Quick viewing(Text Mode)

This Self-Contained, Wide-Bandwidth Fiber-Optic Link Actually Delivers Signal Gain While Maintaining Low System Noise Figure

This Self-Contained, Wide-Bandwidth Fiber-Optic Link Actually Delivers Signal Gain While Maintaining Low System Noise Figure

PERIODICALS POSTAGE PAID A PENTON PUBLICATION APRIL 2000 USPS/BPC POSTAL-PAK 881 Approved Poly FOR DESIGNERS AT HIGHER FREQUENCIES

Communications Issue

NEWS DESIGN FEATURE PRODUCT Scanning trends in Practical test methods Diminutive OCXOs fit for CDMA handsets surface-mount designs

Fiber-opticFiber-optic linklink shinesshines atat 1111 GHzGHz

Visit us at: www.mwrf.com

Reprinted with permission, from & RF, April 2000 by Reprint Services. 207711 For reorders call Reprint Services at 651-582-3800. For subscription information, call 201-393-6060. COVER FEATURE

Fiber-Optic Link Connects 10-GHz Signals This self-contained, wide- fiber-optic link actually delivers signal gain while maintaining low system noise figure.

David Krautheimer Director of Marketing and Sales Mustafa Syed and Daniel Sundberg MITEQ, Inc., 100 Davids Dr., Hauppauge, NY 11788-2086; (631) 436-7400, : (631) 436-7430, : http://www.miteq.com.

IBER-OPTIC technology has improved steadily in recent years to small volume becomes even more where optical links are now viable replacements for coaxial lines attractive. An offers even at bandwidths as wide as 10 GHz. The MDD series from MITEQ, several other advantages: FInc. (Hauppauge, NY) is such a family of high-speed, high-bandwidth 1. It is virtually immune to elec- links, offering a 3-dB bandwidth of 1 to 11 GHz with low noise figure and tromagnetic interference (EMI) and generous link gain over single-mode fiber-optic cables. The links feature humidity, which are of paramount 50- input (I) and output (O) interfaces for ease of connection to RF and importance especially in the con- systems. fines of a ship or an aircraft. 2. It is not very susceptible to in- System designers who need to Translating this into a specific ap- terference from . transmit RF and microwave signals plication means that a system de- 3. It is a relatively secure trans- over a distance of more than a few signer can capitalize on these at- mission media. hundred meters will appreciate the tractive features and transmit a A typical application for a fiber- advantages that a fiber-optic link very large volume of data, voice, optic link, which emphasizes the var- has to offer versus a conventional and video signals over a long dis- ious advantages of fiber-optic com- cable transmission. Typically, a sin- tance without regeneration. munications, is in the use of a remote gle-mode has less than With the increased use of com- . An antenna site may con- 0.5-dB per kilometer plex electronic systems on aircraft sist of several receiver antennas and is capable of carrying light sig- and ships where weight and space spaced some distance from the cen- nals that are direct amplitude mod- are at a premium, an optical-fiber tral station. Due to the relatively ulated (AM) to tens of gigahertz. system with its lightweight and high loss of coaxial copper (Cu) ca- ble, it is often necessary to construct an equipment bay station directly • bias loop • bias adjacent to an antenna, which is im- • -control loop • Voltage regulation • Voltage regulation practical in the case of multiple an- RF RF tennas. Low-noise amplifiers input output (LNAs), frequency downconverters, Laser and other types of equipment in the diode LNA LNA Trans- Trans- bay station are required in order to (preamp- impedance Isolator Optical fiber impedance lifier) MMIC Photodiode MMIC process the signal so as to allow the user to operate in the presence of a high-loss . Assuming 3- Receiver dB attenuation per 30 m of cable for a polystyrene coaxial cable at C- 1. The MDD fiber-optic links consist of direct-modulated receiver and band frequencies, even a 50-dB-gain transmitter modules with simple 50- input and output connections. LNA at the front end would lose its COVER FEATURE

than their coaxial counterparts. and a receiver assembly that employ THE LOW-NOISE AND LOW- Other typical applications may in- direct AM. In this design, the bias clude phased-array antennas, delay current of the laser is modulated by TRANSMISSION-LOSS lines, conformal antennas, and point- the incoming microwave signal that, to-point links between facilities. in turn, modulates the amplitude of CHARACTERISTICS OF THE The MDD series of self-contained the laser output. When compared to MDD SERIES OF FIBER- fiber-optic links from MITEQ is a system using an external modula- ideal for many of these tor, the direct- technique OPTIC LINKS MAKES IT applications. They provide for RF- has lower cost and lower optical to-RF connections (Fig. 1) with a 3- insertion loss. WELL-SUITED FOR WIDE- dB bandwidth from 1 to 11 GHz Integrating MITEQ’s wideband BAND POINT-TO-POINT (Fig. 2), noise figures as low as 15 dB LNA technology into the optical (Fig. 3), and overall link gain of typi- transmitter, the firm’s engineers COMMUNICATIONS cally 10 dB. These features along have been able to achieve a signifi- with its small size and ability to be cant improvement in the noise fig- SYSTEMS. easily inserted into a system make ure (see table) compared to similar this product an ideal replacement for types of products. In addition, the effectiveness after a few hundred its coax counterpart. The low-noise MDD series of transmitter/receiver meters. With optical fiber, the atten- and low-transmission-loss charac- modules can be specially configured uation and the attenuation slope (not teristics of the MDD series make it to match specific applications (i.e., to mention shifts in phase and well-suited for wideband point-to- with lower noise figures, higher in- delay), which is quite evident in co- point (Fig. 4). tercept points, etc.). axial cable, is almost nonexistent. By The MDD link operates at a wave- It should be noted that many di- performing an electrical-to-optical length of 1550 nm with peak-to-peak rect-modulation fiber-optic links

