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