Hydrogen Masers with Cavity Frequency Switching Servos

The stability of the free-running hydrogen maser is limited by pulling of the unperturbed hydrogen transition frequency due to instability of the cavity resonance frequency. Zh~o automatic approaches to cavity stabiliz@zzatlon have been used successfully infield operabb atomic hydrogen masers in the past. One method is based upon the '!~pin-exchan@" tuning procedure and was wed in marers constructed at NASA's Goddard Space F1igh.t 2] The other method iv the cavity frequency switching servo wed on Sigma Tau Standards Corporah'on (STSC) hydrogen ma~ers.[~* 4' ' 1 While automatic spinachunge tuning is in principk the more basic and accurate method, the required beam intensity switching and the long servo time constant resuk in reduced stabilify for measuring intervals up to 1 o6 seconds. More importantly, the spin-exchange tuning method requires a second stabh frequenq source as a reference, idealty a second hydrogen maser, to get the best results. The caviry frequemy switching servo, on the other hand, has very litth e$ect on the maser short term stabiliQ, and isjust enough to correct for cavity dr$ while maintuining the cuvity at the spinachange tuned oflet required to minimize instabilio due to beam intensity fluctuations. Not only does the cavity frequency switching servo not require a second stable frequenq source, but the frequency reference is the atomic hydrogen radiated beam signal, so that no extra RF connectiom need be made to the cavity, and externally generakd signah that wouldperturb the hydrogen atom need not be transmitted through the cavity. In this paper we will discuss the operation of the cavity frequenq swilching stabilization method and will i h t ra te the transknt response of the servo and certain other uspects olthe technique that have potentid for achieving improved basic accuracy. We also willgive stability results obtained between masers at STSC and long term stcrbie data obtuined with STSC hydropn masers at the United States Naval Observatory.