An Examination of Recovery Time of an Integrated Limiter/LNA
■
Jim Looney, David Conway, and Inder Bahl
G
aAs monolithic microwave integrated circuits
(MMICs) are widely used in commercial and mili-
tary microwave systems. Due to the fine geometry
used in MMIC transistors, these circuits are susceptible to
damage from high-power spurious electromagnetic (EM)
radiation, either from microwave transmitters or nuclear
electromagnetic pulse. Especially, low noise amplifiers
(LNAs) in the front-end of microwave systems need high
power protection because these amplifiers can sustain only
low input power levels in the range of 10–20 dBm continuous
wave (CW). To protect these circuits and maintain low noise
figure, a high power and low loss limiter is required.
The purpose of this application note is to document the test
methodology employed and test results achieved measuring
the small-signal gain recovery time of a balanced LNA with an
integrated Schottky diode limiter and high power load.
• single chip, no extra components and assembly, low cost
solution
• balanced configuration, built-in couplers
• high-power termination resistor on the chip
• limiter parasitic capacitance part of the LNA’s input
match, improved noise figure with respect to discrete
solutions
• high-output, third-order intercept point
• uses standard, reliable and high performance MSAG
MESFET technology.
A block diagram of the high power limiter/LNA MMIC is
shown in Figure 1. Figure 2 shows the photograph of the two-
stage balanced limiter/LNA. The chip measures 4.6
×3.1
mm.
The design of integrated high power limiter/LNA has been
described in [2].
Test Methodology
Device Under Test
The circuit selected for this investigation was the commercial-
ly available M/A-COM limiter/LNA MMIC [1]. This
limiter/LNA has an operating bandwidth of 8.5 to 12 GHz, a
nominal gain of 16 dB, a noise figure (NF)
<
3 dB, and an
input third-order intercept (TOI) of 13 dBm. Operating bias
voltages and currents are 5 V, 130 mA nominal, –5 V, 4 mA
nominal for the drain and gate, respectively. The active device
employed in this IC is the low noise multifunction self-
aligned (MSAG) metal-semiconductor field-effect transistor
(MESFET). Salient features of this circuit are:
Jim Looney, David Conway, and Inder Bahl (inderb@tycoelectronics.com)
are with M/A-COM, Inc., 5310 Valley Park Drive,
Roanoke, V 24019. USA
A
The limiter recovery time was measured by pulsing the input
RF signal from a small signal level to a high power state
In
Lange Limiter
Coupler
LNA
Lange
Coupler 50
Ω
50-Ω
High
Power
Single-Ended
Amplifier
Out
Figure 1.
A balanced three-stage LNA with limiter.
March 2004
83
(limiter active), then using a detector to measure the response
time as the RF level drops back to normal operating levels.
Due to equipment limitations, a method to pulse the RF from
a small but detectable signal level to a large-signal level was
not readily available.
Consequently, a system to measure recovery time using
two RF tones was developed. By injecting a CW small signal
level at the low end of the device under test’s (DUT’s) band
of operation (F1) and a pulsed high power signal at the
upper end (F2), it was possible to measure the small signal
recovery time by separating the two
signals at the output of the device
using typical components found in
most test labs. The frequencies of the
two signals were chosen to allow the
use of existing low pass filters to
remove the F2 signal so only the
response of the F1 signal would be
measured. For this test, a dual direc-
tional coupler was used to combine
the signals. The F1 signal was injected
Lange Coupler
through the reflected power port of
the coupler and the pulsed F2 signal
at the thru path. This uses the reverse
isolation of the coupler to separate the
small signal F1 tone from the pulsed
high power F2 tone. For this device,
F1 was a 7-GHz, –10-dBm CW signal
and F2 was 12-GHz, 40-dBm pulsed at
10-µS pulsewidth at a 5% duty cycle,
Figure 2.
MA01502D, a commercially available C/X-band integrated LNA/Limiter.
MMIC size: 4.6
×3.1
mm.
and they were separated using 8-GHz
Limiter Recovery Time Measurment System
Pulse Generator
Oscilloscope
F2
12-GHz, 40-dBm
10µS Pulsewidth
5% Duty Cycle
RF Synthesized
Pulse In
Source
RF
OUT
Atten.
