ETC 1307059

Intermodulation in RF Coaxial Connectors
The increased demand from the mobile communication industry to provide greater channel capacity
coupled with the increased sensitivity of receivers has
exposed a condition within RF Coaxial Connectors
referred to as Intermodulation Distortion (IMD). This
condition occurs when non-linearities within the
connectors act as imperfect diodes to generate other
frequencies known as Intermodulation Products (IMP).
Some of these frequencies appear within the receive band
and effectively block the channel. The purpose of this
application note is to outline the basic causes of
Intermodulation and the techniques M/A-COM has
undertaken to minimize this condition.
working in dBc) or -120dBm (when working in dBm),
both with 2 x (+43dBm) tones. The requirement is so
stringent because the connectors are used in postfiltering sections of the transmit path (between the
diplexer and the antenna) and also because the system is
a full duplex system where the multiple-carrier transmit
path is also the receive path. In a truly linear system, the
output is directly proportional to the input, following
the form of y=mx+c (see fig. 2). Coaxial connectors have
traditionally been viewed as following this pattern. In
reality, there have always been non-linearities present in
coaxial connectors. These were not readily apparent as
the resultant IM products were significantly below the
noise floor of the system due to relatively weak carrier
signals. This situation becomes apparent when the
incident power is raised above 30 dBm.
Linear vs. Non-linear Response
Fig. 2
Fig. 3
Fig. 1 M/A-COM Intermodulation Test Lab
IM Basics
Modern developments in base stations for GSM, DCS
1800 and PCS 1900 have necessitated the use of “7-16”,
“4.1/9.5” and “N” connectors due to the increased
power requirements. The requirements for performance
are typically in the order of -160 dBc to -163dBc (when
The small non-linearities have a characteristic similar
to a square-law (see Fig 3). The distortion to the waveform
is evident, the positive 1/2 cycle being significantly
greater in amplitude than the negative 1/2-cycle. When
converted to the frequency domain, this waveform
consists of the desired fundamental plus a decaying series
of related harmonics that, in themselves, interact with
other carriers present on the transmission line.
Specifications Subject to Change Without Notice.
M/A-COM, Inc.
North America:
Tel. (800) 366-2266
Fax (800) 618-8883
Asia/Pacific: Tel.
+85 2 2111 8088
+85 2 2111 8087
Europe: Tel. +44 (1344) 869 595
Fax +44 (1344) 300 020
The effect of this interaction produces additional
frequencies, some of which occur where they are least
wanted (see fig. 4). The 2F1-F2 (3rd order IMP, IMP3),
3F1-F2 (5th order IMP, IMP5) and 4F1-F2 (7th order IMP,
IMP7) products can all manifest in the receive band and,
if sufficiently large, effectively block a channel by
making the base station receiver think that a carrier is
present when one is not.
Remedies for IM in RF Coaxial
To combat the above identified IM sources,
M/A-COM undertakes precautions during the design
and manufacture of the product, as summarized below:
• High quality plating to 6µm for IM-sensitive products
Spectrum of products in a 2-tone system
% Fig. 4
Rx band
2F2-F2 band 2F2-F1
4F2-3 F1
Frequency (MHz)
The plating must also be free from contaminants
and properly passivated with a chromate passivate.
Silver has been the preferred plating material as it
possesses the lowest practical resistivity thereby
minimizing interface contact resistances. M/A-COM
also offers a unique White Bronze plating finish which
provides excellent durability, tarnish resistance and
non-magnetic properties ideal for low intermodulation. During testing with a system noise floor
of -145 dBm, the difference in performance between
silver-plating and M/A-COM’s new White Bronze
finish is not discernible (refer to White Bronze
Application Note ID1014).
• Restrict materials to copper and its alloys.
This ensures maximum plating adhesion and
minimum electrochemical potential difference between the base materials and their over-platings.
Potential Causes of IM in Coaxial
There are numerous factors which can affect
intermodulation performance in RF coaxial connectors.
Identified below are the most likely sources of concern:
• Contaminated plating solution
• Insufficient plating thickness
• Corrosion
• Avoid the use of stainless steel, nickel, ferrites, etc. in
the signal path
Magnetic and para-magnetic materials will only
compound non-linearites and give poorer interface
contact resistances. During experimentation,
M/A-COM discovered a degradation in performance
of 20dB when nickel plate was used. The presence of
magnetic or para-magnetic materials will also cause
the forward IMP figure to differ from the reverse IMP.
• Quality machining
• Dissimilar metals in intimate contact
Surface finish is paramount. The signal propagates
within a “skin” if this skin is too rough, the signal will
repeatedly transition through metal and surface oxide
layers, thereby creating the same effect as a poor panel
contact (see fig. 5). For IM-sensitive designs M/A-COM
ensures 0.4 µm is the maximum.
• Magnetic materials in the signal path
• Low contact pressure
• Less than 360-degree contact
• Contact design
• Poor surface finish
This primarily affects the connector interface.
Repeated matings can generate small amounts of
plating from the individual parts. These oxidize and
interfere with the mechanical (and therefore electrical)
mating of connectors. The oxidized debris gives
further rise to metal and surface oxide junctions and
consequently, higher IM products.
• Debris and dust within the connector
• Convoluted signal path
Specifications Subject to Change Without Notice.
M/A-COM, Inc.
North America:
Tel. (800) 366-2266
Fax (800) 618-8883
Tel. +85 2 2111 8088
Fax +85 2 2111 8087
Europe: Tel. +44 (1344) 869 595
Fax +44 (1344) 300 020
Effect of surface roughness
Most Commonly Asked Questions
Regarding Intermodulation
Fig. 5
Signal flow
• Ensure, by design, a properly defined contact interface
at connector, panel and contact interfaces
Insufficient contact force will give rise to metal to
oxide junctions. The classic rectifiers were metal oxide
by composition.
