example_simple schematic, choose Launch — ADE L to

start the Virtuoso Analog Design Environment.

Action 2-5: (Optional) Choose Session — Load State, select Cellview in Load State

Option and load state “Lab2_IP3_hbac,” and skip to Action 2-12.


Choose….

Action 2-7: In the Choosing Analyses window, select hb in the Analysis field of the

window.

Action 2-8: Set up a swept hb analysis as follows:



Note: Some specific points are added to get smooth curve and more accurate P1dB result.

During sweep analysis, the default value of restart will be no, which means only the first

pss simulation will run tstab to get initial condition. For the other pss run, the initial

condition will be the previous pss result and tstab will not be run. In the complicated PA

design, the simulation will be very difficult to converge at high input power.

In this case, the user can try the following methods:

1. Use more harms in the simulation, or use oversample factor.

2. If it is single point simulation, hbhomotopy=source can be used when input power

level is high.

3. In a sweep analysis, If any specific power level failed to converge, one or two

specific points can be added before this power level to help convergence.

Action 2-9: Make sure Enabled is selected, and click Apply in the Choosing Analyses

form.

Action 2-10: In the Choosing Analyses window, select hbac in the Analysis field. Set

up the form as follows:



Action 2-11: Make sure Enabled is selected, and click OK in the Choosing Analyses

form.




Action 2-12: In the Virtuoso Analog Design Environment window, choose

Simulation — Netlist and Run or click the Netlist and Run icon to start

the simulation.

When the simulation ends, use the following actions to plot the P1dB and IP3 curves.



Action 2-14: In the Direct Plot Form, select hb and set up the form like this:



Action 2-15: Click PORT2 to plot the 1 dB compression point.

The output referred 1dB compression point, which is 19.92dBm in this case, is more

meaningful for PA design.



Action 2-16: In the Direct Plot Form, set Plotting Mode to Replace. Select hbac and

set up the form like this:



Action 2-17: Click PORT2 to plot the Output Referred IP3.



Action 2-18: Close the waveform window. Click Cancel in the Direct Plot form. Close

the Virtuoso Analog Design Environment window.



Lab 3: Stability and S-Parameter Measurements (PSS and PSP)

As pointed out by Gonzalez in [4], stability is guaranteed for the following conditions

Kf >1, ∆ <1

Kf > 1, 22

22

2

111 1 ∆−−+= SSB f >0

To analyze stability for a PA, set up PSS and PSP analyses. The PSP analysis is a

periodic small-signal analysis, so the S-parameter and VSWR results it generates apply

only to the small signal. In some PA data sheets, the S-parameter and VSWR values

specified are large signal characteristics. SpectreRF currently supports large signal SP

(LSSP) analysis as demonstrated in Lab 4.


EF_example_simple in the library RFworkshop



Parameter Value

Resistance 50 ohm

Port Number 1

DC voltage (blank)

Source type sine

Frequency name 1 RF

Frequency 1 fin


Action 3-3: From the EF_example_simple schematic, choose Launch — ADE L to



Option and load state “Lab3_Stability_HBPSP,” and skip to Action 3-

10.


Choose….

Action 3-6: In the Choosing Analyses window, select pss in the Analysis field of the

window and set up the form as follows:



Action 3-7: Make sure Enabled is selected, and click Apply in the Choosing Analyses

form.

Action 3-8: In the Choosing Analyses window, select psp in the Analysis field of the

window and set up the form as follows:




form.






the simulation.



Action 3-12: In the Direct Plot Form, select psp and click Kf in the Function field. The

form looks like this:



Action 3-13: Click Plot.

The following plot appears.



Action 3-14: In the Direct Plot Form, set Plotting Mode to Replace. In the Analysis

field, select psp, and click B1f in the Function field.

The form looks like this:



Action 3-15: Click Plot.

The following plot appears.



Action 3-16: Close the waveform window.

Action 3-17: In the Direct Plot Form, set Plotting Mode to Append. In the Analysis

field, select psp. In the Function field, select SP. In the Plot Type field,

select Rectangular. In the Modifier field, select dB20.

The form looks like this:



Action 3-18: Click S11, S12, S21, and S22.

Action 3-19: In the waveform window, click Strip Chart Mode.


