With traditional phase-measurement systems generally unable to perform beyond 125 GHz and spectrally pure sources increasing in frequency, the race is on to conquer phase-noise testing of terahertz sources.
To ensure the characterization of phase noise for applications that will integrate terahertz components into usable products, many are developing terahertz phase-noise measurement capabilities. The National Institute of Standards and Technology (NIST), for example, is working on phase-noise measurement systems that support 670 GHz, 850 GHz, and 1.05 THz. Recently, the first cross-spectral phase-noise measurement of a spectrally clean terahertz source was presented by NIST’s J.A. DeSalvo, A. Hati, C. Nelson, and D.A. Howe.
Their approach is to combine even-harmonic mixers with a 2.5-GHz frequency comb. The result is a phase-noise measurement system in waveguide (WR1.5), which is achieved by use of cross-spectral and digital phase-noise measurement techniques. At 670 GHz, an upper bound of this system’s noise floor is -20, -40, and -60 dBc/Hz at 1-, 100-, and 10,0000-Hz offsets, respectively. The team also measured a commercial, low-phase-noise, 670-GHz source at offset frequencies from 0.1 Hz to 1.0 MHz.
To tighten the upper bound of the system’s noise floor, the team is now planning to include a residual phase-noise measurement of a WR1.5 harmonic mixer. Plans also are in the works to extend the measurement range to higher carrier frequencies. See “Phase-Noise Measurement System for the Terahertz-Band,” IEEE Transactions on Terahertz Science And Technology, Nov. 2012, p. 638.