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74 lines
2.3 KiB
Plaintext
74 lines
2.3 KiB
Plaintext
Take into account layout considerations for RF
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There are a number of layout considerationg when designing RF systems
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that were not taken into account or not quantified when making the
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first design.
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- transmission line width
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The microstrip [1] transmission line connecting the balun and filter
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circuit with the antenna must be impedance-matched with the antenna.
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The rule of thumb according to [2] is to make its width twice the
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board thickness, in this case 0.8 mm or 31.5 mil.
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The microstrip calculator at [3] also takes into account the
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thickness of the copper, 1 oz, and yields a slightly narrower 57.5
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mil or 1.46 mm.
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A more elaborate calculator can be found at [4].
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- via spacing
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Section 4.2 of [5] recommends a via spacing of no more than
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Lvia = C/sqrt(Er)/Fres
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where
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C = the speed of light, 3*10^8 m/s
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Er = the board's dielectric constant, 4.5 for FR-4
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Fres = the resonance frequency, at least 24.5 GHz
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We thus obtain Lvia = 5 mm.
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- component placing
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[5] places DC blocking, balun, and filter close to the transceiver,
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with only the feed line between the RF circuit and the antenna. Thus,
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no changes are needed.
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- feed line termination
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Point 12 of [6] warns us that we may need to terminate the
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transmission line if it is longer than 20% of the signal's rise time.
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Point 1 of [6] gives the rise time as 1/(10*Fclk), which looks as if
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it's meant for digital signals. But we'll use it anyway.
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[2] gives us the typical propagation delay for a microstrip as
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150 pS/in.
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This means that Lmax = 0.2*tr*v
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with
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tr = 1/24.5 GHz
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v = 1 in/150 pS
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We thus obtain Lmax = 1.4 mm
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[2] suggests that the maximum unterminated stub is L(in) = tr(nS).
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With tr = 1/(10*Fclk), we thus obtain Lmax = 1.04 mm.
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Not sure if all this even applies to antennas. This needs looking to by
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someone who understands about RF.
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[1] http://en.wikipedia.org/wiki/Microstrip
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[2] http://www.hottconsultants.com/techtips/rulesofthumb.html
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[3] http://www.cepdinc.com/calculators/microstrip.htm
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[4] http://mcalc.sourceforge.net/
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[5] http://www.ti.com/litv/pdf/swra236a
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[6] http://www.pcbmotif.com/home/index.php?option=com_content&view=article&id=104&Itemid=137
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Conclusion: the antenna feed line needs to be revised. The via spacing
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of the RF area needs to be examined. The recommended spacing may be
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beyond the capabilities of a DIY process, though.
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