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Theory and PracticeTheory and Practiceofof
Rb Frequency StandardsRb Frequency Standards
Doug Millar K6JEYDoug Millar K6JEY
SBMSSBMS
InformationInformation
This is a general talk about Rubidium This is a general talk about Rubidium Oscillators and meant for those who have Oscillators and meant for those who have just bought their LPRO oscillators. I have just bought their LPRO oscillators. I have had to strike a balance between technical had to strike a balance between technical information and practical information. The information and practical information. The workings of “atomic clocks” are very workings of “atomic clocks” are very interesting. Consult the sources listed interesting. Consult the sources listed below. below.
History of the ProjectHistory of the Project
A couple of months ago Robin WA6CDR and I decided that we A couple of months ago Robin WA6CDR and I decided that we should try to acquire enough units of an inexpensive Rubidium (Rb) should try to acquire enough units of an inexpensive Rubidium (Rb) oscillator so that as many microwavers as possible could put them oscillator so that as many microwavers as possible could put them in their rigs. That way stability and accuracy could be improved for in their rigs. That way stability and accuracy could be improved for the group. We expected our initial order of 30 or so would be the group. We expected our initial order of 30 or so would be enough. We had that many orders in an hour. We are now up to 180 enough. We had that many orders in an hour. We are now up to 180 units and they are nearly spoken for. units and they are nearly spoken for. Each unit is checked for proper operation and lock but not Each unit is checked for proper operation and lock but not calibrated. Real calibration is a labor intensive job. However, we calibrated. Real calibration is a labor intensive job. However, we have found that the units are amazingly close in frequency and we have found that the units are amazingly close in frequency and we are sure that you will not be able to tell one from the other in are sure that you will not be able to tell one from the other in frequency in normal operation. frequency in normal operation. Do not, unless you have experience and a lab, do any adjustment Do not, unless you have experience and a lab, do any adjustment on the oscillator.on the oscillator.Do write down the lamp voltage and crystal voltage and check it Do write down the lamp voltage and crystal voltage and check it once a year. once a year. The connector is very common. The connector is very common.
Comparison of OscillatorsComparison of Oscillators
CesiumCesium RbRb OCXOOCXO
AccuracyAccuracy 5e-135e-13 1e-121e-12 1e-101e-10
AgingAging N/AN/A 5e-11/Mo5e-11/Mo 5e-10/day5e-10/day
Ph NoisePh Noise -130dbc-130dbc -130dbc-130dbc -140dbc-140dbc
Warm UpWarm Up 10min10min 2-30min2-30min 30min30min
StabilityStability 1e-141e-14 3e-123e-12 1e-101e-10
CostCost $60K$60K $60$60 $100$100
Rb Stability ChartRb Stability Chart
The chart is for an HP RbThe chart is for an HP Rb
One hour is 3.6k secondsOne hour is 3.6k seconds
One day is 86.4k secondsOne day is 86.4k seconds
One week is 601k secondsOne week is 601k seconds
Ten days is 864k secondsTen days is 864k seconds
GPS Lock Data GPS Lock Data 1 hour= 3.6k sec,1day = 86K sec1 hour= 3.6k sec,1day = 86K sec
Interesting VariationsInteresting Variations
This chart shows accuracy at 10e-12 and lower This chart shows accuracy at 10e-12 and lower that occur over a period of time but that are not that occur over a period of time but that are not related to drift of the Rb. Some variations related to drift of the Rb. Some variations happen at the same time (midnight!) and may be happen at the same time (midnight!) and may be temperature related.temperature related.
Rb’s are affected at these levels by forces that Rb’s are affected at these levels by forces that are not well known. Some effects have to do are not well known. Some effects have to do with the Rb gas mixes, which limits their with the Rb gas mixes, which limits their accuracy in some situations. accuracy in some situations.
Rb EphemerisRb Ephemeris
There are diurnal effects on frequencyThere are diurnal effects on frequency
Practically stability can be 2e-12/mo.Practically stability can be 2e-12/mo.
