IEEE 802.15.4 subsystem
Sign in or create your account | Project List | Help
IEEE 802.15.4 subsystem Commit Details
Date: | 2010-08-26 20:41:14 (13 years 7 months 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 | ||
---|---|---|
37 | 37 | Things not done yet |
38 | 38 | ------------------- |
39 | 39 | |
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 | ||
45 | 40 | - examine spectrum around carrier frequency and first harmonic to look for |
46 | 41 | obvious distortions. Vary transmit power. |
47 | 42 | |
... | ... | |
60 | 55 | - verify that the Ben can output an a) 16 MHz clock, and b) with +/- 40 ppm |
61 | 56 | |
62 | 57 | - 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) | |
64 | 59 | |
65 | 60 | - check if we really need three DC blocking caps in the RF path |
66 | 61 | |
... | ... | |
80 | 75 | Bugs to fix |
81 | 76 | ----------- |
82 | 77 | |
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 | ||
87 | 78 | - atrf vs. atspi naming is a bit confusing |
88 | 79 | |
89 | 80 |
atrf/ecn/INDEX | ||
---|---|---|
3 | 3 | 0001 Edit Adjust balun component values |
4 | 4 | 0002 Done Add load capacitors to 16 MHz crystal |
5 | 5 | 0003 Edit Replace balun and filter with integrated balun |
6 | 0004 Edit Take into account layout considerations for RF |
atrf/ecn/ecn0001.txt | ||
---|---|---|
1 | 1 | Adjust balun component values |
2 | 2 | |
3 | ||
3 | 4 | Some of the components specified in the schematics were not at hand |
4 | 5 | in my lab and were thus replaced with similar parts. Furthermore, the |
5 | 6 | LED color was changed to ease visual identification: |
atrf/ecn/ecn0002.txt | ||
---|---|---|
1 | 1 | Add load capacitors to 16 MHz crystal (C14, C15) |
2 | 2 | |
3 | ||
3 | 4 | The crystal has a specified load capacitance of 8 pF. The AT86RF230 has |
4 | 5 | an internal capacitor array that can be trimmed in 16 steps from 0 pF to |
5 | 6 | 4.5 pF. |
... | ... | |
41 | 42 | CNTR measurements are repeatable within 1 ppm, so the nominal accuracy |
42 | 43 | appears to be far too pessimistic.) |
43 | 44 | |
44 | These measurements suggest that, combined with parasitic capacitance, | |
45 | load capacitors of 12 pF make the crystal roughly half the trim range | |
46 | faster than 16 MHz. | |
45 | ||
46 | Conclusion: these measurements suggest that, combined with parasitic | |
47 | capacitance, load capacitors of 12 pF make the crystal roughly half the | |
48 | trim range faster than 16 MHz. |
atrf/ecn/ecn0003.txt | ||
---|---|---|
1 | 1 | Replace balun and filter with integrated balun |
2 | 2 | |
3 | (pick suitable balun) | |
3 | ||
4 | We consider the balun needs for the AT86RF230 and the TI/Chipcon | |
5 | CC2520 we may try as a design alternative. | |
6 | ||
7 | For the AT86RF230, Atmel recommend baluns with integrated filter, | |
8 | namely the Wuerth 748421245 and the Johanson 2450FB15L0001, both in the | |
9 | AR86RF230 data sheet. | |
10 | ||
11 | For the CC2520, TI recommend a microstrip-based design for the balun, | |
12 | both in the CC2420/CC243x/CC2480 application note [1] and the reference | |
13 | design [2], without fully characterizing neither the transceiver's RF | |
14 | output nor all the components in the balun. | |
15 | ||
16 | Johanson lists the 2450FB15L0001 [3] and even a 2450BM15B0002 [3] for | |
17 | the TI/Chipcon CC2520, but not even Octoparts is able to find a | |
18 | distributor for these parts. | |
19 | ||
20 | Digging deeper, TI reveal more information about the balun in [5]. | |
21 | ||
22 | Finally, TI somewhat hesitatingly acknowledge that the Murata balun | |
23 | LDB182G4510C-110 can be used for the CC2520 [6], with a 3.9 nH inductor | |
24 | connecting RF_P and RF_N, the balun coupled to GND not directly but via | |
25 | 10 nF, and finally an LC low-pass filter with 1.5 nH and 2.2 pF at the | |
26 | output, for EMI compliance. | |
27 | ||
28 | Note that this also means that CC2520 and AT86RF230 both have an | |
29 | impedance of 100 Ohm. | |
30 | ||
31 | A balun without filter similar to the Murata part would be the Johanson | |
32 | 2450BL15K100. | |
33 | ||
34 | Manufacturer Part number Package Digi-Key Price/Qty | |
35 | --------------- ----------------------- ------- --------------- --------------- | |
36 | Wuerth 748421245 0805-6 732-2230-1-ND 2.15/25 | |
37 | 732-2230-2-ND 0.753/4000 | |
38 | Johanson 2450FB15L0001 0805-6 - - | |
39 | Johanson 2450BM15B0002 0805-6 - - | |
40 | Johanson 2450BL15K100 0805-6 712-1045-1-ND 0.488/10 | |
41 | 712-1045-2-ND 0.225/4000 | |
42 | Murata 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 | ||
53 | Conclusion: the Wurth balun appears to be the safest choice for | |
54 | prototyping the AT86RF230. Due to its high cost, a circuit with a | |
55 | discrete filter may be considered for larger quantities. | |
56 | ||
57 | For the CC2520, it's probably safest to directly try the Muarta balun | |
58 | with the recommended discrete filtering circuit. |
atrf/ecn/ecn0004.txt | ||
---|---|---|
1 | Take into account layout considerations for RF | |
2 | ||
3 | ||
4 | There are a number of layout considerationg when designing RF systems | |
5 | that were not taken into account or not quantified when making the | |
6 | first 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 | ||
71 | Conclusion: the antenna feed line needs to be revised. The via spacing | |
72 | of the RF area needs to be examined. The recommended spacing may be | |
73 | beyond the capabilities of a DIY process, though. |