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Medical imaging

Terahertz Voltage-Controlled Oscillator Incorporates Coupled-Line Topology

Nov. 3, 2015
This common-emitter cross-coupled push-push VCO has been developed with InP technology.

The 300-GHz frequency band has attracted growing interest for applications, such as terahertz (THz) broadband communications and THz imaging. Because an oscillator is a key component in a THz system, THz-band oscillators have received an increasing amount of attention. In recent times, these oscillators have been achieved with Colpitts or common-base coupling topologies. By implementing a new coupled-line structure based on the common-emitter cross-coupled topology, a group of researchers from the School of Electrical Engineering, Korea University have developed a new THz-band voltage-controlled oscillator (VCO). The design was achieved with 0.25-μm indium-phosphide (InP) heterojunction-bipolar-transistor (HBT) technology.

Conventional common-emitter cross-coupled push-push oscillators employ direct-current (DC) blocking capacitors in the feedback path. Metal-oxide-metal (MIM) capacitors are often employed for these applications. But these capacitors also have a parasitic series inductance. In the proposed design, the capacitors are replaced with a pair of coupled lines. Thus, the parasitic inductance from the MIM capacitors is eliminated and the oscillation frequency is increased. The coupled-line implementation also enables the circuit’s size to be reduced. It should also be noted that the oscillation frequency is tuned by varying the base bias voltage in the proposed design.

Two measurement setups were used to characterize the oscillator. A spectrum analyzer performed the task of measuring the oscillation frequency and the phase noise. The oscillation frequency was tuned from 309.5 to 339.5 GHz by varying VBE from 1.0 to 1.4 V. When the oscillation frequency was tuned to 317.5 GHz, the measured phase noise was -86.55 dBc/Hz at 10 MHz offset. Secondly, a power meter was used to measure the oscillator’s output power. This was measured over the entire VBE tuning range from 1.0 to 1.4 V. The results demonstrated a maximum output power level of -6.5 dBm at 334 GHz. In addition, the oscillator’s power dissipation was 13.5 mW.

See “A 310-340-GHz Coupled-Line Voltage-Controlled Oscillator Based on 0.25-μm InP HBT Technology,” IEEE Transactions on Terahertz Science and Technology, July 2015, p. 652.

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