a-4 designing tr module systems with ads -...

Agilent EEsof EDA

Designing TR Module Systems with ADS

Anurag BhargavaApplication Engineer EEsof EDA

Agilent TechnologiesBangalore, India

Agilent EEsof EDA

Objective

• Main objective of this presentation is to….. – Show ADS capabilities as a complete Design Platform for high frequency

applications– Clearly differentiate ADS from some of the other point tools which often rely

on socket based approach resulting in disconnected flow involving various vendors and low efficiency

– Outline key value propositions of ADS over other products in markets to design System, Circuit and Electromagnetic level etc all under one environment

– Demonstrate a very practical approach to design unique systems such as TR (Transmit/Receive) Module which is used in most of the modern day A/D systems e.g. Active Phased Array Radar

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Agilent EEsof EDA

Presentation Scope

• This presentation will mainly focus on the Transmit mode analysis of TR Module due to time constraint but the idea presented here could be very easily extended to analyze Receive mode as well.

• Main idea is to show how ADS can be used as a single design platform to perform some of the very complicated jobs such as System Designs, File Sweeping operations, Antenna Array and Co-simulation of complete system consisting of System/Circuit along with Antenna Array, Beam Steering which is pretty unique to ADS

• There are other things mentioned in further work slides which can be surely done in order to take this design much more closer to real system design/validation job. Those point s could be easily accomplished within ADS without having the need for any 3rd party plugins/sockets etc but outside the scope of the current presentation. Some of the points mentioned such as Amplifier behavioral modeling (P2D/S2D/X-parameters) etc are again unique to ADS which is non-existing in competitive softwares.

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Agilent EEsof EDA

Agenda

• TR Module Overview & Key Components

• TR Module System Simulation

• TR Module with MMIC components

• Pulsed RF Simulation of TR Modules

• Patch Array Antenna

• TR Module with Patch Array Antenna

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Agilent EEsof EDA

T/R module sizing and frequency

T/R modules are sized to fit within the lattice of a phase array, which is a function of frequency.

A good rule of thumb is that within the plane of the array, the modules must stack together to meet a half-wavelength spacing. At 10 GHz this is 1.5 cm, or about 600 mils. Depending on the system design the module might be close to 1/2 wavelength in one dimension, and much less in the other; quite often the module must be mounted to a structural member or heat sink which takes up considerable percentage of the lattice.

Source: http://www.microwaves101.com/encyclopedia/transmitreceivemodules.cfm

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Agilent EEsof EDA

Typical TR Module Block Diagram

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Digital Phase Shifter

Digital Attenuator TR Switch 1

LNA Stage 1 & 2

Driver + PA

Limiter or Rx Protection Switch

Duplexer Coupler

Tx In

Tx Power Monitoring

Tx Out Rx In

Rx Out

Agilent EEsof EDA

Key Components of TR ModuleDigital Phase shifter

The phase shifter supplies the incremental phases to each element that is what drives the beam in different directions. Because phase shift is required in both transmit and receive, it is usually placed in a path that is common. In this case the phase shifter can be a passive reciprocal device (it usually is). It is possible to design an active phase shifter.

Phase shifters have phase errors, they are not perfect. But a not-so-well understood phenomenon of phase shifters is that their the phase errors can increase significantly when presented with a poor, frequency dependent, VSWR.

By taking the “real-world” loads that the phase shifter might be seeing into account early in the design phase, time-to-market and costs can be reduced significantly.

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Agilent EEsof EDA

Key Components of TR Module

Attenuator

The attenuator is used to add an amplitude taper across the array, to reduce side lobes. This is typically only done in receive mode, in transmit you want to splash as much radiation as you can. The attenuator often performs a second function of aligning the amplitudes of the individual elements.

Typically a digital attenuator is used in modern TR module systems.

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Agilent EEsof EDA


Duplexer

The duplexer is what allows the antenna to be shared between transmit and receive. It can be a ferrite circulator, or sometimes just a SPDT switch. In the case of a circulator, this is not a solid-state component, so it doesn't have to be within a hermetic housing. Sometimes we might see the T/R module's circulator outside the housing.

One other issue that the duplexer has to deal with is that at extreme scan angles, the VSWR of the antenna can get ugly. When this mismatch is passed on to the Power Amp, it's power can degrade due to load pull effects (worse than the straight mismatch loss). If the LNA presents a matched load during transmit, this is not a problem.

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Agilent EEsof EDA


Limiter / Receiver Protection Switch (RPS)

The Limiter prevents damage to the low noise amplifier during transmit or whenever stray radiation is present.

