Interconnect Technologies for Ultra Low Loss L-Band Transmission Lines
By Luis Torres and Stan Hardin, Stratos International

When planning L-Band satellite communication ground transmission lines, ultra low-loss fiber optics give engineers the flexibility to locate the transmission equipment facilities at considerable distances (five kilometers and up) from the antenna/amplifier system station. This flexibility is particularly beneficial in locations with mountainous terrain where high-altitude antennas can be linked to equipment centers at the base of the mountain, and where planners want to feed multiple antennas into a single transmission facility.

There are three important issues we see that engineers must contend with when configuring L-Band transmission lines for cost-effective, ultra low-loss performance: clarifying your definition of “L-Band” for proper equipment selection; achieving lowest possible loss in the front end–the antenna-to-amplifier-to-signal processor link; and selecting optical transmission equipment that provides maximum flexibility in planning your long-distance transmission topography.

Not All L-Band Is the Same
As happens frequently in RF transmission, there is inconsistency among various players in the meaning of “L-Band.” The IEEE defines L-Band as the portion of the electromagnetic spectrum ranging roughly from 390 MHz to 1.55 GHz. This frequency range is used by communications satellites and terrestrial digital radio broadcasters, and is “owned” primarily by the military for telemetry applications within the continental US and overseas territories. In commercial satellite communications, the L-Band range is defined as between 950 MHz and 2.15 GHz, and includes applications such as television broadcasting, global positioning satellites, satellite radio and GSM mobile phones.

While it may be obvious that the specific frequency range in which you will operate determines the kind of transmission equipment you will select, we’ve seen examples in other communications applications, like HDTV, where overemphasis on a single specification, like frequency handling ability can obscure other important factors that must be optimized to ensure best performance (“Distorted 3GHz,” Broadcast Engineering, December 2006).

Well Begun is Half Done
When the L-Band carrier approaches the upper end of the frequency range, managing the signal degradation due to distance literally grows outside some coax cable typically used in short run signal transmission. Because the electromagnetic field that higher frequency signals produce radiates in a field pattern outside a coax transmission line, it becomes sensitive to the dielectric material through which electromagnetic energy is moving. To a very large extent, the signal is lossy due to the dielectric, depending on the attenuation properties of the material chosen. This reduction in signal strength, at some point, prevents successful transmission.

Air-dielectric wave guides provide the lowest dielectric coefficient, just slightly higher than pure vacuum (see Table 1), but the structures required for these types of microwave interconnects are bulky, heavy, and inflexible. They are also extremely costly. The best choice for low-loss dielectric material and physical interconnect flexibility for high-power applications is expanded PTFE (polytetrafluoroethelene), with a dielectric constant of 1.7. Flexible high power interconnect cables expand engineers’ ability to physically configure the connectivity between the low-noise amplifier (LNA) and digital signal processing equipment. This is the first critical link in the end-to-end transmission line, because any loss or interference experienced in the first few feet is magnified as the signal moves through optical conversion and the long-distance cable run to the receiving point.

One of our companies, Semflex, uses expanded PTFE dielectric in all high power cable products for best-case energy passage velocity and layout flexibility in aligning and connecting components within the satellite antenna installation. PTFE dielectric cables also enable a wide range of operating temperatures at high frequency/high power.

How Long Must You Run?
Fiber optic transmission lines bring added flexibility to location decisions regarding satellite antennas and transmission equipment. However, there are distance limitations, depending on the type of laser and the microwave-to-optical conversion protocol. The most commonly used lasers in optical networking are the Fabry-Perot (FP) and the distributed feedback (DFB). FP lasers are more prevalent because they deliver sufficient signal integrity at relatively low cost, and can generally support fiber runs between five and ten kilometers. DFB is a type of singlemode laser that has very clean signal output. Though more costly than FP lasers, DFBs can support point-to-point fiber runs over 40 kilometers, and enable design of modulation-scheme-agnostic converter boards, enabling transmission equipment OEMs to produce more versatile products. While initially more costly, choosing DFB laser systems actually produces economies of scale–manufacturers can focus on developing one board, deliver more agility to the customer, and provide a unit that is applicable across the full range of fiber link applications.

Today’s solutions for RF to optical conversion are available in frame card and large throwdown units. In response to growing customer needs for component miniaturization, our Stratos Optical Technologies unit is developing protocol-agnostic L-Band fiber links for broadcast and military satellite uplinks/downlinks. The goal is to leverage more advanced technology to deliver better performance in a smaller footprint. New media converters will be available toward the end of 2007 that will serve L-Band media conversion needs across multiple applications, but will not be restricted to L-Band; they will also be able to serve UHF and VHF applications such as wireless microphones for stage productions, where multiple units are multiplexed to a single fiber feed to the mixing board.

In transmitting L-Band over fiber, isolation and distance are the key criteria. The new converters that Stratos is developing will be “linearly modulated lasers.” Rather than de-modulating, digitizing, transmitting, and reconstituting RF signals, we are overlaying the RF signal onto a laser carrier. Fiber links using these new converters will be quite flexible, both in frequency range and the distance you can support. A single product will support the lower end of IEEE L-Band (390 MHz), the upper end of commercial L-Band (2.15 GHz), and beyond.

This new design uses lightwave intensity modulation to carry the RF signal; changes in the intensity of the light exactly mirror the changes in frequency. The tighter wavelength spectrum of DFB lasers enables this light modulation design approach. The prime benefits are simplicity and flexibility; instead of needing to specify laser type by link length, customers will have one solution to cover a wide range of frequencies and transmission distances. Table 2 shows the key performance specifications for electrical-to-optical (E-O) and optical-to-electrical (O-E) units.

Simpler, More Versatile Solutions
The days of navigating through a smorgasbord of choices to optimize L-Band optical transmission lines are numbered. Turnkey E-O and O-E modules that deliver small form factor (about the size of a cigarette pack), embedded reporting and intelligence, as well as remote diagnostics and control capability will be available to OEMs.
The versatility in frequency range and transmission distance capability of these new turnkey modules will simplify equipment specification and add new levels of topography flexibility. Users will be able to put one module on either end of the fiber transmission line, connect, power them up, and transmit.

About the Authors
Luis Torres is Vice President of Research and Development for Stratos Optical Technologies, a Stratos International Company. He can be reached at 708-457-2349 or
ltorres@stratoslightwave.com.

Stan Hardin is Director of Microwave Products for Semflex Microwave Solutions, a Stratos International Company. He can be reached at 480 282-8877 or stan.hardin@semflex.com.

About Stratos International – RF, Microwave, and Optical Connectivity Solutions
Stratos International includes the Trompeter, Stratos Optical Technologies and Semflex brands. Collectively, these brands encompass a unique, integrated capability in the design and manufacture of copper RF and microwave interconnects and fiberoptic subsystems, components and connectors used by military/aerospace, telecom/enterprise, industrial and broadcast/video customers worldwide.

Stratos International

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