More RF research for the ECNs (balun and layout)

- atrf/ecn/ecn0001.txt, atrf/ecn/ecn0002.txt: changed ECN format to
  separate title from body by two blank lines and, where a conclusions has
  been reached, put it into a section separated from body also by two blank
  lines
- atrf/ecn/ecn0003.txt: researched balun design and availability
- atrf/ecn/INDEX, atrf/ecn/ecn0004.txt: new ECN: Take into account layout
  considerations for RF
- TODO: removed item covered by ECN0002
- TODO: removed bug fixed in f32xbase commit
  79396b17772639cea615d1c87870d55f08c11850
- TODO: added reference to ECN0003

TODO

@@ -37,11 +37,6 @@ inside a Ben.
 Things not done yet
 -------------------
 
-- define values for crystal load capacitors. Measurements with instruments not
-  quite precise enough (+/- 100 ppm) for the task (+/- 40 ppm, preferably
-  < +/- 10 ppm) suggest that we're about 300 ppm off with no capacitors at all
-  and can only correct about 150 ppm with the internal trim capacitors.
-
 - examine spectrum around carrier frequency and first harmonic to look for
   obvious distortions.  Vary transmit power.
 
@@ -60,7 +55,7 @@ Things not done yet
 - verify that the Ben can output an a) 16 MHz clock, and b) with +/- 40 ppm
 
 - replace discrete balun and filter with integrated solution, to reduce BOM
-  size, maybe cost, insertion loss, and PCB space
+  size, maybe cost, insertion loss, and PCB space (see ECN0003)
 
 - check if we really need three DC blocking caps in the RF path
 
@@ -80,10 +75,6 @@ Things not done yet
 Bugs to fix
 -----------
 
-- two of my systems (tv and u1010) flat out refuse to talk to the board's USB
-  application, but have no problem talking to its DFU boot loader. Very
-  strange.
-
 - atrf vs. atspi naming is a bit confusing
 
 

atrf/ecn/INDEX

@@ -3,3 +3,4 @@ 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
+0004	Edit	Take into account layout considerations for RF

atrf/ecn/ecn0001.txt

@@ -1,5 +1,6 @@
 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:

atrf/ecn/ecn0002.txt

@@ -1,5 +1,6 @@
 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.
@@ -41,6 +42,7 @@ Board		Error		Meas. accuracy	C14/C15		Trim
 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.
+
+Conclusion: 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

@@ -1,3 +1,58 @@
 Replace balun and filter with integrated balun
 
-(pick suitable balun)
+
+We consider the balun needs for the AT86RF230 and the TI/Chipcon
+CC2520 we may try as a design alternative.
+
+For the AT86RF230, Atmel recommend baluns with integrated filter,
+namely the Wuerth 748421245 and the Johanson 2450FB15L0001, both in the
+AR86RF230 data sheet.
+
+For the CC2520, TI recommend a microstrip-based design for the balun,
+both in the CC2420/CC243x/CC2480 application note [1] and the reference
+design [2], without fully characterizing neither the transceiver's RF
+output nor all the components in the balun.
+
+Johanson lists the 2450FB15L0001 [3] and even a 2450BM15B0002 [3] for
+the TI/Chipcon CC2520, but not even Octoparts is able to find a
+distributor for these parts.
+
+Digging deeper, TI reveal more information about the balun in [5].
+
+Finally, TI somewhat hesitatingly acknowledge that the Murata balun
+LDB182G4510C-110 can be used for the CC2520 [6], with a 3.9 nH inductor
+connecting RF_P and RF_N, the balun coupled to GND not directly but via
+10 nF, and finally an LC low-pass filter with 1.5 nH and 2.2 pF at the
+output, for EMI compliance.
+
+Note that this also means that CC2520 and AT86RF230 both have an
+impedance of 100 Ohm.
+
+A balun without filter similar to the Murata part would be the Johanson
+2450BL15K100.
+
+Manufacturer	Part number		Package	Digi-Key	Price/Qty
+---------------	-----------------------	-------	---------------	---------------
+Wuerth		748421245		0805-6	732-2230-1-ND	2.15/25
+						732-2230-2-ND	0.753/4000
+Johanson	2450FB15L0001		0805-6	-		-
+Johanson	2450BM15B0002		0805-6	-		-
+Johanson	2450BL15K100		0805-6	712-1045-1-ND	0.488/10
+						712-1045-2-ND	0.225/4000
+Murata		LDB182G4510C-110	0603-6	490-5023-1-ND	0.325/10
+						490-5023-2-ND	0.114/4000
+
+[1] http://www.ti.com/litv/pdf/swra098d
+[2] http://focus.ti.com/docs/toolsw/folders/print/cc2520em_refdes.html
+[3] http://www.johansontechnology.com/images/stories/ip/baluns/Balun_Filter_Combo_Matched_2450FB15L0001_v11.pdf
+[4] http://www.johansontechnology.com/images/stories/ip/baluns/balun_filter_combo_matched_2450bm15b0002_v2.pdf
+[5] http://www.ti.com/litv/pdf/swra236a
+[6] http://e2e.ti.com/support/low_power_rf/f/155/t/15910.aspx
+
+
+Conclusion: the Wurth balun appears to be the safest choice for
+prototyping the AT86RF230. Due to its high cost, a circuit with a
+discrete filter may be considered for larger quantities.
+
+For the CC2520, it's probably safest to directly try the Muarta balun
+with the recommended discrete filtering circuit.

