11/20/13 Transmit/Receive Modules - Microwave Encyclopedia - Microwaves101.com

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Transmit/receive modulesUpdated May 19, 2012

Click here to go to our page on phased arrays

Click here to go to our page on AESAs

Click here to go to our page on RMS errors

Here we'll discuss a concept that is revolutionizing microwave system designs. In no way will webe touching on anything that is classified or ITAR restricted, which is why you don't see any bookson the subject because it limits what can be said. To those readers that point out that Wikipediahas plenty of microwave content, why use Microwaves101? go to your friends at Wikipedia rightnow and look up T/R module. Who's your Daddy?

T/R modules set up system performance in a phased array. Why a phased array? As LeonardCohen stated, everybody knows that mechanically gimbaled systems eventually wear out (perhapsthe rotary joint goes first), and radars are expected to be reliable, especially when public safety ison the line.

Their main three functions are to boost output power of the transmitted signal up to its finalradiated power, establish system noise figure for receive, and provide beam steering control. Butthe Devil is in the details... this is no career killer!

The module shown here was taken from a Austrian web site, it's used in a European radar system,we'll use this photo as an example in some of the description below. It appears to be built on co-fired multi-layer ceramic board (slightly blue) with a small chunk of thin-film alumina on the inputof the power amplifier (probably to realize a Lange coupler, but why didn't they do this on theoutput of the power amp where loss really matters?) The ceramic "mother board" has cut-outs forthe high-power amp (which has a heat spreader under it) and the circulator. A seal ring is brazedonto the ceramic to allow a thin lid to be seam-welded onto it and form a hermetic cavity. If theyare smart they include getter on the lid to prevent hydrogen poisoning. Pretty simple stuff, n'estce pas?

History of T/R modules

The T/R module concept dates back to the 1970s at least, interest was (and continues to be)driven by military applications. But the concept had to wait until the advent of GaAs monolithicmicrowave integrated circuits (MMICs), which appeared in the early 1980s to become practical.

Many major defense contractors developed their own T/R modules during the 1980s, includingHughes, Texas Instruments, Westinghouse, and others, we'll wait for readers to send us further

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Hughes, Texas Instruments, Westinghouse, and others, we'll wait for readers to send us furtherinfo to expand on the topic. It shouldn't take long for some braggart (probably with a Texas drawl)to put us together on this subject!

The classic T/R module that made high-performance X-band phased arrays possible cost on theorder of $1000 each, which prevented widespread adoption of the technology. Various efforts byDARPA have attempted to bring the price down to $100. You don't have to be Nostradamus topredict that at some point T/R modules will develop a consumer application, GaAs will be replacedwith silicon, and the price will come down to just a few bucks, but with reduced performance frommilitary-style T/R modules.

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

The module in the photo above measures 64.5 x 13.5 x 4.5mm according to the web site. The keydimension is 13.5mm. This is a half wavelength at 11 GHz, so it's operating band is somewhere inthat neighborhood.

Phased arrays have been built at many frequencies, but the classic radar band is X-band (8 to 12GHz) so this is where most T/R modules operate.

T/R module block diagram

We'll describe some of the functions that are required within a T/R module here. But first, let's

consider the lyrics to Dem Dry Bones because this is an early description of a block diagram, and acool spiritual which today is sung to a melody penned by James Weldon Johnson, one of the firstAfrican Americans to teach at NYU:

The foot bone connected to the leg bone,The leg bone connected to the knee bone,The knee bone connected to the thigh bone,The thigh bone connected to the back bone,The back bone connected to the neck bone,The neck bone connected to the head bone,Oh, hear the word of the Lord!

OK, that was a little out there, a memory from the Unknown Editor's childhood 45 RPM recordplayer courtesy of UE's Mom who graduated from MIT in the 1940's but certainly followed her owntaste in music rather than the pop charts, but we digress...

Here's an RF block diagram of the sportiest variety of TR module, it has all the bells and whistles.Later we'll show a "low cost" version of a TR module and discuss the issues it brings. Theschematic of this module is available in Electronic Symbols.doc, a free download you can find here.

The world's sportiest T/R module

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Duplexer (Circulator)The duplexer is what allows the antenna to be shared between transmit and receive. It can be aferrite 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 you might seethe T/R module's circulator outside the housing.

IsolatorOne other issue that the duplexer has to deal with is that at extreme scan angles, the VSWR ofthe antenna can get ugly. When this mismatch is passed on to the power amp, its power candegrade due to load pull effects (worse than the straight mismatch loss). An isolator is often usedto solve this problem, it presents a matched load to the antenna (and power amp) no matter whatthe LNA or limiter are doing to the VSWR. The European TR module shown above does not have anisolator.

If the LNA presents a matched load during transmit, this is not a problem. But guess what? TheLNA is switched off during transmit, and presents a big mismatch. This problem is removed byinserting an isolator in front of the LNA.

It is possible to integrate the circulator and isolator functions into a single assemble, which iscommonly called a four-port circulator (the fourth port terminated)

LimiterThe limiter prevents damage to the low noise amplifier during transmit or whenever stray radiationis present.

The limiter often performs a second important function. It provides a termination to the circulatorduring transmit, to absorb power that reflects from the antenna. Significant power can be reflectedat large scan angles. Why terminate it? The power amp needs to see the correct impedance or itspower will drop due to load pull. In the T/R module above there appears to be a resistive loadbelow the front limiter diode to perform this function.

Low noise amplifier (LNA)The LNA sets the noise figure of the system, but all losses between the antenna and the LNA addto the overall noise figure and must be minimized.

