Time -vs- Frequency

Time is the parameter that can be measured with the highest precision

Eliminate Leap Seconds?

I personally don't like the idea because then sundials would not tell the correct time.  A well made sundial today can be accurate to a few seconds since UTC is kept within a fraction of a second of Solar time (i.e. the Earth's rotation).  If we allowed the difference to get as big as 10 seconds, then sundials would have this amount of difference to common time.

Time Transfer

Radio Station Listings

Time Stations by KH6BZF
NPL - Standard Time and Frequency Transmissions -
VNG Australia - 2.5, 5, 8.638, 12.984, 16 MHz - part of the National Standards Commission -
Brooke's Frequency Assignment Table - almost DC to light
Radio Österreich International - list of time stations -

Japanese Time & Frequency Information:

Time & Frequency Standards - all Japanese
Department of Standard Time and Frequency - English & Japanese
New 40 kHz time station - Japanese - more 40 kHz stuff -

WWVB - 60 kHz

WWVB can be used for two different purposes:
(1) as a phase reference to determine the stability of an oscillator
(2) as a source of amplitude modulated digital time code data to set a clock that includes leap years and leap second data.
The leap second data is done with 2 bits in such a way that the clock can make the jump at the proper time.
Starting in the 1990s there are a number of low cost clocks and watches that set themselves called "atomic clocks" based on this service.
Note that GPS is a world wide system that does NOT have any leap year or second data.  It does have a UTC to GPS time differential that will allow a clock to display the correct UTC time to within a second within about 12.5 minutes of the correct time to make the jump.  GPS does not have any bits for DST.

HP 117A WWVB Receiver & Phase Comparator

I bought the HP 117A VLF Comparator from EIP Microwave since they had replaced it with a Loran-C based system.  It uses Nuvistors in the loop antenna and the 60 kHz amplifier block that are hard to find.  I have replaced them and now it works well.  I would like to use a PIC microcontroller to decode the time signal.  The stock unit only looks at the phase difference between the external signal and the phase of WWVB.  You can see the hourly 45 degree phase shifts on the WWVB carrier as well as the dinural variation in phase of the signal from WWVB.  Note that WWVB is in the process of increasing their power.

Looking for a source of contact type strip chart paper: width = 2 1/2", plot width = 1 15/16", sprocket holes on both sides of paper = 0.1 ID on one side and slots 0.1 x 0.15" with a 1/4" c-c pitch.  along the edge is says: HEWLETT-PACKARD RECORDING CHART NO. 9281-0081
Example plot showing adjustments being made to an HP 107 oscillator.  You can see the hourly phase shifts on the WWVB carrier.
The strip chart drive in the 117 may be the same as an Amprobe model DB81 that uses Amprobe model 850D chart paper.

WWVB Time Receivers

NIST is increasing the power of their time station WWVB on 60 kHz so that the East coast of the U.S. will have a stronger signal all year.  There are other time stations in England MSF 60 kHz , GermanyDCF77 77.5 KhZ, and Japan JG2AS 40 kHz.
The Time Machine - A battery powered clock made by Oregon Scientific. This is a 60 kHz receiver.  Radio Shack sells the 63-970 version.
Temic has a series of Time Code Receivers that can be tuned to the low frequency time signals of most countries and need to be coupled with a microcontroller to decode the digital output into time information.  They have changed their web page and I can no longer find information on the U 4223 B-CFS.  It is packaged in an SSO20 surface mount package, the only other version is a raw chip.  The SSO20 package has 0.025" pitch connections which is 4 times closer than a DIP with 0.1" pitch.  So far this has been a problem in building a prototype.
Temic acquired by Atmel - multimedia - Radio-controlled clock receiver ICs - as of Jun 2001 they stopped shipping
Searching the Temic-semi web site for WWVB returns 5 documents:

