Datasheet

106
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EMI ESSENTIALSEMI ESSENTIALS
DESIGN GUIDE FOR
COMMERCIAL MICROWAVE ABSORBERS
Microwave absorbers have been used in military applications
for several decades. They have been traditionally used for EMI
reduction, antenna pattern shaping and radar cross reduction.
More recently with the rise of wireless electronics and the
movement to higher frequencies microwave absorbers or “noise
suppression sheets” (NSS) are used to reduce electromagnetic
interference (EMI) inside of the wireless electronics assemblies.
Two types of NSS are used for these types of applications.
They are described below:
MAGNETIC ABSORBERS
These are thin (.1 to 3 mm) polymeric materials lled with
magnetic particles. These materials have both high permeability
(magnetic loss properties) and high permittivity (dielectric loss
properties). This combination of properties makes these materials
very effective in eliminating high frequency EMI
Laird has two product types that are used for commercial
applications:
• Q-Zorb HP (high permeability) uses novel magnetic llers to
achieve extremely high permeabilities at low frequencies. This
allows for relatively thin materials to provide EMI reduction at
frequencies below 2 GHz. This material comes in thicknesses
of .15 mm and .5 mm.
• Q-Zorb HF (high frequency) is the optimum choice for cavity
resonance problems from 2-18 GHz and higher. The material
is available in thicknesses from .5 mm to 3.2 mm and is
supplied in sheets or as die cut components. Both materials
are UL-VO and ROHS compliant. They can be supplied with
pressure sensitive adhesive (PSA) for ease of installation.
FOAM ABSORBERS
These absorbers are based upon open celled foam impregnated
with a carbon coating. The carbon coating makes the resultant
product lossy at microwave frequencies, indeed acting like a
free space resistor to incoming electromagnetic energy. These
foam products range from 3.2 mm to 6.4 mm for internal cavity
applications and can be several centimeters thick for outdoor
applications. Two main product types are offered by Laird
• RFLS- Lossy sheets are uniformly loaded with the carbon
coating and used at 3.2 mm and 6.4 mm thick. They are
supplied as sheet materials and may have PSA applied and
re retardant coatings.
• RFRET- is a reticulated foam based absorber. The materials are
thicker ranging from 3/8” to 2” in thickness. They can be used
for air ltration and EMI, or on the inside of cabinet doors for
broadband EMI attenuation.
ABSORBER APPLICATIONS
Electronic operating at high frequencies can have problems with
emission of high frequency noise. Once put inside an enclosure,
the energy will add in phase at certain frequencies to cause
resonances which will hinder the performance of the device.
A good example of this phenomenon is seen in Figure 1.
The amplier was measured in the condition shown and
subsequently remeasured inside of its enclosure. When put inside
the enclosure the performance was severely degraded due to
cavity resonances inside of the enclosure. The measured data is
shown below in Figure 2.
Figure 1. Variable Gain Broadband Amplier 100 MHz to 12 GHz
0 2 4 6 8 10 12
S21 (magn) [dB]
Frequency (GHz)
Amplier Performance
24
23
22
21
20
19
18
17
16
Figure 2. Performance of Amplier in Open Condition
and Inside Enclosure
To improve the performance of the device, Laird Q-Zorb
microwave absorber was put on the inside cover of the enclosure.
This is a standard way in which absorbers are used. Q-Zorb is
supplied with a pressure sensitive adhesive to allow for ease of
installation. The cover is shown below in Figure 3.
Figure 3. Enclosure with Q-Zorb PN 2238 Installed
The amplier was then measured inside the enclosure with the
Q-Zorb in place. The performance of the amplier now mirrored
the open condition by absorbing the internal reections and
surface currents. Figure 4 shows this measurement.
0 2 4 6 8 10
S21 (magn) [dB]
Frequency (GHz)
Variable Gain Amplier Performance
24
23
22
21
20
19
18
17
16
Figure 4. Performance of amplier inside enclosure showing performance
of Absorber Material (PN 2238)
MAGNETIC ABSORBER VS. LOSSY FOAM
In these enclosure applications the designer needs to understand
the tradeoffs between using the magnetic absorber vs the lossy
foam type absorber. The foam absorber is a much more cost
effective solution, provided it will meet the performance goals.
However there are a couple major reasons to use the magnetic
sheet absorbers (Q-Zorb)
• Foam absorbers can breakdown over time causing resistive
particles to drop onto to circuit traces
• Energy along the side walls of cavity are H eld (magnetic)
dominated currents. The E eld is at a minimum along the
walls and materials with only dielectric loss (resistive foams)
will not perform very well in these applications. The magnetic
absorbers have high H eld loss and attenuate these currents.
Figures 5 and 6 below do show the use of a lossy foam absorber
inside of an amplier cavity.
Figure 5. Amplier and Enclosure with Lossy Foam Inside
15
0 5 10
S21 (magn) [dB]
Frequency (GHz)
Mini Circuits Amplier Performance
13.0
12.5
12.0
11.5
11.0
10.5
10.0
Figure 6. Performance in Enclosure and Using Foam Absorber
DESIGN GUIDE FOR
COMMERCIAL MICROWAVE ABSORBERS