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Schematic of the CryoMech low-vibration cryostat20 housing the sapphire resonator on the cold finger cooled by about a liter of liquid helium in the enclosed liquid/gas helium space. The CryoMech PT407-RM pulse-tube condenser constantly maintains the liquid helium bath near 4 K. A heater, sensor, and a Lakeshore 340 temperature controller maintains the temperature at the sapphire crystal within about 10 K in the short-term by controlling the temperature at the resonator frequency-temperature turnover point.
A block diagram of the measurement technique used to compare two cryocooled oscillators (cryoCSO1 and cryoCSO2) with a Symmetricom 5125 A phase noise test set (attenuators and DC blocks not shown). The principle components used were; a Watkins Johnson M14A mixer, Mini-Circuits amplifiers ZFL-500LN+, and a divide by 112 chain. The latter was constructed from the series connection of Hittite dividers: the HMC364 divide by 2, the HMC365 divide by 4, and the HMC705LP4 programmable divider which was set to divide by 14. LPF = low pass filter.
The SSB phase noise of the 100 MHz divider chain (curve 1), and the SSB phase noise for a single oscillator at 11.2 GHz (curve 2). Curve 3 represents the phase noise of the Endwave JCA microwave amplifier used in the loop oscillator, measured at 11.2 GHz with an incident power level of −30 dBm, which is approximately the loop oscillator operating condition. Curve 4 is a best fit for the phase noise of the locked oscillator for Fourier frequencies Hz.
The fractional frequency stability (Allan deviation ) for a single 11.2 GHz crycooled sapphire oscillator as a function of integration time, , with a linear frequency drift /day subtracted (curve 1) and with the drift included (curve 2). The broken line fit (curve 1) is described by Eq. (2). Curve 3 is the stability for a single 11.2 GHz cryogenic oscillator using the same resonators as in this paper but cooled in a large liquid helium bath.13 Curve 4 is the fractional frequency stability of a 10 GHz microwave signal derived the NIST photonic generator.28 The values for for s of curve 1 and all of curve 3 are estimated from a -counter. Curve 4 includes drift, but its impact on stability for s is negligible.
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