Practical RF Circuit Designing

Anurag Bhargava

Application ConsultantApplication Consultant

Agilent EEsof EDA

Agilent Technologies

Email: anurag_bhargava@agilent.com

You Tube: www.youtube.com/user/BhargavaAnurag

Blog: http://abhargava.wordpress.com

Agenda

• What is RF?

• Approaches to RF circuit designs

• Practical considerations

• Successful methodology for RF circuit designs

Agenda

• What is RF?

• Approaches to RF circuit designs

• Practical considerations

• Successful methodology for RF circuit designs

What is RF?

Radio frequency (RF) is a rate of oscillation in the range of about 3 kHz to 300 GHz,

which corresponds to the frequency of radio waves, and the alternating currents

which carry radio signals.

Electric currents that oscillate at radio frequencies have special properties not shared

by direct current or alternating current of lower frequencies. The energy in an RF

current can radiate off a conductor into space as electromagnetic waves (radio

waves), this is the basis of radio technology.

RF current does not penetrate deeply into electrical conductors but flows along their

surfaces; this is known as the skin effect.

Scope of current discussion

• RF Circuit design is very wide topic and we shall limit

ourselves to discuss about Lumped/Discrete and Distributed

circuit design aspects in this presentation.

• Techniques which are discussed in this Webinar can be

applied to any type of RF/uWave circuits i.e Active or Passive. applied to any type of RF/uWave circuits i.e Active or Passive.

• All the simulations have been carried out using Agilent ADS (Advanced Design System) software.

Agilent EEsof EDA Solutions

ADSADS– Signal Integrity

ADSADS– MMIC, RF Board, SiP,

Printed Antenna, Arrays

SystemVueSystemVue– ESL

GenesysGenesys– RF Board

ICIC--CAP CAP – Device Modeling

EMProEMPro– 3D EM (FEM & FDTD)

Golden GateGolden Gate– RFIC

Agilent ADS for MMIC, SiP and RF Board Co-Design

ADS Main FeaturesADS Main Features

1.1. Complete FrontComplete Front--toto--Back RF Board DesignsBack RF Board Designs

2.2. Complete FrontComplete Front--toto--Back MMIC DesignsBack MMIC Designs

3.3. BestBest--inin--class simulators for Active and class simulators for Active and Passive Circuit/System DesignsPassive Circuit/System Designs

4.4. Superior capabilities for High Speed Signal Superior capabilities for High Speed Signal Integrity AnalysisIntegrity Analysis

5.5. Links to 3Links to 3rdrd party tools like Cadence, Mentor party tools like Cadence, Mentor 5.5. Links to 3Links to 3rdrd party tools like Cadence, Mentor party tools like Cadence, Mentor Graphics, Zuken etcGraphics, Zuken etc

6.6. Direct Links to Agilent EMPro for full 3D EM Direct Links to Agilent EMPro for full 3D EM analysis and Agilent SystemVue for RFanalysis and Agilent SystemVue for RF--Digital Digital system design & Verificationsystem design & Verification

7.7. Widest support for Vendor Component ModelsWidest support for Vendor Component Models

8.8. Biggest market share in RF/Biggest market share in RF/uWaveuWave Design Design segmentsegment

Day in Life of a RF Designer…..few examples!!

Designed

Measured

Example 1: Low Pass Filter Design

Day in Life of a RF Designer…..few examples!!

Example 2: MMIC Amplifier Design

NF Spec: 0.7 dB

Example 3: MIC LNA Design

Day in Life of a RF Designer…..few examples!!

NF Spec: 0.7 dB

Measured: 8 dB

FET

Some familiar statements (Do you hear them often?)….

• RF/uWave is black magic….

• i.e. you don’t know why things work and why they don’t

• RF/uWave is too tough to handle…

• i.e. I am better off in some other field

• No matter how much work you do in software, you still need to take out

Day in Life of a RF Designer…..

• No matter how much work you do in software, you still need to take out

screw driver (just an analogy..!!!) and fix it on the measurement table to make RF circuits work….

• i.e. Don’t waste your time using RF design software

• Design software is really bad, it showed me good response but

measurement results are really bad, not sure what to do….

