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Transmit/Receive Modules

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Here we'll discuss a concept that is revolutionizing microwave system designs. In no way will we be touching on anything that is classified or ITAR restricted, which is why you don't see any books on the subject because it limits what can be said. To those readers that point out that Wikipedia has plenty of microwave content, why use Microwaves101? Go to your friends at Wikipedia right now 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 Leonard Cohen stated, everybody knows that mechanically gimbaled systems eventually wear out (perhaps the rotary joint goes first), and radars are expected to be reliable, especially when public safety is on the line.

Their main three functions are to boost output power of the transmitted signal up to its final radiated power, establish system noise figure for receive, and provide beam steering control. But the 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 input of the power amplifier (probably to realize a Lange coupler, but why didn't they do this on the output of the power amp where loss really matters?) The ceramic "mother board" has cut-outs for the high-power amp (which has a heat spreader under it) and the circulator. A seal ring is brazed onto the ceramic to allow a thin lid to be seam-welded onto it and form a hermetic cavity. If they are smart they include getter on the lid to prevent hydrogen poisoning. Pretty simple stuff, n'est ce pas?

Transmit-Receive Modules

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 monolithic microwave 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, including Hughes, Texas Instruments, Westinghouse, and others, we'll wait for readers to send us further info 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 the order of $1000 each, which prevented widespread adoption of the technology. Various efforts by DARPA have attempted to bring the price down to $100. You don't have to be Nostradamus to predict that at some point T/R modules will develop a consumer application, GaAs will be replaced with silicon, and the price will come down to just a few bucks, but with reduced performance from military-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. 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.

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

Phased arrays have been built at many frequencies, but the classic radar band is X-band (8 to 12 GHz) 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 a cool spiritual which today is sung to a melody penned by James Weldon Johnson, one of the first African 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 record player courtesy of UE's Mom who graduated from MIT in the 1940's but certainly followed her own taste 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. The schematic of this module is available in Electronic Symbols.doc, a free download you can find here. The two images represent the transmit and receive state switch routing.

 

TR Receive State

 

TR Transmit State

The world's sportiest T/R module, transmit and receive states

Duplexer (Circulator)
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 you might see the T/R module's circulator outside the housing.

Isolator
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, its power can degrade due to load pull effects (worse than the straight mismatch loss). An isolator is often used to solve this problem, it presents a matched load to the antenna (and power amp) no matter what the LNA or limiter are doing to the VSWR. The European TR module shown above does not have an isolator.

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

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

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

The limiter often performs a second important function. It provides a termination to the circulator during transmit, to absorb power that reflects from the antenna. Significant power can be reflected at large scan angles. Why terminate it? The power amp needs to see the correct impedance or its power will drop due to load pull. In the T/R module above there appears to be a resistive load below 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 add to 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 limiter diodes, right above the power amp.

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. Adding a termination function to the LNA eliminates the need for the isolator, which was there to ensure that the power amp always sees 50 ohms.

In the future, the LNA might migrate from GaAs technology to GaN, which could eliminate the need 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 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 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 phase shifter between well-matched components (usually one side of the phase shifter is connected to the attenuator which always provides a nice match).

High-power amplifier
The high-power amplifier is often the biggest and most expensive part of a T/R module (the circulator/isolator can get expensive too). 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 combining is that the impedance looking into the combined device 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 power amplifiers of moderately higher power can shrink to smaller footprints, which provides opportunities for cost savings, and the amount of DC current that has to be dealt with is greatly reduced because operating drain voltage is much higher (30 volts instead of 7 for example). Who knows, maybe the controversy of gate modulation versus drain modulation will be revisited... the biggest problem with gate modulation in a GaAs power amp is that in order to pinch off the power amp 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 our block 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 invent an amplifier that has commandable phase states!

More on CLC (hopefully..) on this page

Attenuator
The attenuator is used to add an amplitude taper across the array, to reduce sidelobes. This is typically only done in receive, 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.

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

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

Modulation circuitry
T/R modules must be switched from transmit to receive quickly. The transmit gain path is turned off 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 almost never done, probably because any noise on the gate due to settling time of the modulation waveform 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 combination of low on-resistance (just a few milliohms!) and no weird power supplies such as an N-channel MOSFET might need.

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

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. Check out our page on charge storage calculation!

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!) you need to carefully consider what to use. Composites such as aluminum silicon carbide (AlSiC) were all the rage for a few years, but they are not without problems and could be considered a career killer.

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?" To which someone else must have 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't test for everything, but the one thing that probably will fail the fastest is the power amplifier, due to its self-heating. If you look at the T/R module above, there seems to be a coupler circuit between 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 for that "Liquid-Cooled Module Experiment"...

Way way more to come!

Author : Unknown Editor

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