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.

comparator1
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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DIY High Stability Timebase Hack for ~$25. Why? Frequency Stability Matters!

DIY High Stability Timebase OCXO

If you have an old “Racal-Dana 199x” frequency counter or similar 10 MHz internally referenced gear with a poor tolerance “standard quartz crystal oscillator” or bit better “temperature compensated crystal oscillator” (TCXO) you could upgrade to a high stability timebase “oven controlled crystal oscillator” (OCXO) for under $25. [Gerry Sweeney] shares his design and fabrication instructions for a DIY OCXO circuit he made for his Racal-Dana frequency counter. We have seen [Gerry] perform a similar upgrade to his HP 53151A, however, this circuit is more generic and can be lashed up on a small section of solderable perf board.

Oven controlled oscillators keep the crystal at a stable temperature which in turn improves frequency stability. Depending on where you’re starting, adding an OCXO could improve your frequency tolerance by 1 to 3 orders of magnitude. Sure, this isn’t as good as a rubidium frequency standard build like we have seen in the past, but as [Gerry] states it is nice to have a transportable standalone frequency counter that doesn’t have to be plugged into his rubidium frequency standard.

[Gerry’s] instructions, schematics and datasheets can be used to upgrade any lab gear which depends on a simple 10 MHz reference (crystal or TXCO). He purchased the OCXO off eBay for about $20 — it might be very old, yet we are assured they get more stable with age. Many OCXO’s require 5 V, 12 V or 24 V so your gear needs to accommodate the correct voltage and current load. To calibrate the OCXO you need a temperature stable variable voltage reference that can be adjusted from 1 to 4 volts. The MAX6198A he had on hand fit the bill at 5 ppm/°C temperature coefficient. Also of importance was to keep the voltage reference and trim pot just above the oven for added temperature stability as well as removing any heat transfer through the mounting screw.

You can watch the video and get more details after the break.

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Parts: ChronoDot RTC Module (DS3231)

ChronoDot

Macetech’s ChronoDot is a Real Time Clock module for projects requiring highly accurate time keeping and measurement. The ChronoDot uses the DS3231 chip, which features a TCXO to compensate for variations in temperature which affect normal oscillators, like the ones in most microcontrollers. The DS3231 uses simple I2C commands and registers for storing and retrieving time, but also features a variable output that goes all the way down to 1.000 hz for low power, interrupt style timekeeping applications. With the provided watch battery, the ChronoDot can keep time in idle mode for up to 8 years.

Normally the ChronoDot comes mostly assembled, requiring you to only solder on the watch battery. However, due to a manufacturing mistake, Macetech is selling a version with the header pins on the wrong side they call the ChronoDoh. This module is currently nearly half off the regular price of $14.99, which makes it a great low cost addition to a project. Macetech has sent us a couple of these modules to demonstrate how functional they still are.

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