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May 2002 edition of the Amphenol RF Connection v 1.3
In this issue:
Amphenol's 75 Ohm Connectors Provide Optimal Router Performance
Dave the Engineers Q & A: What is Passive Intermodulation Distortion?
Amphenol's Over-Mold Jumper Assemblies Reduce Passive Intermodulation Distortion
Amphenol's 75 Ohm Connectors Provide Optimal Router Performance
When using the Internet or any networking system, a router is a device in a computer that
determines the next network point to which a packet (or data package) should be
forwarded toward its destination. Unlike a bridge whose only purpose is to split
the physical network being shared by many computers into smaller segments, the router
serves an active role in the network. Since it understands the pertinent protocols
being used to carry this packet, the router uses the appropriate rules to decide what
to do with a specific piece of data. The router is connected to and joins at least
two networks, passing information from one to the other. It decides which way to send
each information packet based on its current understanding of the state of the networks
to which it is connected, even to the point, in some cases, of executing translations
of various protocols between the two networks. Because of this, routers are useful in
linking multiple networks that are used for different purposes or by different
organizations.
A router, then, has two separate but related jobs. First, the router ensures that
information doesn't go where it's not intended. This is critical for keeping large volumes
of data from clogging the connections of all systems users. Second, the router makes sure
that information successfully makes it way to the intended destination. It also protects
these networks from one another, preventing the traffic on one system from unnecessarily
spilling over onto the others. As the number of networks attached to one another grows,
the logistics for handling traffic among them grows, and the processing power of the
router is increased. As the Internet is one huge network made up of tens of thousands of
smaller networks, its use of routers is an absolute necessity.
In order to handle all the users of even a large private network, millions and millions of
data packets must be sent simultaneously. Some of the largest routers are made by Cisco
Systems, Inc., who specializes in networking hardware. Cisco's Gigabit Switch Router 12000
series of routers is the sort of equipment that is used on the backbone of the Internet.
These routers use the same sort of design as the most powerful supercomputers in the world.
Beyond the computing power of the processors, these routers can handle so much information
because of their highly specialized nature. Relieved of the requirements of displaying 3-D
graphics, processing input from a mouse and other such tasks, modern processors and software
can cope with amazing amounts of information. All of this happening in a tiny fraction of
a second, millions of times a second, 24 hours a day, every day.
In order to support the requirements for speed and to maintain
the integrity of the data during these exchanges, look to Amphenol's
75 ohm product offering to supply the optimal connector interface
on these routers. Typically, these routers will have 75 Ohm
BNC printed circuit board receptacles or, to provide smaller
connectorization, 75 Ohm SMB receptacles. Among the Amphenol
designs already in use in these systems are the vertical BNC
PC receptacle 031-71062-4060, the right angle BNC PC receptacle
03-70221, the BNC edge mount PC receptacle 031-6009, and the
vertical SMB PC receptacle 903-581J-71P. To connect to any of
these receptacles on the routers, Amphenol offers a full range
of 75 ohm BNC and SMB cable plugs. You can find any of these
connectors online by using our product
search.
Q: What is Passive Intermodulation Distortion?
A: Until the use of cellular phones became widespread, Passive Intermodulation Distortion, or PIM,
was of little concern to the connector designer. PIM was well known to the satellite
communications and microwave trunking industries, but most connector engineers had never heard
of the phenomenon. Now, due to the frequency plans of today's modern base stations, the use of
higher transmitter power levels and more sensitive receivers, PIM has surfaced as a problem for
cellular and other wireless services.
PIM is the unwanted signal or signals generated by the non-linear mixing of 2 or more frequencies
in a passive (or linear) device such as a connector or cable. If the circuit has some unwanted
non-linear characteristics, then the fundamental frequency components will become distorted in
the time domain and generate a decaying series of higher order harmonic frequency components in
the frequency domain. If these generated harmonic frequency components fall within the receive
band and are of sufficient magnitude, they can effectively block a channel by making the base
station receiver think that a carrier is present when one is not.
Generally, the components of concern are 3rd, 5th and 7th order where the third order is of
greatest signal strength, and therefore, of primary concern. If the fundamental frequencies are
f1 and f2, then the possible intermodulation frequency components can be described by the following
equation, where the sum of m and n is the product order:
Fim = mf1 + nf2
For example, (2f1-f2), where m = 2 and n = 1 is a 3rd order intermodulation product. If f1 =
930 Mhz and f2 = 955 Mhz, then Fim = 905 Mhz.
In a future issue, we will discuss what causes PIM in connectors
and how Amphenol designs low PIM connectors. For more information
on related topics, click on our RF
Made Simple section.
Amphenol's Over-Mold Jumper Assemblies Reduce Passive Intermodulation Distortion
Amphenol RF has developed a line of high performance cable assemblies utilizing corrugated
cable and 7/16 and Type N interfaces for jumper applications in the base station market.
The cable assemblies offer superior loss and intermodulation characteristics due to their
design features and process control. Typical performance requirements are -30 dB return
loss up to 2 GHz and passive intermediation better than -110 dBm with two +43 dB carriers.
The key design features of the cable assemblies that result in their high performance are a
complete solder attachment and a plastic injected over-mold. The solder attachment provides
a superior electrical connection that resists resistance changes when subjected to vibration
or harsh environments. The plastic over-mold provides a strain relief to the cable attachment
to reduce the effect of cable movement on the cable attachment.
Because the assemblies are produced under tight quality control in a factory environment, the
performance is very consistent. When jumper assemblies are terminated in the field, they become
susceptible to process inconsistencies that result in reduced performance. Factory-built
assemblies are produced in a clean, controlled environment and then tested and certified for
performance.
The construction of an over-mold style connector uses fewer and simpler components than a
traditional field attachable connector. This results in a lower cost solution. The cable
assemblies must be ordered in predetermined lengths to fit specific applications. With careful
system planning, the assemblies offer significant system cost reduction.
Please contact us for more information about the
entire product line or to discuss custom applications.
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