• i.e. blame it on the software

Let’s think……

Did you account for all things which shall affect circuit performance during the design phase?

i.e. Did you….

• Account for parasitic behavior of Inductors, Capacitors etc at RF?

• Account for component tolerances and its effect on your circuit performance during your design phase? (Our focus today…!!)

• Account for radiation, parasitic coupling effects of transmission lines at higher frequencies?

• Account for Etching tolerances in fabricating your PCBs/MICs?

Day in Life of a RF Designer…..

• Account for Etching tolerances in fabricating your PCBs/MICs?

• Account for Parasitic effects of transmission lines at higher frequencies?

• Simulated your virtual design as close as to the real life assembly, accounting for most of the

things which might make difference in the performance?

Most often you will find that we missed something during the design phase, let’s try to understand these things better so that we have better predictability in our RF circuit performances after fabrication……

Agenda

• What is RF?

• Approaches to RF circuit designs

• Practical considerations

• Successful methodology for RF circuit designs

Approaches to RF Circuit Design

RF Circuit Implementation Techniques

There are various implementation techniques for RF circuits

e.g:

a. Lumped / Discrete components based circuits

b. Distributed Lines (e.g. Microstrip, Stripline etc)

c. Waveguide based structures

Approaches to RF Circuit Design

How to select implementation technique for RF Circuits?

Implementation technique for a RF circuit can be selected by considering various factors:

a. Frequency of Operation:

a. Circuits under 500MHz are usually designed using Lumped / Discrete components

b. Circuits between 1 GHz – 30 GHz can be designed using distributed lines such as Microstrip etc

c. Circuits over 30 GHz are usually designed using waveguide based components

b. Power Handing Capacity:b. Power Handing Capacity:

Another important aspect of RF circuit design is power handing or operating power of the circuit which may call for hybrid implementation technology.

c. Q-factor requirements (usually referred for filters)

Sometimes high-Q structures (e.g. Sharp rejection) can be implemented using waveguide based

techniques even at lower frequencies e.g. Cavity Filters

Agenda

• What is RF?

• Approaches to RF circuit designs

• Practical considerations

• Successful methodology for RF circuit designs

Practical Considerations for Discrete Elements

What could make difference in real world?

Component ‘Q’: Also known as Unloaded ‘Q’ which introduces loss in circuit

HF Parasitics: Each component has some HF equivalent circuit which could affect circuit performance (see next slide…)

Layout parasitics: These parasitics could play a role in changing circuit performance and they originate due to dispersive nature of transmission lines which are used as a connecting medium for layoutconnecting medium for layout

Component Tolerance: All the discrete components will have some inherent tolerance associated and it is not practically possible to produce a discrete component without any tolerance and actual component value can vary within their specified tolerance range.

HF equivalent circuit of Inductor and Capacitor

Cmain -> Main Capacitance Lmain -> Main Inductance

Lseries -> Series Inductance caused due to bonding at both side of Capacitor internal assembly, causes Series resonance (SRF)

Rseries -> Equivalent Series Resistance

Cpar -> Capacitance caused due to parasitics between capacitor plates and assembly metals in SMT comps., causes parallel resonance (PRF)

Rpar -> Parasitic parallel resistance

Rseries -> Equivalent Series Resistance (ESR)

Cpar -> Capacitance caused due to parasitics between Inductive coils and assembly metals in SMT comps.

Rpar -> Parasitic parallel resistance, typically varies with frequency of operation

Guidelines for selection of Inductor and Capacitor

• Before selecting any component from Inductor and Library it is always preferable to consult manufacturer manual to note their ‘Q’ and SRF specifications.

• Ideally Frequency of operation should be <= 20% - 30% of SRF of each component, which poses the upper limit on the Discrete component values which can be used for circuit design.

• Meeting these SRF guidelines ensures some kind of • Meeting these SRF guidelines ensures some kind of flexibility in design process so that the parasitic effect don’t dominate the natural response of the components, resulting in more tolerant design to account for component value variation due to manufacturing limitations.

• If it is not possible to restrict component values as per SRF guidelines then it is preferable to choose discrete components with tighter tolerance levels as far as possible to ensure yield variations are kept to minimum and components value do not vary much due to various reasons and this will help in producing high yield manufacturing circuits.

