The figure shows a biasable subharmonic mixer designed to operate in the frequency range from 520 to 600 GHz. This mixer is a prototype of low-power mixers needed for development of wide-band, high-resolution spectrometers for measuring spectra of molecules in the atmospheres of Earth, other planets, and comets in the frequency range of 400 to 700 GHz.

Three considerations dictated the main features of the design:

  • It is highly desirable to operate the spectrometers at or slightly below room temperature. This consideration is addressed by choosing Schottky diodes as the frequency-mixing circuit elements because of all mixer diodes, Schottky diodes are the best candidates for affording sufficient sensitivity at or slightly below room-temperature range.
  • The short wavelengths in the intended operating-frequency range translate to stringent requirements for precision of fabrication and assembly of the circuits; these requirements are even more stringent for wide-bandwidth circuits. This consideration is addressed in two ways: (1) As much as possible of the mixer circuitry is fabricated in the form of a monolithic integrated circuit on a GaAs membrane, employing a modified version of a process used previously to fabricate a non-subharmonic mixer for a frequency of 2.5 THz and frequency multipliers for frequencies up to 2 THz. (2) The remainder of the circuitry is precision machined into a waveguide block that holds the GaAs integrated circuit.
  • Generation of a local-oscillator (LO) signal having sufficient power to pump a mixer requires more DC power as the LO frequency increases; this is because the only wide-band LO sources available in this frequency range are Schottky-diode frequency multipliers, and their efficiencies decrease with frequency. This consideration is addressed in two ways: (1) Unlike the prior 2.5-THz GaAs-membrane mixer, this mixer is subharmonically driven, meaning that the LO operates at half the frequency of the incoming signal to be measured [denoted the radio frequency (RF) in traditional frequency mixer parlance]. (2) The diodes are arranged so that they can be biased to operate closer to their switching voltage so that less LO power is needed to switch the diodes between the conducting and nonconducting states. This switching is what makes the diodes act as a frequency mixer.

This Biasable Subharmonic Mixer for the RF range of 520 to 600 GHz consists of a monolithic integrated circuit on a GaAs membrane held in a precision-machined waveguide block.
The Schottky diodes are fabricated in an antiparallel configuration, using beam leads, such that one electrode of each diode is grounded. One diode is AC grounded through a capacitor to allow the diodes to be biased. A simple probe picks up the LO signal from a waveguide shown on the left side of the figure. The LO signal bypasses an RF filter comprised of two vertical stubs and is coupled into the mixer diodes. Similarly, another probe picks up the RF signal from a waveguide shown on the right side of the figure and the RF signal flows leftward to the diodes.

The on-chip circuitry also conveys the lower-frequency mixer output signal [also denoted, variously, as the intermediate-frequency (IF) signal or the down-converted version of the RF signal in traditional frequency-mixer parlance] to an off-chip circuit board on the right side. The stub filter to the left of the diodes prevents the leakage of the RF signal past the diodes to the LO waveguide. Leakage of the LO signal into the RF waveguide is inherently blocked as it is below the cutoff frequency of the RF waveguide. There is also a filter in the output channel, implemented as shunt capacitors (not shown here), to prevent leakage of RF and LO signals to the off-chip circuitry that processes the IF signal.

This work was done by Erich Schlecht, Peter Siegel, Imran Mehdi, John Gill, James Velebir, Alejandro Peralta, Raymond Tsang, John Oswald, and Robert Dengler of Caltech for NASA’s Jet Propulsion Laboratory.


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Biasable Subharmonic Membrane Mixer for 520 to 600 GHz

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This article first appeared in the March, 2009 issue of NASA Tech Briefs Magazine.

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