Feature: Clock and timing electronics


can measure 171,000 seconds with 6.66ns accuracy, including a factor of two for the round trip. Tis ignores effects including solar plasma, the ionosphere, the movement of the Earth during the two days, and relativity, but it is reasonably close to the real-world problem. To ensure the final measurement is


primarily due to the distance, and not the clock error, an additional factor of ten can be imposed on the clock requirements to derive a hypothetical specification: “Te maximum time interval error for the reference clock over an interval of 170,000 seconds should be no greater than 660ps”. Te good news is that such a clock exists – the active hydrogen maser.


The hydrogen maser A hydrogen maser (acronym of “microwave amplification by stimulated emission of radiation”) is an advanced atomic clock and frequency standard that uses the natural hyperfine transition of hydrogen atoms to produce exceptionally stable microwave signals. It was first demonstrated by a team at Harvard that included Nobel laureate Norman Ramsey. A group at NASA led by Harry Peters introduced the first field-deployable version in 1969. Dr Peters went on to create his own company in Tuscaloosa, the US, and successfully introduced the first commercially-available hydrogen maser in 1985. Over forty years later, because of the


demonstrated performance and reliability of field-deployed hydrogen masers, the demand for the clocks continues to grow far beyond space applications into exciting new fields like quantum computing. To service the demand, there are now two other companies worldwide manufacturing commercial hydrogen masers. Dr Peters’s original operation outgrew itself and this year moved into a larger facility. Te active hydrogen maser is the


microwave equivalent of a laser. Te hydrogen atom consists of a proton with one electron. Both subatomic particles possess a charge and spin property and act like tiny magnets. Due to the laws of quantum mechanics, the spins of the electron and the proton can only take


Imagine what surveyors (Mount


Fuji), civil engineers (the Panama Canal) and structural engineers (the Burj Khalifa) could do with measurement tools of this precision


one of two relative alignments, pointing in parallel or antiparallel orientations to each other; see Figure 2. Atomic hydrogen in an unexcited state has an equal distribution of these orientations. Te atom can change alignment via an absorption or emission of a photon at the hyperfine transition frequency of 1.420405751768GHz. Te effective bandwidth of the transition in hydrogen is only 1Hz wide, which is what makes a hydrogen maser such an effective clock. In an active hydrogen maser, molecular


hydrogen is split into atoms and passed through a magnetic field. Because the different orientations have slightly different magnetic properties, they


interact differently with the field. Te parallel atoms move into a microwave resonant cavity, while the antiparallel atoms are ejected. Te atoms that enter the cavity bounce around a Teflon-coated bulb inside the cavity for one second, travel one kilometre, and leave the cavity. At any given moment, the cavity contains about one trillion hydrogen atoms. Because the cavity’s resonant frequency


itself is designed to be close to the hyperfine transition, at room temperature there is a small amount of energy close enough to the transition frequency to induce stimulated emission in a few parallel atoms. Tese emit their own photons at the exact transition frequency as they drop into the antiparallel state. Aſter the first interactions, the emitted


photons cause other atoms to transition, creating more coherent photons in a cascading reaction that leads to a barely detectable picoamp current at the hyperfine transition frequency. Te maser electronics amplify the signal. A continuous flow of parallel atoms into the maser ensures sufficient atoms are in the correct state for stimulated emission to continue. Electronics surrounding the maser work to thermally stabilise the cavity and minimise any fluctuations in the output frequency caused by the cavity itself. In simpler terms, the maser tears apart hydrogen molecules, uses magnets to get


Figure 2: An electron and a proton can spin in parallel or antiparallel orientations www.electronicsworld.co.uk November 2026 27


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