Action 3-20: In the Direct Plot Form window, set Plotting Mode to Append. In the

Analysis field, select psp. In the Function field, select SP. In the Plot

Type field, select Z-Smith.



Action 3-21: Click S11. A waveform window appears.

Action 3-22: In the waveform window, click New Subwindow.



Action 3-23: In the Direct Plot form, click S22.

S11 and S22 are plotted in the form of Smith Charts.


Action 3-25: In the Direct Plot Form, in the Function field, choose VSWR (Voltage

standing-wave ratio). In the Modifier field, select dB20. Click VSWR1,

then VSWR2.

You get the following waveforms:



Lab 4: Large Signal S-Parameter Measurement (LSSP Wizard)

The small-signal S-parameter characterization of an RF circuit is well established.

However, for circuits with either large nonlinearity or frequency translations, small-signal

S-parameters are not sufficient for design purposes. This is especially true for designs

such as those that use power amplifiers and mixers.

As a natural extension of small-signal S-parameters, large-signal S-parameters can be

defined as the ratio of reflected (or transmitted) waves to incident waves. Because small-

signal S-parameters are based on the simulation of a linearized circuit, small-signal S-

parameters are independent of input power.

Large-signal S-parameters are based on large-signal steady state simulation techniques

such as the SpectreRF PSS analysis with its shooting Newton method or harmonic

balance simulators. Large-signal S-parameters are sensitive to input power levels.


EF_example_LSSP in the library RFworkshop.



Parameter Value

Resistance 50 ohm

Port Number 1

DC voltage (blank)

Source type sine

Frequency name 1 RF

Frequency 1 fin




Parameter Value

Resistance 50 ohm

Port Number 2

DC voltage (blank)

Source type sine



Frequency name 1 RFout

Frequency 1 fout

Amplitude 1 (dBm) pout

Make sure you are using PORT. SpectreRF currently only supports PORT

for LSSP simulation.


Action 4-5: From the EF_example_LSSP schematic, choose Launch — ADE L to


Action 4-6: In the Virtuoso Analog Design Environment window, choose Tools —

RF — LSSP …

Action 4-7: In the Large Signal S-Parameter Wizard window, select Port1 in the

Define Input/Output field.

Action 4-8: Change Type to Input.

Action 4-9: Select Port2 and change type to Output.

Action 4-10: In the Large Signal S-Parameter Wizard window, choose Amplitude in

Sweep field.

Action 4-11: Set up the form as follows:



Action 4-12: In the Large Signal S-Parameter Wizard window, click OK to close the

window.






the simulation.

After the simulation ends, the waveform window appears.

Action 4-14: In the waveform window, place a marker in curve mag(S21) at Pin=-5

dBm by choosing Marker — Place — Trace marker. It shows that

S21=23.29dB at Pin=-5 dBm.



Action 4-15: In the Virtuoso Analog Design Environment window, choose Variable —

Edit. The Editing Design Variable window appears.

Action 4-16: In the Editing Design Variable window, click pin -10, change its value

to -5. Click Change.

Action 4-17: In the Editing Design Variable window, click pout 10, change its value to

18.29. Click Change.

The PA output is -5+23.29=18.29 dBm when pin=5dBm.

Action 4-18: Click OK in the Editing Design Variable window.

Action 4-19: In the Virtuoso Analog Design Environment window, choose Tools — RF

— LSSP ...

Action 4-20: Set up the Large Signal S-Parameter Wizard form as follows:



Action 4-21: In the Large Signal S-Parameter Wizard window, click OK to close the

window.






the simulation.

After the simulation ends, the waveform window appears.

Note: If it does not happen by default, you might want to change the graph to strip mode

to get individual graphs in each sub window.




the Virtuoso Analog Design Environment window. Close the

EF_example_LSSP schematic.



Lab 5: Load-Pull Measurements (Swept hb)

A load pull analysis is a systematic way to measure large signal impedance matching. In

a load pull analysis, the output reflection coefficients are swept; SpectreRF measures the

output power and plots it as a function of the complex load seen by the transmitter.

Because the complex load requires two axes, the results are plotted as constant power

contours on a Smith chart. The contours show how the output power increases as the load

impedance reaches its optimum value, Zopt.

