Initial Commit - Copy from Altus Metrum AltOS

doc/frequency-cal.inc

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+[appendix]
+== Frequency Calibration
+
+All products that have radio interfaces require calibration of the radio
+frequency.  Normally, this calibration is done once during the production
+process and the resulting cal value is saved into non-volatile memory.  The
+procedure decribed here should only be used outside of the factory if you 
+are really convinced the radio calibration is bad, and you have access to
+the required tools to do the calibration.
+
+Because this procedure is only rarely needed in the field, we have not 
+written any fancy user interface for doing it .. some interaction with 
+and careful typing in a command-like style interface are required!
+
+=== Background Information
+
+The radio system on each board uses a quartz crystal to control
+a frequency synthesizer that can be programmed to a range of operating
+frequencies.  While these crystals are very stable, they have an accuracy
+specification that means once the base frequency they set is multiplied up
+to the typical operating range of our products, any variation also gets
+multiplied.  The objective of the calibration process is, indirectly, to
+measure the actual operating frequency of the crystal and adjust the way
+the frequency synthesizer is programmed to account for this variation.
+
+The frequency may shift a few tens of Hz over the full operating temperature
+range, and it may also shift a bit over time as the crystal ages.  But once
+properly calibrated, none of those changes are likely to ever cause any 
+operational problem, as the shift in operating frequency due to these factors
+is tiny compared to the bandwidth of our transmitted signal.
+
+=== Required Equipment
+
+The calibration process requires the ability to precisely measure the actual
+frequency of a steady CW carrier on or about the intended operating frequency
+in the vicinity of 435 MHz.  
+
+In production, we use an HP 5385A that is locked to a 10 MHz reference that 
+is in turn locked to GPS, which provides a highly accurate calibration.  Any 
+reasonably accurate frequency counter is likely to be sufficient.
+
+You also need a computer with terminal program and USB cable to attach to
+the board in question, along with a battery and power switch suitable for
+powering the board up.
+
+=== RF Calibration Procedure
+
+Using the terminal program, connect to the board over USB.  You will find 
+that you are now interacting with a command interpreter on the board.  Using
+'?' will show the available commands.  Of interest for this process are the
+'C' command which turns on a steady transmitted carrier on the currently
+selected operating frequency, and the 'c' subcommands that allow interaction
+with the saved configuration.
+
+Use the 'c s' command to discover and note the current radio calibration 
+value, and the operating frequency the board is configured for in kHz.
+
+Set up your frequency counter with a suitable antenna near the board's 
+antenna and use the 'C' command to turn on a steady carrier.  Let the
+frequency stabilize, and note what it is to as many digits as are steady
+on your counter's display.
+
+To calculate the new calibration value, the equation is:
+
+.  (intended_frequency / measured_frequency) * current_cal_value
+
+Set the new calibration value using 'c f <value>', then use 'c w' to save
+that cal value into non-volatile memory.  You can use the 'C' command again
+to confirm the operating frequency is now within a few 10's of Hz of the
+intended operating frequency.
+