Root/ecn/ecn0004.txt

Source at commit 49e7c83796bc04941e9dbcec69bc0751563ff4d4 created 2 years 5 months ago.
By Werner Almesberger, atusb/: use ""VDD" symbol from kicad-libs
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.
74

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