atrf/ecn/ecn0004.txt

@@ -0,0 +1,73 @@
+Take into account layout considerations for RF
+
+
+There are a number of layout considerationg when designing RF systems
+that were not taken into account or not quantified when making the
+first design.
+
+- transmission line width
+
+  The microstrip [1] transmission line connecting the balun and filter
+  circuit with the antenna must be impedance-matched with the antenna.
+  The rule of thumb according to [2] is to make its width twice the
+  board thickness, in this case 0.8 mm or 31.5 mil.
+
+  The microstrip calculator at [3] also takes into account the
+  thickness of the copper, 1 oz, and yields a slightly narrower 57.5
+  mil or 1.46 mm.
+
+  A more elaborate calculator can be found at [4].
+
+- via spacing
+
+  Section 4.2 of [5] recommends a via spacing of no more than
+  Lvia = C/sqrt(Er)/Fres
+  where
+  C = the speed of light, 3*10^8 m/s
+  Er = the board's dielectric constant, 4.5 for FR-4
+  Fres = the resonance frequency, at least 24.5 GHz
+
+  We thus obtain Lvia = 5 mm.
+
+- component placing
+
+  [5] places DC blocking, balun, and filter close to the transceiver,
+  with only the feed line between the RF circuit and the antenna. Thus,
+  no changes are needed.
+
+- feed line termination
+
+  Point 12 of [6] warns us that we may need to terminate the
+  transmission line if it is longer than 20% of the signal's rise time.
+
+  Point 1 of [6] gives the rise time as 1/(10*Fclk), which looks as if
+  it's meant for digital signals. But we'll use it anyway.
+
+  [2] gives us the typical propagation delay for a microstrip as
+  150 pS/in.
+
+  This means that Lmax = 0.2*tr*v
+  with
+  tr = 1/24.5 GHz
+  v = 1 in/150 pS
+
+  We thus obtain Lmax = 1.4 mm
+
+  [2] suggests that the maximum unterminated stub is L(in) = tr(nS).
+
+  With tr = 1/(10*Fclk), we thus obtain Lmax = 1.04 mm.
+
+  Not sure if all this even applies to antennas. This needs looking to by
+  someone who understands about RF.
+
+[1] http://en.wikipedia.org/wiki/Microstrip
+[2] http://www.hottconsultants.com/techtips/rulesofthumb.html
+[3] http://www.cepdinc.com/calculators/microstrip.htm
+[4] http://mcalc.sourceforge.net/
+[5] http://www.ti.com/litv/pdf/swra236a
+[6] http://www.pcbmotif.com/home/index.php?option=com_content&view=article&id=104&Itemid=137
+
+
+Conclusion: the antenna feed line needs to be revised. The via spacing
+of the RF area needs to be examined. The recommended spacing may be
+beyond the capabilities of a DIY process, though.