Discrete Components Tolerance Reference Table

Tolerance Level Value Comments

B +/- 0.1 Absolute Value

C +/- 0.2 Absolute Value

D +/- 0.5 Absolute Value

F +/- 1 % -------F +/- 1 % -------

G +/- 2 % -------

J +/- 5 % -------

K +/- 10 % -------

M +/- 20 % -------

Distributed Circuits

• At sufficiently high frequencies it is not possible to design

circuits using discrete components as their physical size

become appreciable fraction of wavelength

• All the components used for circuit design needs to be realized

using printed techniques, still the basis of all circuits would be L,

C, R but realized using printed techniquesC, R but realized using printed techniques

• Designers have option to select printed transmission line

technology of his choice, few of them are highlighted on the next

slide.

• Inductor can be realized with High Impedance (Narrow width)

line, Capacitor can be realized with Low impedance (Wider

width) line, resonators can be realized using coupled

transmission lines.

RF/uWave Printed Transmission Lines

Microstrip Lines: Easy to fabricate, Easy tuning

after fabrication, Easy mechanical assembly,

Relatively Low ‘Q’, Quasi TEM mode

Striplines: Complex assembly, No Tuning,

Higher ‘Q’ than microstrip lines, TEM mode

Suspended Stripline: Complex assembly,

Mechanical Intensive, High ‘Q’ , No Tuning

Picture source: www.microwaves101.com

Microstrip Lines

• Microstrip lines have been extensively used for RF/Microwave circuit design over the years and they are still preferred in most of the cases because of their flexibility

• Substrate selection is an important criteria in designing circuits based on Microstrip technology

• Electric field needs to be confined in the substrate for proper circuit operation else it will face radiation problem (Antenna is circuit operation else it will face radiation problem (Antenna is reverse where we need only radiation)

• Lot of references available on theory of Microstrip lines, one of the most popular being the book: Foundations for Microstrip Circuit Design – T.C. Edwards

Selecting Substrate for Microstrip Designs

There are few key parameters which helps in deciding right substrate for the

RF/Microwave designs:

a. Substrate Height: This needs to be selected as per the frequency of operation, it can cause Surface wave radiation. Higher the frequency of operation lower should be the height of substrate

b. Relative Permittivity / Dielectric Constant: Usually referred as dielectric constant (Er) is the measure of opposition placed by Dielectric substrate towards any electric field.Dielectric substrate towards any electric field.

c. Loss Tangent: It is the quality measure of dielectric purity (depicts lossy nature of dielectric). Lowest possible value is always desirable to ensure that dielectric is lossless. Represents the ‘Q’ factor of the dielectric. Lesser the better………

d. Conductor Material: Conductivity of the conductor used in Microstrip design contribute to the transmission losses. (e.g. Gold: 4.1E7, Copper: 5.8E7, Silver: 6.1E7, all units in Siemens/meter)

** Substrate performance can be controlled/optimized with proper selection of Height and Er

Practical Considerations for Distributed Circuit Designs

What could make difference in real world?

Boundary conditions: All the printed line components are modeled using Analytic equations in schematic/ circuit design environment which may not always represent “true” behavior of transmission lines and they can be truly characterized in EM (Electromagnetic) domain using EM solvers.

Parasitic Cross Couplings: All the transmission line structure can observe cross coupling between adjacent & non-adjacent sections and it is almost impossible to take care of these parasitic coupling by any of the circuit impossible to take care of these parasitic coupling by any of the circuit simulators as all the lines/sections drawn in schematic are independent of each other.

Radiation Problems: Circuit simulator will have limitations on accurate prediction of the radiation problems which might occur due to the circuit layout size (Space wave radiations) or because of Substrate properties (Surface wave radiations).

Does it mean circuit simulators are useless for Distributed Circuit Designs?

