Number: 80
Date: August 3, 1994
From: Martin Levine
Multiple Wavelength Optical Transmission for the SMA
1. INTRODUCTION
Early this year, a decision was made to reconfigure the antenna siting pattern from a set of concentric circles to a set of similar circles roughly tangential to a single point near the control building. As a consequence, the vault building which stood at the center of the concentric pattern has been eliminated, and all LO reference generation and correlator equipment consolidated into the control building. These changes offer an opportunity to rethink the physical and logical layout of the Local Oscillator Reference Distribution system, and perhaps to improve upon the original design. Each Gunn oscillator is phaselocked to the sum of two reference signals. The first of these signals is an unmodulated carrier in the 6 to 8 GHz range which is common to all antennas in the array. (Since there are two independently-tuned receivers at each antenna site, two such carrier signals are generated and transmitted). The second is a phase- and frequency-modulated signal at approximately 191 MHz which is generated by an upconverted direct-digital synthesizer (DDS). The output of the basic DDS is at about 9 MHz; the 191 MHz signal is the result of a single-sideband upconversion using a common 200 MHz local oscillator. There are a total of twelve independent direct digital synthesizers, one for each active receiver.
The Gunn oscillators must be phase modulated to accommodate fringe rotation, to suppress dc offsets and crosstalk, and to separate the images from the desired sidebands. In the SMA, the fringe rotation and Walsh function switching will be applied to the DDSs as digital words. A precise time reference is needed to synchronize the phase and frequency switching of the DDS synthesizers across the array. This "heartbeat" signal operates at approximately 100 pps with stability and repeatability of the zero crossing in the order of 5 to 10 ns.
1.1 The Original Reference Distribution Concept
In the original LO reference distribution concept, two direct digital synthesizers and the associated upconverters are located in each antenna cabin. This arrangement was selected to maximize the efficiency of the transmission facilities. The two microwave signals are modulated onto a single optical carrier which serves all antennas. In addition, a 50 MHz clock signal for all DDSs is required as well as a 200 MHz local oscillator signal for all DDS upconverters. The DDS clock and DDS local oscillator signals are common to all antennas and therefore could be transmitted by electrically multiplexing the signals onto the same optical carrier which distributes the 6 to 8 GHz reference signals. This approach required no additional optical transmitters, receivers or fibers, but there was a potential intermodulation distortion problem.
Despite some obvious virtues, locating the DDS synthesizers int the antenna cabin introduces two major problem areas. The first is a severe shortage of space within the antenna enclosure and, in particular, in the antenna electronics rack. The second problem area is in the setting, maintenance and verification of the synchronization of the DDS phase across the array. Of special interest is the phase relationship between the zero crossings of the "heartbeat" and the 50 MHz DDS clock signal. Phase (and frequency) switching of the DDS is synchronous with the DDS clock; therefore, a relative time shift of the heartbeat and the clock of greater than 10 ns (assuming that the initial timing offset is optimized) will perturb the DDS output phase by one full clock cycle, or as much as 65 degrees. This change, in turn, will be directly reflected in the phase of the Gunn oscillator at 90 GHz. Although it should be possible to maintain the necessary timing stability to minimize this problem, it is not easy to positively confirm that physically separated synthesizers are operating coherently. Identifying and localizing problems in this aspect of the system could be very troublesome and time consuming.
1.2 Alternative DDS Design
It is possible to both reduce the antenna enclosure space requirements and to greatly simplify the synchronization problem by physically moving the DDS synthesizers from the antenna sites to the control building. Note that this entails a major change in design philosophy. In the original baseline design, all distributed LO reference signals are common to all antennas; those signals which are unique to a particular antenna site are generated within the antenna. The alternative design still requires two signals for the Gunn oscillator phaselock loop and the characteristics of these signals are identical to the those of the baseline design. The difference is in the point of origin of the signals; the 6 to 8 GHz carrier originates in the control building and is common to all antennas in the array. The frequency- and phase modulated 191 MHz signal is specific to a particular receiver at a particular antenna site and is generated by one of 12 dedicated DDS synthesizers in the control building.
Two options for DDS distribution in the alternative architecture have been considered:
A. Optically multiplexing one DDS signal onto each of the two fibers feeding the IF signals from the antenna sites to the control building. This approach requires twelve high-power (but relatively narrow bandwidth) optical transmitters and twelve optical directional couplers in the control building. Two low-cost optical receivers and two optical directional couplers are needed at each antenna. Note that the two DDS signals are at approximately the same frequency and must be spatially separated for transmission purposes. (It is possible to separate the two DDS signals in the frequency domain, but subtle problems arise when the corresponding receivers are operated at the same sky frequency for dual-polarization observations.)
B. Transmitting the signals over dedicated single- or multi-mode fibers in the teleme try/control cable. Again, twelve optical transmitters are needed in the control building and two low-cost optical receivers are required at each antenna.
All other things being equal, the 191 MHz signals are about 300 to 500 times less sensitive to time delay perturbations than the 6 to 8 GHz references (which must be further multiplied into the 300 to 800 GHz range). Nevertheless, this factor of 300 may be insufficient to avoid significant phase-coherency perturbations if conventional or multimode optical fiber and fiber components are used in the transmission path. Therefore, it has been decided to proceed with the more conservative option A and to share the temperature-compensated Sumitomo fibers with the IF signal paths.
