Date:2010-08-26 20:41:14 (10 years 1 month ago)
Author:Werner Almesberger
Commit:2406f674f5641addb9d78fa013daf169555ae009
Message: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
Files: TODO (3 diffs)
atrf/ecn/INDEX (1 diff)
atrf/ecn/ecn0001.txt (1 diff)
atrf/ecn/ecn0002.txt (2 diffs)
atrf/ecn/ecn0003.txt (1 diff)
atrf/ecn/ecn0004.txt (1 diff)

Change Details

TODO
3737Things not done yet
3838-------------------
3939
40- define values for crystal load capacitors. Measurements with instruments not
41  quite precise enough (+/- 100 ppm) for the task (+/- 40 ppm, preferably
42  < +/- 10 ppm) suggest that we're about 300 ppm off with no capacitors at all
43  and can only correct about 150 ppm with the internal trim capacitors.
44
4540- examine spectrum around carrier frequency and first harmonic to look for
4641  obvious distortions. Vary transmit power.
4742
...... 
6055- verify that the Ben can output an a) 16 MHz clock, and b) with +/- 40 ppm
6156
6257- replace discrete balun and filter with integrated solution, to reduce BOM
63  size, maybe cost, insertion loss, and PCB space
58  size, maybe cost, insertion loss, and PCB space (see ECN0003)
6459
6560- check if we really need three DC blocking caps in the RF path
6661
...... 
8075Bugs to fix
8176-----------
8277
83- two of my systems (tv and u1010) flat out refuse to talk to the board's USB
84  application, but have no problem talking to its DFU boot loader. Very
85  strange.
86
8778- atrf vs. atspi naming is a bit confusing
8879
8980
atrf/ecn/INDEX
330001 Edit Adjust balun component values
440002 Done Add load capacitors to 16 MHz crystal
550003 Edit Replace balun and filter with integrated balun
60004 Edit Take into account layout considerations for RF
atrf/ecn/ecn0001.txt
11Adjust balun component values
22
3
34Some of the components specified in the schematics were not at hand
45in my lab and were thus replaced with similar parts. Furthermore, the
56LED color was changed to ease visual identification:
atrf/ecn/ecn0002.txt
11Add load capacitors to 16 MHz crystal (C14, C15)
22
3
34The crystal has a specified load capacitance of 8 pF. The AT86RF230 has
45an internal capacitor array that can be trimmed in 16 steps from 0 pF to
564.5 pF.
...... 
4142CNTR measurements are repeatable within 1 ppm, so the nominal accuracy
4243appears to be far too pessimistic.)
4344
44These measurements suggest that, combined with parasitic capacitance,
45load capacitors of 12 pF make the crystal roughly half the trim range
46faster than 16 MHz.
45
46Conclusion: these measurements suggest that, combined with parasitic
47capacitance, load capacitors of 12 pF make the crystal roughly half the
48trim range faster than 16 MHz.
atrf/ecn/ecn0003.txt
11Replace balun and filter with integrated balun
22
3(pick suitable balun)
3
4We consider the balun needs for the AT86RF230 and the TI/Chipcon
5CC2520 we may try as a design alternative.
6
7For the AT86RF230, Atmel recommend baluns with integrated filter,
8namely the Wuerth 748421245 and the Johanson 2450FB15L0001, both in the
9AR86RF230 data sheet.
10
11For the CC2520, TI recommend a microstrip-based design for the balun,
12both in the CC2420/CC243x/CC2480 application note [1] and the reference
13design [2], without fully characterizing neither the transceiver's RF
14output nor all the components in the balun.
15
16Johanson lists the 2450FB15L0001 [3] and even a 2450BM15B0002 [3] for
17the TI/Chipcon CC2520, but not even Octoparts is able to find a
18distributor for these parts.
19
20Digging deeper, TI reveal more information about the balun in [5].
21
22Finally, TI somewhat hesitatingly acknowledge that the Murata balun
23LDB182G4510C-110 can be used for the CC2520 [6], with a 3.9 nH inductor
24connecting RF_P and RF_N, the balun coupled to GND not directly but via
2510 nF, and finally an LC low-pass filter with 1.5 nH and 2.2 pF at the
26output, for EMI compliance.
27
28Note that this also means that CC2520 and AT86RF230 both have an
29impedance of 100 Ohm.
30
