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Frequency Measurement
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
The Q of a rubidium oscillator is about 107. The shifts in the resonance frequency are mainly caused by collisions between the rubidium atoms and other gas molecules. These frequency shifts limit the long-term stability. Stability at τ = 1 s is typically less than 1 × 10−11 and near 1 × 10−12 at τ = 1 day. There is generally no guaranteed specification for accuracy, but after a warm-up period of a few minutes, a rubidium oscillator will typically be accurate to within parts in 1010 or less, and some devices might be as accurate as 5 × 10−12. However, if an application has an accuracy requirement of parts in 109 or smaller, a rubidium oscillator will need to be regularly measured and adjusted, because accuracy better than about 5 × 10−9 cannot be assumed. With regular frequency adjustments, a rubidium can maintain average frequency to within a few parts in 1011 or 1012 over periods of months or years. The adjustments are made to compensate for the aging and frequency drift that changes the rubidium frequency slowly over time. Manufacturers typically specify the aging rate as less than 5 × 10−11 per month, but this is sometimes conservative, as the frequency of a well-behaved rubidium standard might change by less than 1 × 10−11 over the course of a month. Even so, the frequency change can exceed 1 × 10−10 if left unadjusted for a year, which is unacceptable for some applications [2,6–8].
Chirped-pulse millimetre-wave spectrometer for the 140–180 GHz region
Published in Molecular Physics, 2018
C. Lauzin, H. Schmutz, J. A. Agner, F. Merkt
The FID radiation is received by a conical horn antenna and directly drives the radio-frequency port of a subharmonic mixer (Virginia Diodes, WR5.1 SHM). The local-oscillator port of the mixer is fed by the up-converted and sextupled (Oleson, SM10-AG) signal from the receiver arm. The lower sideband at the intermediate-frequency port of the mixer, with a centre frequency of 2.88 GHz and a maximum span of 4.8 GHz, is preamplified with a gain of 38 dB and recorded on a digitiser (LeCroy, WP 760Zi). At a sampling rate of 20 GS/s for 10 µs, 2 samples are averaged directly on the digitiser. The phase coherence necessary to accumulate the FID signal in the time domain is achieved by synchronising all timing signals to a GPS-disciplined 10 MHz rubidium standard reference clock (Stanford Research, FS725 and Spectrum Instruments, TM-4). The repetition rate of the experiment is controlled through the master clock and has to be a subharmonic of the reference clock. The digitiser is triggered by a synchronous timing signal of the AWG, which has marker outputs for this purpose. The burst generator in Figure 1 allows for multiple measurement sequences (chirped pulses with subsequent FID recording) during a single gas pulse. The burst typically consists of five pulses generated at a repetition rate of 50 kHz. The digitiser acquisition memory can be split in segments (segmented memory) to reduce the dead time between trigger events. The maximum overall trigger rate reaches 250 Hz and is only limited by the averaging capability of the digitiser. The chirp rate of the different measurements ranged from 0.05 to 2 MHz/ns depending on the intensity of the recorded signal. Once recorded, the FID signal is Fourier transformed using the Hanning apodisation function provided by the “signal” library [25] of the R project software [26].