here to go to our page on phased arrays
here to go to our page on AESAs
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
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,
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?
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 4.5mm according to the web site. The key
dimension is 13.5mm. 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
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
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
sportiest T/R module
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.
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
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)
The limiter prevents damage
to the low noise amplifier during transmit or whenever stray radiation
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
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.
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
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).
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
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
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
on CLC (hopefully..) on this page
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.
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
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
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.
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.
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
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
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!