Attenuators
Updated
September 30, 2008
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Aten, the
Egyptian Sun God
that you attenuate with SPF-45!
Here's a clickable index
our growing material on attenuators:
Introduction
to attenuators
Fixed
attenuators
L-pad
attenuators (separate page, new for February 2007!)
Temperature-compensating
fixed attenuators
Power
dissipation in attenuators (separate page)
Attenuator
math (separate page)
Switchable
attenuators (separate page)
Switched-network
attenuators
Switched-element
attenuators
Digital
attenuators (stepped attenuators)
Variable
attenuators (separate page)
Gain
equalizers (separate page)
Introduction
to attenuators
Attenuators are passive
resistive elements that do the opposite of amplifiers, they
kill gain. Why would you want to do that? Suppose your design
specification calls for 10 dB gain, with a 1.2:1 maximum VSWR.
You search the amplifier vendors, and locate an amplifier
in your frequency band, but it has 14.5 dB gain and a lousy
2.5:1 match on the input. By adding an attenuator to the input,
you can bring the gain down to 10 dB, and you will be improving
the input match. Two things to consider when you play this
game: don’t add an attenuator to an amplifier’s input if you
are concerned with the amplifier’s noise figure, every dB
of attenuation you put on the input will raise the noise figure
by the same amount. Similarly, don’t add an attenuator to
a power amplifier’s output without considering what it will
do to your output power, or what the RF output power of the
power amp might do to your attenuator.
There are five common attenuator
topologies used in microwave circuits, the tee, the pi, the
bridged tee, the reflection attenuator and the balanced attenuator.
The tee, pi and bridged tee each require two different resistor
values, while the reflection and balanced attenuators need
only a matched pair of resistors. This allows both the reflection
and balanced topologies to be used as variable attenuators
with a single control voltage or control current. There are
two variations of the reflection attenuator, depending on
whether the terminations R1 are less than or greater than
the system characteristic impedance Z0.
When would you use a tee
versus a pi versus a bridged tee? Here's some examples. When
you are designing a fixed-value 3-dB attenuator on a thin
film circuit, with a sheet resistivity fixed at 100 ohms per
square, the 8.5 ohm value of R1 for a tee might be a little
hard to accurately etch, and the pi might be a better choice.
On the other hand, if your sheet resistivity was 10 ohms per
square, you'd need 29 squares to create R1 for the pi, and
that might prove to be too inductive to work at high frequency.
With thick film circuits, you can take your pick because there
are different resistivity values available.
The bridged tee can be
thought of as a modified pi network. The attraction to the
bridged tee comes when you are making a variable attenuator,
with PIN diodes or FETs. Here are two reasons you might consider
it over the pi. First, it only needs two variable resistors
(pi and tee need three). Second, the bridged tee uses the
full range of resistor values, from zero to infinity, for
both R1 and R2. For the pi attenuator, R1 never goes below
Z0 (50 ohms) so some of the diodes' useful resistance range
is wasted. Finally, the bridged tee has a tendency to match
itself to Z0 at high attenuation values, because of its two
fixed resistors. In practice, the pi may give you higher attenuation
range. The resistor R2 can be a "sneak path" in
the bridged tee because the diode (or FET) never reaches zero
ohms.
  
 
The table below provides
equations for solving for the attenuator resistive elements:
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Attenuator equations
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Configuration
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R2
vs. R1
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R1
vs. Attenuation
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R2
vs. Attenuation
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Tee
Attenuator
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Pi
Attenuator
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Bridged
Tee Attenuator
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Reflection
Attenuator,
R1
<Z0
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Reflection
Attenuator,
R1>Z0
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Balanced
Attenuator
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Click on the calculator
icon ()
to check out our calculator where you'll be able to enter
your desired attenuation parameters and we will calculate
the resistors for you.
If you use the above equations
to calculate R1 and R2, be sure to observe whether "ATT"
is divided by 10 or 20. For those of you too busy or lazy
to calculate your own resistor values, the table below should
be useful (Z0 is assumed to be 50 ohms, but you
can scale the values if necessary).
Fixed
attenuators
There are many commercially
available fixed attenuators, including chip, coaxial, and
waveguide implementations. Two considerations you will need
to consider are frequency of operation, and power handling.
Chip attenuators can dissipate
hundreds of milliwatts up to a few watts. The higher power
units will be realized on thermally conductive substrates
such as beryllium oxide and aluminum nitride.
If you are designing your
own attenuator, the resistive elements can be realized with
either thick film or thin film processes. While the thick
film process can use different "inks" for different
resistors, providing different ohm/square values, the thin
film process uses one sheet resistance for all resistors.
Having different inks solves the problem of making compact
resistors of two radically different values, for instance,
a 1 dB tee attenuator would require an R1 of 2.9 ohms and
an R2 value of 433 ohms. Large value thin film resistors can
be made compact by using a meandered layout. In both thick
and thin film processes, the "as-fired" tolerance
is usually no better than 15%. Both types of resistors can
be trimmed with a laser to tolerances of less than 1% (see
also
thin film resistors).
There are two opposing
concerns in microwave attenuator design: the high frequency
of operation requires that resistive elements be kept small
to be considered "lumped
elements" (i.e. less than 1/10 wavelength), and power
handling requires that resistors be made as large as possible
to dissipate heat. The heat issue can be dealt with for small
resistors if attention is paid to the material properties
of the resistors’ substrates and attachment methods, i.e.
beryllium oxide for a substrate instead of alumina and solder
attachment instead of conductive epoxy (see also material
properties and heat
dissipation).
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Rule
of thumb: a chip attenuator is good for at least 1/16
watt if it is mounted to a circuit card such as Duroid
or FR-4, 1/4 watt if mounted to metal with conductive
epoxy, and 1/2 watt if it is attached with solder to a
metal heatsink. |
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