The Limiter/RPS often performs a second important function. It provides a termination to the duplexer/circulator during transmit, to absorb power that reflects from the antenna. Significant power can be reflected at large scan angles.

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Agilent EEsof EDA


Low Noise Amplifier (LNA)

The LNA sets the noise figure of the system, but all losses between the antenna and the LNA add to the overall noise figure and must be minimized.

In the picture shown above, two LNAs are used in series.

In order to maximize the sensitivity of the T/R module, every effort is made to locate the first LNA and the power amp as close as possible to the antenna to minimize attenuation of long transmission lines.

Sometimes an LNA is designed so that it provides a good impedance match when it is biased off.

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Agilent EEsof EDA


High-Power Amplifier (HPA)

The high-power amplifier is the biggest and most expensive part of a T/R module. It also is the primary source of waste heat that you have to dump overboard.

Often the power amp uses two chips and combines them with quadrature or in-phase Wilkinson couplers. The attraction of quadrature is that the impedance looking into the combined device is well matched.

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Agilent EEsof EDA


Modulation circuitry

T/R modules must be switched from transmit to receive quickly. The transmit gain path is turned off during receive cycle, and the receive amplifier path is biased off during transmit. This is almost always done by circuitry that turns off the drain current to the amplifiers that must be turned off. It is theoretically possible to modulate the amplifiers using the gate voltage, but this is almost never done, probably because any noise on the gate due to settling time of the modulation waveform will have a much bigger effect than ringing on the drain voltage.

P-channel MOSFETs are usually used to turn the amplifiers on and off. These offer a combination of low on-resistance (just a few milliohms!).

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Agilent EEsof EDA


Charge storage capacitance

Because the T/R element must be quickly switched, and the power supply is electrically far away, charge storage capacitors are used to maintain the amplifier bias voltages during the pulse.

An acceptable voltage droop for a power amplifier during pulsed operation is 5%, which will drop the power by a similar amount (5%, or about a quarter of a dB).

Question: What charge storage do you need for a 10 usec pulse to power a 10W power amp running at 8 volts and 5 amps peak?

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Answer: 125 uF

Agilent EEsof EDA


Beam steering digital circuitry

The phase shifters in the array must be set to specific values to control the beam position, this is no easy task and usually takes an distributed computer to get it done quickly and efficiently. This is often called the beam steering computer.

Housing

The housing that surrounds the T/R module is usually hermetic to assure a long and healthy life. The material is usually chosen to match the thermal expansion coefficient of the materials that are used within (i.e GaAs, silicon, various ceramics). This is one of the cost drivers of the technology.

The housing is usually the single biggest contributor to the mass of the overall T/R module. This is not a problem for ground based systems, but for airborne applications (or space!)

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Agilent EEsof EDA


Substrates

T/R modules typically use microstrip interconnects, by CPW and even stripline are possible. The substrates inside the module are usually ceramic, often a form of alumina is used.

Built-in test (BITE)

About an hour or two after the first phased array went to test, someone must have asked “There's a problem with the array, how do we know which module is bad?”

And so the T/R module usually has some form of built-in test circuit to verify its health. We can't test for everything, but the one thing that probably will fail the fastest is the power amplifier, which may be due to it's self-heating.

If you look at the T/R module block diagram on slide 17, you will notice a coupler circuit after Duplexer, this is for built-in test.

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Agilent EEsof EDA

Agenda







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Agilent EEsof EDA

TR Module - System Level Analysis

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Digital Phase Shifter

Digital Attenuator

TR Switch

LNA Stage 1 and 2 Rx Protection Switch

Driver and Power Amp

Tx Power MonitoringDuplexer

Agilent EEsof EDA

TR Module – System Level Results

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Agilent EEsof EDA

TR Module – Ideal Digital Attenuator Sweep

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• Digital Attenuator model is swept from 0 – 31.5 dB in 0.5 dB steps (64 states)

• As expected system O/P power is decreasing linearly with attenuator value

Agilent EEsof EDA

TR Module – Ideal Phase Shifter Sweep

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• Digital Phase Shifter model is swept from 0 – 90 deg in 5.625 deg steps (16 states) just to characterize system performance

• In actual system we will use 6-bit Digital Phase Shifter which will make it possible to have phase shift from 0 - 354.375 deg

Agilent EEsof EDA

Agenda







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Agilent EEsof EDA

Replacing Digital Phase Shifter and Digital Attenuator system blocks with MMIC chips

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Agilent EEsof EDA

Challenges

• How to simulate / analyze system with MMIC chip data?

