50-to-110-GHz high-performance millimeter-wave source modules

Hewlett-Packard Journal, April, 1991 by Mohamed M. Sayed, Giovonnae F. Anderson

This problem is overcome by changing the substrate from 0.010-inch sapphire ([epsilon][.sub.r]= 9.6) to 0.010-inch fused silica ([epsilon][.sub.r] = 3.2) and reducing the height of the waveguide. The choice of material and substrate thickness is a compromise between the need to suppress the LSE modes, the impedance requirements for achieving the proper input match to the diodes, and the feasibility of fabricating the thin-film capacitors necessary for diode biasing. The waveguide height is reduced from 0.074 inch to 0.070 inch to ensure that the cutoff frequency is above-band. The doubling device for both the R-band and the V-band doublers is a 40-[micro]m[.sup.2] Schottky diode on an n liquid phase epitaxy layer on a GaAs substrate.

Placing the dielectric circuit into the waveguide is mechanically difficult. For this reason, a split block approach is employed. As shown in Fig. 14, the waveguide housing is in two parts, and one part includes machined cavities above and below the waveguide. These cavities are one quarter wavelength (at midband) from the waveguide and filled with polyiron. The polyiron ensures that no modes can exist in the cavities. The cavities themselves present an open circuit, which transforms to a short circuit at the edge of the waveguide.

R-Band Amplifier Doubler

The input signal of 12.5 GHz to 18.75 GHz (maximum input power = 15 dBm) is first amplified by an MMIC (microwave monolithic integrated circuit) amplifier. This amplifier is a reactively matched two-stage MESFET amplifier designed to give 22-dBm output power from 12 GHz to 20 GHz. The amplifier roll-off below 12 GHz provides rejection for the lower harmonics coming from the source. Like many amplifiers, this one had a tendency to oscillate if the input and bias leads were of any significant length. Therefore, several large monoblock capacitors were added around the circuit to keep it from oscillating. The multiplier operates on the previously discussed frequency doubling principle. The circuit is realized on a 0.010-inch sapphire substrate. The frequency-doubled signal propagates down the slotline and through a transition (transformer) into microstrip line. The slotline-to-microstrip transition and its model are shown in Fig. 15. It is formed by overlapping the slotline and the microstrip line.

Amplifier Multiplier Performance

Typical output power performance data for the W-band amplifier tripler, the V-band amplifier doubler, and the R-band amplifier doubler is shown in Fig. 16. The different technologies, devices, and thin-film parameters used in component development for the HP 83557A and HP 83558A millimeter-wave source modules are shown in Table III.

Source Module System Evaluation

Once the individual components were developed and met their specifications, the source module was assembled with the component bias board and the source interface board. A few challenging issues were discovered and resolved by the project team for the source and the system to meet specifications.