31A balun without filter similar to the Murata part would be the Johanson
322450BL15K100.
33
34Manufacturer Part number Package Digi-Key Price/Qty
35--------------- ----------------------- ------- --------------- ---------------
36Wuerth 748421245 0805-6 732-2230-1-ND 2.15/25
37                        732-2230-2-ND 0.753/4000
38Johanson 2450FB15L0001 0805-6 - -
39Johanson 2450BM15B0002 0805-6 - -
40Johanson 2450BL15K100 0805-6 712-1045-1-ND 0.488/10
41                        712-1045-2-ND 0.225/4000
42Murata LDB182G4510C-110 0603-6 490-5023-1-ND 0.325/10
43                        490-5023-2-ND 0.114/4000
44
45[1] http://www.ti.com/litv/pdf/swra098d
46[2] http://focus.ti.com/docs/toolsw/folders/print/cc2520em_refdes.html
47[3] http://www.johansontechnology.com/images/stories/ip/baluns/Balun_Filter_Combo_Matched_2450FB15L0001_v11.pdf
48[4] http://www.johansontechnology.com/images/stories/ip/baluns/balun_filter_combo_matched_2450bm15b0002_v2.pdf
49[5] http://www.ti.com/litv/pdf/swra236a
50[6] http://e2e.ti.com/support/low_power_rf/f/155/t/15910.aspx
51
52
53Conclusion: the Wurth balun appears to be the safest choice for
54prototyping the AT86RF230. Due to its high cost, a circuit with a
55discrete filter may be considered for larger quantities.
56
57For the CC2520, it's probably safest to directly try the Muarta balun
58with the recommended discrete filtering circuit.
atrf/ecn/ecn0004.txt
1Take into account layout considerations for RF
2
3
4There are a number of layout considerationg when designing RF systems
5that were not taken into account or not quantified when making the
6first design.
7
8- transmission line width
9
10  The microstrip [1] transmission line connecting the balun and filter
11  circuit with the antenna must be impedance-matched with the antenna.
12  The rule of thumb according to [2] is to make its width twice the
13  board thickness, in this case 0.8 mm or 31.5 mil.
14
15  The microstrip calculator at [3] also takes into account the
16  thickness of the copper, 1 oz, and yields a slightly narrower 57.5
17  mil or 1.46 mm.
18
19  A more elaborate calculator can be found at [4].
20
21- via spacing
22
23  Section 4.2 of [5] recommends a via spacing of no more than
24  Lvia = C/sqrt(Er)/Fres
25  where
26  C = the speed of light, 3*10^8 m/s
27  Er = the board's dielectric constant, 4.5 for FR-4
28  Fres = the resonance frequency, at least 24.5 GHz
29
30  We thus obtain Lvia = 5 mm.
31
32- component placing
33
34  [5] places DC blocking, balun, and filter close to the transceiver,
35  with only the feed line between the RF circuit and the antenna. Thus,
36  no changes are needed.
37
38- feed line termination
39
40  Point 12 of [6] warns us that we may need to terminate the
41  transmission line if it is longer than 20% of the signal's rise time.
42
43  Point 1 of [6] gives the rise time as 1/(10*Fclk), which looks as if
44  it's meant for digital signals. But we'll use it anyway.
45
46  [2] gives us the typical propagation delay for a microstrip as
47  150 pS/in.
48
49  This means that Lmax = 0.2*tr*v
50  with
51  tr = 1/24.5 GHz
52  v = 1 in/150 pS
53
54  We thus obtain Lmax = 1.4 mm
55
56  [2] suggests that the maximum unterminated stub is L(in) = tr(nS).
57
58  With tr = 1/(10*Fclk), we thus obtain Lmax = 1.04 mm.
59
60  Not sure if all this even applies to antennas. This needs looking to by
61  someone who understands about RF.
62
63[1] http://en.wikipedia.org/wiki/Microstrip
64[2] http://www.hottconsultants.com/techtips/rulesofthumb.html
65[3] http://www.cepdinc.com/calculators/microstrip.htm
66[4] http://mcalc.sourceforge.net/
67[5] http://www.ti.com/litv/pdf/swra236a
68[6] http://www.pcbmotif.com/home/index.php?option=com_content&view=article&id=104&Itemid=137
69
70
71Conclusion: the antenna feed line needs to be revised. The via spacing
72of the RF area needs to be examined. The recommended spacing may be
73beyond the capabilities of a DIY process, though.

Archive Download the corresponding diff file



interactive