Amazing piece of technology. Its successor, the 5071A (now made by microsemi) is still the most common primary frequency standard. HP used to be such a great company. Now they make crappy consumer printers and network cables with DRM chips.
The atomic clock product line was sold off before the latter event in 2005 to Symmetricom, which was then acquired by Microsemi in 2013, and Microsemi itself was itself acquired by Microchip a year or two ago!
The 5071A (the evolution of the product in the video) is still in the product line:
These things used to be critical elements in optical transmission for circuit switched telephone networks (SONNET/SDH). The old "5 9's" Reliability commitment ment your Stratum clock was working across the whole ring. Now you just throw a GPS antenna on every base station !
You still need low phase noise local oscillators. Both atomic standards and GPSDOs use ovenized crystal oscillators (double ovenized if they’re fancy).
It blows my mind that at the end of the day, the lowest noise method of single frequency electrical signal generation we have is a mechanical system.
I believe that the commenter you responded to is referring to the fact that a quartz resonator is literally a small piece of quartz which mechanically vibrates at a specific frequency:
At one point I had a good reference but I don’t have it on hand.
Quartz oscillators don’t have very good long term stability (see: Allen deviation) compared to atomic oscillators. Atomic oscillators are fascinating, and have something to do with the geometry of the atomic orbitals. See the section “Electron placement on the periodic table” and the “Orbitals table” in the “atomic orbitals” wikipedia[1]. Note that rubidium and caesium are the heaviest non radioactive alkali metals, thus have the largest s valence orbital. I believe this is the key feature to them producing a stable frequency.
We can do even better though. We put a few of atomic oscillators in orbit and have them talk to each other to agree on the time and location of each other, taking into account time of flight of signals and relativistic effects. You can use the phase offset between the different satellite signals to find your position and time very accurately on the ground. This is GPS and it is the current gold standard for time on Earth, outside of NIST and other global authorities on time.
Both GPS and atomic oscillators are very stable, but require the use of a quartz oscillator. These sources discipline a quartz oscillator in a feedback circuit (phase locked loop, PLL). PLLs allow you to synchronize two clocks (in this case, the quartz oscillator is being synchronized to the more stable source). PLLs have a loop cutoff frequency at which the tracking begins. Above this cutoff frequency, no tracking occurs. It is important that the local oscillators we use do not have a lot of phase noise (aka jitter, I literally wrote my thesis on the stuff. Phase noise manifests itself as edges coming too soon or too late compared to an ideal clock, but at rates above 10 Hz (an arbitrary boundary defined between jitter and wander). Quartz oscillators, in this context, most typically run at 10 MHz. It’s a very useful frequency for a huge number of applications. This frequency can be multiplied by any integer by using PLLs, but phase noise that seemed trivially quiet at 10 MHz is the volume of a rock concert at 1 GHz. So, we use quartz oscillators for a reason: they have low phase noise. The amount of phase noise in quartz oscillators can be decreased by compensating for temperature (TCXO) or controlling the temperature (OCXO). You can put your oscillator in an oven and put that oven in an oven for the lowest phase noise oscillator that humans make: the double ovenized cystal oscillator (DOCXO). The reason temperature affects quartz oscillators is because they work by applying a constant voltage to a crystal that outputs a resonant frequency. The physical size and electrical behavior of this crystal changes with temperature. Importantly, it also has a strong electrical signal compared to the noise. Atomic oscillators have a small signal that is amplified up, and it brings a lot of noise with it. Atomic oscillators work by modulating a bright light by a small amount[2].
"Transition frequency" seems to be a common source of confusion here. In the context of the SI definition it means "frequency corresponding to the energy difference between the two states". This is the frequency of radiation that can _most efficiently_ drive transitions between the states. How fast the atom transitions between the states depends on the frequency, polarization, and power of the driving radiation and can in principle take any value. To avoid confusion this is called the "Rabi frequency" in atomic physics.
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The 5071A (the evolution of the product in the video) is still in the product line:
https://www.microsemi.com/product-directory/cesium-frequency...
It blows my mind that at the end of the day, the lowest noise method of single frequency electrical signal generation we have is a mechanical system.
https://en.wikipedia.org/wiki/Crystal_oscillator
Quartz oscillators don’t have very good long term stability (see: Allen deviation) compared to atomic oscillators. Atomic oscillators are fascinating, and have something to do with the geometry of the atomic orbitals. See the section “Electron placement on the periodic table” and the “Orbitals table” in the “atomic orbitals” wikipedia[1]. Note that rubidium and caesium are the heaviest non radioactive alkali metals, thus have the largest s valence orbital. I believe this is the key feature to them producing a stable frequency.
We can do even better though. We put a few of atomic oscillators in orbit and have them talk to each other to agree on the time and location of each other, taking into account time of flight of signals and relativistic effects. You can use the phase offset between the different satellite signals to find your position and time very accurately on the ground. This is GPS and it is the current gold standard for time on Earth, outside of NIST and other global authorities on time.
Both GPS and atomic oscillators are very stable, but require the use of a quartz oscillator. These sources discipline a quartz oscillator in a feedback circuit (phase locked loop, PLL). PLLs allow you to synchronize two clocks (in this case, the quartz oscillator is being synchronized to the more stable source). PLLs have a loop cutoff frequency at which the tracking begins. Above this cutoff frequency, no tracking occurs. It is important that the local oscillators we use do not have a lot of phase noise (aka jitter, I literally wrote my thesis on the stuff. Phase noise manifests itself as edges coming too soon or too late compared to an ideal clock, but at rates above 10 Hz (an arbitrary boundary defined between jitter and wander). Quartz oscillators, in this context, most typically run at 10 MHz. It’s a very useful frequency for a huge number of applications. This frequency can be multiplied by any integer by using PLLs, but phase noise that seemed trivially quiet at 10 MHz is the volume of a rock concert at 1 GHz. So, we use quartz oscillators for a reason: they have low phase noise. The amount of phase noise in quartz oscillators can be decreased by compensating for temperature (TCXO) or controlling the temperature (OCXO). You can put your oscillator in an oven and put that oven in an oven for the lowest phase noise oscillator that humans make: the double ovenized cystal oscillator (DOCXO). The reason temperature affects quartz oscillators is because they work by applying a constant voltage to a crystal that outputs a resonant frequency. The physical size and electrical behavior of this crystal changes with temperature. Importantly, it also has a strong electrical signal compared to the noise. Atomic oscillators have a small signal that is amplified up, and it brings a lot of noise with it. Atomic oscillators work by modulating a bright light by a small amount[2].
1. https://en.wikipedia.org/wiki/Atomic_orbital
2. https://youtu.be/ymV9LwhD0W0
I had no idea how Cesium clocks work, other than the SI definition of a second is 9xxxx transitions between two states of a cesium atom.