Time-Code Receiver with TC Output - U4226B

Time-Code Receiver with Digitized Serial Output - U4224B

Low-Cost Time-Code Receiver - T4225B

Time-Code Receiver with A/D Converter - U4223B

Time-Code Reception -

Zeit - makes a number of clocks that work by receiving LF time signals
A Radio-controlled Digital Clock - PIC based with LCD display
Elmer's Guide to Atomic Clocks for the Home and Office -
Klockit - sells a movement to drive conventional hands.  This ships with a pin to lock the hands than must not be removed until it is installed and the hands attached.  I used one to convert a Wal-Mart clock to WWVB controlled. (You might look for an ALL plastic clock for better reception.) Klockit now has a complete kit in addition to the movement.   When I built mine I needed to order the second hand separately, be sure to get all three hands with the raw movement.
B Roehrig, "Most Accurate Frequency Standard, Parts 1,2,3", 73 magazine, Jan/Feb/Mar 1994.Circuit boards available from FAR Circuits  This is a frequency standard and needs additional circuitry to demodulate and decode the time signals.
Quartzlock Instruments - WWVB receiver and precision time standards
Spectracom Corporation - 8182 NTP sync receiver & time of day, precise frequency, telecommunications products
Ultralink, Inc - OEM TTL version, RS-232 and one with analog signal strength meter
    UL333 Antenna & RF decoder , Data Decoder & Display  it's now a UTC only time display, but might have be upgradable to local time
Radio Shack's Radio Controlled Clock to SDR Interface - info on Blob cable
Agilent 4395A -  If an active external antenna like the Dymec DA-100, that can pick up the 60 kHz signal from WWVB is connected and the center frequency is set to 60 kHz, span to 100 Hz, RBW to 100 Hz, then the sweep time is 45.71 ms.  The display jumps up and down 10 dB in step with the time code modulation on WWVB.  The number of points is set automatically to 5.  4395A Spectrum Plot 50 to 150 kHz.  The triangular peak centered on 100 kHz is LORAN-C.
Precitel Co. - Time Products for VLF and GPS including public clock movements
Universal Time Company - custom clocks as art
AMRAD - LF project -  10 kHz to 30 MHz active whip with high intermod tolerance
Radio Clock - a PC sound card based software package that decodes the audio WWVB signal from your receiver and sets the PC clock.  Made to support BeaconSee for ham radio propagation monitoring.  They also have a free applet for GPS PC clock synch.

HF Time Stations WWV -  WWVH - WWVS

The WWV and WWVH stations can be used in a number of ways:
(1) audible voice announcement of the time every minute with audible second ticks
(2) audible voice announcements about a number of topics at specified minutes past the hour
(3) AM modulated digital data on a 100 Hz subcarrier with time code data similar to WWVB.  The Heathkit GC-1000 used this signal.
It had a bug in that the switch to/from standard time/DST happened when Colorado changed, not in your time zone although there were DIP switches for time zone as well as propagation delay in ms.  In California it was wrong by an hour or two twice a year if set to display local time.

WWVS is a time system that is (was?) on two sats, one one the East coast and one on the West coast.  The sat was nominally geosnychronous, but there were variations in the orbits that showed up as time variations.

In my T&F rack  there is an HP 1148BR Time Comparator and HP 120 oscilloscope and an HP 113AR Frequency Divider and Clock.
The 114 takes in the audio from a receiver tuned to one of the WWV stations.  It feeds that audio to the 120 scope Vert. and a 1 PPS to the Hor.
The 1 pps can be moved in 1 ms steps using the 3 thumb wheel switches on the 114.

ESE - ES-180A WWV master clock -
Austrailian Standards Leaflets - Radio VNG 2.5, 5.0, 8.638, 12.984 and 16.0 MHz.

LORAN-C

Loran-C replaced WWVB as a phase standard for stability measurements.  This was because the 100 kHz ground wave pulses could be sampled with more precision than the WWVB signal.  Since LORAN is a navigation system there is no time code data for setting clocks.

International Loran Association - Prior to GPS Loran-C was the best available broadcast time signal (except for an expensive satellite system)

Chirp Sounders

There are many H.F. radio transmitters that sweep from 2 to 30 MHz for the purpose of studying the ionosphere.  Many of them now use GPS to keep their start times very accurate and could be used as time transfer transmitters. RCS-5 based and DSP based receivers.

