Abstract
This paper introduces recently result on passive hydrogen maser of Shanghai Astronomical Observatory by single frequency modulation. Digital servo system and analog servo system have been experimented. DDS is used to generate frequency modulation in the RF interrogation chain in both systems. The algorithm separating two errors is used in digital system. The traditional integrator and digital phase shift are used in analog system. An Allan standard deviation of two systems has been tested based on same physical part. Both of them have achieved at σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> (τ) <; 1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> τ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1/2</sup> (1s≤τ≤10000s) and σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> (day) <;2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-14</sup> .
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