15 25

12 20

9 dB dB — 15 —

Gain 6 10 Noise figure

3 5

0 1234567891011 0 1234567891011 Frequency—GHz Frequency—GHz

2. The 3-dB bandwidth of the MDD fiber-optic links is 1 3. The noise figure of the MDD fiber-optic links is 14 dB to 11 GHz. at 4 GHz. conversion close to the feed of the group delay of only 0.1 ns for a 1-km suffer from high insertion loss and antenna, the signal can be transmit- link distance. The transmitter limited dynamic range. This is in ted a few kilometers over fiber with- achieves optical output power of 4 to part due to the large impedance out significant attenuation. This 9 mW with power-supply require- mismatch between the approximate technique allows all signal-process- ments of 270 mA at +10 VDC, 12 mA 10- impedance of a and ing equipment to be located in a at 10 VDC, and 750 mA (maxi- the typical 50- impedance of a mi- single facility that is virtually inde- mum) at +4 VDC. The receiver crowave system. To improve the pendent of its geographic location. portion of the link requires 150-mA dynamic range of the link, a gal- Fiber-optic links also simplify the current at +10 VDC and 10-mA cur- lium-arsenide (GaAs) monolithic- logistics of building a large earth sta- rent at 10 VDC. The link, which is microwave-integrated-circuit tion, which have multiple antennas designed for operating tempera- (MMIC) transimpedance amplifier that have to be well-separated in or- tures of 30 to +60°C, features a was developed by Thomson-CSF der to avoid side-lobe interference. nominal output third-order intercept Detexis (Paris, France) in order to Fiber-optic links are easy to install point (IP3) of +7 dBm at 4 GHz. optimally match the RF system and are less expensive to maintain The link consists of a transmitter with the laser diode. COVER FEATURE

The normally diverging optical output of the laser diode is first colli- The MDD fiber-optic link at a glance mated to pass through an optical iso- Operating 1550 nm lator and then focused into the fiber 3-dB bandwidth 10 GHz (1 to 11 GHz) . The optical alignment and con- nectorization of the modules is per- Input/output transfer characteristics 10 dB (nominal) formed by Diamond S.A. using their Noise figure 14 dB (nominal) at 4 GHz* E-2000 plug-in optical connector. To make the MDD link easy to Output third-order intercept 7 dB (nominal) at 4 GHz** use, the modules have been de- Group delay (peak-to-peak) 0.1 ns (nominal) signed to be self-contained. The transmitter module has built-in Input VSWR 1.25:1 (nominal) thermal stabilization using a ther- Output VSWR 1.25:1 (nominal) moelectric cooler (TEC). This cooler maintains the laser diode at a con- Input/output impedance 50 stant temperature by using feed- Transmitter optical power 4 to 9 mW back from a thermistor that is on the Nominal DC power Transmitter *** laser carrier. Laser-diode thermal 10 VDC, 270 mA stabilization helps minimize laser 10 VDC, 12 mA wavelength drifting. A good heat 4 VDC, 750 mA (maximum) sink is necessary for the transmitter Receiver module to allow the TEC to shunt 10 VDC, 150 mA the heat effectively. 10 VDC, 10 mA In addition, a second feedback loop Optical input/output connectors DIAMOND E-2000 in the control circuit senses the out- put of a photodiode that is located at RF input/output connectors SMA female the back facet of the laser to main- Physical dimensions 1.43 2.47 0.48 in. (3.63 6.27 1.22 cm) tain the laser optical power at a con- Operating temperature 30 to 60¡C stant level through the laser aging process. Both of the modules are Storage temperature 54 to 125¡C provided with built-in regulators, re- verse voltage polarity protection, * Lower noise figure is available. ** Higher IP3 units are achievable. and they are available in a hermeti- *** Transmitter requires heat sinking. cally sealed package. Fiber-optic links are often associ- dia for analog communications sys- dle the transport of complex multi- ated with digital communications tems. Analog fiber-optic links have channel signals. More recently, and fast data rates, although they traditionally been used in cable- these links are widely used within are also powerful transmission me- (CATV) systems to han- buildings and office campus envi- ronments to distribute cellular sig- 40 nals without the deleterious effects of phase , signal loss, and 20 Third-order intercept point +7 dBm EMI. Due to the minimal group de- lay of fiber-optic links (nominally 0 0.1 ns peak-to-peak for the MDD se- ries) and low loss, in-building cellu- Fundamental 10-dB gain slope lar signals carried by optical cables dBm –20 do not require the additional equal- — ization and gain commonly used in –40 coaxial lines. The link is designed for analog –60 Third-order intermodulation slope

Output power applications to 11 GHz. It offers all of the advantages of fiber-optic Output-power –80 noise level –89 dBm technology, with simple 50- inter- faces for ease of –100 with existing microwave systems. –100 –90 –80 –70 –60 –50 –40 –30 –20 –100 10 20 MITEQ, Inc., 100 Davids Dr., Input power—dBm Hauppauge, NY 11788-2086; (631) 486-7400, FAX: (631) 4. The wide dynamic range of the MDD fiber-optic link derives from MITEQ’s 436-7430, Internet: http:// strong background in LNA design. www.miteq.com.

Top View