Measuring The Accuracy Of A Rubidium Standard

A rubidium standard, or rubidium atomic clock, is a high accuracy frequency and time standard, usually accurate to within a few parts in 1011. This is still several orders of magnitude less than some of the more accurate standards – for example the NIST-F1 has an uncertainty of 5×10-16 (It is expected to neither gain nor lose a second in nearly 100 million years) and the more recent NIST-F2 has an uncertainty of 1×10-16 (It is expected to neither gain nor lose a second in nearly 300 million years). But the Rb standard is comparatively inexpensive, compact, and widely used in TV stations, Mobile phone base stations and GPS systems and is considered as a secondary standard.

[Max Carter] recently came into possession of just such a unit – a Lucent RFG-M-RB that was earlier in use at a mobile phone base station for many years. Obviously, he was interested in finding out if it was really as accurate as it was supposed to be, and built a broadcast-frequency based precision frequency comparator which used a stepper motor to characterise drift.

Compare with WWVB Broadcast

WWVB Receiver
WWVB Receiver

The obvious way of checking would be to use another source with a higher accuracy, such as a caesium clock and do a phase comparison. Since that was not possible, he decided to use NIST’s time/frequency service, broadcasting on 60 kHz – WWVB. He did this because almost 30 years ago, he had built a receiver for WWVB which had since been running continuously in a corner of his shop, with only a minor adjustment since it was built.

Comparator Circuit Installed in a Case

His idea was to count and accumulate the phase ‘slips’ generated by comparing the output of the WWVB receiver with the output of the Rb standard using a digital phase comparator. The accuracy of the standard would be calculated as the derivative of N (number of slips) over time. The circuit is a quadrature mixer: it subtracts the frequency of one input from the other and outputs the difference frequency. The phase information is conveyed in the duty cycle of the pulses coming from the two phase comparators. The pulses are integrated and converted to digital logic level by low-pass filter/Schmitt trigger circuits. The quadrature-phased outputs are connected to the stepper motor driver which converts logic level inputs to bi-directional currents in the motor windings. The logic circuit is bread-boarded and along with the motor driver, housed in a computer hard drive enclosure which already had the power supply available.

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Jaw-Dropping Atomic Clock Build

You could cruise the Internet bazaars for a talking clock but you’ll never find one as awesome as this. Just look at it… even if it didn’t work it would be awesome.

[Art] certainly lives up to his username. His Rubidium-standard atomic real-time clock is surely an example of hardware art. The substrate is a collection of point-to-point soldered perfboard modules. Each laid out meticulously. What does such layout call for? A gorgeous enclosure which doesn’t obscure your view of the components. For this he went with a copper tube frame and a custom fabricated aluminum chassis pan.

For the circuit itself [Art] tells us he wanted to build something akin to the old HP nixie frequency counters so he went with logic chips. The pictures and a few video annotations are the only clues we have for how this works. Hopefully your encouragement in the comments will help prompt him to share more about that.

Oh, and the talking clock part that we referred to earlier? Every minute you get a readout of the time thanks to a PIC playing back audio using [Roman Black’s] BTc sound compression algorithm.

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Master Clock Keeps Time for All Other Clocks

[Brett] just finished construction and long-term testing of this extremely accurate timepiece. It keeps such great time by periodically syncing with the atomic clock in Mainflingen, Germany.

The core of the project is an ATMega328 which uses the new DCF77 library for decoding the signal broadcast by an atomic clock. The libraries written by Udo Klein significantly increase the noise tolerance of the device reading the signal, but they will not work with any project that use a resonator rather than a crystal.

In the event of a complete signal loss from the atomic clock, the micro driving the clock also has a backup crystal that can keep the clock running to an accuracy of within 1 second per day. The clock can drive slave clocks as well, using pulses with various timings depending on what [Brett] needs them to do. The display is no slouch either: six seven-segment displays show the time and an LCD panel reads out data about the clock. It even has chimes for the hour and quarter hour, and is full of many other features to boot!

