Evaluation of passive component short-term stability via use in low loop delay oscillators

Measurement of the PM noise generated by passive components operated at HF carrier frequencies was reported by NIST at the 1998 Frequency Control Symposium. The results were obtained using a two channel, cross-correlation measurement system. The results indicated that the flicker portion of short-term reactance fluctuations of inductors, capacitors, and varactor diodes could result in carrier signal spectral degradation, especially when the components were used in moderate and narrow bandwidth tuned circuits (i.e., bandpass filters. In this paper, the results of similar measurements made in the VHF range are reported. Determination of the short-term reactance stability of several types of inductors, capacitors, varactor and PIN diode-based attenuators and helical resonators has been made. The measurements were made by using these components in simple tuned circuits and multipole filters that are used as the frequency-determining element in low loop delay oscillators. Using this method, the near-carrier phase noise of the oscillator signal is poor enough as to be easily measured. Further, if the oscillator loop delay is known, the equivalent open loop phase noise of the oscillator components can be accurately determined from the oscillator signal measurements. Using modular amplifiers having very low 1/f PM noise as the oscillator sustaining stage and DUT circuit bandwidths of 10% or less, it has been possible to measure reactive element short-term stability down to a measurement limit at or slightly below PSD(ΔX/X)²=10^-16/f from f=1 Hz to 1 KHz. In virtually every case, the measured levels of oscillator flicker-of-frequency and white frequency noise were almost entirely traceable to the sustaining stage amplifier itself. The conclusions drawn from this work are that “active” components such as amplifiers, mixers, diode and transistor frequency multipliers and dividers, as opposed to the types of passive components measured, constitute the primary contributor to VHF and UHF carrier signal 1/f PM noise