Keep in mind that you are sweeping output reflection coefficients by changing a linear

load. The large signal output reflection coefficients computed in this manner equal the

small-signal, or incrementally computed, load reflection coefficients. However, for input

reflection coefficients, this is no longer true. You are actually computing the large signal

reflection coefficients at the fundamental frequency.

You might not always be able to achieve the optimal output power due to other design

goals, such as stability concerns. Those goals are generally posed as constraints in the

reflection coefficients. SpectreRF allows you to overlay the reflection coefficients on top

of the constant power contours to facilitate your design choices.

However, a constant power contour does not equal a constant power gain contour. You

should plot the input power contours both to verify that the input impedance of the PA

does not change significantly as the load impedance changes and to ensure that you have

achieved a reasonable power gain.

Action 5-1: Open the schematic view of the design EF_example_loadpull in the

library RFworkshop.

The following figure shows the modified EF_example_simple schematic for load pull

calculations.

The input port in the above testbench has the following parameters:

Parameter Value



Resistance 50 ohm

Port Number 1

DC voltage (blank)

Source type sine

Frequency name 1 RF

Frequency 1 fin


The output port is set up with the following parameter values:

Parameter Value

Resistance 50 ohm

Port Number 2

DC voltage (blank)

Source type dc

Action 5-2: From the EF_example-loadpull schematic, choose Launch — ADE L to

start the Virtuoso Analog Design.


Option and load state “Lab5_LoadPull_hb” then skip to Action 5-10.


Choose….


window.

Action 5-6: From MMSIM10.1, a new compact load-pull analysis option is

introduced,Port Adaptor in the testbench is not needed. In hb, this new

load-pull option is setup instead of sweep, simplifying the simulation

setup greatly. In the load-pull form, click Select for Load Instance form,

click the PORT2 on the schematic, click Variable for rho(Z) form, and set

up the form like this.



Click Variable for phi(Z) form, and set up the form like this.

Fill 50 in the Z0 form.

Now the setup form should look like this.



(Optional): If you are using old version IC, in order to do load pull analysis, an instance

of a PortAdaptor is needed to connect to the load. The testbench should look like this:

The PortAdaptor is set to have the following properties:

Frequency 1 G

Phase of Gamma theta

Mag of Gamma mag

Reference Resistance r0 (this value must be equal

to the load)

In the hb form, set up two the form for sweep 1 and 2 as follows:




form.

Action 5-8: In the Virtuosos Analog Design Environment window, choose Outputs —

To be Saved — Select on Schematic.

Action 5-9: In the schematic, select the input terminals of PORT1 and PORT2. Press

the ESC key to end the selection process.






the simulation.

Action 5-11: After the simulation runs, in the Virtuoso Analog Design Environment

window, choose Results — Direct Plot — Main Form.

Action 5-12: In the Direct Plot Form, select hb, and choose the Power Contours

function. Make sure Select is toggled to Single Power/Refl Terminal,

select fundamental (harmonic 1) as the output harmonic. The form looks

like this:



Action 5-13: In the schematic window, select the PORT2 plus terminal. If you want,

click Close contours. The plot shows the contours of constant output

power.



From the above plot, the maximum power of 0.72mW is located at the center of the

smallest constant power contour.

You might want to maximize load power subject to a constraint on the magnitude of the

amplifier’s input reflection coefficient. Such a constraint can prevent unstable

interactions with the preceding stage.

You can overlay the output power contours with contours of constant input reflection

coefficient magnitude. The optimal load corresponds to the reflection coefficient that lies

on the largest power load-pull contour and also lies on a constant input reflection

coefficient contour that is within the constraint. Here, largest power means the contour

corresponding to the largest amount of power delivered to the load.



Action 5-14: In the hb Direct Plot form, choose the Reflection Contours function, then

toggle Select to Separate Refl and RefRefl Terminals. Select the input

port (PORT1) of the PA first, and then select the output port (PORT2).

This plots the constant input reflection contours in the Smith chart of the

output reflection coefficients.

The Direct Plot form looks like this:



The following plot shows the constant input reflection coefficient contours overlaying the

output power contour:

Action 5-15: Change the Plotting Mode to Replace, choose the Power Contours

function, and select the terminal of the input port to plot the input power

contour. If the contour shows that the input power does not vary

significantly over the output reflection coefficient sweep, then the constant

power contour is very close to the constant gain contour.



