Clock modules introduction 

Are you using GPS as a timing reference? 

Are you using some other timing source as a reference? Does this result in a 1 pulse per second (1PPS) signal? 

What happens when you lose GPS, or your reference source? 

Clock modules, also known as GPS disciplined oscillators GPSDO’s, are here to maintain timing accuracy (or 1PPS phase synchronisation) in the event that you lose GPS, or your reference source. This is called holdover mode. 

IQD offers a range of clock modules 

Some (the IQCM-100 and IQCM-200) are designed to receive your existing 1PPS signal and output their own 1PPS signal. This is an in-line solution for your system. The phase alignment of this output 1PPS signal will be maintained in the event that the input 1PPS signal is lost. 

Some of our clock modules offer an in-build GPS receiver (the IQCM-110). This means that you do not need to be involved in any GPS signal processing. For these you must connect an aerial, and the clock module will output a 1PPS signal which is locked to the GPS. In the event that GPS signal is lost, the clock module will maintain the accuracy of the 1PPS output.

The clock modules contain an OCXO with an output divided down to 1PPS, this is locked to the input 1PPS signal. the clock modules run a Kalman filter algorithm which monitors the corrections necessary to keep the output locked to the input. 

The more variations seen during holdover the harder this is to maintain. Therefore, the internal OCXO is very high specification. This means that the cost of the clock module can vary significantly depending on the specification you choose.

FIND IQD'S FULL RANGE OF DISCIPLINED OCXO HERE

How accurately do you need to maintain holdover, (the phase synchronisation of the 1PPS output)?

The best we can offer at the moment is ±1.5µs. This meets the timing requirements for LTE-TDD (we are currently working on improvements to the design in readiness for the next generation at ±0.9µs and beyond) 

If this is more than you require then you could reduce the cost significantly by asking for a more relaxed specification, for example ±8µs.

For how long must you maintain this accuracy?

The best we can offer is a 24hours period, again, this is in line with the requirements for LTE-TDD.

If this is more than you require then you could reduce the cost significantly by asking for a more relaxed specification, for example 8hours.

Would you really be running for a whole day before some other back up system was implemented? 

What temperature change will be seen during the holdover period?

The best we can offer is ±15°C. This means that whatever the temperature at the time GPS is lost it may vary by ±15°C during the holdover period. 

If this is more than you require then you could reduce the cost significantly by asking for a more relaxed specification, perhaps ±2°C.

Many applications are running in air-conditioned rooms where the temperature change is very small.

Additional functionality

The clock module gives a very stable 10MHz output derived from the OCXO which is locked to the input, if using GPS as your reference the 10MHz output is accurate to E10-12

The modules also incorporate a lock status output pin which gives instant logic status as to whether the module is in lock mode or holdover mode. 

The modules give a status input pin, which allows you to force the unit into holdover mode. 

Clock modules incorporate a communication port, information is available via the com port to allow your software to react to changes in the clock module.

Evaluation board

We offer evaluation boards for the clock modules designed to assist your design team in the early stages of development and testing. The clock module is plugged into the evaluation board. The evaluation board provides easy access to inputs and outputs via BNC and D type connectors, there are switches to allow various modes of the clock module to be selected and LEDs for visual indication of the status. We provide all the necessary cables and power supplies with the evaluation board so your design team can focus immediately on both the hardware testing and software design. 

Evaluation boards are available with or without on-board GPS receivers. This option might help if you lack a 1PPS signal in your development environment.  

Alternative applications for GPS DOCXO

Originally developed to meet the timing requirements of the telecommunications industry such as LTE-TDD, since introducing the clock modules range, we have found a number of unanticipated applications for example:

• Synchronisation of units too far apart for any other form of synchronisation.  
• A filter on the GPS timing signal. Since the 1PPS output of the clock module is derived from the internal OCXO with a lock to the input signal, it provides a form of filtering against some short-term noise from the input signals. 
• Short term synchronisation with no GPS. Systems which are keep locked to GPS until use, then immediately during use the system is placed in an environment with no GPS signal, but the clock modules maintain the synchronisation of the units.
• Low-cost reference signal. Using a clock module on an evaluation board you have created a 1PPS and 10MHz signal locked to the GPS, with holdover capability to cover GPS outage. For some engineers this has provided a low-cost reference signal for their laboratory.
• The parameters of the clock module were optimised for the telco industry application. However, we can, and have, modified these parameters in our software to optimise for specific application, call our Application Support team to discuss your requirements.

This application note introduces IQD Frequency Product’s range of advanced clock modules which provide electrical timing functionality for distributed network systems. These units primarily revolve around the 1PPS (Pulse Per Second) timing synchronisation signal produced by a GPS receiver which is derived from transmitted timing information received from GPS satellite systems.

Application Considerations

The main objective of these modules is to maintain the 1PPS (Pulse Per Second) signal of the GPS system and is used in communication network systems to synchronise timing. If your system relies on the GPS signal to maintain accuracy and the GPS signal fails due to loss of lock, bad weather, jamming or other issues then these clock modules are able to keep the 1PPS signal maintained until GPS lock is restored, enabling your network system to remain within specification.

What specification module do I require?

In order to decide which module is suitable for your specific application then various parameters need to be considered such as:

a. What holdover spec does my system require?

b. Over what time period will I be without the GPS signal?

c. What environmental conditions will the module see when experiencing the loss of GPS signal?

Why 1.5μs in 24hrs?

To fully meet the requirements of some specific telecoms protocols such as LTE, your product will have to show that it can maintain accuracy to within ±1.5μs over 24hours if the GPS signal is lost. However, some systems may not require this level of holdover and thus if this value can be relaxed then the unit cost can be reduced. Remember not to over-specify this parameter if it is not required.

Environmental change during holdover mode

These clock modules use an OCXO in combination with software algorithms to maintain their 1PPS accuracy during periods of no GPS signal and so it is important to understand what temperature changes will be seen by the module during loss of GPS. If your equipment will be used in an air-conditioned room then the temperature changes may be very small, perhaps ±2°C. However if the module is sited at for example the base of an aerial then it may experience a much wider temperature change during this no GPS period.

It is important to understand that the holdover temperature specification is not the same as the operating temperature range. The operating temperature range could be 0 to 70°C but the holdover change of temperature during no GPS signal may be only ±2°C to achieve the required 1.5μs over 24hr limit. This means that the module can be operated at any specific temperature within it’s operating range but when GPS signal is lost, the module must then remain within it’s much smaller holdover temperature range of for example ±2°C in order to stay within 1.5μs over 24hr.

Since the holdover temperature needs to be compensated for, the larger the change of temperature during loss of GPS then the harder the compensation becomes thus affecting unit cost. 

Holdover time period during loss of GPS signal

For applications where the GPS signal will not be lost for a full 24hrs, e.g. 8hrs, then there is no need to specify the clock module with its full holdover time. This relaxation in specification affects the unit cost so remember not to over-specify this parameter if it is not required.

For more information on Disciplined OCXO specification, read the Extended App Note below: