V
VK2RK
Guest
Frequency Sub Standard for the Amateur Radio Shack
Preamble.
Time and frequency measuring instruments require calibration insuring long term accuracy, this is done by comparing against a frequency / Time standard.
The calibration process provides certification for purposes of traceability, a requirement of industry or government when absolute accuracy is mandatory.
(Refer to NATA certification)
The cost of instrument calibration is not small, the certification is done by laboratory permitted to perform this task, further your very expensive device has to travel to and from the laboratory adding risk of damage in the transport process.
All of the above in the past as now is outside the reach of electronics hobbyists, unless very rich, however today things have changed, there are gadgets available to take some of the calibration assurances within the grasp of the hobbyist.
This article deals with Frequency and time measurement, so there is no consideration given to any other accuracy measurement task.
Frequency measurement.
Before the technologies that are available today , we only had one method assuring reasonable accuracy of either frequency or time, this was done by comparing against radio signals from Time Standard stations like WWV, a method often used by Amateur radio operators offering a degree of accuracy by zero beating the test signal against the received signal, usually requiring a marker oscillator, the accuracy in this method is only as good as the marker frequency itself, but for the purposes of operation this was close enough with an approximate accuracy of +/- 10 Hz
The use of a frequency counter would also suffice as long as the instrument was calibrated at some stage to conform to its long term stability requirements.
At best what was available to the hobbyist was comparing to a radio time standard station such as WWV or if worked in the industry the comparison against the instruments at work.
Today many alternatives are on offer with the availability of disposal professional service equipment.
Radio service monitor are now in reach of the serious hobbyist, providing all that is required for receiver / transmitter repair and alignment, the same thing can be said for high resolution frequency counters, in all cases these second hand instruments require the calibration to be looked at insuring accuracy.
Frequency accuracy requirements.
Manufacturers are now adding external frequency reference inputs to the amateur radio transceivers, this is becoming a requirement for some digital modes that mandate a stable and accurate frequency that is locked to an external reference signal.
Test equipment manufacturers have adopted for a very long time the use of an external reference input, with this providing accuracy and stability for all instruments in use to be as good as the reference signal.
Cesium beam frequency standard.
The Cesium beam clock frequency accuracy is that of 2 to 3 parts in 10 to the 14th, i.e. 0.0002 Hz; this corresponds to a time measurement accuracy of 2 nanoseconds per day or one second in 1,400,000 years. (https://en.wikipedia.org/wiki/Atomic_clock)
Long term stability of this kind offers the best frequency / time reference, used by commercial radio stations such as the BBC and any service requiring very high precision timing.
(Note that the GPS system requires high timing accuracy, all the satellites clocks are corrected by an earth station using a Cesium Clock)
Rubidium Frequency Standard. (https://en.wikipedia.org/wiki/Rubidium_standard)
The rubidium clock offer an accuracy of ±0.05 ppb (±5×10^11)
It can be considered as a substandard to the Cesium clock, offers very good short term stability
with a long term stability of <5×10^10 (yearly)
(Note that the Cesium clock long term stability (1,400,000 years) makes this the standard reference to the Rubidium Clock)
In the case of GPS satellites the on board Rubidium Clocks are adjusted every 12 hours, this overcomes the issue of time dilation (See Einstein Theory of Relativity) This correction has nothing to do with either long term or short term clock stability of a Rubidium clock.
GPS worst case time dilation.
Time dilation effect causes an error by a factor of 5×10^6, or about 45.9 μs/day.
This gravitational frequency shift is noticeable and requires correction to maintain the GPS positional accuracy. (Not a real issue in the application of a GPSDO)
Oven Controlled Oscillator. (OCXO)
Having described what frequency and time references are, I made some references to GPS signals, I did this because I am about to describe what I consider to be the best thing after a Rubidium Reference that costs many thousands of dollars.
Devices have appeared that utilise a very accurate short term stable oscillator, usually 10MHz, this kind of device is held at a constant temperature insuring that the crystal oscillator frequency remains constant in the short term with a long term frequency aging. 1 parts per Million (ppm): 1 ppm means 1/10^6 part of a nominal frequency.
Even this will require calibration assurance for long term accuracy.
GPS disciplined oscillator. (GPSDO)
Having considered the varied accurate frequency systems, what provides the required degree of accuracy is out of the reach of the hobbyist, or is it?
We have an oscillator that provided short term accuracy (OCXO) now all we need to do is keep the device calibrated against some high accuracy standard.
As usual some other technology that required high accuracy clocks come to the rescue, the mobile phone service is such technology.
