Another significant advantage of having one module that includes the transformer, choke, and low-pass filter is that they can be easily fine-tuned to work together. Generally, when the low-pass filter and/or the magnetics consist of discrete components, they must be fine-tuned after populating the board.
This causes a degree of difficulty because the interaction between discrete magnetics and filters is difficult to predict. But with the integration of the magnetic components, the transformers, and the filter, the end user won’t have to worry about such complexity, allowing for quicker design and the ability to use a package as a single device with more predictable behavior.
Also of note, when both parts are integrated within the same module, parasitics can be fine-tuned in advance, ensuring they are IEEE compliant. Fine-tuning can be accomplished with minimal changes, and the IC can be fine-tuned to each individual customer’s board with minimal modifications. It follows that having an integrated solution allows direct connection with the PHY, which reduces the traces and yields better performance.
Furthermore, for many IC manufacturers, each PHY must have a fixed value for low-pass filters. In the early stages of development, the PHY exhibits limited capabilities for implementing corrections. Standalone filters come with a set value, and there is no flexibility. Using a filter module with a built-in transformer and a low-pass filter offers flexibility because values are changeable when the system is fine-tuned.
Users can employ simulation software to determine the ultimate values required for the board while the manufacturer can measure the IC and test it again before board layout begins. Some systems need 5 Pole filters while others need 7 Pole. Therefore, new modules with integrated transformers and filters offer both pole options.
KEY CHANGES IN THE 10G ARENA
Since the inception of IEEE 802.3an, magnetics manufacturers have made significant strides in improving return loss on their transformers. First-generation 10G systems specified a return loss of –8 dB, and the second generation improved to –12 dB. Today’s 10G transformers (Fig. 2) packing highly tunable low-pass filters are performing at –20-dB levels at 400 MHz for discrete magnetics.
It is imperative to understand that some of the key parameters found in 10G differ from its predecessors. For example, inductance (OCL) has changed. In 100BaseTX Ethernet applications, a minimum of 350 µH of inductance is necessary. But in 10G Ethernet, there is no minimum inductance requirement.
Also, as transmit frequencies increase, it is important to be able to minimize noise. This is achievable by selecting good common-mode chokes in a discrete magnetics package and better shielding in an integrated package.
Switching to 10G Ethernet and using copper instead of fiber was very promising when the standard first went into effect because copper is one-third the cost of fiber. Although the overall power necessary to run fiber is higher than copper, the high 12-W per port range of the first-generation copper ICs set some limits on viable applications.
Now, the power wattage of a 10G copper PHY IC is at least 50% less. Some device manufacturers claim that newer PHYs consume only 3.5 W, making them an excellent fit for peripheral component interconnects. There is also less heat generation, which allows for the use of a smaller heatsink, making the chips viable for use in high-density, 48-port 10G switches.
FLEXIBLE DESIGN SOLUTIONS
ICs perform differently, depending on the magnetics. Some IC manufacturers have customers who prefer to have a choke first in the primary circuit and then the transformer, also known as a C+T configuration. Other manufacturers prefer the transformer and then the choke, a la T+C. Filter modules (Fig. 3) that include the transformers and filters can be used for either of these configurations.
When the low-pass filter and transformer are integrated together, it becomes apparent to the designer that any schematic configuration can be arranged and wires can be connected and/or reconnected in any design configuration deemed necessary by the IC manufacturer for the whole system. OEMs and device manufacturers can work with the magnetics manufacturer to customize the system configuration.
Yet another advantage for designers of these 10G systems is having the flexibility for device configuration and system design by having available to them a choice of filter modules that have the same overall footprint as the previous transformer modules, but also incorporate the low-pass filter necessary for the design. This offers the designer greater placement options for the transformer on the circuit board. Now achieving the smaller size, the designer can improve board density or trim the board’s dimensions.
Accounting for all of these factors, the move to 10G has become easier and more viable. The new filter modules with built-in transformers and low-pass filters offer greater flexibility in system design with better performance and reliability. IC manufacturers and OEMs now have more options in using chokes, transformers, and magnetics in a variety of combinations, resulting in custom board designs that work best for each company’s application.