8–12.4-GHz
Circulator
12.4-GHz
Low Pass
Filter
Power Meter
Power Sensor
Atten.
Atten.
4–8-GHz
Circulator
4–8-GHz
Circulator
DUT
Dual
Directional
Coupler
Directional
Coupler
(10 dB)
25-W 10-W
Atten Load
12GHz
TWT
RF Synthesized Source
RF
OUT
F1
7GHz
CW
–10 dBm
8 GHz
Low
Pass
Filter
8-GHz Low
Pass Filter(s)
Detector
(+ Output)
DC Supply
Figure 3.
A LNA/limiter recovery time test set-up.
84
March 2004
RF Response Time
F1
RF
Level
low pass filters. At the output of the device, a directional
coupler was used to sample the combined signals and low
pass filters were used to filter out the pulsed F2 tone. The
recovery time measurement was then made using a positive
output voltage detector connected to a high frequency oscil-
loscope. Refer to Figure 3 for the test set-up block diagram.
Test Results
Figure 4 represents the RF levels of the two frequency tones
versus time. When the high power F2 signal is on (“high”),
the Schottky diodes of the limiter are effectively short circuit-
ed to ground. The CW F1 signal is subsequently attenuated
and goes low, as shown in Figure 4.
Figure 5 shows a typical RF recovery time plot from the
oscilloscope using the internal “rise time” measurement capa-
bility to determine the 10–90% rise time (in this example 36.6
ns). By using a negative detector to look at the F2 pulse (at
incident port of dual directional coupler), one could also mea-
sure the delay between the two signals. This plot was taken at
the full rated power of the limiter, 10 W, CW. A limiter recov-
ery time acceptance limit of
<
100 ns could easily be achieved
based on this data.
Figure 6 indicates how the limiter recovery time improves
with a lower incident power level. In this plot, the power tone
F2 is no longer detectable. The small-signal tone F1 is still
clipping, but not as much as in Figure 4. Under these condi-
tions, the 10–90% rise time is
<
5 ns.
When this testing was conducted several test “issues”
were noted and are listed below:
1) To avoid damaging test components and equipment, be
absolutely sure that the low power signal path is suffi-
ciently isolated from the high power signal and that all
other components are able to withstand the high power
levels.
2) Be sure to use an oscilloscope with sufficient band-
width to measure the response times. By going from a
150-MHz scope to a 500-MHz scope, the measured
recovery time decreased by approximately 50%.
3) Use the “cleanest” pulse generator possible to pulse the
RF source. Typically the falling edge of a pulse is noisi-
er than the rising edge, this leads to “noise” on the
falling edge of the pulsed high power signal, which
leads to “noise” in the recovery time measurement.
4) Experiment with different types of pulse generators (even
different units of the same type) to get the best signal.
5) Watch for thermal problems. Even though our DUT
was attached to a heatsink, the RF signal level dropped
as the RF level of the pulsed signal was increased (orig-
inally 100-µS wide). By decreasing the pulsewidth, it
was possible to maintain the normal small signal gain
levels of the DUT.
F2
Time (nS)
Figure 4.
A representational plot of relationship between F1 (CW,
small-signal) and F2 (pulsed, large-signal) tones.
Figure 5.
A LNA small-signal recovery time plot of F1 tone
(Channel 1) under high-power pulsed stimulus (F2 tone, P
out
~10
W, Chan. 2, negative detector). Channel 4 is pulse generator signal
used to trigger F2 source and oscilloscope.
Figure 6.
An LNA small-signal recovery time plot of F1 tone
(Channel 1) under moderate power pulsed stimulus (F2 tone,
Channel 2, negative detector). F2 tone no longer detectable. F1
tone weakly attenuated.
86
References
[1] Microwave MMIC Products, M/A-COM, Roanoke, VA.
[2] I.J. Bahl, “10W CW broadband balanced limiter/LNA fabricated using
MSAG MESFET process,”
Int. J. RF and Microw. Computer-Aided Eng.,
vol. 13,
pp. 118–127, Mar. 2003.
March 2004
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