Daily average frequency can be parts in Daily average frequency can be parts in 10e-13.10e-13.
Some Advantages of an Rb Over Some Advantages of an Rb Over an OCXOan OCXO
Less need for calibrationLess need for calibration
Much greater stability in the field.Much greater stability in the field. Less affected by temperature and vibrationLess affected by temperature and vibration Less sensitivity to voltage changes.Less sensitivity to voltage changes. Used Rb’s a better gamble than used OCXO’sUsed Rb’s a better gamble than used OCXO’s
How does an Rb Work?How does an Rb Work?
Simply, the “atomic package” has a lamp Simply, the “atomic package” has a lamp with Rbwith Rb8787 gas in it that is heated by RF. gas in it that is heated by RF.
The Rb radiation is sent through a The Rb radiation is sent through a chamber with Rbchamber with Rb8787 in it . in it .
A photo cell on the far side detects the A photo cell on the far side detects the amount of light reaching it.amount of light reaching it.
When the chamber is fed with RF at When the chamber is fed with RF at
6.834,685Ghz the light level changes.6.834,685Ghz the light level changes.
The photocell is servoed to the oscillator and the The photocell is servoed to the oscillator and the oscillator is kept at the null in intensity. A 10mhz oscillator is kept at the null in intensity. A 10mhz oscillator is linked to the servo and the stability oscillator is linked to the servo and the stability of the null is reflected in the 10mhz output. of the null is reflected in the 10mhz output. The physics package has to be calibrated when The physics package has to be calibrated when it is built and has to be recalibrated as factors it is built and has to be recalibrated as factors change in the physics package.change in the physics package.Depending on how robust you make the physics Depending on how robust you make the physics package the oscillator and lamp will last longer package the oscillator and lamp will last longer and be more tolerant of outside forces like and be more tolerant of outside forces like magnetism and temperature. magnetism and temperature. The calibration mechanism is to change the C The calibration mechanism is to change the C field, which is a coil around the package that field, which is a coil around the package that effects the Rb gas resonance. effects the Rb gas resonance.
Block Diagram of a PRS 10Block Diagram of a PRS 10
In practice Rb’s are so tough some have In practice Rb’s are so tough some have been space qualified. been space qualified.
They have an ordinary life of 15-20 years. They have an ordinary life of 15-20 years.
By keeping track of the lamp voltage, one By keeping track of the lamp voltage, one can chart how old the Rb is getting. can chart how old the Rb is getting.
Other than the lamp itself little else seems Other than the lamp itself little else seems to go wrong.to go wrong.
Now Some Details about Now Some Details about Three Types of Rb’sThree Types of Rb’s
Efratom FRK – Efratom FRK – Old (1970’s technology)Old (1970’s technology)
Efratom LPRO- Efratom LPRO- Modern and inexpensiveModern and inexpensive
Stanford Research Associates PRS 10-Stanford Research Associates PRS 10- New and well engineered. New and well engineered. ExpensiveExpensive
Efratom FRKEfratom FRK
The FRK series were the best of the original The FRK series were the best of the original types. All analogue, brute force physics package types. All analogue, brute force physics package and well shielded. All discreet components. Mine and well shielded. All discreet components. Mine still works fine. They are big in comparison to the still works fine. They are big in comparison to the LPRO. TTL or Sinewave output.LPRO. TTL or Sinewave output.
They are probably the most stable Rb’s next to They are probably the most stable Rb’s next to the PRS 10. the PRS 10.
FRS-C is a miniature version and is more FRS-C is a miniature version and is more susceptible to magnetics and a little less stable.susceptible to magnetics and a little less stable.
The LPRO RbThe LPRO Rb
Trimble originally made them to be Trimble originally made them to be inexpensive, robust and maintenance free.inexpensive, robust and maintenance free.They designed it for 10 year full time They designed it for 10 year full time service life. service life. They have an ultra small physics package They have an ultra small physics package and a lesser amount of Rband a lesser amount of Rb8787 loaded in loaded in them.them.Cost is very reasonable. Eg. If yours dies, Cost is very reasonable. Eg. If yours dies, get another one. get another one.
