here to learn about designing for high peak power (new for
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New for September 2008!
Limiters are used to protect weak components such as low noise amplifiers
(LNAs) from stray signals. The power transfer characteristic (shown
below) behaves somewhat like an amplifier with just a little loss
rather than gain. Above some critical input power, the output power
can behave quite strangely, often described as having a "kink"
to it. From the plot you can read off the small signal loss (insertion
loss), one-dB compression point, and the flat leakage.
The limiter is used most often
to protect a low-noise amplifier in a receiver chain. LNAs are carefully
designed for low noise figure, and use very small devices to accomplish
this; small devices can't handle high input power. Two mechanisms
can kill an LNA, heat from the RF input signal, or overstress due
to the RF voltage that appears on the input transistor. Often, an
LNA's performance can be changed slightly from high input power,
without causing it to fail. Having a damaged LNA might be worse
that having a destroyed LNA, you never know when it might fail.
In a radar the stray signal
that most likely will damage an LNA comes from the transmitter,
so it is a pulsed signal. Damage threshold of an LNA might change
somewhat with duty factor of a pulsed signal, but we recommend that
you perform LNA survival testing with CW signals because it is easier,
and it will give you a worst case result.
One method of evaluating an
LNA damage threshold is a stepped stress test, which we'll explain
later. There doesn't seem to be any accepted standards for survival
testing across the industry, but at least we'll all soon have one
method to refer to.
Flat leakage of a limiter refers
to the CW output signal that bleeds through it under high input
power conditions (see figure above). Guess what? The term "flat"
is a misnomer, in real limiters the leakage will have a slope to
it at any power level.
Spike leakage refers to the very
short part of a high-power pulse that blasts through a limiter,
before it clamps down on the signal (there is some delay in turning
on PIN diodes). The spike leakage is often referred to in units
of energy, not power. For example, if he limiter allows a spike
of 1 watt for 10 nanoseconds, the spike leakage would be 10 nano-Joules.
We said spike
leakage, not Spike Leekage
Terminating or reflective limiters
Terminating limiters will attempt
to provide an impedance match at any power level. This is a trickier
design to pull off than a reflective design, and it won't have as
good a response (more insertion loss, or higher flat leakage for
example) Plus, you might have to think about sizing the load termination
for all the power you might have to dissipate, which could be an
issue if you are designing a limiter on a MMIC.
another correct adjective for a limiter that absorbs power is "absorptive".
If you say "absorbtive" or "absorbative" your
smarter coworkers will snicker and say mean things behind your back...
Solid state limiters are most
often comprised of PIN diodes, but Schottky diodes, FETs and other
devices have been used. A shunt PIN diode acts like a small lumped
capacitance to small signals, and matching networks or pairs of
diodes separated by a quarter-wavelength
can bring the network back to fifty ohms.
PIN diode limiters can be realize
monolithically (as a MMIC), but the best performance comes from
a chip-and wire limiter. In this case, silicon PIN diodes can be
used (they perform better than GaAs believe it or not). One problem
with assembling limiters is getting a wirebond connection to a diode
where the mesa is list 1 mil in diameter... if your wirebond smooshes
out beyond the mesa, you've just increased the capacitance of the
More to come.