Populated load capacitors of the crystal. Started organizing issues in ECNs.

- atrf/ecn/INDEX: ECN index
- atrf/ecn/ecn0001.txt: Adjust balun component values (pending)
- atrf/ecn/ecn0002.txt: Add load capacitors to 16 MHz crystal (C14, C15)
- atrf.sch, atrf/wpan-atrf.brd: changed load capacitors from NC to 12 pF
- atrf/ecn/ecn0003.txt: Replace balun and filter with integrated balun
  (pending)
- TODO: one more CNTR-related task

atrf/ecn/INDEX

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+Number	Status	Description
+------	-------	---------------------------------------------------------------
+0001	Edit	Adjust balun component values
+0002	Done	Add load capacitors to 16 MHz crystal
+0003	Edit	Replace balun and filter with integrated balun

atrf/ecn/ecn0001.txt

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+Adjust balun component values
+
+Some of the components specified in the schematics were not at hand
+in my lab and were thus replaced with similar parts. Furthermore, the
+LED color was changed to ease visual identification:
+
+Component	Norminal	Board 1		Board 2
+---------------	---------------	---------------	-------------------
+LED (D1)	red		red		red-orange
+C6/C7		0.82 pF		0.7 pF		0.9 pF
+L2/L3		4.3 nH		4.7 nH		4.7 nH
+
+The resulting spectrum should be examined to determine which variant
+performs better. Note that the discrete balun circuit shall be
+replaced with an integrated balun later on.

atrf/ecn/ecn0002.txt

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+Add load capacitors to 16 MHz crystal (C14, C15)
+
+The crystal has a specified load capacitance of 8 pF. The AT86RF230 has
+an internal capacitor array that can be trimmed in 16 steps from 0 pF to
+4.5 pF.
+
+The crystal has a nominal tolerance of 15 ppm at 25 C plus a temperature
+tolerance of 15 ppm. The following deviations from 1 MHz at CLKM were
+measured with the CNTR board, roughly at 20-25 C:
+
+Board		Error		Meas. accuracy	C14/C15		Trim
+		(f, ppm)	(ppm, nom.)	(pF)		(pF)
+---------------	---------------	---------------	------------	--------
+2, "orange"	+268		 99.6		NC		0
+		+266		 99.5		NC		0
+		+266		 40.0		NC		0
+		+148		 99.8		NC		3.0
+		+147		 99.7		NC		3.0
+		+147		 40.0		NC		3.0
+		+111		100.0		NC		4.5
+		+110		 99.8		NC		4.5
+		- 13		 99.9		15		0
+		- 14		100.0		15		0
+		+ 11		 99.9		12		0
+		+ 11		 99.6		12		0
+
+1, "red"	+263		 99.7		NC		0
+		+263		100.0		NC		0
+		+107		 99.3		NC		4.5
+		+108		100.0		NC		4.5
+		+ 24		 99.8		10		0
+		+ 24		100.0		10		0
+		-  9		 99.7		10		4.5
+		-  9		100.0		10		4.5
+
+(Multiple measurements to assess CNTR performance and drift. Note that
+CNTR measurements are repeatable within 1 ppm, so the nominal accuracy
+appears to be far too pessimistic.)
+
+These measurements suggest that, combined with parasitic capacitance,
+load capacitors of 12 pF make the crystal roughly half the trim range
+faster than 16 MHz.

atrf/ecn/ecn0003.txt

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+Replace balun and filter with integrated balun
+
+(pick suitable balun)