TeraHertz Schottky-Diode Balanced Mixers

We report on the first THz balanced mixer/upconverter using a Schottky diode MMIC chip. Using an optically pumped laser at 1562 GHz as an LO source with a coupled power of about 1 mW, and 1 mW input at an IF frequency of 10 GHz, we obtained a sideband output power of 23 uW (sum of two sidebands). As a mixer, at an LO of 1621 GHz, we obtain a conversion loss of 12.4 dB DSB and a noise temperature of 5600 K DSB. Response is believed to be similar over a band 1250-1650 GHz. New diodes have been designed for easier application as mixers up through 3 THz, and a new wafer run is in process. INTRODUCTION In the THz range there is a need for room temperature mixers to act as both downconverters and upconverters using laser local oscillators. In these applications the relatively high LO power required by Schottky diodes is not a problem. Such mixers primarily use whisker contacted Schottky diodes in cube-corner mixers, originally reported 30 years ago . While the cube-corner mixer is very simple, it has a very poor beam pattern and is compatible only with whiskered diodes. This paper reports on the first THz balanced mixer built using a planar diode MMIC in waveguide. This development was motivated by a need for a frequency agile sideband generator at 1.5 THz with >10 μW output power which would use a laser as one input and a 5-40 GHz microwave synthesizer as the other. The typical cube corner in this application can produce only 1 μW and impedance matching is difficult with high offset frequencies. MIXER DESIGN The application was a proof of concept, so it had to use existing devices. The MMIC used in this work was designed as a frequency doubler for 1.5 THz, developed at JPL for the Herschel HIFI program. THz varactor diodes are essentially the same as mixer diodes since high doping (5x10/cm) is needed to minimize carrier velocity saturation losses, and breakdown voltage is low (~4V) since large drive voltage amplitude is not practical. The device is built on 3 μm thick GaAs with beam lead contacts for ground and bias. The doubler circuit geometry is the same as that for a balanced mixer as shown in Figure 1, except that there is no external terminal for the IF port. The IF port was added by contacting the input waveguide probe with a wire which then is brought to an external coaxial port. A balanced mixer has the advantage of separate LO and signal ports, meaning that no diplexer is required at the mixer input. With good symmetry there is high isolation between these ports. The doubler chip as used in the mixer is shown in Figure 2. In this mixer circuit, the coaxial input line of Fig 1 transitions into a waveguide probe in the LO waveguide. Fig 1. Schematic diagram of a balanced mixer with coaxial input on the right and waveguide output on the left. The doubler circuit is identical except that the input frequency is half of the output. Terahertz Technology and Applications II, edited by Kurt 9. Linden, Laurence P. Sadwick, CrAidhe M. M. ODSullivan, Proc. of SPIE Hol. 7215, 721508 O P 2009 SPIE O CCC code: 0277-786U/09/W18 O doi: 10.1117/12.807505 Proc. of SPIE Hol. 7215 721508-1