Action 5-16: Close the waveform window. Click Cancel in the Direct Plot form.

To draw the load pull contour for the compression points, three variables; power, phase

and magnitude need to be swept. The power sweep is necessary to obtain the

compression points for the specified phase and magnitude.


Choose …


window. Set up the form for sweep 1 as follows:



(Optional): If you are using old version IC, you can set up sweeps as below:



Action 5-19: Click OK in the Choosing Analyses form.






the simulation.



Action 5-22: Select Compression Point for Function. Select Points for Plot. Select 1

for Gain Compression (dB). Select 1 for 1st Order Harmonic. Select

Loadpull Contour.



Action 5-23: In the schematic window, click the PORTT2 port.

Action 5-24: (Optional): If you are using old version IC, setup the direct plot form like

this:



In the schematic window, click the PORTT2 port.

In the Virtuoso Analog Design Environment window, choose Tools — Calculator …,

select the load pull function. Copy the following expression to the Signal form.

compressionVRI((v("/RFOUT" ?result "hb_fd") - 0.0) '1 ?rport

resultParam("PORT2:r" ?result "hb_fd") ?gcomp 1)

In the Calculator window, click OK and plot the load pull contour for the compression

points.

Action 5-25: In the Graph Window, choose Graph — Display Type — Impedance



Close the Virtuoso Analog Design Environment window.



Lab 6: Envelope Analysis (ENVLP and ACPR Wizard)

The envlp analysis, designed to generate an efficient and accurate prediction of the

envelope transient response of circuits to different modulation schemes, is used to

measure inter-channel interference. The circuits are generally driven by carrier-

modulated signal, with carrier frequency orders of magnitude larger than the modulating

signal. A classical transient approach is too expensive, and neither PSS nor QPSS work

because the modulation signal is neither periodic nor quasi-periodic.

Envelope analysis reduces simulation time without compromising accuracy, exploiting

the behavior of circuits to a fixed high frequency clock. In particular, the envelope of the

high-frequency clock can be followed by accurately computing the circuit behavior over

occasional cycles. This accurately captures the fast transient behavior. The slow varying

modulation cycle is accurately followed by a piecewise polynomial.

To do envlp analysis, you can use either the shooting engine or the harmonic balance

engine. This lab shows you how to use the Virtuoso®

Spectre®

RF Envelope with the

harmonic balance engine to design and analyze transmitters.

Action 6-1: Open the schematic view of the design EF_example_envlp_acpr in the

library Rfworkshop.

The power amplifier is driven by modulation signals. CDMA I/Q baseband chip streams

are fed into an ideal QPSK modulator.

Action 6-2: View the object properties of I36. ACPR_source, which is available in

rflib starting from IC5141ISR141 and IC613ISR17, re-designed from

IC614ISR8, is a behavioral modulator that allows users to specify I/Q

PWL files, carrier frequency, input power level in one single instance,



thus greatly simplify the ACPR simulation setup. To eliminate the noise

introduced by the round-off error in I/Q PWL files, normalized raised

cosine (nrc) filter is used to filter the PWL data. The filter's bandwidth

is same as the channel bandwidth (1.2288M) and roll-off factor is chosen

to be 0.23.

.

You can also get an equivalent input by setting up a port because it can output carrier-

modulated signal for I/Q paths, configure the port as follow:




Action 6-4: From the schematic window, choose Launch — ADE L to start the

Virtuoso Analog Design Environment.




Option and load state “Lab6_ENVLP_hb,” then skip to Action 6-11.

Action 6-6: In the Virtuoso Analog Design Environment window, click the Choose

Analyses icon.

Action 6-7: In the Choosing Analyses form, select the envlp analysis and choose the

harmonic balance engine. Set Stop Time to 1.667m, Fund Frequency to

1G, and Number of harmonics to 3.

3 harmonics are enough in this case. If the circuit is strongly nonlinear, more harmonics

are needed. In cases where the circuit has a square carrier, nine harmonics are sufficient.

But for some cases, twenty or more harmonics may be required.

Action 6-8: Set the Time Step Control field to Fixed and set Step Period to

2.03451e-7.

