Power levels in CPW circuits can be measured without packaging.
Power amplifiers comprising InP-based high-electron-mobility transistors (HEMTs) in coplanar-waveguide (CPW) circuits designed for operation at frequencies of hundreds of gigahertz, and a test set for on-wafer measurement of their power levels have been developed. These amplifiers utilize an advanced 35-nm HEMT monolithic microwave integrated-circuit (MMIC) technology and have potential utility as local- oscillator drivers and power sources in future submillimeter-wavelength heterodyne receivers and imaging systems. The test set can reduce development time by enabling rapid output power characterization, not only of these and similar amplifiers, but also of other coplanar-waveguide power circuits, without the necessity of packaging the circuits.
One of the amplifiers designed and tested at 330 GHz is shown in Figure 1. It is a three-stage unit containing one HEMT in the first stage, two HEMTs in the second stage, and four HEMTs in the third stage, with 1:2 CPW power splitters between the HEMT stages. The outputs of the third-stage HEMTs are coupled via a 4:1 CPW power combiner. Each HEMT is a two- finger device having an output periphery of 10 μm per finger, so that the total output periphery per HEMT is 20 μm. Hence, the total output periphery of all four third- stage HEMTs is 80 μm. Because the layout is so extremely compact that individual biasing of each stage cannot be accommodated, the gate and drain bias conductors of all seven transistors are tied together.
Figure 2 schematically depicts the power test set as configured for characterizing this amplifier or another device at 330 GHz at different input power levels. A Gunn oscillator generates a 110-GHz signal, which is then fed via a W-band amplifier and a variable attenuator to a frequency tripler. The output of the frequency tripler is a 330- GHz power source used as the input signal for the amplifier or other device under test (DUT). A calorimeter measures the power output of the DUT. Input and output coupling of the 330-GHz signals between the test set and the DUT is effected by means of commercially available waveguide probes of WR3 cross section (a standard rectangular cross section for a nominal frequency range of 220 to 325 GHz) with transitions to CPW contacts at one end. For measurement of the DUT-input 330-GHz power, the output of the frequency tripler is coupled directly via a waveguide to the calorimeter. For measurement of power levels used to correct for losses in the probes, a standard section of CPW is substituted for the DUT. In tests performed thus far, the amplifier exhibited an output power of 1.6 dB, with a maximum low power level gain of 7 dB.