• 6-bit MMIC components will have 64 nos. of S-parameter files

• Changing each file manually would be extremely tough because of total number of combinations (64 X 64 = 4096)

• Having real measured data included in simulation will allow designers to get more accuracy in the system analysis as this would allow us to capture frequency dependent behavior of MMIC chips

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Agilent EEsof EDA

Using Data Access Component (DAC) in ADS

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DAC allows us to sweep multiple files in ADS by using simple indexing as shown below.

Att_Datafile.txt

By sweeping AState variable we can read different files in ADS simulation which allows us to verify system performance with different S-Parameter files

Agilent EEsof EDA

6-Bit Digital Attenuator (Measured MMIC results)

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Agilent EEsof EDA

6-Bit Digital Phase Shifter (Measured MMIC results)

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Agilent EEsof EDA

TR Module System with MMIC attenuator

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Agilent EEsof EDA

TR Module System Results with MMIC Attenuator

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Agilent EEsof EDA

TR Module with MMIC Ph. Shifter and Attenuator

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Agilent EEsof EDA

Results with MMIC Ph. Shifter and Attenuator

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Agilent EEsof EDA

Agenda







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Agilent EEsof EDA

Ptolemy Pulsed-RF TR Module Co-simulation

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Agilent EEsof EDA

Pulsed RF Response

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Agilent EEsof EDA

Agenda







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Agilent EEsof EDA

Designing Patch Antenna Array

• Start with a Single Patch

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Optimized Patch Response

Initial Patch S11 response

Antenna Result Snapshot

Agilent EEsof EDA

Building Patch Array

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3D View of Antenna Array

Antenna Far Field PatternAntenna Array Snapshot

Agilent EEsof EDA

Cross Coupling in Patch Array Antenna

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Agilent EEsof EDA

Agenda







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Agilent EEsof EDA

Challenges

• It is essential to combine TR Module system with Antenna Array so that we can characterize the beam steering, loading effect of Antenna on TR Module system etc.

• Challenging part however is to find a way whereby we can integrate Antenna Array (characterized in EM simulator) and TR Module System (or with actual MMIC chips, Circuits etc which is mainly characterized in Schematic domain)

• This is the point where most software would fail and designer won’t be able to combine these 2 important parts of complete system to visualize the complete integrated system performance.

• More often than not designer end-up bringing S-Parameters of designed Antenna thru multi-port Touchstone/S-parameter files and use it as a load to TR Module system (or any other type of systems being designed).

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Agilent EEsof EDA

ADS Solution

• ADS offers Momentum Far Field Design Kit which can be downloaded freely from Knowledge Center

• This design kit allows designers to integrate Circuit/System level components with part designed in EM environment as a sub-circuit.

• This utility directly launches Radiation Pattern Utility and automatically update amplitude and phase data at each feed points as shown in next slide animation and live demo.

• Another benefit is that there is no manual intervention required from designers side no matter how many feed points are involved in this unique System-Circuit-Antenna co-simulation.

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Agilent EEsof EDA

Combine TR Module System with Antenna Array

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Agilent EEsof EDA

Co-simulate TR Module System with Antenna Array

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Agilent EEsof EDA

Steer the Antenna Beam

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Agilent EEsof EDA

Further Work

• Non-Linear Amplifier modeling could be easily accomplished by connecting ADS to Agilent Network Analyzers to derive P2D/S2D/X-Parameter models for better non-linear performance prediction and great accuracy in system analysis.

• We could easily replace system level behavioral blocks with Designed Circuits for Bottoms-up verification which will allow us to check designed circuit’s compatibility on a system level design/analysis.

• TR Module Board layout can be done pretty easily in ADS and whole system could be EM co-simulated to take care of layout parasitic which might affect the system performance because of interconnect performance at these high frequencies.

• Performing Board layout EM simulation would also help in predicting some level of EMI issues as well to locate/analyze probable stray radiations.

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Agilent EEsof EDA

Summary

• ADS provides single software solution to perform System/Circuit/Electromagnetic analysis

• System/Circuit could be integrated with Electromagnetic components such as Antenna array shown in the present case

• ADS cuts work of hours to few seconds and makes it easy to sweep data included in multiple files such as for 6-bit Phase Shifter and Attenuator in the present system whereby it is needed to handle 64 S-parameter file for each components

• Circuit level designs could be placed at the System Level to check for complete system performance along with the Antenna etc and check for some critical parameters such as Beam Steering, phase/amplitude match at various elements etc all in one environment without having to use 3rd party plug-in/sockets etc.

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