2. DETAILED IMPLEMENTATION
2.1 System Description
Briefly recapitulated, the recommended LO/IF Distribution System includes:
2.1.1 Two reference signal generators (RSG) located in the control building, each in the frequency range 6 to 8 GHz, which are the primary references for the harmonic mixers in the Gunn oscillator phaselock loops. The two RSG signals are common to all antennas in the array and are transmitted by a single, wideband, high-power laser carrier over six Sumitomo fibers, one to each antenna site.
2.1.2 Two wideband laser transmitters in each antenna, one for each receiver in the antenna, modulated by one 4 to 6 GHz IF signal. Two Sumitomo IF fibers run from each antenna site to the control building.
2.1.3 Twelve wideband IF photodiode receivers colocated with the correlator first downconverters in the control building, one for each active receiver in the array. An optical patch panel is provided in the vault beneath the correlator analog room to select the fibers from the populated ring of antenna sites.
2.1.4 Twelve phase- and frequency-modulated direct digital synthesizers located in the control building; each generating a unique phase sequence at a nominal 191 MHz center frequency. A narrowband laser transmitter is associated with each DDS and the pair of outputs for a given antenna is transmitted over the same pair of fibers which carries the IF signals from the antenna to the control building. The specific technology for sharing these transmission lines is described in the following section.
2.1.5 Two narrowband DDS photodiode receivers in each antenna, one for each receiver in the antenna. The DDS receivers share the same Sumitomo fibers which carry the IF signals in the opposite direction; again, the specific technology is described below.
2.2 Wavelength Division Multiplexing
Two-way full-duplex operation on a single optical fiber is possible using directional couplers at both ends of the link to separate out the two directions of transmission. This mode of operation is analogous to full-duplex operation at RF and suffers from the same limitations; half of the (optical) power is lost at each directional coupler and the directivity of the couplers is finite, resulting in crosstalk and reflections. However, there is an option, analogous to an RF diplexer, available in fiberoptic communications which is much more difficult to implement at RF or microwave frequencies. It is relatively straightforward to fabricate optical wave division multiplexers (WDM) which separate the two directions of transmission based on the wavelength of the optical carrier. The insertion loss is relatively low, typically less than 0.5 dB, and the directivity is in the order of 50 to 55 dB. WDMs are surprisingly simple and inexpensive passive structures comprised of two or more fibers fused into a single, monolithic bundle. By careful selection of the length of the coupled fiber length the insertion loss can be precisely tailored to two wavelength windows. For single mode transmission, the two wavelength windows of greatest interest are centered at 1310 and 1550 nm; suitable transmitters, receivers and multiplexers can be readily found for both wavelengths.
Sumitomo temperature-compensated fiber, and most "standard" fibers, are optimized for transmission at 1310 nm. Optimized, in this case, refers to chromatic dispersion which goes through a zero at about 1310 nm. Attenuation is actually lower at 1550 nm than at 1310, but dispersion at the longer wavelength is excessive for long-haul, broadband communications applications. However, dispersion is not a problem for the relatively narrow frequency band centered at about 191 MHz required to transmit the upconverted DDS signals.
2.3 Control Building Implementation
It is convenient to start the system description at the Control Building IF termination. In Figure 1, the two fibers shown at the left side of the diagram originally carried only the IF signals in the band 4 to 6 GHz, modulated onto a 1310 nm optical carrier, from one antenna to the corresponding optical receivers. The only change in this path is the insertion of the AOFR, Inc WDMs which provide a sidearm for the insertion of a 1550 nm optical carrier directed towards the antenna. The 1550 nm optical transmitters will be similar to the devices widely used for analog cable television distribution; the specific model has not been selected. Each optical transmitter is modulated by the sum of two RF carriers: the 191 MHz DDS signal and a 100 MHz reference for the antenna YIG oscillator phaselock loop.
2.4 Antenna Enclosure Implementation
If we follow the IF fibers back to the originating antenna, as shown in Figure 2, they can be seen to terminate at the Antenna Optical Transmitter Box. The four signals carried by the 1550 nm carriers (two on each fiber) are stripped off at this point; the 191 MHz upconverted DDS signals going to the Gunn phaselock loops and 100 MHz references to the YIG oscillator lock loops.
2.4.1 Antenna Optical Transmitter. The internal details of the Optical Transmitter Box are shown in Figure 3. The two IF fibers from the Control Building enter the diagram on the right-hand edge. The AOFR wave division muliplexers, used in the reverse sense of the Control Building application, couple the outputs of the wideband 1310 nm IF laser transmitter to the fibers. The sidearm separates out the 1550 nm carrier travelling in the opposite direction and directs it to corresponding photodiode receiver. Again, the specific 1550 nm receiver model has not been selected, but will be similar to devices used in CATV distribution. The 100 and 191 MHz signals are amplified and separated by an RF diplexer.
The wideband IF signal processing within the Optical Transmitter Box has been described in the Design Study; only the most basic elements are shown in Figure 3.
2.4.2 Antenna Optical Receiver, The Reference Signal Generator (RSG), which is described in detail in the Design Study, is physically housed within the Optical Receiver Box. Figure 4 shows a simplified block diagram of the RSG, which is basically unaffected by the changes described in this Technical Note.
3. CONCLUSIONS
Wavelength division multiplexing appears to be a effective means to more fully utilize the optical fiber network planned for Mauna Kea without compromising performance or flexibility. The major drawback is the relatively high cost to the analog transmitters and receivers; expanded use of fiber technology in CATV distribution should sharply decrease the cost of these items within the next few years.
WDM techniques also open up other possible applications within the SMA. JPL, for example, has proposed using WDM for closed-loop phase error correction in LO reference distribution. Using two different optical wavelengths in the two directions of transmission minimizes the potential for reflection problems which can seriously degrade a conventional approach.