In the picture, two LNAs are used in series, these are to the right of the circulator and limiterdiodes, right above the power amp.

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

Sometimes an LNA is designed so that it provides a good impedance match when it is biased off.Adding a termination function to the LNA eliminates the need for the isolator, which was there toensure that the power amp always sees 50 ohms.

In the future, the LNA might migrate from GaAs technology to GaN, which could eliminate theneed for the limiter. GaN LNAs routinely are reported to withstand 10 watts peak incident power,you are lucky if you can get a GaAs LNA to withstand 100 mW.

Phase shifterThe phase shifter supplies the incremental phases to each element that is what drives the beamin different directions. Because phase shift is required in both transmit and receive, it is usuallyplaced 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 phenomenonof phase shifters is that their phase errors get worse if they see an crummy VSWR! When you

design a T/R module you need to take this into account; first off you should place the phaseshifter between well-matched components (usually one side of the phase shifter is connected tothe attenuator which always provides a nice match).

High-power amplifierThe high-power amplifier is often the biggest and most expensive part of a T/R module (thecirculator/isolator can get expensive too). It also is the primary source of waste heat that youhave to dump overboard.

Often the power amp uses two chips and combines them with quadrature or in-phase Wilkinsoncouplers. The attraction of quadrature combining is that the impedance looking into the combineddevice is well matched. It can also add an important degree of immunity to load-pull effects.

Today the highest-power, most expensive modules will use gallium nitride for power amps,replacing gallium arsenide. The promise of 10X improvement in output power won't be realized,because a 10X improvement in ways to remove waste heat is not on the horizon. But poweramplifiers of moderately higher power can shrink to smaller footprints, which providesopportunities for cost savings, and the amount of DC current that has to be dealt with is greatly

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opportunities for cost savings, and the amount of DC current that has to be dealt with is greatlyreduced because operating drain voltage is much higher (30 volts instead of 7 for example). Whoknows, maybe the controversy of gate modulation versus drain modulation will be revisited... thebiggest problem with gate modulation in a GaAs power amp is that in order to pinch off the poweramp with the full drain voltage on, you are getting dangerously close to gate-drain breakdown.Now that 100 volts breakdown is routine, hmmmm, maybe this deserves another look!

Common-leg circuit (CLC)The phase shifter and often the attenuator are used in both transmit and receive paths. IN ourblock diagram, we have configured an amplifier and the phase shifter in the common-leg circuit.Note that when the phase shifter is in the common leg, it does not have to be a reciprocal device.Phase shifters usually reciprocal devices, but maybe you can exploit the block diagram and inventan amplifier that has commandable phase states!

More on CLC (hopefully..) on this page

AttenuatorThe attenuator is used to add an amplitude taper across the array, to reduce sidelobes. This istypically only done in receive, in transmit you want to splash as much radiation as you can. Theattenuator often performs a second function of aligning the amplitudes of the individual elements.Typically a digital attenuator is used.

Power conditioning Voltage regulators are used to clean up the voltages that are supplied to the array. Often the DCcurrent to the array is a very high value, and the distribution network that brings the T/R modulebias currents. Linear regulators take in noisy voltages burn off perhaps 1.5 volts, and provideclean outputs.

The voltages to a T/R module usually include a drain voltage for the power amps, a drain voltagefor the LNA, and and a gate voltage that is used by all amplifiers. The gate voltage to an amplifieris negative, and is usually a very low current, consequently

Modulation circuitryT/R modules must be switched from transmit to receive quickly. The transmit gain path is turnedoff during receive, 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 almostnever done, probably because any noise on the gate due to settling time of the modulationwaveform will have a much bigger effect that ringing on the drain voltage.

P-channel MOSFETs are usually used to turn the amplifiers on and off. These offer a combinationof low on-resistance (just a few milliohms!) and no weird power supplies such as an N-channelMOSFET might need.

For some reason the International Rectifier trademark name HEXFET has stuck in the industry tomean "MOSFET" the same way Xerox means copy, probably because IR parts kick ass. Do yourselfa favor and recognize the difference or you'll sound like an ignoramus to people that can tell thedifference.

Charge storage capacitanceBecause 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. Checkout our page on charge storage calculation!

Beam steering digital circuitryThe phase shifters in the array must be set to specific values to control the beam position, this isno easy task and usually takes an distributed computer to get it done quickly and efficiently. Thisis often called the beam steering computer.

HousingThe 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 thatare used within (i.e. GaAs, silicon, various ceramics). This is one of the cost drivers of thetechnology.

The housing is usually the single biggest contributor to the mass of the overall T/R module. Thisis not a problem for ground based systems, but for airborne applications (or space!) you need tocarefully consider what to use. Composites such as aluminum silicon carbide (AlSiC) were all therage for a few years, but they are not without problems and could be considered a career killer.

SubstratesT/R modules typically use microstrip interconnects, by CPW and even stripline are possible. Thesubstrates 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

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About an hour or two after the first phased array went to test, someone must have asked "there'sa problem with the array, how do we know which module is bad?" To which someone else musthave said, "aw shucks, we have to pull them all out and test them all!"

And so the T/R module usually has some form of built-in test circuit to verify its health. You can'ttest for everything, but the one thing that probably will fail the fastest is the power amplifier, dueto its self-heating. If you look at the T/R module above, there seems to be a coupler circuitbetween the power amp and circulator, this is for built-in test.

Advice from the Unknown Editor... if you try to launch an IRAD project such as the"Built-in Test Equipment Module Experiment", don't reveal the acronym until you get it

totally approved and turned on, then play dumb when someone figures it out. Ditto forthat "Liquid-Cooled Module Experiment"...

Way way more to come!

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