A MMIC-Based Transceiver for Satellite Ground-Terminal Applications: Part I
By R.W. Alm, Ph.D; R.E. Adams; N.A. Luque and D.J. Donoghue; <%=company%>
Contents
Electrical design considerations
Mechanical design considerations
As the twentieth century concludes, the global telecommunications industry finds itself involved in a "space race" of extraordinary proportions. Efforts to deploy a variety of satellite data communications systems are now manifesting themselves as the driving forces behind technology development, cost-effective design and strategic planning throughout the telecommunications industry as a whole.
A variety of satellite communications systems are currently being deployed with many others in development. Geostationary (GSO) systems such as SES Astra in Europe and Hughes Spaceway in the U.S.; medium Earth orbit (MEO) systems, and Global low Earth orbit (LEO) systems like Teledesic and Skybridge will all require ground terminals in the allocated K and Ka-Band frequency bands. In addition, terrestrial based LMDS/LMCS systems will require much of the same technology for both base-station as well as individual subscriber terminals.
A 19/29 GHz dual-channel transceiver prototype module aimed at the ground-terminal requirements for any number of currently planned K/Ka-Band satellite data communication systems has been designed. Built as a proof-of-concept vehicle to prove the viability of an MMIC-based approach, this module is designed to evaluate a variety of low-cost packaging techniques at mm-wave frequencies, and to demonstrate a "design for manufacturability" development approach. The goal was to demonstrate a functional transceiver module that would be compatible with high volume, low cost manufacturing methods.
Electrical design considerations
Table 1 summarizes the design choices that were made in defining the prototype dual-channel transceiver module. The concept outlined in Table 1 provides for a module that demonstrates as many technology points as possible as applicable to both terrestrial and ground-terminal applications. As a single mechanical module with dual RF I/Os for the RF and IF signals, the assumption is that the prototype module could be evaluated in one or two antenna systems by "plumbing" the necessary RF paths to the antennas.
Figure 1 shows the block-diagram of the dual-channel transmitter that was realized. The internal Dielectric Resonator Oscillator (DRO) provides an LO frequency of 13.5 GHz for the sub-harmonically pumped n=2 up-conversion mixer. The sub-harmonically pumped mixer is attractive in that it allows the entire LO chain to be constructed using low-cost plastic surface-mount components, and there is no need for a frequency doubler, a post-doubler filter or a post-doubler gain stage.
The post-mixer filter is required to remove the adjacent conversion "lines" separated by only 1.45 GHz at the low-end of the band. For purposes of the prototype, a simple edge-coupled filter was realized on a separate Alumina substrate, but a final design would require more elements to bring the adjacent conversion products into compliance with ETSI and/or FCC requirements.
A dual-conversion approach was studied which would help to alleviate the filtering requirements, but the added complexity of the additional mixers, gain stages and filters made the approach undesirable from a cost standpoint. The goal was to provide 1-Watt of output power with a nominal -30 dBm input IF level. This translates to 60 dB of transmit channel gain as shown in Table I.
Figure 2 shows a similar block diagram for the dual-channel 19 GHz receiver portion of the transceiver module. A separate DRO is included, which again is used with a sub-harmonically pumped down-conversion mixer for the same reasons listed above. In the case of the receiver, the filtering requirement is simpler, but the separate Alumina filter is required to remove the image frequency. The goal was to provide 45 dB of gain in the receive path with an applied input level of -75 dBm as shown in Table I.
Mechanical design considerations
The three main goals of the mechanical design approach include:
- The physical separation of the transmit and receive functions to maximize T/R isolation.
- Incorporation of a soft-substrate approach that would utilize a single-substrate for an entire transmitter or receiver function.
- To create a design that demonstrates compatibility with high-volume, low-cost manufacturing techniques.
To these ends, the approach shown in Figure 3 was adopted.
Dual-channel transmit and receive functions were realized in two essentially separate modules which can function separately or joined to form a single module as shown in Figure 3. The unique feature of this design is that all RF electronics are placed on a single soft-substrate circuit board which is attached via sheet epoxy to the back-side of the "cover" plate, as shown in Figure 4.
NOTE: Figure 4 shows the transmitter side of the module. Although the receiver function on the opposite side is not shown, the circuitry and mechanical packaging is realized in an almost identical manner.
The "body" of each module contains the DC bias circuitry (underneath) and the compartment walls that provide needed isolation for circuit elements on each side, also as shown in Figure 4. This strictly planar RF assembly eliminates walls during the assembly process, and allows for easy assembly of all "pick-and-place" surface mount components —a proven high-volume manufacturing technique.
Also visible in Figure 4 is the local oscillator (DRO) and it's associated cavity. The RF waveguide launches are printed on the substrate, and the ground plane is removed behind the launch to form a dielectric window launch, which also seals the module at the waveguide port. The alumina filters are seen to be epoxied into place, and are large as a variety of designs were evaluated. In the "body" of the module, Eccosorb patches are seen above the 60 dB of RF gain provided by three cascaded MMIC amplifiers, and adjustable waveguide backshorts can be seen for tuning the waveguide launches.
About the authors:
You can reach the authors at Raytheon RF Components, 362 Lowell St., Andover, MA 01810; Phone: 978-470-9450; Fax: 978-470-9891; e-mail: Roberto_W_Alm@me.raytheon.com.