The Step Period parameter is equal to the sample interval of the baseband data (I/Q data

in instance I36). If you check the PWL files cdma_2ms_idata and cdma_2ms_qdata, the

sample interval of these data is 0.203451us, i.e. the sample frequency is 4.9152MHz. To

achieve better accuracy and better simulation speed, the Step Period parameter should be

set to the sample interval of these data. Because the sample period is 0.203451us, the

spectrum bandwidth is -4.9152M/2 to 4.9152M/2, and the toal number of samples is

1.667m/ 0.203451us=8193.

Action 6-9: Set the Accuracy Defaults (errpreset) field to moderate.

Action 6-10: Click OK in the Envelope Options form and then click OK in the

Choosing Analyses form.





the simulation.

Notice the output log file when the simulation ends. The speed-up factor in this case is

203.

.



Action 6-13: Select Voltage for Function. Select time for Sweep.



Action 6-14: In the schematic window, click the RFOUT net.

The voltage waveform appears in the waveform window.



Action 6-15: In the waveform window, double click the X-axis and set the Min and Max

values to 116u and 126u, respectively. Click OK.

The waveform window appears as follows.



The plot displays a number of vertical lines with a wavy line running through them. The

vertical lines are the points at which detailed calculations are performed and the wavy

line connects these points. The simulation runs much faster than a Virtuoso Spectre

Transient Analysis simulation (as envelope skips carrier cycles whenever possible) and

still satisfy numerical tolerances.

Action 6-16: To get a closer look, zoom in on any one of the vertical lines.

You can see the detailed simulation for one complete cycle.



The modulation riding on the RF carrier is the baseband signal - the information to be

transmitted. The baseband signal determines the amplitude and phase of the RF carrier,

and can be extracted at any point in the design. It is important to determine how the

transmitter might alter the baseband signal.

Action 6-17: In the Direct Plot form, set these options:

a. Select Replace for Plotting Mode.

b. Select Voltage for Function.

c. Select harmonic time for Sweep.

d. Select Real for Modifier.

e. Select 1 for Harmonic Number.

Action 6-18: In the schematic, click the adder output.

A plot for the real portion appears in the Waveform window.



Action 6-19: In the Direct Plot form, select Append for Plotting Mode and Imaginary

for Modifier.

Action 6-20: In the schematic, click the adder output.

A plot for the imaginary portion is added to the waveform window.

Action 6-21: In the waveform window, click the Strip Chart Mode icon .

Action 6-22: Set the X Axis to 45u to 57u so that you can see both the real and

imaginary parts clearly.



The baseband waveforms recovered from the modulated RF carrier (as displayed in the

figures above) do not directly reveal much about how the transmitter affects them. The

steps below indicate how to display the associated trajectory, which is the plot of one

waveform against the other. The trajectory reveals much more about the kind of

distortion introduced by the transmitter. The steps below first display the input baseband

trajectory and then the output baseband trajectory. A comparison of the two trajectories

reveals whether the power amplifiers in this example are really distorting the signal.

Action 6-23: In the waveform window, double click the X Axis and set the Range to

Auto.

Action 6-24: In the Plot vs. field (at the bottom of the form), select h = 1; v /RFIN ;

envlp re(V), and click OK.

The plot below appears in the waveform window. This is the input baseband trajectory,

undistorted by the power amplifiers.



Action 6-25: Close the Waveform window, then repeat the steps that you used to

display the plot for /RFIN, but substitute the /RFOUT net for /RFIN.

The plot you create in the waveform window looks like this.



The entire trajectory is scaled linearly and rotated. The output baseband signal is the

input baseband signal, multiplied by a complex constant. The input and output

waveforms look different because of the rotation, not because of non-linear distortion. A

common non-linear distortion, such as saturation, makes the outer edges of the trajectory

lie on a circle.

The adjacent channel power ratio (ACPR) is a common index of how much power a

transmitter emits outside its allotted frequency band. To measure ACPR, first obtain the

power spectral density of the transmitted signal. This section describes how to plot the

transmitted power spectral density.

To simulate ACPR measurement, we drive the transmitter with realistic baseband signals.

