Design of Mixed Signal Power Divider for Fiber Optic

Applications

Kwon-Seob Lim, Hyun Seo Kang

Optical Communication Research Center, ETRI, 1110-6 Oryong-dong, Buk-gu, Gwangju 500-480, Korea

kaide.lim@etri.re.kr, hskang87@etri.re.kr

Abstract: A simple monolithic power divider for digital/RF mixed signal is designed for fiber-optic

applications. The insertion and return loss between input port and RF output port are 0.2dB and 31dB at

the center frequency. Almost 30dB of isolations between digital port and RF port are achieved each other.

1. Introduction

The rapid and explosive expansion of internet and data communication has accelerated the growth of optical

communication. The future networks providing services such as telecommunication, broadcasting, and internet

access seamlessly will have to deal with the optical, RF, and digital signals in an unique platform [1]. In this

paper, a simple digital/RF mixed signal power divider is designed for hybrid fiber-optic communication

applications as shown in Fig. 1 (a). Schematic views of the proposed power divider are depicted in Fig. 1 (b).

(RF_) P3 P2 RFDigital)

IEEE802.11 a IEEE802.11 aWLAN (5.6GHz) WLAN (5.6GHz 1 (gitai)

Top view

PRBS NRZ PRBS NRZ

(up to 4.25Gbps) (up to 4.25Gbps)Bottomn view

(a) (b)Fig. 1 (a) Diagram of digital/RF mixed signal fiber optic system and (b) schematic views of divider

2. Design Considerations and Performances

A coupled line directional coupler has excellent directivity and isolation properties as long as the impedance

matching condition is satisfied. But this type of coupler has a weak coupling property. To achieve the tight

coupling requires lines that are too close together to be practical. In order to overcome the weak coupling of

conventional coupled line coupler, we modified the conventional single-section coupled line coupler using a

coplanar waveguide (CPW) resonator which is formed on the bottom side of the substrate [2]. The CPW

resonator is a quarter-wavelength long at the center frequency and coupled to two microstrip open-circuited stubs

on the other side of a common substrate. The mixed signal power divider is comprised of the modified coupled

line coupler for the RF signal and the 9th order Bessel low pass filter (LPF) for the broadband digital signal. The

divider is optimized for a gigabit-Ethernet signal (up to 4.25Gbps, PRBS) and the IEEE802.11 a WLAN signal

(5.6GHz) directly and constructed on a single dielectric substrate which has a relative dielectric constant of 3.0

and a thickness of 0.5mm using a 3D electro-magnetic field solver [3]. The designed divider has properties of

band pass filter itself that can transmit the whole band of IEEE802.11 a WLAN signal as shown in Fig. 2 (a). We

obtained very low insertion loss for RF signal and excellent isolation characteristics between two output ports.

The insertion and return loss between input port and RF output port are 0.2dB and 31dB at the center frequency

respectively. The Bessel LPF has not only a good group delay response but also makes a role to reflect the RF

signal, which leads to almost 30dB of isolations between digital port and RF port are achieved each other. The

clear eye diagram of 4.25Gbps PRBS NRZ signal at the digital output port is depicted in Fig. 2 (b).

0 1.2

-10 s < 1.0

-30

Sm432 X S12 -_2-5 0 2 \/0.26X-50~~~~~~~~~~~~~~~~~~0

-70 {).2-60 0.0 ______

0 2 4 6 8 10 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45Freq., GHz Time, ns

(a) (b)Fig. 2. Simulation results for the mixed signal power divider. (a) S-parameters (b) Eye diagram of 4.25Gbps

3. Conclusion

In this paper, a simple monolithic digital/RF mixed signal power divider is designed for hybrid fiber-optic

communication applications. The insertion and return loss between input port and RF output port are 0.2dB and

31dB at the center frequency respectively. Almost 30dB of isolations between digital port and RF port are

achieved each other. The clear eye-opening for digital signal of 4.25Gbps is also obtained. This proposed divider

is easily integrated on any kind of platform monolithically because it does not have any via and can be also used

as mixed signal power combiner as well without any degradation.

4. References

[1] Rao R. Tummala et. al., "The SOP for miniaturized, Mixed-Signal Computing, Communication, and Consumer Systems of the Next

Decade," IEEE Transaction on Advanced Packaging, vol. 27, No. 2, pp. 250-267 (2004).

[2] Neil Thomson, and Jia-Sheng Hong, "Compact Ultra-Wideband Microstrip/Coplanar Waveguide Bandpass Filter," IEEE Microwave and

Wireless Components Letters, vol. 17, No. 3, pp. 184-186 ( 2007).

[3] Computer Simulation Technology, Inc., "Microwave Studio 2006B"