VNA Measurements for Terahertz Wireless Systems

Overview:

As demands for wireless data increase exponentially, the Terahertz (THz) band has become a target for future allocation for system architectures that will have data throughput on the order of Tbps. To assess performance of THz transmitted and received signals covering distances on the order of 5-10m, vector network analyzers (VNA) are normally used with probes running to the transmitter and receive frequency extenders. Transporting VNA frequencies up to 40 GHz is prohibitive over copper coaxial cabling for longer distance because of extensive losses. Active fiberoptics can transport the 40 GHz RF signals over much longer distances, from 100s of meters to even 10s of kilometers if necessary.

“Our group conducted the first long-distance double-directional THz channel measurements, however, our experience made it clear that we should switch to integrated RFoF for higher performance. After considering a number of RFoF products, we selected OZ based on their lower NF and amplifier customizations. We have had a very positive experience with their products, which we have extensively used for our channel measurements. Apart from the products, the team at OZ is quite competent and they have been really helpful and prompt in their service.”

Dr Naveed A. Abbasi, USC’s lead for THz Channel Measurement and Modelling”

Challenges:

• 40 GHz RF signal transport over distances > 10-100 meters and beyond
• Maintain RF signal level through fiberoptic link
• Maintain signal fidelity, SFDR, NF
• Linearity

Solution:

The Optical Zonu broadband (1 – 40 GHz) OZ1840 fiberoptic link (transmitter + receiver) meets the transport distance and fidelity requirements of the Terahertz I/F VNA signal transport.

The fiberoptic transmitter consists of a high-power source laser, LiNbO3 modulator, and linear RF circuitry. The receiver is designed around a high performance InGaAs photodiode and low noise amplification.

To achieve an end-to-end RF gain of +5 dB through the fiberoptic link, a 35 dB RF amplifier is integrated into the fiberoptic transmitter. The resulting transmitter receiver pair, transporting through > 100 meters of singlemode fiber has a spur free dynamic range of 109 dB/Hz2/3 and a Noise Figure of 15 dB (TBR).