LPRO Insides
PRS 10 AssetsPRS 10 Assets
It has a 20 year lamp lifeIt has a 20 year lamp life
Has an internal CPU that estimates a Has an internal CPU that estimates a 72hour hold-over. Turn it off and on and it 72hour hold-over. Turn it off and on and it comes right back to frequency.comes right back to frequency.
-130db/hz @10hz Phase noise-130db/hz @10hz Phase noise
RS 232 communications to and from RbRS 232 communications to and from Rb
Cost $1400 ($400 surplus)Cost $1400 ($400 surplus)
The PRS 10 Inside is BusyThe PRS 10 Inside is Busy
PRS 10 Without BoardsPRS 10 Without Boards
Lamp PhysicsLamp Physics Pkg
What is the Difference?What is the Difference?
From the other station’s point of view, all From the other station’s point of view, all of them will keep you within 10hz for a of them will keep you within 10hz for a month at 10,368.125,000mhz.month at 10,368.125,000mhz.
The LPRO’s are cheap and plentiful, The LPRO’s are cheap and plentiful, others are more expensive and/or older.others are more expensive and/or older.
So, use of any Rb will dramatically So, use of any Rb will dramatically improve your rig’s stability, but the LPRO improve your rig’s stability, but the LPRO does represent an inexpensive choice.does represent an inexpensive choice.
Care and FeedingCare and Feeding
Make sure your power supply is filtered. Make sure your power supply is filtered. Keep it away from strong magnetic fields.Keep it away from strong magnetic fields.Consider a sun shade if used portable. Consider a sun shade if used portable. Use a heat sink. Almost any base plate will Use a heat sink. Almost any base plate will do. The internal regulator is 17v, so a do. The internal regulator is 17v, so a supply of about 20v will dissipate less heat supply of about 20v will dissipate less heat than higher voltages.than higher voltages.Write down the lamp voltage and crystal Write down the lamp voltage and crystal voltage for trend analysis.voltage for trend analysis.
In OperationIn Operation
When the Rb locks it will be about 1e-10. After 30 When the Rb locks it will be about 1e-10. After 30 minutes it should be 1e-11. that means that you will be minutes it should be 1e-11. that means that you will be within 50hz at 10ghz at lock, and stabilize to better than within 50hz at 10ghz at lock, and stabilize to better than 10hz in the next few minutes. 10hz in the next few minutes. Do not attempt to adjust it Do not attempt to adjust it unless you have a good time lab. unless you have a good time lab. The LPRO’s lock after more than 2 minutes and go to The LPRO’s lock after more than 2 minutes and go to better than 10e-10 at that time. Have a lock light to tell better than 10e-10 at that time. Have a lock light to tell you when you are locked. A schematic for one is on the you when you are locked. A schematic for one is on the SBMS website.SBMS website.You might want to test your IF rig for stability to see how You might want to test your IF rig for stability to see how much it contributes to the total drift. Eg.You might want much it contributes to the total drift. Eg.You might want to get a TCXO for your FT 817. to get a TCXO for your FT 817.
Further InformationFurther Information
For more information here are some resources. For more information here are some resources. A good spot is this site- go to the PRS 10 ad A good spot is this site- go to the PRS 10 ad and look for the data sheet and manual. and look for the data sheet and manual. www.thinksrs.com/products/PRS10.htmwww.thinksrs.com/products/PRS10.htmSome of the graphs were taken from:Some of the graphs were taken from:
www.febo.com/time-freq/index.htmlwww.febo.com/time-freq/index.html Tom’s website has excellent info. Tom’s website has excellent info.
The SBMS website has the LPRO manual and a The SBMS website has the LPRO manual and a lock light design: lock light design: http://www.ham-radio.com/sbmshttp://www.ham-radio.com/sbms
