Of course, it turns out there aren't that many home applications for an atomic clock, other than collecting precision metrology equipment. And if you're running a data centre and want higher precision than NTP, chances are you'll choose a PTP grandmaster clock at the high end, or a GPS receiver with a 1PPS output at the low end, in rather than buying second-hand parts from ebay.
For all the people who have a rubidium standard at home, 70% are using it for electronic lab, amateur radio, or NTP at home. But the remaining 30% is another large userbase too - audiophiles. Many audiophiles claim jitter and phase noise in the clock signal significantly affects audio quality, and the most extreme audiophiles use a lab grade frequency standard, rubidium or better, and feed it to all their Hi-Fi gears.
P.S: Given all the factors that affect audio quality, is the phase noise from a PLL synthesizer of reasonable quality really a major factor? Even if it is, does a rubidium standard really have any benefit beyond diminishing return over a crystal oven? Well, of course, these are the questions that are never answered by audiophiles.
> You can buy a used rubidium frequency standard on ebay
There is a continuous source of used rubidium standard coming from retired telecommunication and lab equipment, they are the cheapest atomic standard available. The catch is that the rubidium inside the discharge lamp will eventually get depleted during operation, usually within 10 years, once its life ends, it's useless and needs to be rebuilt completely by the manufacturer. So read the manufacturing date if you are going to power it 7x24.
The real expensive ones are the Cesium Standard, such as the HP 5061A. Recently, the Hydrogen Standard also saw some uses.
Cesium oscillators on the other hand do deplete their cesium (by design), as mentioned by CuriousMarc in the video. The tube in his unit was probably replaced at least twice, at a cost of >80k. The high performance tubes fare even worse because they "burn" through their cesium at a 3x rate.
The 5071 successor by HP-Agilent-Symmetricom-Microsemi is the most frequently used COTS clock that contributes to UTC.
Thanks for the tip, that's interesting. I never know that they can be renewed in this way.
> Cesium oscillators on the other hand do deplete their cesium (by design), as mentioned by CuriousMarc in the video. The tube in his unit was probably replaced at least twice, at a cost of >80k.
Sad story. Recently I was browsing a web store that sells decommissioned U.S. military equipment. I was here to search for some cheap RF power meters, and I was surprised to see a HP 5071 in the listing. The vendor noted that he knew nothing about the equipment and information was welcomed. For a moment, I was thinking about sending him a service manual from the HP archive, before I realized it was simply impossible to get it up and running again without replacing the tube :(
> The 5071 successor by HP-Agilent-Symmetricom-Microsemi
Aha, the old Hewlett-Packard was truly a unique company. Its electronic technology has at least three separate chains of succession.
Test Equipment: HP-Agilent-Keysight
Semiconductor: HP-Agilent-Avago-Broadcom (Broadcom ended production of most HP parts, so now it's mostly dead, RIP...)
Frequency Standard: HP-Agilent-Symmetricom-Microsemi
Frequency Standard: HP-Agilent-Symmetricom-Microsemi-Microchip
Ideally, the clock frequency should be as stable as possible. Unfortunately, all clock oscillators have inherent short-term instabilities, long-term instabilities, and non-zero temperature coefficient. Short-term instabilities, known as phase noise (in the frequency domain), or jitter (in the time domain) is a particular concern. As long as the audio system is properly optimized and characterized, jitter is not an issue. A good DAC will normally have a jitter around -90 dBc or lower, and it's probably negligible. NwAvGuy (an audio engineer known for his criticism of baseless audiophile practice), has a good explanation of clock jitter: https://nwavguy.blogspot.com/2011/02/jitter-does-it-matter.h...
But of course, some audiophiles want to power their Hi-Fi gears with the best oscillator available, so that it'll have the lowest jitter and the minimum absolute frequency error and temperature coefficient, even if the benefits is dubious to other people. And they realized that a rubidium frequency standard is the best clock oscillator they can find. First, it's possible to DIY. All you needed to do is understanding how the digital part of the Hi-Fi system works and find the crystal responsible for generating the system/ADC/DAC clock. You simply remove the crystal and inject an external clock signal from the rubidium standard to the chip. 10 MHz is a common output frequency from a standard oscillator, if the audio circuit also has a 10 MHz clock, the rubidium standard output can be used directly. If it's not, if the frequency radio of the two is a integer, it's possible to use a frequency multiplier or a divider to generate the needed clock frequency from the rubidium standard output. If it's not an integer, a PLL frequency synthesizer can be used.
Second, in an electronic lab, many gears have a "10 MHz Reference Input" port, their internal timings can be derived from the reference input, so that all signal generators, frequency counters, spectrum analyzer to a master clock oscillator (can be crystal oven, WWVB radio, GPS, or atomic) for consistency, so that all equipment's frequencies don't drift to different directions, all get a good clock with constant performance, and there won't be a disagreement between equipment. In some professional audio equipment for music production, it also allows an external reference to be used for the same purpose. One can simply plug the 10 MHz standard oscillator, including a rubidium standard, into the BNC connector of the audio gears.