Wideband IF-Integrated Terahertz HEB Mixers: Modeling and Characterization

This paper presents a detailed study of the IF properties of phonon-cooled NbN hot electron bolometer (HEB) terahertz mixers and receivers, performed over the widest intermediate frequency range reported so far. First, a broadband technique is used to obtain the IF small-signal output impedance of HEB mixers using network analyzer measurements. The impedance is measured over an IF range of 300 kHz to 8.5 GHz under the presence of dc bias and local oscillator (LO) illumination. The LO frequencies (
f

_LO
) used in this investigation range from 694 GHz to nearly 2 THz, covering frequencies below and above the superconducting bandgap frequency of the NbN film for the first time. The impedance (real and imaginary parts) can be fitted to the Standard model for HEB devices over almost the entire range of frequencies measured. The impedance data are then used in conjunction with a computer-aided design model for the monolithic micorwave integrated circuit IF amplifier to design and analyze the performance of integrated receivers (mixer/IF amplifier combinations). The measured and predicted variations of the receiver noise, mixer gain, and mixer output noise with IF are in good agreement. A record IF noise bandwidth of 8 GHz (obtained for
f

_LO
= 694 GHz) is demonstrated for a receiver designed using the technique described in this paper.