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--- as3935 [2024/04/11 08:32] – river
+++ as3935 [2024/07/25 22:33] (current) – [Calibration] river
@@ Line 47: / Line 47: @@
 A badly designed, pulled, aged, uncompensated crystal oscillator might be 50ppm off, so much better than needed as a calibration standard for this application. Therefore can use a µC to calibrate it automatically. I don't see this done in the Sparkfun library even though it seems obvious.
-The chip can the calibrate its clocks from the above value, so antenna should be trimmed first.
+The chip can then calibrate its clocks from the above value, so antenna should be trimmed first.
 Clocks calibration must be repeated after power loss.
@@ Line 80: / Line 80: @@
 I wrote a program to test and tune it. Initially found:
+<file>
 [016412] [D] measure_frequency(): counts: 33133, time: 999837, frequency: 530214 Hz
 [016412] [D] Trimcap: 0pF, Error: 30214 Hz, 6.0%
@@ Line 112: / Line 113: @@
 [031442] [D] measure_frequency(): counts: 31516, time: 1000023, frequency: 504244 Hz
 [031442] [D] Trimcap: 120pF, Error: 4244 Hz, 0.8%
-[031444] [D] Lowest error: 0.848800%. Using 120pF.
+[031444] [D] Lowest error: 0.848800%. Using 120pF.''
+</file>
 Added 2x 6.8pF NPO caps, soldered on top of and beside the existing caps. Then measured:
+<file>
 [020013] [D] measure_frequency(): counts: 31876, time: 1000755, frequency: 509631 Hz
 [020013] [D] Trimcap: 0pF, Error: 9631 Hz, 1.9%
@@ Line 149: / Line 152: @@
 [035045] [D] Trimcap: 120pF, Error: -13223 Hz, -2.6%
 [035047] [D] Lowest error: 0.034200%. Using 48pF.
+</file>
 Cleaned off flux and got:
+<file>
 [021200] [D] measure_frequency(): counts: 32138, time: 1000670, frequency: 513863 Hz
 [021200] [D] Trimcap: 0pF, Error: 13863 Hz, 2.8%
@@ Line 185: / Line 190: @@
 [036237] [D] Trimcap: 120pF, Error: -10133 Hz, -2.0%
 [036239] [D] Lowest error: 0.144200%. Using 64pF.
+</file>
 I don't know why added the caps seemed to add way more capacitance then expected. It looks like I added more like 80pf rather than the 13.6pF expected.
 The better tuning increased the rate of QRM detections from fairly frequent to constant. Running in my lab with otherwise default settings.
+Over the next hours I noticed the capacitance drifted back down again, needing more from the trimcaps. Maybe board drying out or cooling down? The next day it was using 96pF or 88pF trimcaps.
+Next day, added 51pF. Now it gets: Trimcap: 0pF, Error: -134 Hz, -0.0%
+Also added 22uF ceramic to power socket pins, and 1nF from IRQ to ground. Removed electrolytic. Also modified µC board - added bigger 3.3V regulator with ceramic caps and 470uF input cap.
+Get occasional QRM reports but nothing like without any caps that started with.
+===== Noise =====
+It's very sensitive to interference / noise. It was constantly reporting disturbers.
+I tried adding more caps to the power supply and put a ferrite round the leads. The biggest difference seen so far came from soldering a 47uF electrolytic to the back of the connector on the sensor board. This seems to have stopped the disturber reports. I also have a ferrite on the leads but this makes less / no difference I think.
+==== Probe disturbance ====
+Touching any of the pins on the sensor board causes spurious reports. That is, one multimeter lead touched to any pin, e.g. ground, with the other lead connected to nothing, or the other end of the lead disconnected, causes report of disturber, or lightning, or an error. Very often an I2C error is reported by the µC, and the interrupt register reads 15. Not sure if the 15 is coming from the sensor or I2C lib on a failed read.
+This problem was resolved with the cap mentioned above.
+===== Errata? =====
+Interrupt pin does not go high on detection after initial configuration / cap trimming, until a read is made to the interrupt register. After that it works normally.
+(Not extensively tested, maybe reading other registers works too.)
+I read that some boards have wrong tuning caps. I couldn't find anything about cap values in the datasheet, but one website suggested 680pF + 270pF = 950pF, which looks about right, maybe a bit small. My DFRobot board has 470pF x 2 = 940uF, according to the schematic anyway, which seems to be a bit too little.
+===== Links =====
+  * [[https://bitbucket.org/christandlg/as3935mi/|AS3935MI Library]]. Looks much better than the Sparkfun one I tried.
+  * [[https://sites.google.com/view/as3935workbook/home]] - Very comprehensive testing / tuning of the sensor.
+  * [[https://github.com/PlayingWithFusion/PWFusion_Lightning_Emulator/tree/master|Open Source Lightning Emulator]]
+  * [[https://www.digikey.ca/en/articles/incoming-storm-a-lightning-detector-from-ams]]
+  * [[https://www.eevblog.com/forum/microcontrollers/define-mmsbyte-for-as3935-lightning-detector/|Info on energy bits]]