AN-1104

Application Note: GPS Disciplined Sources

For more information, contact Diarcy Technologies ■ 3804 Tamarack Trail, Austin, TX 78727 USA

+1.512.238.6840 ■ Skype: diarcy.technologies ■ contact_us@diarcy.com ■ www.diarcy.com

Rev AN-1104a

Page 3

If one were to monitor a typical Signal Generator output from a cold start to thermal steady-state

conditions, one would see a significant frequency shift (many kilohertz). Even with a stable OCXO

Source, the frequency error over time would look something like the red trace shown here. Although

the OCXO warms up quickly, the absolute error can still be

off from 5 ppm to 1 ppm depending on the model. At a

1 ppm error, the frequency error is still too large to be used

Our FR-Series Frequency Standards “shift” this error by

calibrating the various errors seen by the oscillator as

shown in the blue trace. This “correction factor” is stored

within each unit and is unique to each system. Thus, every

time the FR-Series Frequency Standard is turned on, the

unit’s STBY LED is active to indicate to the user that the

The only error that remains after calibration of an FR-Series

Frequency Standard is the aging of the internal OCXO. This

error is common to all oscillator systems. However, the

error is bounded by the specifications of the OCXO, typically

less than 4.5 ppm over a 10 year period. Thus, depending on

the application, the FR-Series may not need recalibrating. However, if it does, Diarcy Technologies

Our DS-Series Frequency Standards take a different approach as shown in the green trace. Since GPS

signals are available, the DS-Series Frequency Standards are constantly computing the error in the

oscillator system. Every time a new correction factor is computed, it is applied to the oscillator

system and also stored in memory. This stored value is used on subsequent power-ups. This can also

be useful if the GPS system is not active since the latest correction factor has already been applied.

Any long-term aging of the internal OCXO is accounted for in the constantly updating algorithm. Even

after 10 years, the internal oscillator system continues to be updated as long as the GPS signals are

Since the DS-Series and FR-Series take different approaches to removing the frequency and thermal

errors, different calibration algorithms are used within each series. In both cases, the GPS system

error must also be taken into account. This error is inherent in the GPS system, usually dominated by

the satellite geometry at a given time. However, over a long period of time (hours or days), the GPS

Thus, in the FR-Series Frequency Standard, the calibration used takes many hours, sometimes up to

5 to 10 hours. Whereas in the DS-Frequency Series Frequency Standard, the integration time is less.