Mobile phone towers use very accurate clocks to track a signal from a mobile comparing one tower to the other using the signal time difference to determine the mobile direction of travel and hand over form one tower to the other, this requires a very accurate clock.
So a device was developed that would keep an OCXO corrected by the GPS time signal.
In a worst case situation due to time dilation the largest error in the system would be that of 5x10^6 or 45.9 us due to GPS time signal dilation. The OCXO would be phase corrected every one second, making this a very accurate reference.
What has all of the above to do with Amateur Radio?
With the high reliability requirements along with the evolution of mobile phone systems many of the clocks used get replaced as part of preventative maintenance, the modules appear as disposals for a fraction of the cost price.
Enterprising amateurs developed solutions using these modules turning them into frequency references with long term stability that is useful as a shack standard.
Like all things many other profiteering individuals have jumped on the GPSDO band wagon offering very cheap and nasty devices. One has to be very careful when purchasing such a device that in fact uses the kind of technology required to obtain the required stability.
GPSDO control systems.
The disciplining of the OCXO can be done using one of two methods, one is by comparing a 1 second pulse from a GPS module with the divided frequency to 1 second pulse from the OCXO, generating a correction voltage as is the case in a PLL system, usually this system will produce some jitter that is a detriment to the accuracy or the use as a reference.
The more expensive GPS modules use digital correction that is the OCXO takes the signal from a DAC (Digital to Analogue convertor) this is then applied to the oscillator to correct the error.
The Trimble modules are one that uses this kind of control system providing no jitter.
When new these modules fetched a very high price, I am told over $1000.00 but I take this with a grain of salt, today a second hand unit can be had for a fraction of the new price. The quality is such that had been manufactured to provide a very long service life, making them ideal for our purposes.
Chinese made GPSDO.
I recently acquired a unit that is not cheap but for under $300.00 offers three 10 MHZ Sinusoidal ports with three 10MHz Square wave ports, along with two 1 sec pulse ports.
I purchase this with an expectation that I may need to return it if the quality was not up to my expectation as many Chinese items often are.
To my delight the unit arrived and was supplied with quality components that included:
One USB to RS232 Adaptor cable.
One D9 to D9 RS232 Serial Cable.
Eight BNC to SMA adaptors.
One GPS Antenna.
One 240V to 12 Volt 48 Watt plug pack.
The build consisted of a Trimble GPS module as used in 4G phone towers, a power supply regulator board to power the Trimble module and all was mounted on a the 10 MHz distribution mother board that included its own power supply regulators.
In the construction a large effort was made to provide stable power distribution providing best case isolation and decoupling of the power sources within the stages of the device.
I have soaked tested the unit for three days and the device ran with no issues at all.
The person that makes these units is a radio amateur in china, his specific purpose was to provide a reference for amateur radio associated equipment of sufficient quality at a low cost.
If I had to provide any criticism is that there is a standing DC voltage of 2.5 volts with no load on the ports, I further traced the circuit and he uses low pass filtering on each port, the second Harmonic (20MHz) is just a tad under the 17 dBm output to be 30 dB below, this is of no concern.
The DC voltage is, one has to insure that the input of the devices to be referenced have a DC blocking capacitor in place. Good engineering practices usually provide such blocking, but not in all cases.
Overall I found the device to be 100% satisfactory.
Understanding the Trimble Module.
Navigation GPS units have incorporated in them memory that stores the satellites constellation, this to provide fast response when cold starting the device.
Sometimes when a navigation GPS is not used for some time the Almanac (Satellite Constellation) is too old and the unit will download a new Almanac from any acquired Satellite. This explains why sometimes you get very fast mapping after turning the unit on and at other times seems slower. All to do on how old the Almanac is and what needs updating.
With the Trimble device, there is no real time mapping taking place in fact are usually in a stationary location as the case would be in a land survey or in the case of a Phone tower permanently fixed.
Thus cold start time is not really a consideration, it can take up to 5 minutes to acquire and lock the OCXO to the Satellite.
However the unit remembers the last OCXO adjustment and upon cold start will output a reference frequency that is temperature adjusted to be as precise as possible all done within the OCXO unit (Why these things are so expensive).
Should the device loose satellite lock it will still retain the very last adjusted frequency, providing the reference signal until the satellites are once again acquired and the disciplining resumes.
Remember that the short term stability of the OCXO is such that it does not require adjustments for at least 6 months, having said this the error will creep at the worst case figure of 45.9 us/day, in reality your equipment will be referenced at better than a .001 Hz
Once the lock is resumed, the OCXO will immediately get disciplined (if required). How often? Once every second.