In most cases, the baseband signals pass through a digital filter before driving a

transmitter. The digital filter compresses the spectrum outside the channel

bandwidth. However the distortion in the transmitter may cause the spectrum re-growth

at the adjacent channels.



Action 6-26: In the Direct Plot form, set these options:

a. Select Replace for Plotting Mode.

b. Select ACPR for Function.

c. Select 1 for Harmonic Number.

d. Select IS-95 for Method.

e. Select 0 for From.

f. Select 2.03451e-7 for Strobe Period.

g. Select 1024 for Window Size.

h. Select 8 for Repetitions.



The ACPR GUI is for the following post-processing function,

************************************************************

db10((psdbb(real(harmonic(v("/RFOUT" ?result 'envlp_fd) 1))

imag(harmonic(v("/RFOUT" ?result 'envlp_fd) 1)) START_TIME STOP_TIME

TOTAL_POINTS ?windowSize NFFT ?windowName "Cosine4" ?detrending "none")))

************************************************************

The psdbb( ) function calculates the baseband PSD of the "RFOUT" node. The time

domain waveform is taken from time START_TIME to time STOP_TIME with total

sample points TOTAL_POINTS. NFFT is window size and it should be a power of 2.

FFT is repeated N times and the PSD is the average value of these results, where N is the

integer part of TOTAL_POINTS/NFFT.

The sample period Ts is (STOP_TIME - START_TIME) / TOTAL_POINTS and the

frequency resolution Fres is 1 / (NFFT*Ts). The psdbb function strobes the waveform on

the net RFOUT with strobe period of T_strobe.

To miminize the aliasing error, it should be consistent with the step-period specified in

envlp analysis form and the data rate in I and Q PWL files.

In this lab, the data rate of input baseband signal is 2.03451e-7. Thus the step-period

in envelope analysis is set to be 2.03451e-7, and the strobe-period (T_strobe) in ACPR

post-processing is also 2.03451e-7.

Once the ACPR strobe period T_strobe is determined, other parameters in psdbb ( )

function can be deduced. Suppose we take NFFT=1024, which means the frequency

resolution is 4800Hz (Fres= 1 / (NFFT* T_strobe)). NFFT can be changed according to

the Fres of interest. If FFT is repeated 8 times, the TOTAL_POINTS should be

1024*8=8192. Note that in real simulation, TOTAL_POINTS can be slightly different.

The simulation time should be 1.666667m (TOTAL_POINTS* T_strobe). We should

choose the stop time larger than 1.666667m. In this example, we choose 1.667m.


The ACPR waveform appears in the waveform window.



Note: The PSD plot is the normalized result for ACPR. If you use IC 5.1.41 USR6, the

normalized PSD is plotted, while the un-normalized PSD is plotted when using IC 5.1.41

USR5 or before.

The lower ACPR is -88.6971dB, and the upper ACPR is -88.9014as shown in the plot

above.

Action 6-28: In the Direct Plot Form, click Main Channel Power, setup the form as

follow:




The Main Channel Power waveform appears in the waveform window.




Using the ACPR Wizard

In this quick exercise, you rerun the previous ACPR demonstration using the Spectre RF

ACPR Wizard.

Action 6-31: Open the ACPR Wizard in one of two ways.

• In the Simulation window, choose Tools - RF – ACPR …

or

• In the envlp Choosing Analyses form, press Start ACPR Wizard.

In either case, the ACPR Wizard displays.



Action 6-32: Click on Update From Hierarchy to get Clock Name from the testbench.

Set the following:

Net /RFOUT

Channel Definitions IS-95

Symbol Start (Sec) 0

Strobe Period 2.03451e-7

Window Size 1024

Repetitions 8

The Strobe Period parameter is related to the sample rate of the baseband data (I/Q data

in PORT0 or PORT1). If you check the PWL files cdma_2ms_idata and

cdma_2ms_qdata, the sample interval of these data is 0.203451us, i.e. the sample

frequency is 4.9152MHz. To obtain the real values on these sample points, the Strobe

Period parameter should be set to the sample interval of these data.

The number of repetitions is set to 8, which gives a reasonable simulation time and

accuracy. Increasing the number of repetitions provides better accuracy at the cost of a

longer simulation time.

The main part of ACPR Wizard form looks like this:



Action 6-33: In the ACPR Wizard form, click Apply.

