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Ultra-Broadband Passives Enable Broadband Communications

Thu, 05/16/2013 - 10:02am
Edgardo Menendez, Field Applications Engineer, AVX

Mobile phones, tablets, and laptops have accelerated global connectivity, simultaneously driving the need for a fast and reliable Internet infrastructure.

The fiber optics market has experienced a significant increase in demand over the last decade, largely driven by the rise, availability, and affordability of sophisticated portable electronics, such as mobile phones, tablets, and laptops. These devices have accelerated global connectivity, simultaneously driving the need for a fast and reliable Internet infrastructure.

Over the years, major data centers around the world have implemented faster technologies to maximize their throughput, upgrading from 1G to 10, 40, and 100 G speeds to improve the efficiency of their networks. Such routine enhancements have been critical to providing Internet users with the bandwidth necessary to seamlessly browse the Internet, download data-heavy content, play online games, conduct research, store data, execute business initiatives, and socialize like never before.

Facilitating networks’ ability to operate at ultra-high frequencies, while maintaining the fidelity of incident signals are extremely reliable for high-performance modern broadband technologies. Small Form Factor Pluggable transceivers (SFP modules) and 10 Gb Small Form Factor Pluggable transceivers (XFP modules), act as the interconnect points for the fiber optic cables that carry the data signals for optical applications. This includes the ultra-high-speed data transfers conducted by data centers, which store and move information within the cloud, as well as computational research labs, in which real-world scenarios and various complex structures are modeled in super computers.

SFP and XFP modules are transceivers that utilize broadband transimpedance amplifiers (TIA) to convert signal voltages to current that powers laser diodes.

These diodes emit the light signal that is transmitted through the fiber optic cable to the receiving module, which then converts it back to an electrical signal. Data rates range from 10 up to 100 Gbps. The ultra-high data rates are split into 4 or 10 channels within a module, the combination of which supplies the overall data rate. Typical data rates for broadband TIAs in data center applications are 25 and 10 Gbps. Consequently, ultra-broadband amplifiers require components that present very low loss and high performance at frequencies greater than a given application’s rate of data transfer.

Ultra-Broadband Capacitors

Ultra-broadband capacitors have an extremely high resonance point (often ≥40 GHz) and a low insertion loss. To be useful in modern broadband applications, capacitors must be able to operate at frequencies well above 20 GHz, while also exhibiting low insertion loss with no resonance point beyond a circuit’s operating frequency.

DC blocks are used to block DC in RF lines. To block DC from an RF signal, a capacitor with a high capacitance (10 to 100 nF) is placed in series between the signal source and its destination. This requires DC blocks to be placed at the input and output of an RF amplifier. For the capacitor to successfully block DC, it must not reach resonance (the point at which ESR and impedance are at a minimum) within the span of the circuit’s operating frequency range. If the DC block’s self-resonant frequency (SRF) is equal to or less than that of the operating frequency, the capacitor will essentially look like an inductor and it will pass DC. The capacitor’s SRF must be high enough to allow it to block DC throughout the span of the circuit’s operating frequency range.

Ultra-broadband capacitors are the go-to solution to act as DC blocks in fiber optics applications, in which data rates can reach up to 100 Gbps, due to their high-resonance point (often ≥40 GHz) and low insertion loss. To achieve such high performance, capacitor manufacturers have introduced innovative modern designs that reduce parasitic inductance and loss; for example, the GX Series of ultra-broadband capacitors from AVX, which features a unique internal electrode geometry that helps lower ESL and terminations that help reduce loss, and thus also reduce noise. This unique topology achieves <0.5 dB of insertion loss out to 40 GHz. Other factors that influence the parasitic elements of a capacitor include: electrode metal composition, electrode pattern geometry, electrode count and spacing, termination metal composition, termination dimensions, case thickness, and the dielectric’s relative permittivity and loss tangent characteristics over frequency.

Ultra-broadband capacitors also provide designers working at lower frequencies with the ability to utilize a single device where several capacitors with different resonant points would have been required in the past. In low frequency applications (>12 kHz), ultra-broadband capacitors can be used as DC blocks in circuits with many different resonant points. This approach reduces cost, simplifies designs, improves performance, and simplifies BOMs to one part number for designs that would have previously required a BOM consisting of different capacitor values.

Ultra-Broadband Inductors

RF amplifiers require DC biasing to set the rails for operation. Biasing RF amplifiers requires RF signals to be blocked from the power supply pin in order to prevent noise from being incorporated into the power supply, as that would affect other parts of the circuit that depend on the DC source. Inductors are chosen to block RF, because their impedance to RF frequencies increases as the frequency through the component is increased (|Z|=2πfL), blocking RF signals while passing DC current until resonance is reached.

In a typical RF application in which operating frequencies range from 3 MHz to 6 GHz, an RF inductor with a resonant point one magnitude above the frequency of operation will sufficiently block RF. This level of performance is achieved with commodity inductors. When the frequency of operation becomes higher than an RF inductor’s self-resonant frequency, the inductor’s parasitic capacitance will take over, causing the component to act as a capacitor, passing RF to the DC power supply.

Composed of either a conical or pyramidal three-dimensional geometry with a ferrite core and relatively high inductance of several microhenries (µH), ultra-broadband inductor SRFs are well above 40 GHz, allowing them to block DC for a wide range of frequencies without the effects of parasitic capacitance.

Ultra-broadband inductors have been available as wire bondable devices with fly leads for quite some time. SMT packages have become necessary for mass production of optical transceiver modules, as SMT devices can be placed on a board more consistently using pick and place machinery, reducing both assembly cost and production time for manufacturers.

The primary challenge for ultra-broadband inductor manufacturers is to create an SMT package that will not increase the inductor’s parasitic components, as these have the potential to compromise signal integrity.

Conclusion

Due to the ever-expanding dependence on the Internet, steadily increasing ultra-broadband passives must demonstrate extremely high performance and reliability to help both sustain and improve Internet infrastructure. Ultra-broadband components will continue to evolve and become more prevalent as high-speed Internet access continues to be adopted in all worldwide regions. Such widespread and incessant growth of Internet accessibility and wireless communications crowds the electromagnetic spectrum, which forces RF designers to invent new devices capable of utilizing the unclogged, higher-frequency sections of the spectrum, and subsequently drives the need for more and better ultra-broadband active and passive devices.

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