A circuit that can function as a phase switch, frequency mixer, or frequency multiplier operates over a broad frequency range in the vicinity of 100 GHz. Among the most notable features of this circuit is a grounded uniplanar transition (in effect, a balun) between a slot line and one of two coplanar waveguides (CPWs). The design of this circuit is well suited to integration of the circuit into a microwave monolithic integrated circuit (MMIC) package.
One CPW is located at the input end and one at the output end of the top side of a substrate on which the circuit is fabricated (see Figure 1). The input CPW feeds the input signal to antiparallel flip-chip Schottky diodes connected to the edges of the slot line. Phase switching is effected by the combination of (1) the abrupt transition from the input CPW to the slot line and (2) CPW ground tuning effected by switching of the bias on the diodes.
Grounding of the slot metal to the bottom metal gives rise to a frequency cutoff in the slot. This cutoff is valuable for separating different frequency components when the circuit is used as a mixer or multiplier.
Proceeding along the slot line toward the output end, one encounters the aforementioned transition, which couples the slot line to the output CPW. Impedance tuning of the transition is accomplished by use of a high-impedance section immediately before the transition.
The uniplanarity of this transition is key to the utility of the circuit because it enables the top-side placement of all components [except via holes and the metal (ground plane) on the bottom side of the substrate], thereby making the circuit compatible with other circuits, components, and processes used to fabricate MMICs.
Figure 2 shows some results of tests of the circuit as a phase switch. Referring to the upper part of this figure, the pronounced increase in insertion loss at 91 GHz is attributed to a resonance of a substrate electromagnetic mode associated with the via holes. Subsequent modification of the via holes changed the frequency of (but did not eliminate) this resonance, making it possible to limit the insertion loss to approximately 5 dB only over the frequency range from 90 to 110 GHz. As shown in the lower part of Figure 2, the phase difference introduced by the switch remained within ±10° of the desired value of 180° over all but a small lower-edge portion of the frequency band from 90 to 110 GHz.
In a test of its performance as a balanced fundamental-frequency mixer, the circuit operated at a 12-dB conversion loss, with an intermediate-frequency bandwidth of 25 GHz, at input frequencies from 78 to 110 GHz. This mixer performance is comparable to that of commercially available waveguide mixers, which, unlike this circuit, are not compatible with MMICs.
This work was done by Todd Gaier, Mary Wells, and Douglas Dawson of Caltech for NASA’s Jet Propulsion Laboratory.