Filters that suppress electromagnetic interference (EMI) on signal cables connected to cryogenic electronic equipment can be made from cores consisting of high-permeability materials. The basic principle of operation of these filters is the same as that of the ferrite-core common-mode EMI filters now commonly used on cables that connect computers with peripheral equipment. An inductive EMI filters blocks EMI due to its impedance to high frequency EMI signals. Since the impedance is proportional to the permeability, a material with high permeability forms the core of such a filter

Chui, Talso

Quach, Hung

NASA Technical Reports Server

8 NASA Tech Briefs, May 2006
of 13 dB in the frequency range from
31.8 to 32.3 GHz.
The amplifiers would be identical to
commercially available GaAs monolithic
microwave integrated circuits (MMICs).
Accompanying the QOPA, on the same
circuit board, there would be two ar-
rays of antenna elements: a drive array
(a planar array of identical input an-
tenna elements) and a transmitting
array (a planar array of identical out-
put antenna elements). The drive array
would be fed via a hard horn, provid-
ing uniform illumination to each array
element. By use of microstrip transmis-
sion lines, all of equal length, the input
and output terminals of the MMIC am-
plifiers would be connected to the cor-
responding drive and transmitting an-
tenna elements, respectively. This
QOPA design would offer the following
advantages (among others):
• The separation of the input and out-
put drive arrays helps eliminate the
problem, encountered in prior QOPA
systems, of  oscillation and allows the
use of high-gain amplifiers.
• Unlike a TWTA, the MMIC amplifiers
would not necessitate a high-voltage
power supply.
• The array of MMIC amplifiers could be
actively cooled from its back side; un-
like in prior QOPA arrays, it would not
be necessary to rely on edge cooling,
which is less effective and thus limits
the achievable power to a lower level.
This is important for future inclusion
of wide band-gap devices such as GaN.
• The failure of a single amplifier would
not be catastrophic: as long as the
other amplifiers continued to operate,
the loss in performance would be rela-
tively small. For maximum efficiency,
the independent bias lines allow indi-
vidual modules to be turned off as out-
put power demands change.
The system would include a fre-
quency-selective surface (essentially, a
radio-frequency dichroic reflector) in-
tended to reflect the transmitted beam
while passing the received monopulse
beam. The FSS would provide between
40 and 60 dB of isolation between the
transmitted and received beams.
This work was done by Abdur Khan, Dan
Hoppe, Larry Epp, and Raul Perez of Caltech
for NASA’s Jet Propulsion Laboratory.
Further information is contained in a TSP (see
page 1).
NPO-30559
EMI Filters for Low-Temperature Applications
Unlike ferrite-core filters, these should work well under cryogenic conditions.
NASA’s Jet Propulsion Laboratory, Pasadena, California
Filters that suppress electromagnetic
interference (EMI) on signal cables con-
nected to cryogenic electronic equip-
ment can be made from cores consisting
of high-permeability materials. The
basic principle of operation of these fil-
ters is the same as that of the ferrite-core
common-mode EMI filters now com-
monly used on cables that connect com-
puters with peripheral equipment.
The ferrite-core filters are effective at
room temperature but not at low tem-
peratures, because their relative perme-
abilities decrease from ≈15,000 at room
temperature to ≈20 at a temperature of
4 K. In cases of cables that connect cryo-
genic electronic equipment with room-
temperature electronic equipment, it
has been common practice to place the
ferrite filters at the room-temperature
ends of the cables. This makes it neces-
sary for the filtered signals to traverse
the cables; during such traversal, cross-
talk with other cables can cause the fil-
tered signals to become recontami-
nated with EMI before they reach the
cryogenic equipment. Hence, it would
be preferable to place the EMI filters at
the cryogenic ends of the cables. The
present development makes this a vi-
able option.
An inductive EMI filters blocks EMI
due to its impedance to high frequency
EMI signals.  Since the impedance is pro-
portional to the permeability, a material
with high permeability forms the core of
such a filter.   Several metallic alloys like
Cryoperm 10 and VITROVAC are known
to have relative permeability exceeding
14,000 at low temperature.   However,
their relative permeabilities decrease rap-
idly at frequencies higher than a few hun-
dred hertz due to eddy current, which
prevents the magnetic field from pene-
trating the material.   Because ferrite is an
insulator, eddy current is not present.
Therefore it works at high frequencies.
However, all known materials with high
permeabilities at low temperatures are
metallic. Therefore, for the purpose of
constructing cores for low-temperature
EMI filters, it is desirable to prepare the
high-permeability materials in the form
of thin foils or fine powders to reduce the
effects of eddy currents. Preliminary
Relative Permeability was measured with a superconducting quantum interference device (SQUID)
magnetometer at 4 K.
100 101 102 103 104 105
Frequency, Hz
R
el
at
iv
e 
Pe
rm
ea
b
ili
ty
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7 x 104 
NPO-30901 
ABPI
 01-06-04 LE
https://ntrs.nasa.gov/search.jsp?R=20100025740 2019-08-30T09:59:43+00:00Z
NASA Tech Briefs, May 2006 9
