ID1018
V3.00
Intermodulation in RF Coaxial Connectors
Introduction
The increased demand from the mobile com-
munication 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 post-
filtering 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
12
10
8
6
4
2
0
Fig. 1 M/A-COM Intermodulation Test Lab
Linear
120
100
80
60
40
20
0
Fig. 3
Non-linear
IM Basics
Modern developments in base stations for GSM, DCS
1800 and PCS 1900 have necessitated the use of 鈥?-16鈥?
鈥?.1/9.5鈥?and 鈥淣鈥?connectors due to the increased
power requirements. The requirements for performance
are typically in the order of -160 dBc to -163dBc (when
Specifications Subject to Change Without Notice.
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.
M/A-COM, Inc.
North America:
Tel. (800) 366-2266
Fax (800) 618-8883
s
1
Asia/Pacific: Tel.
Fax
+85 2 2111 8088
+85 2 2111 8087
s
Europe: Tel. +44 (1344) 869 595
Fax +44 (1344) 300 020
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