One of the most annoying things about timekeeping is daylight savings time corrections, and this clock handles that with a manual switch. This can truly take care of all of your timekeeping needs!

THP Entry: Atomic Space Time

LCD featuring HaD logo

Accurate time is all around us. Streaming down from satellites thousands of miles in space, UTC time information is at all of our fingertips. You just have to know how to reach out and grab it. [hkdcsf] not only knows how to do this, he does it in style.

Tipping his hat into The Hackaday Prize contest, [hkdcsf]’s atomic clock is masterfully crafted. Not only does it get time information from GPS satellites, it also has the ability to grab the infomation from the DCF77 transmitter. And if ever it’s in a position where neither signal can be found, an RTC crystal keeps the time and date accurate.

His design is based on a PIC18F25K20, and bristles with so many features that it might make you dizzy. So be warned – you might want to be in a seated position before taking a look at this project. [hkdcsf] does a great job at detailing exactly how his clock works, and his efforts to provide this level of detail will surely help other hackers to add similar features to their future projects.

SpaceWrencherThe project featured in this post is an entry in The Hackaday Prize. Build something awesome and win a trip to space or hundreds of other prizes.

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A weekend trip to verify general relativity

8 years ago, for the 100th anniversary of the theory of relativity [Tom] decided to test the general theory of relativity.

As he was going to Mt Rainier (5400ft high) with his children for the weekend, he brought in his van 3 cesium clocks while leaving other atomic clocks at his home for comparison. The theory behind the test is that if you’re are at higher altitudes, then your speed (in a galactic coordinate system) is higher than the one you’d have at sea level and therefore time would go “slower” than at lower altitudes.

[Tom] brought 400 pounds of batteries, 200 pounds of clocks and left his car turned on during his 2 days stay in the ‘Paradise Lodge’. He used 120V DC to AC converters and chose to bring 3 cesium clocks to have a triple redundant  setup. When he came back home, he had the good surprise of finding a time difference of 23ns. This is a great application for those rubidium sources you’ve been scavenging.

[Thanks Indyaner via Reddit]

Atomic skull clock reminds us we’re dying


Whether you like it or not, every second that passes brings you one step closer to your own demise. It’s not a comforting topic to dwell upon, but it’s reality. This art installation entitled ‘Memento Mori’ is a haunting reminder of just that. Even with all the advanced technology we have today, we still have absolutely no way of knowing just when our time will come.

[Martin] cast a real human skull, then added a 4 digit LED display that’s attached to a rubidium atomic clock (running a FE-5680A frequency standard). The display counts down a single second over and over, measured in millisecond-steps, from 1.000 to 0.001. He built a custom electronic circuit to convert the 10 MHz sine wave into a 1 kHz pulse signal, and used ATmega8 chips running an Arduino sketch to do the rest of the dirty work.

Watching the video after the break, with that smooth mysterious music in the background, one can’t help but ponder our mortality. On a personal note, this totally feels like something you’d find in a video game.

[Thanks Martin]

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DCF77 signal filtering and decoding


[Udo] decided to build a clock using the DCF77 radio module seen above. This of course has been done before: the hardware draws a clock signal from the atomic clock in Braunschweig, Germany. So he grabbed a library for Arduino and got to work. But he was getting rather poor results and upon further investigation realized that the library had been written for 20 Hz modules and his operates at 300 Hz. This means better accuracy but the drawback is that the hardware is more susceptible to noise.

So began his journey to filter, process, and decode the DCF77 protocol. That link goes to the project overview. It will be in several parts all of which will be linked on that page. So far he has applied a low-pass filter and coded some exponential smoothing. He has yet to write the other four parts, but does mention that early testing shows that this technique will make the reception better than what is achieved with commercially available clocks. He was able to lock onto a signal that had more than 80% noise ratio. That’s impressive!

Just want to see a clock that uses a DCF77 module? Check out this PIC-based atomic clock.