IEEE 802.15.4 subsystem
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IEEE 802.15.4 subsystem Git Source Tree
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| Source at commit 07b5bfddc052ab98a23cc55c060d07e037f35aae created 6 years 9 months ago. By Werner Almesberger, atusb/atrf.sch: move invisible footprint test, to avoid confusing eeshow | |
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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 | |