This action loads the output section of the ADE window with your selected values.

The ADE window now looks like this:



the simulation.

The waveform window automatically opens after simulation and you can read ACPR

number in the Output session in the Analog Design Environment.



Now the Virtuoso Analog Design Environment window looks like this:



Action 6-35: Close the waveform window. Click Cancel on the Direct Plot form. Close




Lab 7: Envelope Analysis (fast-mode)

Starting from MMSIM10.1, a new envlp analysis mode is added, which is the fast envlp

analysis. It can complete the simulation in minutes, does not depend on designers’

specified stop time and has a great speed-up compared with regular envelope analysis.

Meanwhile, fast envlp analysis realizes good compromise between computational

efficiency and accuracy; the circuit is automatically calibrated and replaced by the

accelerated mathematical representation without user intervention.

This lab shows you how to use the Virtuoso® Spectre® RF fast envelope with the

harmonic balance engine to design and analyze transmitters.

Action 7-1: Open the schematic view of the design EF_example_envlp_acpr in the

library Rfworkshop.

Action 7-2: View the object properties of I36. ACPR_source, which is available in

rfLib starting from IC5141ISR141 and IC613ISR17, re-designed from

IC614ISR8, is a behavioral modulator that allows users to specify I/Q

PWL files which are necessary in order to do fast envlp analysis, carrier

frequency, input power level in one single instance, greatly simplify the

ACPR simulation setup.







Option and load state “Lab7_ENVLP_fastmode_hb,” then skip to

Action 7-11.


Analyses icon.


harmonic balance engine. Set Stop Time to 1.667m, Fund Frequency to

1G, note fast envlp requires a larger number of harms than regular envlp

does because it uses hb, and must have enough harms for nonlinear circuit,

in this lab, set Number of harmonics to 7.




Action 7-9: Select fast envlp levels in the fast envlp mode field, the parameters are

explained below:

Off: fast envlp is off, use regular envelope analysis (default).

level1: fast envelope analysis in level 1 (short memory).

level2: fast envelope analysis in level 2 (more accurate) .

Modulation source I: The name of I source, which corresponds to the

real part of base-band signal. Only PWL type is supported.

Modulation source Q: The name of Q source, which corresponds to the

image part of base-band signal. Only PWL type is supported.

output: nodes to be saved in fast-mode envelope analysis. Multiple nodes

can be selected.

Level 2 may be needed in the simulation where more accurate results are

required. Then Modulation bandwidth must be specified, by experience

this can be set as twice times of the main channel bandwidth, in this lab

we use level 1.

Action 7-10: In the Modulation source I, click select button and then click I36 on the

schematic, /I36/PORT0 is added automatically, note /I36/PORT1 is added

for Modulation source Q as well. In the output nodes, click select button

and then click on the output net and ACPR source output net on the

schematic, /RFOUT and /RFIN appear in the form.

Now the envlp setting should look like this:



Click OK in the Choosing Analyses form.





the simulation.

Note the simulation finished very fast.

Now you can repeat actions from Action 6-17 to Action 6-25 to plot net voltages etc.,

however, we only show ACPR function for fast envlp analysis below. For conception of

ACPR, refer to page86.

Action 7-22: In the Direct Plot form, set these options: a. Select Replace for Plotting Mode.

b. Select ACPR for Function.

c. Select 1 for Harmonic Number.

d. Select IS-95 for Method.

e. Select 0 for From.

f. Select 2.03451e-7 for Strobe Period.

g. Select 1024 for Window Size.

h. Select 8 for Repetitions.




The ACPR waveform appears in the waveform window.

Note: The PSD plot is the normalized result for ACPR. If you use IC 5.1.41 USR6, the

normalized PSD is plotted, while the un-normalized PSD is plotted when using IC 5.1.41

USR5 or before.

The lower ACPR is -87.3384dB, and the upper ACPR is -87.5928dB.