Electromagnetic vs. Circuit Simulations

EM simulators

• Analyzes physical structure based on EM theory (Maxwell’s Equations)

• Considers all coupling• Passive components only• Generally slower

Circuit Simulators

• Analyzes circuit schematic using built-in models

• Considers explicit coupling• Passive & Active

components• Generally faster simulations• Generally slower

simulations• Optimization is not easy

• Generally faster simulations• Optimization is very easy• Designs can be tuned in real

time

1

L= 10 mil

W= 10 mil

L= 15 mil

W= 10 mil

2L= 10 mil

W= 10 mil

L= 15 mil

W= 10 mil

L= 120 mil

W= 10 mil

L= 15 mil

W= 10 mil

S= 15 mil

L= 10 mil

W= 10 mil

L= 10 mil

W= 10 mil

3

4

It is recommended to start with Circuit Simulators and later turn to EM for more accurate analysis. ADS offers hybrid simulation to combine Circuit and EM analysis.

Agenda

• What is RF?

• Approaches to RF circuit designs

• Practical considerations

• Successful methodology for RF circuit designs

Recommended Lumped / Discrete Circuit Design Flow

Start with circuit design obtained by manual calculations /ADS Designguide

/ Genesys Synthesis

Replace ideal components with actual vendor Models or use Q-

factor or Spice Models or S2P fileYield

Run Optimization

Run Optimization

Step 1:

Step 2:

Statistical (Yield) Analysis

Prepare Layout

Generate Gerber / DXF etc

Yield Optimization

EM/Ckt co-simulation

Step 3:

Step 4:

Step 5:

Recommended Distributed Circuit Design Flow

Start with circuit design obtained by manual calculations /ADS Designguide

/ Genesys Synthesis

Prepare Layout and run EM simulations (Mandatory for

accurate prediction at high freq)

Yield

Run Circuit Optimization

Run EM Optimization (if

possible)

Step 1:

Step 2:

Statistical (Yield) Analysis

Prepare Final Layout

Generate Gerber / DXF etc

Yield Optimization

Step 3:

Step 4:

Step 5:

Note: Type of EM simulation technology can be selected as per the circuit under design. Designers

usually have choice of using Method of Moments (MoM), Finite Element Method (FEM) or Finite

Difference Time Domain (FDTD), each technology has their own advantages and disadvantages….

Step 1: Initial Circuit Design

• We start our circuit design by calculating element values using equations.

• Following circuit shows the circuit component values obtained after

optimization to meet the desired S11 and S21 specifications

Step 2: Circuit Design with Q-factor included

• As a next step we change the ideal L & C components to the models

with Q-factor so that we can take lossy nature of L & C into account

• We can see that in this case there is no appreciable change in circuit

performance with inclusion of Q-factor so we are good to proceed to next

step

Step 3: Yield Analysis

• Yield Analysis is a 3 step process:

• Set the Goals / Performance Criteria to be met by the circuit

• Set the tolerance in the components

• Set Yield Controller for desired number of iterations

Inductors are having +/- 10%

tolerance and Capacitors are

having +/- 5% tolerance

Step 3: Yield Analysis• Yield analysis shows that this circuit has a poor yield of 45% that means only 45%

of the circuits fabricated will meet the desired specifications and rest 55% will

fail…….

• From the simulation results we can observe that S21 rejection criteria (>40dB) and

S11 (<-15dB) is getting worse due to component tolerances hence causing lower

Yield.

What’s wrong with the circuit…..??

Questions:

a. Why circuit yield is so bad?

b. Which component or components in my design are creating

problems?

c. How to correct the problem and improve the yield?c. How to correct the problem and improve the yield?

Sensitivity Histograms in Agilent ADS S/W

Sensitivity Histograms in ADS are very useful to find out how each component is

making a effect on a specific Yield Goal or specification.

We can place Sensitivity Histogram measurement functions/equations from Yield

library palette in ADS as shown below:

• Equation shown here is for checking C1’s tolerance effect on S21’s rejection

goal from 250MHz – 500 MHz

• Similarly, we can place these measurement functions for all different type of

specifications we have in our design e.g. S11, S21 etc

• Remember these equations apply equally for active circuits performance criteria

to check Output Power, Noise Figure, Power Added Efficiency etc

Sensitivity Histogram Result for S21 response

How to read histogram results: •Y-axis is Yield % and X-axis is the component value with its +/- tolerance range

•If we closely look at these plots we can notice that which component is affecting

performance metrics for S21

•From the graphs we can conclude that component values will need to re-adjusted to

improve the Yield performance…..

