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Frequency Measurement
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
An active hydrogen maser works by sending hydrogen gas through a magnetic gate that only allows atoms in certain energy states to pass through. The atoms that make it through the gate enter a storage bulb surrounded by a tuned, resonant cavity. Once inside the bulb, some atoms drop to a lower energy level, releasing photons of microwave frequency. These photons stimulate other atoms to drop their energy level, and they in turn release additional photons. In this manner, a self-sustaining microwave field builds up in the bulb. The tuned cavity around the bulb helps to redirect photons back into the system to keep the oscillation going. The result is a microwave signal that is locked to the resonance frequency of the hydrogen atom and that is continually emitted as long as new atoms are fed into the system. This signal keeps a quartz crystal oscillator in step with the resonance frequency of hydrogen (Figure 42.9).
Phase Noise Impact on the Short-Term Frequency Stability of a Frequency Source
Published in IETE Journal of Research, 2023
Md. Tosicul Wara, M. S. Bhuvaneshwari, M. R. Raghavendra, Usha Bhandiwad
The frequently used measuring instrument like “Allan deviation meter” mainly works on the phase meter method. The Allan deviation meter is having two inputs – the reference input and the test input. The Device Under Test (DUT) i.e. the frequency source – output is connected to the test input of the phase meter. On the other hand, the reference input is fed from a standard reference source (whose frequency stability is at least one order better than the DUT). The phase meter output provides the instantaneous phase values of the frequency source (DUT) which is processed offline inside the in-built processor for Allan deviation computations. Usually, during frequency stability/Alan deviation measurements of Rubidium or Cesium atomic clocks, Passive Hydrogen Maser (PHM) or the Active Hydrogen Maser (AHM) is used as the standard reference source.
Cavity-enhanced double resonance spectroscopy of HD
Published in Molecular Physics, 2022
M.-Y. Yu, Q.-H. Liu, C.-F. Cheng, S.-M. Hu
Absolute frequencies of pump and probe lasers are obtained by monitoring the beat frequencies between lasers and an optical frequency comb (OFC). The OFC is disciplined by a hydrogen maser with a fractional frequency accuracy better than . One of the beat signal is sent to a home-made frequency-lock circuit and the feedback is applied to the cavity PZT to lock the laser frequency with the comb. Therefore, the stabilised optical cavity provides short-term frequency stability to the laser, and transfers the absolute accuracy of the frequency standard. A function generator is implemented to tune the beat frequency and consequently scan the laser. Because both lasers are locked to the same cavity, the other laser is scanned simultaneously and its beating signal is monitored by a frequency counter.
Measurement, Analysis, and Understanding of the Error Vector Magnitude (EVM) of Navigation Signals
Published in IETE Journal of Research, 2018
Md. Tosicul Wara, Raghavendra M. R., Kodandaram M., Bhuvaneshwari M. S.
The 10 MHz reference clock is used at the ACMU input and includes a Passive Hydrogen MASER (PHM), a Rubidium Atomic Frequency Standard (RAFS), and a Temperature Compensated Crystal Oscillator (TCXO). Out of all these sources, the PHM is the highly stable frequency having lowest phase noise. Consequently, the EVM values of the navigation payload with PHM as the reference clock will be the minimum. On the other hand, the TCXO is having poor frequency stability and poor phase noise. Consequently, the EVM values of the navigation payload with TCXO as the reference clock will be the maximum. Hence, one could conclude that poorer the phase noise of the navigation carrier, poorer will be the EVM values. However, from Tables 3 and 4, it is clear that for the navigation signals with very high carrier to thermal noise density ratio (C/No), the EVM values are almost independent of the carrier phase noise.