Axial maximum material condition at the
connector interface is critical in order to ensure
minimum mismatch and maximum potential of a buttcontact. Panel interfaces generally concern the
physical contact of the connector body to the panel. In
this case, it has been determined that a protruding
feature as close as possible to the body bore will give
the best IM performance. The applied mounting force
is concentrated in the surface area of the protrusion
which, on engagement with the panel, punctures the
existing oxide layer to give a metal-to-metal, gas-tight
Why is intermodulation such a concern for cellular
infrastructure equipment?
The primary concerns for cellular service providers
today are channel efficiency and clarity of transmission.
Growth in demand for mobile communications has
created a need to operate equipment at greater capacities
and reliability to service the competitive market.
Intermodulation degrades or limits the ability of the
service provider to operate at optimal levels of
performance and may ultimately cause subscribers to
experience poor call quality. Intermodulation has
become an important factor in system selection to ensure
the best possible network service.
Where is intermodulation most likely to occur in
cellular infrastructure equipment?
Intermodulation is typically of greatest concern
between the filtering elements of the system and the
antenna. The introduction of higher power levels for the
transmit side of the equipment creates greater potential
for intermodulation to occur. This is why the majority of
focus for intermodulation concerns 7-16, type N, SMA
and 4.1/9.5 connector interfaces.
• Avoidance of crimps
Crimps, by nature, can only give multiple pointcontact rather than 360-degree contact and also cause a
variability in the position of electrical contact during
dynamic testing. IM products will therefore be greater.
It has been found that soldered center contacts and
clamp/solder outer contacts give the best static and
dynamic IM performance.
Improving IM Connector Design
M/A-COM continues to pursue design techniques
which improve intermodulation performance to address
emerging telecommunication market needs. A state of
the art intermodulation test facility and participation on
the international (IEC SC46D WG5) committee to
develop standard test practices ensures our commitment
to the understanding of intermodulation characteristics.
This applied technology base is instrumental in
developing innovative low intermodulation products for
7-16, Type N, SMA and OSP interfaces.
Is intermodulation a recent development?
Intermodulation has always been inherently present
in RF coaxial connectors but may be relatively
imperceptible in some devices for a variety of reasons.
The amount of power applied to an RF connector
determines the relative IM threshold which can be
observed. Intermodulation is therefore more likely to
cause concern in a higher power system, for example,
utilizing a 7-16 connector interface rather than an
equivalent low power OSX solution. The trend toward
higher power digital cellular systems creates the need
for greater intermodulation sensitivity.
Specifications Subject to Change Without Notice.
M/A-COM, Inc.
North America:
Tel. (800) 366-2266
Fax (800) 618-8883
Asia/Pacific: Tel.
+85 2 2111 8088
+85 2 2111 8087
Europe: Tel. +44 (1344) 869 595
Fax +44 (1344) 300 020
What is the best method of cable attachment for
IM sensitive cable assembly applications?
Soldering and clamping are preferred methods due
to the 360-degree point of contact created at the cable to
connector interface. Such intimate contact improves the
overall contact resistance leading to improved IM
characteristics. In addition, it is also better to solder the
center conductor of the connector to the cable versus
crimping due to the improved contact resistance path
and elimination of voids.
Are there ways to test for intermodulation in an
RF coaxial connector?
Yes, very sophisticated methods are needed to test for
intermodulation in RF connectors. The test system must
utilize extremely sensitive filtering or clean
amplification so that the equipment itself has a very low
intermodulation noise floor. There is not yet a
standardized approach to testing, although an
international committee has been formed in the
connector industry to address the situation. M/A-COM
has a state of the art test facility where our designs are
optimized for low intermod performance and where
further analysis on the effects of this phenomena can be
IMP3 in mixers follows a 3dB/dB relationship.
What is it for connectors?
The relationship is identical. Taking the 3rd order
(2F1-F2): varying the power of F2 gives an IMP3
relationship of 1dB/dB whereas varying the power of F1
gives a relationship of 2dB/dB as the IMP is derived
from the 2nd harmonic of F1. This gives a total of
3dB/dB when symmetrically varying both carrier
I am buying a complete cable assembly from
M/A-COM. How do I interpret the IMP result now?
With caution! It is M/A-COM’s policy when testing
devices to move away from the normal static test to a
dynamic test where the cable termination interfaces are
mechanically exercised during live IM conditions. It is
also a good indicator to customers of the build quality of
the assemblies. A dynamic evaluation has shown 15dB
degradation in IMP performance for poor assemblies
and even as much as 50dB for bad ones.
It is therefore strongly advisable that IM performance
figures are stated in the context of a dynamic
Is intermodulation in coaxial connectors frequency
No. Because coaxial connectors are broadband
devices there is no frequency dependency. Some
apparent variability can be detected during testing but
this is not due to the connector. The impedance matches
of the output diplexer/triplexer and terminations are the
causes of the variations and should not be incorrectly
attributed to the connector/assembly. M/A-COM
Interconnect Business Unit has demonstrated that by
varying the impedance match of the test station
termination, a DUT can show 15dB better IMP3 than
exists in reality.
Printed in U.S.
North America
Central America
South America
Europe/Middle East/Africa
44 (1344) 869 595
44 (1344) 300 020
85 2 2111 8088
85 2 2111 8087
Specifications Subject to Change Without Notice.
M/A-COM, Inc.
North America:
Tel. (800) 366-2266
Fax (800) 618-8883
Tel. +85 2 2111 8088
Fax +85 2 2111 8087
Europe: Tel. +44 (1344) 869 595
Fax +44 (1344) 300 020