Initial Value Modified Value

C1-> 43pF 48pF

C2-> 43pF 48pF

L1-> 75nH 83nH

L2-> 139nH 174nH

L3-> 70nH 88nH

S21 Performance Correction

As suggested by Histograms, components values were readjusted and after

Yield Analysis we find that circuit performance has improved and we are able

to achieve compliance for S21 (rejection criteria) of >40dB in 250-500 MHz

band so remaining problem can be thought of coming from S11 failures….

Still S11 results seems to be giving problem

S11 Sensitivity Histograms

• By including Measurement functions for S11 histograms, we can notice that L1 and L2

inductors are still causing S11 performance to degrade and we will need to reduce the

tolerance of these 2 components.

• Change the tolerance of L1 and L2 inductors to 5%

Yield Optimization in ADS

• Now as we fixed one specification and reduce tolerance of few components to

take care of the other specification, we can use Yield Optimization to centre our

components values in one direct way to achieve design centering for various

performance criteria together in a more reliable manner .

• Yield Optimization is different than normal circuit optimization as it centers the

components values using Statistical Method and we should have optimized circuit

before we run this kind of optimization.

• Also, remember that Yield Optimization can take longer time because it performs

Yield Analysis at each iteration to check the circuit Yield and centers the design

performance based on the tolerance of the components

Steps for Yield Optimization

1. Define the component values to be Optimizable: So that ADS has the right to

change the values. We can always set the min and max of the values under

which ADS changes the component values

2. Set the Yield Optimization Controller and number of iterations

Circuit Yield after Yield Optimization

• We can see that RF circuit’s Yield improves dramatically after our component

value modifications and Yield Optimization.

• We can now proceed for Layout generation and perform EM-Circuit co-

simulation for checking if Layout parasitics will affect our circuit performance

Step 4: EM-Circuit Cosimulation (Virtual PCB Assembly…!!)

• EM – Circuit cosimulation is like performing virtual PCB assembly whereas ADS will simulate PCB layout using EM simulator (Momentum i.e. Method of Moments or Finite Element Method (FEM) based on the selection and then hybrid simulation with Circuit simulator.

• It allows designers to take care of the parasitic effect which may be caused by interconnecting PCB lines etc on top of usual electrical component designs. We can include transistors etc as well….

EM-Circuit cosimulation results

EM-Circuit cosimulation result here show very close matching with our final

design hence our circuit is ready for fabrication….

Example: Yield Analysis on GaN Power Amp

As we stated before, technique which we learn can be applied on all type of RF circuits i.e.

Active or Passive e.g. Power Amplifier, Mixer, Oscillators etc..

Here is one example of GaN Power Amplifier where widths of the transmission lines have etching tolerance of +/-35um based on the printing technology used and we can observe the

variation in PA’s Output Power and Power Added Efficiency (PAE) as shown below.

What we learnt today…..?

• Successful RF circuit design is the result of careful design process

• Nothing can be taken for granted…..simulate when in doubt !!

• Agilent ADS software provides all the key capabilities to make you successful RF

circuit designer

• Simulation tools are usually as good as designers (barring their own inherent

limitations…...)limitations…...)

• Software tools are different in their capabilities, accuracy and algorithm

implementations so designers need to make the choice according to what they need.

• Agilent ADS is a mature RF design software with many years of unmatched innovations in technology, features etc and it provides best in class technology for RF designers to achieve 1st pass design success

Looking Ahead..!!

Kindly send your requests on what we can cover in our future

webinars to:

Mukul Pareek

Marketing Engineer, Agilent Technologies

Email: mukul_pareek@agilent.comEmail: mukul_pareek@agilent.com

Resources to help you..….

• Subscribe to the Blog: http://abhargava.wordpress.com to receive latest news, tips &

tricks on RF design and industry news..

• Subscribe to the You Tube Channel: www.youtube.com/user/BhargavaAnurag to

see various ADS tutorial videos and learn the ADS concepts better

• Agilent EEsof Home Page: www.eesof.com

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