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
Frequency Error Correction
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
in RF communication systems.
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
internal circuitry is still in the “warm-up” phase.
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
can re-calibrate the unit for a nominal cost.
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
available.
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
system is very accurate.
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.
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