GPS

See my write up on how Sputnik led to the Transit sat nav system that lead to GPS.  GPS does not need the precision clock that was required for Transit.
This is done by requiring 4 satellites, 3 for position (X, Y and Z) and a forth to remove the requirement for a precision clock in the receiver.  There are no bits in the GPS data for leap seconds, leap years or the U.S. DST to/from standard time.  There is an offset number that describes the number of seconds that GPS time differs from UTC.  This is needed because the GPS system does not have any leap seconds and UTC does.

GPS is now the best time transfer method available for most applications replacing LORAN-C.

PIC Microntroller Based Motorola Oncore VP+ Binary Time & Date Display

This is a PIC16F84 based circuit that reads the binary data stream from the VP+ and formats the data into DOW, DOM, Month as a 3 letter name, YYYY, hh:mm:ss and displays the data on any standard 44780 based alphanumeric LCD.  Since the display is only reading the data it will automatically show leap seconds.  It will show local or UTC time depending on how the VP+ was programmed by an external computer.  I plan to add dip switches so that you can select the number of columns and number of rows.

Brooks Shera's GPS-Controlled Frequency Standard

This is a PIC microcontroller used to steer an oscillator based on the 1 PPS from a Motorola GPS receiver.
CNS Systems - commercial TAC32 clock - GPS Time - Low Cost High Accuracy GPS Time paper from ION
Horita - GPS based Video Time Code generators
HP GPS based timing products & app notes
KEK GPS Clock System - TACGPS backup module -
New Zealand and Australia Time Resourses - sub second timing for astronomical use
Symmetricon - is taking over the HP (Agilent) GPS based timing products 58333A, 58540A - I think these used the discontinued Motorola Encore VP series OEM boards.
W7CQ page on the Shera setup with onfo on the HP(Agilent) 10544A
Zyfer - GPS diciplined 1PPS & 10 MHz -

Network Time Synchronization

Network Time Protocol (NTP) - The Electrical Engineering Computer Information Systems department of the Univ. of Delaware is the center of action for NTP.  The internet requires time synchronization in order to operate and this is handled by NTP.  You can also set your computer clock using NTP type software (but not the time server software on this server which is intended for the server rather than the client.  I am using a shareware program called SocketWatch to set my computer's clock.
Symmetricom, Inc - (NASDAQ: SYMM) -multimode time sync including GPS
Proposed new <time.h> for ISO C 200X -

Frequency Standards

Mechanical

There have been many mechanical time keepers.  Some used sand or water, later the pendulum clock was highly developed.
Harrison spent most of his life developing a chronometer that would work at sea so that sailors would know their longitude.
Knowing latitude is relatively simple by measuring the elevation angle of the Sun or stars, but knowing the longitude requires knowing what time it is.  Great Britain had a standing reward of 10,000 pounds sterling for someone to produce a workable way to know the longitude at sea.

Crystal

Quartz crystals are probably the most common devices used to control the frequency of electrical oscillators. They are used in wrist watches, clocks and most microcontroller based devices.  There are many ways to slice a quartz crystal from the piece that's found in nature or grown and these different "cuts" yield different properties.

There are a number of environmental conditions that effect the frequency of a quartz based oscillator.  A crystal will change resonant frequency when the temperature changes depending on the cut of the crystal and the actual temperature.  HP used to offer a precision temperature meter that used a crystal optimized to vary with temperature.  Crystals change frequency with time, called aging.  It turns out that this has a lot to do with how the crystal is packaged.  If there are any molecules that can land on the crystal they will change it's frequency. The acceleration due to gravity will effect crystal frequency.  See the plot on my PRS-10 web page showing the effect of gravity on it's internal 10 MHz crystal. Atmospheric pressure and humidity also can have an effect on crystal frequency.

The highest precision crystal oscillators place the crystal inside an oven (OXO) that keeps the temperature constant.  Another way to compensate for the temperature dependence of the crystal is to measure the temperature and use that information to change the frequency of the oscillator (TCXO).

There is a ceramic resonator technology that has performance slightly poorer than a simple crystal and costs less that is used in high volume low cost microcontroller applications although my guess is that overall the crystal is still the most used frequency determining element.

