An X-band, High Power, MMIC-based Microwave Amplifier
Isle of Wight, UK
When established performance ranges are extended, it is often a simple case of developing and building on the original technology. Not so for the AS 80110/30 microwave amplifier, which has been produced to extend the frequency range of the Series 2000 broadband Class A power amplifiers to supply 30 W of saturated output power between 8 and 11 GHz. Its development marks a sea change in technology, for while existing models in the series utilize discrete transistors in order to achieve the higher frequencies, the new X-band, high power amplifier offers a MMIC-based solution.
Forming the basis of the amplifier are 5 W, GaAs MMIC amplifier devices, with each bare die mounted into a module assembly that is tested and treated as a plug-in block. The module itself, shown in Figure 1 , is machined aluminum. Inside, the MMIC die is eutectic glue bonded to moly-copper shims, which, in turn, are soldered down to the gold-plated block. 50 W tracks fabricated onto alumina tiles route the RF signals to the SMA connectors.
Fig.1 The 5 W module assembly.
Bias connections enter the block via feed-through filters and connect directly to small PCBs mounted inside the inner edge that contain decoupling capacitors. The PCB connections provide wire-bonding points for the MMIC supply connections. Eight MMIC amplifier modules are arranged as four balanced amplifiers in parallel to achieve 30 W saturated power over the frequency range.
Each module has independent bias circuitry consisting of sequencing circuitry, negative voltage generator and drain voltage regulator. The complete RF system is protected against overheating via temperature sensing of the heat sink and the current is set by adjusting the gate voltage.
A critical component is the divider/combiner with the two main design criteria being to achieve low loss and phase matching. For the former it is crucial that the substrate material has a low loss tangent and low dielectric constant. Ideally the combiner should be air spaced. This is not practical at such high frequencies, however, so the circuit features are created in microstrip.
The material thickness is also important. At X-band the quarter-wavelength is approximately 5 mm in low dielectric constant materials, so the 50 W track widths must be 1 mm or less to avoid moding effects in the tracks, which determines the substrate thickness. The substrate selected was Metclad polytetrafluoroethylene (PTFE)-based with a dielectric constant of 2.45 and substrate thickness of 15 mm. The distance from each input to the combined output is critical to the overall combiner loss. Previous combiners used physically parallel inputs, but this results in long signal paths, so the X-band amplifier had to improve on this, as the losses would be unacceptable at high frequencies. Figure 2 shows the combiner layout.
Fig. 2 The combiner layout.
To sum the power from eight amplifier modules, the signal is first divided into eight paths, each one amplified and summed together in the combiner. The phase change in each path is matched exactly as any differential results in power lost in the combiner load resistors. The amplifier modules are combined using a quadrature-balanced topology in order to produce amplifier blocks with good input and output matching and low distortion. The 90° couplers are implemented as three-stage branchlines providing the required bandwidth.
The output coupler provides sampled ports for monitoring of forward and reverse output power. One pair of ports is used to drive bar graph displays, the other provided as front panel RF outputs for user monitoring.
Mechanically, the divider combiner assembly is stacked vertically, with the MMIC module outputs connected directly to the combiner inputs. The divider outputs, which are directly above the associated combiner inputs, are routed to the MMIC inputs via semi-rigid cable assemblies. A specific benefit of the design approach is that by using MMIC technology and combining eight 5 W MMICs, a very flat output power (signal gain flatness is typically ±2 dB or ±4 dB maximum) is achieved across the entire bandwidth. The alternative would be to use internally matched transistors that have narrow bandwidths. Also, a gain of 45 dB permits the amplifier to be driven from standard signal sources.
Fig. 3 The complete RF assembly.
The complete RF assembly is shown in Figure 3 . Modularity of design, which is useful for maintenance and upgrading, is a feature of the entire Series 2000, and so the AS 80110/30 amplifier has shared features with the other models. These include output power monitoring and built-in test (BIT) capability, together with the facility to be controlled either directly or by the use of a GPIB/ RS232 extension box.
An important feature is the addition of a safety interlock that permits inclusion of the amplifier in safety circuits. Under conditions when a measurement has been set up and the amplifier is to be operated remotely, a software feature enables the front panel to be locked out.
Physically the unit fits into a 19-inch rack or a bench case with the facility to have connectors either on the front panel or rear panel and the option of having RF and DC sample ports. The front panel, shown in Figure 4 , has provision for up to four N-type and four SMA connectors while the rear panel will accommodate two N-type and two SMA connectors.
Fig. 4 The AS 80110/30 amplifier's front panel.
This X-band amplifier has been developed specifically to meet the needs of the medical, communications, laboratory and defense communities. For the medical market, for example, the broader bandwidth means that for experiments to examine how tissues react to different frequencies the power provided by the X-band unit over the full bandwidth (Figure 5 ) means only one amplifier is required. For laboratories carrying out immunity testing it is possible to reduce the gain necessary from traveling-wave tubes because they can be driven with higher powers. Also, it has applications for checking amplifier performance in the development of transistors. In the telecommunications and defense fields the unit will cover the communications band at 7.9 to 8.4 GHz and the radar band at 9 GHz.
Fig. 5 Output power vs. frequency.
The new microwave amplifier's extended frequency range opens up its use for a large number of applications. On the outside the AS 80110/ 30 amplifier looks the same as the other models in the series, with identical features. However, it is inside the unit and the use of MMIC technology where the significant difference lies. The result is an X-band amplifier that not only performs the standard functions expected but also offers increased levels of performance and functionality that open up new avenues of operation. The standard product is 30 W at 8 to 11 GHz. However, it is scaleable, so that for specific applications, 50, 10 or 5 W models can be produced in the same housing.
Milmega Ltd. Isle of Wight, UK +44 (0) 1983 618000
Circle No. 301