IEEE 802.15.4 subsystem
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IEEE 802.15.4 subsystem Git Source Tree
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Source at commit e587d39d3de00ff5e40ec80b9683c9a32778f722 created 7 years 7 months ago. By Werner Almesberger, atusb/usb.sch, atusb.pro: replace CONN_1 with TESTPOINT | |
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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. |
74 |