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A microstrip circuit question.

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neil1981

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Questions are in the pic.
 

It is for tuning purposes. Usually done with silver paint.
 

Silver paint is old-school. Now people either wirebond across the gaps, or solder a small wire or ribbon across. In production, if you have variations that have to be tuned out, often the circuit is assembled with all the gaps wirebonded over, and during tuning the technician pluck the wirebonds away (starting with the ones closest to the end of course) until the unit meets spec.
 

i still cant understand/
 

Transistor parameters does not have precise values, they have significant tolerances. The same thing is with PCB material and fabrication. To acheieve optimal parameters (gain, matching, noise figure...) of circuit it is often necessary to tune it. It can be done by adjusting strip length on specific places.
 

Are the gaps equivalent to capacitros?
 

No. They can vary the length of the open stubs.
They are negligible if not connected to the rest of the strip.
The rest has been clearly explained earlier.
Mazz
 

As a matter of fact gaps can be considered as series capacitors. But in this case they have no practical meaning.
 

When we wirebond across the gaps, or solder a small wire or ribbon across.how to consider the discontinuity??or no consideration?
 

I guess I would design the circuit to theoretically work with half of the gaps bridged over. Then I could add or subtract some length to the open circuited stubs during tuning. If I were going to use wirebonds, I would model the wirebond as a short length or wire, or 200 ohm transmission line 5 mils long, or whatever your analysis software has available in its library.

If I were soldering over the gap, I would not model the gap, just extend the length of the open circuited stub to the the length of main stub, gap length, short pad length, and the fringing capacitance at the open circuited end.

Its not rocket science, you just farble around empirically until it works best. If it does not work at all, how the best set up of the stubs might indicate what is wrong and how to respin the board.

One other trick. Where the open circuited main stub meets the main thru microstrip line: You can wirebond across the valley, on the device side, or farthest away from the device, to "move" the location of the stub a little closer or further from the device.
 

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