This is it, hope you find this article informative 73’s VK2RK
Preamble.
Time and frequency measuring instruments require calibration insuring long term accuracy, this is done by comparing against a frequency / Time standard.
The calibration process provides certification for purposes of traceability, a requirement of industry or government when absolute accuracy is mandatory.
(Refer to NATA certification)
The cost of instrument calibration is not small, the certification is done by laboratory permitted to perform this task, further your very expensive device has to travel to and from the laboratory adding risk of damage in the transport process.
All of the above in the past as now is outside the reach of electronics hobbyists, unless very rich, however today things have changed, there are gadgets available to take some of the calibration assurances within the grasp of the hobbyist.
This article deals with Frequency and time measurement, so there is no consideration given to any other accuracy measurement task.
Frequency measurement.
Before the technologies that are available today , we only had one method assuring reasonable accuracy of either frequency or time, this was done by comparing against radio signals from Time Standard stations like WWV, a method often used by Amateur radio operators offering a degree of accuracy by zero beating the test signal against the received signal, usually requiring a marker oscillator, the accuracy in this method is only as good as the marker frequency itself, but for the purposes of operation this was close enough with an approximate accuracy of +/- 10 Hz
The use of a frequency counter would also suffice as long as the instrument was calibrated at some stage to conform to its long term stability requirements.
At best what was available to the hobbyist was comparing to a radio time standard station such as WWV or if worked in the industry the comparison against the instruments at work.
Today many alternatives are on offer with the availability of disposal professional service equipment.
Radio service monitor are now in reach of the serious hobbyist, providing all that is required for receiver / transmitter repair and alignment, the same thing can be said for high resolution frequency counters, in all cases these second hand instruments require the calibration to be looked at insuring accuracy.
Frequency accuracy requirements.
Manufacturers are now adding external frequency reference inputs to the amateur radio transceivers, this is becoming a requirement for some digital modes that mandate a stable and accurate frequency that is locked to an external reference signal.
Test equipment manufacturers have adopted for a very long time the use of an external reference input, with this providing accuracy and stability for all instruments in use to be as good as the reference signal.
Cesium beam frequency standard.
The Cesium beam clock frequency accuracy is that of 2 to 3 parts in 10 to the 14th, i.e. 0.0002 Hz; this corresponds to a time measurement accuracy of 2 nanoseconds per day or one second in 1,400,000 years. (https://en.wikipedia.org/wiki/Atomic_clock)
Long term stability of this kind offers the best frequency / time reference, used by commercial radio stations such as the BBC and any service requiring very high precision timing.
(Note that the GPS system requires high timing accuracy, all the satellites clocks are corrected by an earth station using a Cesium Clock)
Rubidium Frequency Standard. (https://en.wikipedia.org/wiki/Rubidium_standard)
The rubidium clock offer an accuracy of ±0.05 ppb (±5×10^11)
It can be considered as a substandard to the Cesium clock, offers very good short term stability
with a long term stability of <5×10^10 (yearly)
(Note that the Cesium clock long term stability (1,400,000 years) makes this the standard reference to the Rubidium Clock)
In the case of GPS satellites the on board Rubidium Clocks are adjusted every 12 hours, this overcomes the issue of time dilation (See Einstein Theory of Relativity) This correction has nothing to do with either long term or short term clock stability of a Rubidium clock.
GPS worst case time dilation.
Time dilation effect causes an error by a factor of 5×10^6, or about 45.9 μs/day.
This gravitational frequency shift is noticeable and requires correction to maintain the GPS positional accuracy. (Not a real issue in the application of a GPSDO)
Oven Controlled Oscillator. (OCXO)
Having described what frequency and time references are, I made some references to GPS signals, I did this because I am about to describe what I consider to be the best thing after a Rubidium Reference that costs many thousands of dollars.
Devices have appeared that utilise a very accurate short term stable oscillator, usually 10MHz, this kind of device is held at a constant temperature insuring that the crystal oscillator frequency remains constant in the short term with a long term frequency aging. 1 parts per Million (ppm): 1 ppm means 1/10^6 part of a nominal frequency.
Even this will require calibration assurance for long term accuracy.
GPS disciplined oscillator. (GPSDO)
Having considered the varied accurate frequency systems, what provides the required degree of accuracy is out of the reach of the hobbyist, or is it?
We have an oscillator that provided short term accuracy (OCXO) now all we need to do is keep the device calibrated against some high accuracy standard.
As usual some other technology that required high accuracy clocks come to the rescue, the mobile phone service is such technology.
