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I am building a "DIY Cellphone" project (http://2loo.com/ax). I have ordered all the components in the Bill Of Materials (http://2loo.com/ay). I put together almost everything when I stumbled upon a component that I don't know what it is. It is the component in the figure below (Taken from: http://2loo.com/aw). I think it says "D" or "0" on it. Is that a capacitor or a jumper?

Unknown component in red circle

According to the cellphone schematics (http://2loo.com/az), one capacitor and two coils are supposed to be there. However, those are missing from the bill of materials and are also missing from the figure above.

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Hello Dimme, I'm really eager to know that where have you bought that micro-strip antenna?(that micro-strip blue antenna in the picture) –  Roh Mar 24 at 6:19
    
Use an ohmmeter or multimeter. You can usually tell a jumper from a capacitor that way (though not always in RF circuits!) –  Brian Drummond Mar 24 at 9:45
    
This is the antenna: antenova-m2m.com/documents/download/… –  Lior Bilia Mar 24 at 12:32
    
Roh: I bought the antenna here: in-time.se You can also buy it at Digi-Key: digikey.com/product-detail/en/A10340/627-1022-1-ND/2108083 –  Dimme Mar 24 at 13:50
    
Unshortened link for the project: web.media.mit.edu/~mellis/cellphone/index.html –  Adam Davis Mar 24 at 16:45

4 Answers 4

up vote 10 down vote accepted

Looks like a "0 ohm resistor" (a jumper in some standard size such as 0603).

From Yageo's catalog:

enter image description here

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It is resistor. It's value is 0 ohm, so neutral to the trace. It is used in place of a jumper or wire to bridge two traces while allowing a few features. It can easily replace (or be replaced by) an actual resistor, if it was added then decided that the resistor was not needed. It allows a second signal to pass underneath. And it can be used for bootstrapping, as in only added in when needed, on a single board that can have multiple configurations.

Furthermore, due to the nature of production, smd pick and place machines can deal with a smd resistor a lot easier than it can a wire or jumper. And allows keeping the through hole part down, also making it cheaper for production.

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doesn't look like a substitute would be a resistor though, perhaps Adam has a better explanation, that alternatively this 0-ohm resistor is not populated and some LC chain is. –  user17857 Mar 24 at 15:08
    
@qarma for me it looks more like that unpopulated item is for an external antenna connector, which would make a series resistor likely. But a chip inductor or chip capacitor is just as likely as a chip resistor. –  Passerby Mar 24 at 22:26
    
@qarma Then again, it looks like its labeled c8, so a series cap? –  Passerby Mar 24 at 22:32
    
That is quite confusing actually, as there's a second pair of pads right next to the component, but those are clearly connected in the top layer. Which makes me wonder if those connections were intended to be scratched through if required, but if so, why bother with smd jumper? –  user17857 Mar 25 at 13:13
    
@qarma those are vias, most likely to test points, or to another part of the circuit to monitor power output? (I can't check the schematic or board printouts at the moment) –  Passerby Mar 25 at 23:23

The purpose of the capacitor and inductors in that part of the circuit is to act as a filter. Such filters are often added to designs at the antenna in order to deal with problems after the design is built, but are not always needed. So the 0 ohm resistor effectively removes this filter from the circuit acting as a jumper.

If the final design is shown to have emissions that fail FCC, CE, or other regulatory testing, it may be that adding the correct parts in place of the jumper will resolve the problem quickly without having to redesign the PCB. If testing is satisfactory without a filter in place at that location, then a 0 ohm resistor is very, very cheap.

So there's little risk in adding it, and if things don't work as well as planned (perhaps due to a mismatch between the antenna and transceiver) then a few parts can be added in a matter of hours or days rather than waiting days or weeks to re-spin the PCB.

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Adam's and Passerby's answers make a lot of sense, but let me try an alternative interpretation:

BU-SMA-V component is not populated, and that's an antenna jack. Perhaps 0Ohm resistor is there only "tell" the chip that the external antenna is missing?

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No, its hard to see, but the center of the SMA jack pinout, also goes past it, to the blue surface mounted antenna. The SMA jack is optional/alternative, but the trace has to be connected to an antenna to work. –  Passerby Mar 25 at 23:25

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