There are a number of companies that make test equipment to measure the electrical parameters of quartz crystals.  It turns out that the 32,768 Hz crystals used in watches are extremely difficult to measure because they have high Q which translates into measuring impedance in the 1 to 10 Meg Ohm range.  The HP (Agilent) 4194A can make this measurement

Heathkit Crystal Calibrator

Radios made prior to the introduction of digital synthesizers typically used an LC (inductance capacitance) controlled oscillator as the tuning element and did not have the ability to tune to a station directly, you needed to tune up and down the dial until you heard the station.  To help find stations a crystal calibrator could be used.  It generated harmonics spaced by the 100 kHz crystal frequency and when you heard the harmonic yo know you were tuned to some multiple of 100 kHz.

Sultzer

This design shows up in the book  "Evolution of Naval Radio-Electronics and Contributions of the Naval Research Laboratory" (NRL Report 7600, January 1976) on page 289 is a photo showing the Sultzer labs unit at the top of the Omega nav receiver rack as a major improvement in crystal oscillator technology.  It has a cylindrical shape and uses a proportional oven control coupled with a large thermal mass.  This was a big improvement over the prior art which used bang-bang type temperature control and low mass.  The crystal frequency was 2.5 MHz and it was the first all transistor frequency standard.

HP

The Sultzer design was the basis of the HP 105 crystal oscillator and the cylindrical crystal oscillator used in many HP time and frequency products.  HP used a 5 MHz crystal for all their versions.  This is a fairly large and heavy unit.

Gibbs

This unit used a double proportional oven surrounding a Bliley glass enclosed 10 MHz crystal.  The inner oven is set at the turnover temperature of the crystal to minimize the effect of any temperature change.  By plotting the oscillator frequency vs. temperature you can find the sweet spot.  The size and weight of the complete oscillator assembly (excluding the power supply) was much smaller and lighter than the Sultzer design.

It was designed so that the lead acid batteries were in the same rack space as the double oven crystal oscillator.  The acid fumes etched the PC boards inside the oven and it died.  I removed the batteries, and rebuilt the boards.  It ran for a few more years and I learned a lot about precision crystal oscillators.  I have replaced the old double oven unit with a new Stanford ResearchSC-10 crystal oscillator.  It has both mechanical trim as well as external electrical frequency control.

The power supply uses two 723 based voltage regulators in series.  The first converts normal 120 VAC mains power down to about 20 volts to charge 3 each 6 Volt gel cell batteries.  The battery voltage goes through another 723 based power supply that drives the heaters and electronics using separate circuits.  When the mains power fails there is no switch over since the batteries are always feeding the final supply.  When the mains power comes back on the batteries charge.  If the blackout lasts too long a relay will disconnect the batteries to prevent battery damage from over discharge.

Stanford Research

My Gibbs got to where it was not economical to repair and was replaced by the SC 10 crystal oscillator.
Stanford Research was founded and still is based on physics rather on electrical engineering.  For me, their products offer very good value and performance for the money.  The SC 10 has both mechanical coarse frequency setting and a number of different options for how the electronic fine tuning will work.

By adding a printed circuit board that Radio Shack used to sell that has a number of decades of division the 10 MHz output can be divided down to a 1 PPS signal.  This can be compared to the 1 PPS from a GPS receiver.  At this time SA was turned on and the 1 PPS had about 100 nS of random jitter (although there was always one sat with SA turned off).  I used this combination to try and use the saw tooth correction feature on the Motorola VP GPS receiver, but there was a glitch in the Motorola saw tooth.

About this time I had written a Lab VIEW program to compare the SC 10 to GPS over a few hours and could fit a parabolic curve to the difference and solve for the aging of the SC 10.

Quartz Crystal Resonators and Oscillators for Frequency Control and Timing Applications by Rakon

Rubidium

The Rb and Cs "standards" are not absolute, they are just like a crystal in that their frequency needs to be set.  What they offer is a lower aging rate i.e. they are more stable.