Mobile phone towers use very accurate clocks to track a signal from a mobile comparing one tower to the other using the signal time difference to determine the mobile direction of travel and hand over form one tower to the other, this requires a very accurate clock.
So a device was developed that would keep an OCXO corrected by the GPS time signal.
In a worst case situation due to time dilation the largest error in the system would be that of 5x10^6 or 45.9 us due to GPS time signal dilation. The OCXO would be phase corrected every one second, making this a very accurate reference.
What has all of the above to do with Amateur Radio?
With the high reliability requirements along with the evolution of mobile phone systems many of the clocks used get replaced as part of preventative maintenance, the modules appear as disposals for a fraction of the cost price.
Enterprising amateurs developed solutions using these modules turning them into frequency references with long term stability that is useful as a shack standard.
Like all things many other profiteering individuals have jumped on the GPSDO band wagon offering very cheap and nasty devices. One has to be very careful when purchasing such a device that in fact uses the kind of technology required to obtain the required stability.
GPSDO control systems.
The disciplining of the OCXO can be done using one of two methods, one is by comparing a 1 second pulse from a GPS module with the divided frequency to 1 second pulse from the OCXO, generating a correction voltage as is the case in a PLL system, usually this system will produce some jitter that is a detriment to the accuracy or the use as a reference.
The more expensive GPS modules use digital correction that is the OCXO takes the signal from a DAC (Digital to Analogue convertor) this is then applied to the oscillator to correct the error.
The Trimble modules are one that uses this kind of control system providing no jitter.
When new these modules fetched a very high price, I am told over $1000.00 but I take this with a grain of salt, today a second hand unit can be had for a fraction of the new price. The quality is such that had been manufactured to provide a very long service life, making them ideal for our purposes.
Chinese made GPSDO.
I recently acquired a unit that is not cheap but for under $300.00 offers three 10 MHZ Sinusoidal ports with three 10MHz Square wave ports, along with two 1 sec pulse ports.
I purchase this with an expectation that I may need to return it if the quality was not up to my expectation as many Chinese items often are.
To my delight the unit arrived and was supplied with quality components that included:
One USB to RS232 Adaptor cable.
One D9 to D9 RS232 Serial Cable.
Eight BNC to SMA adaptors.
One GPS Antenna.
One 240V to 12 Volt 48 Watt plug pack.
The build consisted of a Trimble GPS module as used in 4G phone towers, a power supply regulator board to power the Trimble module and all was mounted on a the 10 MHz distribution mother board that included its own power supply regulators.
In the construction a large effort was made to provide stable power distribution providing best case isolation and decoupling of the power sources within the stages of the device.
I have soaked tested the unit for three days and the device ran with no issues at all.
The person that makes these units is a radio amateur in china, his specific purpose was to provide a reference for amateur radio associated equipment of sufficient quality at a low cost.
If I had to provide any criticism is that there is a standing DC voltage of 2.5 volts with no load on the ports, I further traced the circuit and he uses low pass filtering on each port, the second Harmonic (20MHz) is just a tad under the 17 dBm output to be 30 dB below, this is of no concern.
The DC voltage is, one has to insure that the input of the devices to be referenced have a DC blocking capacitor in place. Good engineering practices usually provide such blocking, but not in all cases.
Overall I found the device to be 100% satisfactory.
Understanding the Trimble Module.
Navigation GPS units have incorporated in them memory that stores the satellites constellation, this to provide fast response when cold starting the device.
Sometimes when a navigation GPS is not used for some time the Almanac (Satellite Constellation) is too old and the unit will download a new Almanac from any acquired Satellite. This explains why sometimes you get very fast mapping after turning the unit on and at other times seems slower. All to do on how old the Almanac is and what needs updating.
With the Trimble device, there is no real time mapping taking place in fact are usually in a stationary location as the case would be in a land survey or in the case of a Phone tower permanently fixed.
Thus cold start time is not really a consideration, it can take up to 5 minutes to acquire and lock the OCXO to the Satellite.
However the unit remembers the last OCXO adjustment and upon cold start will output a reference frequency that is temperature adjusted to be as precise as possible all done within the OCXO unit (Why these things are so expensive).
Should the device loose satellite lock it will still retain the very last adjusted frequency, providing the reference signal until the satellites are once again acquired and the disciplining resumes.
Remember that the short term stability of the OCXO is such that it does not require adjustments for at least 6 months, having said this the error will creep at the worst case figure of 45.9 us/day, in reality your equipment will be referenced at better than a .001 Hz
Once the lock is resumed, the OCXO will immediately get disciplined (if required). How often? Once every second.
This is it, hope you find this article informative 73’s VK2RK