Frequency Calibrator Documentation page
The Frequency Calibrator have the following characteristics
Is powered with a USB cable.
Comes with a GPS antenna with 3m cable.
Outputs any frequency from 1Hz to 10MHz with high accuracy.
Controlled by an Ardino Pro mini running open source firmware.
Configuration using a PC software (free)
0.5W (5V 100mA)
The Frequency Calibrator is using the GPS network to phase lock an internal local oscillator to a frequency that can set by the user. Frequencies from 1Hz to 10MHz can be set by a PC configuration program and automatically output on it's RF port every time the GPS have a good position fix.
The Frequency Calibrator can be operated standalone without a connection to a PC once it has ben configured. It then only needs 5V from it's micro USB port and connection to a GPS antenna.
The calibrator comes delivered with an indoors GPS antenna that can be put near a window for easy operation but for best performance a roof mounted outdoors GPS antenna should be used.
Se picture below for description of the connectors
The front connections
The back connections
The configuration software is free and runs on a PC with Windows.
Configuration made in the software can be stored in the deceive and used at startup so so the deceive can be run standalone without the need to having it connected to a computer.
The software includes a GPS signal quality meter that is useful to verify proper antenna position.
The signal meter should indicate 70% or more for best performance.
Choosing an output frequency for use in calibration.
As the output is typically compared to an external oscillator that needs to be calibrated the natural inclination would be to set the output frequency of the Calibrator to the same frequency as the external oscillator, but that is not always the best option as we will see.
E.g if a 10MHz oscillator is to be calibrated one would set the output to 10MHz
However the output jitter (short term frequency variations) is better when the Calibrator is set to output some specific frequencies.
The reason for that is due to the internal construction of the GPS and it's use of a internal Phase Locked Loop that has a 48MHz reference oscillator.
Therefore if the output frequency has a relationship to 48MHz that can be expressed with a division using an integer (as contrasted to an division with a fractional number) the output frequency will have the lowest possible jitter.
Please note that the output will always have quite a lot of jitter compared to say a crystal oscillator but by using a 48MHz integer division the output jitter is only due to the GPS system and not due to to fractional PLL spurs.
A few examples:
8MHz output is low jitter as 48 divided by 8 is the integer 6.
7MHz out is not low jitter as 48 dived by 7 is not an integer.
6MHz is OK as 48/6=8
5MHz is not OK as 48/5 is a not an integer.
4MHz is OK as 48/4 =12
In view of this it might be better to calibrate a 10MHz external oscillator by comparing it with a 2MHz output from the Calibrator. The 10MHz and 2MHz signals can easily be compared with an oscilloscope that has the two signals to its two signal inputs. Se picture for example.
The oscillator would be adjusted until no sideways drift can be seen on the oscilloscope
If low jitter is not important then any frequency can be used and general speaking - high jitter can be combated with longer averaging times of the output.
Regardless of the output frequency It is best practice to average the output over a few seconds for best precision.
Frequency precision down to or better than 1 part per billion is possible if averaging is used.