Stanford Research Systems PRS10 Rb Source with built in time tag & GPS lock plus extensive RS-232 communications for only $1,495 is single qty!!!
TrueTime Inc - has a broad line of timing products, they are in Santa Rosa, near my location

Cesium

HP 5060A Cesium Beam Frequency Standard Option H21
Theory of Operation -

Allan Variance

The Allan variance is a statistical measure of the stability of an oscillator.  The slope of the Allan variance vs. sampling time plot will tell you about the kind of noise that is disturbing the oscillator.  The modified Allan variance adds another noise type and is preferred by some as a more inclusive characterization.  Typically Allan variance is measured on 1 PPS type of data using a time interval counter that has no dead time, i.e. it measures every tick.  The data is stored in a computer and then processed with different decimation intervals.

A similar characterization can be done in the frequency domain.  There has been a lot of high powered math done demonstrating that the Allan variance information in the time domain can be converted back and forth into the frequency domain.

Time Interval Metrology Enterprise - Allan's web page - New Unified Field Theory - Gravity is proportional to energy density, not simply density
Agilent (HP) app note 1289 The Science of Timekeeping (pub# 5965-7984E)
USNO - Clock Performance and Performance Measures.
Hamilton Technical Services - stability software and a number of on line papers

Sundials

Test Equipment

Links

There is a 9 second discrepancy between the voice announcement of the time from the USNO and an accurate clock.  The USNO says that it is caused by the audio streaming of the internet, but who knows?
 
 

The Time & Frequency Division of NIST maintains atomic clocks that are set from USNO observations.  Has a page for radio station WWVB (60 kHz) and another one for WWV & WWVH (2.5, 5, 10, 15 & 20 MHz).  There are a number of other quality pages at this site.  Their primary focus is on stable Frequency whereas the USNO is concerned with Time. Atomic Web Clock -Java clock showing both computer & Nist time
 
 
 
 
 

The PTTI is an organization that has meetings and publishes papers related to time and time interval.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

The IEEE Ultrasonics, Ferroelectrics and Frequency Control (UFFC) Society has annual meetings, publishes a periodical and papers.  This page also has a  large number of links.

The National Earth Orientation Service was organized to coordinate, collect, analyze, and distribute data from the various operational U. S. programs that monitor variations in the orientation of the Earth. It serves as the Sub-Bureau for Rapid Service and Predictions of the International Earth Rotation Service

Brief History of the Development of Ultra-precise Oscillators for Ground and Space Applications by Norton, Cloeren & Sulzer
 
 
 
 
 

IERS was created in 1988 by the International Union of Geodesy and Geophysics (IUGG) and the International Astronomical Union (IAU). It replaced the Earth rotation section of the Bureau International de l'Heure ( BIH ), and the International Polar Motion Service ( IPMS ). It is a member of the Federation of Astronomical and Geophysical Data Analysis Services (FAGS ).
 
 
 
 
 

Frequency Standards Lab - The Frequency Standards Laboratory houses the National Aeronautics and Space Administration's (NASA) Lead Center for Frequency and Time. The Laboratory supplies NASA's Deep Space Network (DSN) with the hardware, expertise and technology for state-of-the-art frequency standards, clocks, distribution networks, and time synchronization to enable deep space navigation and advanced radio science experiments.

Equipment

I have a collection of Hewlett Packard time and frequency instruments that goes back to the late fifty's.
Office Instruments   Time and Frequency Rack
I am using the Stanford Research ST 620 Time Interval Counter.  It seems optimized for working with 1 PPS signals.  It is an older instrument and some of the printing functions are written for Epson format, but the 25 pS one shot time resolution and display with 16 digits makes it ideal for 1PPS work.
HP 5216A 12.5 MHz  Electronic Counter - Nixie tube display, DIP ICs, 4 bits/digit output
eBay "Frequency Standard" - Army 1994 probably audio derived from a 1 MHz crystal, 11 relay controlled frequencies

Links

Brooke's Home Page

This is the 28618th time this page has been accessed since 30 Jan. 2000.

Note if you are seeing a bar at the top of the page you don't have Java enabled. This will also cause the clock at the top of the page to be missing.
You can just jump to this page:http://www.pacificsites.com/~brooke/timefreq.shtml