From e3127155c02cd42ef8aa42d167fc214e8997113f Mon Sep 17 00:00:00 2001 From: Bdale Garbee Date: Wed, 22 Dec 2021 11:35:52 -0700 Subject: [PATCH] doc: add an appendix on frequency calibration to the all-in-one manual --- doc/Makefile.am | 1 + doc/altusmetrum.txt | 2 ++ doc/frequency-cal.inc | 71 +++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 74 insertions(+) create mode 100644 doc/frequency-cal.inc diff --git a/doc/Makefile.am b/doc/Makefile.am index d7d92f2c..5ca61379 100644 --- a/doc/Makefile.am +++ b/doc/Makefile.am @@ -155,6 +155,7 @@ INC_FILES=\ installation.inc \ using-am-products.inc \ updating-firmware.inc \ + frequency-cal.inc \ altosui.inc \ altosdroid.inc \ system-operation.inc \ diff --git a/doc/altusmetrum.txt b/doc/altusmetrum.txt index 4b7699c5..41e79a2b 100644 --- a/doc/altusmetrum.txt +++ b/doc/altusmetrum.txt @@ -63,6 +63,8 @@ Keith Packard ; Bdale Garbee ; Bob Finch; Anth include::updating-firmware.adoc[] + include::frequency-cal.adoc[] + include::flight-data-recording.adoc[] include::specs.adoc[] diff --git a/doc/frequency-cal.inc b/doc/frequency-cal.inc new file mode 100644 index 00000000..7c663051 --- /dev/null +++ b/doc/frequency-cal.inc @@ -0,0 +1,71 @@ +[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 ', 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. + -- 2.30.2