Intro to Cable Modem Termination Systems

As more people turn to the internet, fast internet access is a growing concern for many customers. Currently, there are several ways to access the internet such as traditional phone lines, DSL and satellite. However, one of the most popular access methods is through your local cable television service provider. If you already have cable TV service, adding internet access is affordable and easy to do, and several cable providers in the Broadband Market are rapidly upgrading their systems to accommodate this growing market. One of the most important pieces of equipment necessary for this to happen is the CMTS, or Cable Modem Termination System.

The CMTS, which is located at the head end or service providers location, works with a cable modem located in the customer's home to bring internet access to subscribers. It manages the internet traffic created by local users and acts as a gateway between the user and the internet. Currently, internet traffic is separated into two channels; upstream and downstream and is governed by industry standard protocols like DOCSIS 2.0.

Upstream traffic is a request sent from the local user upstream' to the local ISP (internet service provider) and out to the internet. A response to that request travels back downstream' from the internet, through the ISP and ultimately to the local user. The upstream channel is smaller since most information requests are relatively small in size. However, the downstream channel is considerably larger to handle the potential download traffic generated by local users. Anyone that has tried to download a large file will realize that this can take some time! System architects realized this imbalance and have made the downstream channel larger.

Initially, one or many neighborhoods communicate through one CMTS with a minimum amount of daughter cards. As more users sign on to the service, traffic levels can increase dramatically and performance noticeably declines since everyone is on the same network.

To accommodate this, many CMTS are scalable. Most CMTS are a box or rack with multiple slots for network connections. For low volume traffic needs, it may require only one line card but as usage increases, additional line cards can be added as needed. At this point, RF transmission lines are required to pass data between line cards and the system. These cables and RF connectors allow for communication between the necessary systems components, and are an integral part of the CMTS. The quality of these components determines how much information can be sent, and thus are very important to the overall system performance.

Amphenol RF is a market leader in providing custom RF connector and cable solutions to CMTS manufacturers. As their needs grow and their requirements get tighter, we continue to work with their system engineers to find low cost, flexible and scalable solutions. Our engineering department uses state of the art software modeling techniques and RF analysis tools to develop the right part from the beginning.

If you are interested in finding out more about what we can do for your design, contact the representative in your area or click here to contact the factory.

Amphenol RF Expands Capacity in China

To increase localization efforts and grow our global manufacturing capacity, Amphenol RF has built a new factory in Shenzhen, China. Scheduled to open in September, 2004, the new facility has approximately 120,000 square feet of manufacturing floor space, which is twice the size of our previous manufacturing space in Shenzhen.

Amphenol RF's RoHS Compliance Plan

On July 1, 2006 a new European Union based directive will become effective titled, Directive 2002/95/EC of the European Parliament and of the council of 1/27/03 more commonly referred to as the RoHS directive or Lead free directive. The term RoHS stands for Restriction of Hazardous Substances. It will, by design, limit the use of specified hazardous materials commonly used in electrical and electronic products.

Although the directive is intended for the European market, its effects are far reaching and basically will apply to all electronic and electrical products shipped world wide. Several countries are planning similar legislation.

The directive restricts 6 materials that are commonly used in electronic and electrical products. They are: Lead; Cadmium; Mercury; Hexavalent Chrome; PBB; PBDA.

Amphenol RF is primarily impacted by the lead and chrome materials that are commonly used for plating, in the case of lead and as a tarnish protectant for silver in the case of the Hexavalent Chrome material. We have identified alternate materials for these materials that fully comply to the directive. Pure Tin is generally accepted as a plating replacement for lead/tin, however, it's tendency to grow metallic whiskers has added caution to its widespread replacement of Tin/lead as a plating. Pure tin replacements that do not have the undesirable property of whisker growth are just now becoming available.

While the four remaining materials are not common to our products, we will be required to prove that we conform to the directive in products that we ship, for all 6 materials.

We plan to be fully RoHS compliant by May 2005.

We are currently upgrading our Quality Overview section of the web site to give our customers a simple method to ask questions, view the entire RoHS Directive and eventually allow them to view compliant product directly. The upgrades will be completed in Q4.

Q: "How does Amphenol RF use ANSOFT HFSS to design non-traditional products?"

A: I've spent several recent issues of Ask Dave describing how we use ANSOFT HFSS at Amphenol RF to design connectors accurately and quickly. Since I've already covered general topics such as Power, Intermodulation Distortion, Return Loss and Insertion Loss, I will use this column to highlight some of our products and describe how we use HFSS to help us design products other than traditional connectors.

Terminations are a natural extension of traditional connector design. Sooner or later, designers are going to have situations where they have unused ports that need to be terminated. Some typical applications are on switches, power dividers and combiners.

The actual termination is a resistor whose value is normally equal to the impedance of the system into which it is inserted. However, just soldering a resistor inside a connector will generally not result in a termination that will exhibit good Return Loss at high frequencies. Once again, ANSOFT HFSS is used by Amphenol RF to design our terminations for the best possible RF performance. How do we use HFSS to optimize the designs of resistor terminations?

There are generally 3 types of resistors used in terminations. They are chip, rod, and disc style resistors. Each one exhibits different impedance characteristics at high frequencies (generally above 1 GHz) and therefore, the termination design must be such that it matches the impedance discontinuities of the resistor.

Disc resistors are usually limited to lower frequencies because they tend to exhibit a very large capacitance where they terminate the RF signal. One good use for disc resistors is in a device called a Feedthru termination. This is not a termination in the traditional sense since it has 2 ports, instead of one. It is often used as a matching device on oscilloscope probes.