mmWave Traceability for Signal Generation and Measurement R&D Topics

Errors in circuits that generate radiofrequency signals generally scale with frequency. Manufacturing tolerances that produce a few degrees of error at today’s operating frequencies near 700 megahertz grow to tens of degrees at 7 gigahertz and hundreds of degrees at 70 GHz and above, which is where many next-generation 5G wireless systems could well operate. Because similar types of technology are used for both signal generation and measurement, both suffer from similar errors that scale with frequency, including frequency response, impedance effects, interleave errors and other nonlinear effects. These all must be accurately characterized and then corrected by use of digital signal processing. NIST CTL’s mmWave traceability for signal generation and measurement work aims to characterize mmWave semiconductors and device ways to address their weaknesses.

Applications for calibrated real-time mmWave signal generation and measurement are many, including improved channel sounding and calibrated electromagnetic environment sensing and emulation, all in multi-gigahertz bandwidths. To this end, we’re working in three primary areas.

1) Enhancing the NIST on-wafer elecro-optic sampling (EOS) system, which can provide traceability for waveform measurements from 200 MHz into the terahertz range. Due to limits in the speed of today’s optoelectronic sources and broadband coaxial connectors, though, the system tops out at about 70 GHz. To speed up the EOS system so it can fulfil its potential as an enabler of next-generation 5G wireless, CTL is:

• Establishing a capability for making s-parameter measurements in a single sweep from 10 MHz to 145 GHz on a 0.8 mm coaxially connectorized devices and on-wafer devices.
• Developing higher-bandwidth sources with 0.8 mm connectors (single-mode up to 145 GHz) and rectangular waveguide output to extend the upper frequency range to support accurate signal measurements in the mmWave regime.
• Demonstrating optoelectronic devices capable of generating electrical impulses with more than 200 GHz bandwidth.
• Demonstrating an optoelectronic impulse source with a 0.8 mm coaxial connector and at least 145 GHz bandwidth and provide uncertainties for these sources in the NIST Microwave Uncertainty Framework.
• Investigating the use of phase-locked loop (PLL) timing schemes to characterize high-power electrical sources that generate wider classes of waveforms than is possible with current optoelectronic sources. 

2) Improving methods of testing millimeter-wave sources using rectangular waveguides at frequencies above those at which oscilloscopes are useful. Millimeter-wave sources are typically connectorized in rectangular waveguides and operate above the frequencies where oscilloscopes function. Both these issues pose metrological problems. CTL is:

• Developing the capability to measure modulated electrical sources using asynchronous sampling.

• Developing a theory for modulated signals in rectangular waveguide where the “correct” characteristic impedance is still a matter of debate.
• Constructing, calibrating, and disseminating calibrated sources for use at 70 GHz and above.

3) Developing free-field measurements to verify device performance, with a focus on establishing tests methods using reverberation chambers, which can establish key performance metrics in a repeatable, cost-effective way. Prior NIST CTL work has developed reverberation-chamber methods for mmWave frequencies to capture simple metrics such as radiated power. We are extending these efforts to include other figures of merit, such as error-vector magnitude, bit-error rate, and data throughput. These metrics are more complicated because they require demodulation of the received signal. The work involves:

• Developing reverberation chamber configurations for characterizing error vector magnitude (EVM), bit-error rate (BER), and data throughput of wireless devices at mmWave frequencies.
• Providing uncertainty analysis and traceability through the NIST Microwave Uncertainty Framework.

NIST Teams involved
Metrology for Wireless Systems Group
Waveform Metrology Project
High-Speed Electronics Project