Action 7-24: Close the waveform window. Click Cancel on the Direct Plot form. Close




Lab 8 EVM functions for OFDM signals

The error vector is defined as a vector in the I-Q plane between the ideal constellation

locations and the signal points sent by a transmitter or received by a receiver. If the

transmitter or the receiver is ideal, all the signal points are at the ideal locations in the

constellation and the error vector is 0. However, due to the non-idealities such as carrier

leakage, phase noise of the system, the actual constellation points deviate from the ideal

locations. EVM(Error Vector Magnitude) is a measure used to quantify such non-

idealities by averaging the power of the error vector and normalizing to the signal power.

The following actions show you how to use the fast envelope to plot EVM functions.

Action 8-1: Open the schematic view of the design EF_example_envlp_EVM in the

library Rfworkshop.

Action 8-2: View the object properties of I7. rfVsource ,which is a behavior modulator

that allows users to specify OFDM signal standards, carrier frequency,

input power in one instance, greatly simplify the EVM simulation setup.







Option and load state “Lab8_ENVLP_EVM_hb,” then skip to Action 8-

12.


Analyses icon.



The following table is reference of how to configure the envlp settings:

From the table, we can choose stop time of envlp analysis and step period according to

various signal standards, other parameters are used for post-processing later.


harmonic balance engine. Set Stop Time to 200u, note this should be less

than DataLength item in the table, otherwise the simulation results would

be incorrect. Set Fund Frequency to 2.4G. Note fast envlp requires a

larger number of harms because it uses hb, and must have enough harms

for nonlinear circuit, in this lab, set Number of harmonics to 7.

Action 8-8: Set the Time Step Control field to Fixed, since we are using 802_11n_

20M_Green, from the table, set Step Period to 8.33333333e-9.


Action 8-10: In the fast envlp mode field, select level1, the meanings of the parameters

are explained in Action7-9.

Action 8-11: In the Modulation source I field, click select button and then click I7 on

the schematic, /I7/V0 is added automatically, note /I7/V1 is added for

Modulation source Q as well. In the output nodes, click select button and

then click on the output net and rfVsource source output net on the

schematic, /RFOUT and /RFIN appear in the form.

Now the envlp setting should look like this:



Click OK in the Choosing Analyses form.



the simulation.



Action 8-14: Select EVM for Function, harmonic time for sweep, Percent for

modifier, in the Error Vector Magnitude Parameters field, select

OFDM for Modulation, in the Signal Standard list, select

802_11n_20M_Green, fill other fields according to the table in Action8-

6.

Now the direct plot form should look like this:



Action 8-15: Click net RFIN on the schematic, the input EVM plots appears.

Action 8-16: In the direct plot form, set New Win for Plotting Mode, click net RFOUT

on the schematic, the output EVM plots appears.



From the results, we can see the EVM increases after the signal transmitted by the

transmitter.

To learn how EVM varies with input power, we can sweep input power using parametric

sweep.

Action 8-17: In the ADE window, select Tools-Parametric Analysis. Configure plo as

the sweep variable, sweep from -30dBm to -10dBm, total steps is three,

the parametric analysis window looks like:



Action 8-18: Click Analysis-Start All to start the simulation.

Action 8-19: In the direct plot form, set New Win for Plotting Mode, click net RFOUT

on the schematic, the output EVM versus input power plot appears.

From the result, we can see the EVM increases with the input power.

Action 8-20: Close the waveform window, the Direct Plot form, and Virtuoso Analog

Design Environment window.



Conclusion

This workshop describes how to use SpectreRF for RF power amplifier designs. The

workshop

• Presents the typical PA design parameters and describes how to build testbenches

and perform measurements within the Virtuoso Analog Design Environment.

• Covers in detail how to set up SpectreRF analyses and perform measurements

related to PA design.

• Displays and interprets the simulation results.

Reference [1] B. Razavi, RF Microelectronics, Prentice Hall, 1998.

[2] T. Lee, The Design of CMOS Radio Frequency Integrated Circuits, Cambridge

University Press, 1998.

[3] Ken Kundert, “Predicting the Phase Noise and Jitter of PLL-Based Frequency

Synthesizers”, The Designer’s Guide, www.designers-guide.com, 2005

[4] M. Hella, RF CMOS Power Amplifiers: Theory, Design and Implementation,

Kluwer Academic Publishers, 2002.

cadence pa design using spectrerf

Documents

pa design

design example

specific design

spectrerf purpose

spectrerf mmsim

new hb analysis

pa measurements

power related measurement