How to Beat the High Cost of High Speed Interconnects

Data transmission has forever changed from the single-ended TTL (transistor-transistor logic) era to the differential era. Low Voltage Differential Signaling (LVDS) is now one of the preferred digital interfaces. This family of logic offers high speed, low power consumption, good noise immunity and low emissions of radiated EMI.

Why Differential Signaling (for Digital)?

INTRODUCTION

Data transmission has forever changed from the single-ended TTL (transistor-transistor logic) era to the differential era. Low Voltage Differential Signaling (LVDS) is now one of the preferred digital interfaces. This family of logic offers high speed, low power consumption, good noise immunity and low emissions of radiated EMI.

As with most technologies, LVDS has a downside; it requires two signal conductors to carry a bit stream instad of one.

This differential pair concept is not something new. CAT-5 cables connecting desktop computers to servers have been using unshielded differential pairs for over a decade. Telephone companies were using twisted pairs prior to 1900. It is a well proven technology.

Additionally, there are differences between a twisted pair and a differential pair. A twisted pair not only has the two conductors in close proximity, but the pair rotates along their length. This is the best case without shielding. A differential pair only has the two conductors in close proximity. You still receive many advantages of a twisted pair by using this approach, such as the use of non-impedance controlled connectors, such as ZIF connectors, to be used without a significant electrical penalty, while offering significant cost savings.

SYNOPSIS

This article describes Miraco's effort to fabricate controlled impedance (Zo) cable assemblies from standard polyester based flat flexible cables (FFC) that are designed to mate with off-the-shelf ZIF (zero insertion force) connectors. The ZIF connectors and test cables had conductors on a pitch of 0,5 mm.

Coplanar waveguide transmission lines, which do not use ground planes, were used in the FFC to control the characteristic impedance of the signal traces. Both single ended signals and differential pairs were accommodated, with emphasis on the differential pairs. The coplanar waveguide approach kept all conductors in one plane or layer, which resulted in a lower cost cable. Several material stackups of FFC were tested for their characteristics impedance (Zo).

OBJECTIVE and GOALS

The primary objective of this effort was to use standard polyester based FFC to make moderately high speed digital interconnections at low cost. The goals of this effort were:

1. Fabricate differential pair transmission lines with a
   characteristic impedance (Zo) close to 100 Ohms;
2. Fabricate single ended transmission lines with a
   characteristic impedance (Zo) close to 50 Ohms;
3. To have high frequency losses, based on the dissipation factor of the materials used, comparable to the losses incurred in epoxy glass printed circuit boards (PCBs) and in regular flexible printed circuits (FPCs) using polyimide film with acrylic adhesive.

CONCLUSION

The testing has shown that it is possible to build "low cost" controlled impedance cable assemblies using standard FFC material stackups. In comparison, FFCs offer a significant cost savings over FPCs with comparable characteristic impedance.

Some examples would include FFCs under twelve inches versus a FPC typical savings of 50% to 100% are realized. When considering FPC's twelve inches and longer savings of 150% to 250% are common. An additional benefit to FFC designs is size is not a limiting factor. Cable lengths can be from under one inch to 100 feet and longer. The FPC world manufacturers are typically limited by panel size -18 1/2" X 24". When the manufacturers can build longer FPCs it is at a premium.

When examining your design requirements, consider the options of FFC versus FPC. For a more indepth look at How to Beat the High Cost of High Speed Interconnects, click here.