measurements and calculations have
shown that when foil thicknesses or parti-
cle sizes are reduced to <25 µm, eddy cur-
rents become unimportant.
We have performed low-temperature
test (see figure) of a cobalt-based mag-
netic material made by Honeywell
called Meglas 2714A, which has very
high permeability at room temperature
and is available in form of tape-wound
cores of various sizes. These cores are
wound from 18-µm thick ribbons to re-
duce eddy current for high-frequency
operations. The relative permeability is
higher than 10,000 at frequencies up to
100 kHz, the limit of capability of our
measurement. EMI filters made from
this material should work at low tem-
perature.
This work was done by Talso Chui and
Hung Quach of Caltech for NASA’s Jet
Propulsion Laboratory. Further informa-
tion is contained in a TSP (see page 1).
NPO-30901
“Micro-CPI” (wherein “CPI” signifies
“cloud-particle imager”) is the name of a
small, lightweight electronic camera that
has been proposed for use in research
on clouds. The Micro-CPI would be in-
corporated into a small autonomous or
remotely piloted airplane of a type that
is now used in meteorological research
and that is capable of remaining aloft for
times long enough (typically about 30
hours) to collect statistically significant
sets of data.
According to a preliminary design, the
Micro-CPI would have a mass <1.5 kg and
would consume less than 7 W of electric
power. It would acquire and digitize
high-resolution (3-µm-pixel) images of
ice particles and water drops at a rate up
to 1,000 particles (and/or drops) per
second. The Micro-CPI incorporates a
particle detection laser that triggers the
camera imaging laser, and also counts
and sizes very small (<1-µm) aerosol par-
ticles and cloud drops up to about 100
µm in diameter. The Micro-CPI could
record data for an observation time of
more than 30 hours and could operate
autonomously.
Although a quantitative estimate of
the cost of the Micro-CPI is not yet
available, it has been projected that the
cost would be low, relative to the costs
of cameras of conventional design that
could offer the same imaging capabili-
ties. This is fortunate because there
could be a potential need in the com-
ing years to launch hundreds or even
thousands of small uninhabited aircraft
carrying cameras of Micro-CPI design
as part of an effort to measure proper-
ties of clouds on a global scale. There
are also potential applications in the
measurement of drop-size distributions
in sprays, especially in the agricultural
and painting industries.
This work was done by Paul Lawson of
SPEC Inc. for Goddard Space Flight Cen-
ter. Further information is contained in a
TSP (see page 1).
GSC-14950-1
Lightweight Electronic Camera for Research on Clouds
This camera would rapidly acquire image data on aerosol particles.
Goddard Space Flight Center, Greenbelt, Maryland
Pilot Weather Advisor System 
Cockpit displays of weather affecting flight are updated every five minutes. 
John H. Glenn Research Center, Cleveland, Ohio
The Pilot Weather Advisor (PWA) sys-
tem is an automated satellite radio-
broadcasting system that provides
nearly real-time weather data to pilots of
aircraft in flight anywhere in the conti-
nental United States. The system was de-
signed to enhance safety in two distinct
ways: First, the automated receipt of in-
formation would relieve the pilot of the
time-consuming and distracting task of
obtaining weather information via voice
communication with ground stations.
Second, the presentation of the infor-
mation would be centered around a
map format, thereby making the spatial
and temporal relationships in the sur-
rounding weather situation much easier
to understand.  Starting in the early
1990s, the PWA system was developed by
ViGYAN, Inc., under the NASA SBIR
program.  The system recently became
commercially viable and was sold to
WSI, a leading provider of weather serv-
ices in aviation. The system is now mar-
keted under the brand name “WSI In-
Flight.”
The PWA system includes a ground
processor (see figure), wherein a com-
puter running special-purpose software
converts, compresses, and schedules the
weather data. The compressed data are
then transmitted through a ground sta-
tion to a geosynchronous satellite, from
whence they are broadcast to cover the
continental United States.  The signal is
acquired by a light, low-drag antenna
mounted on a subscriber’s aircraft, and
is then interpreted by an equally light-
weight receiver. In the cockpit of each
InFlight equipped aircraft, the data are
processed, by use of other special-pur-
pose software and hardware, into an
easy-to-interpret graphical display. The
display is presented on a portable or
panel-mounted unit. The data, which
include graphical meteorological avia-
tion reports (METARs), terminal aero-
drome forecasts (TAFs), and Next Gen-
eration Weather Radar (NEXRAD)
images, as well as other weather prod-
ucts, are updated every five minutes.
Accessibility of the system to light gen-
eral aviation was a design goal because
such airplanes are more susceptible to
changes in weather than are larger,
higher-flying airplanes. The lightweight,
low-drag nature of the PWA airborne
components and the relatively low cost of
acquiring and using the equipment make
the PWA system affordable for incorpora-
tion into lower-cost single-engine air-
planes, which constitute the largest seg-
ment of the aviation market. Hence,


EMI Filters for Low-Temperature Applications

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