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[SOLVED] Tx Module long VDD traces

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Junior Member level 3
Aug 26, 2011
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Hi Everyone,

My PCB design will need to incorporate a 433 MHz OOK tx module @ 5V

I am designing a PCB for a case I have found with pre machined cut outs for switches etc. Due to posistioning of the components to line up with the holes I have came across a problem.

I have had to make the VDD trace to the tx module longer than I would like. A few questions have come up during this design that I would love an answer to. The ground connection is a solid ground plane on a double sided board so very low ground impedance.

1. How will the length of the VDD trace effect the Tx module, is there anyway to cancel out the effects of the long trace with capacitance? Is this even required.
2. Will using vias to jump other signals over the Tx VDD line cause any catastrophies
3. What about running the VDD next to a relay?

Are their any rules of the thumb for this kind of thing

Thanks in advance for any help guys

Does the board have a ground plane or not? If it has a ground plane, you can just put a 0.1 uF capacitor at the Vdd pin to ground and not worry about 433 MHz traveling down the Vdd trace. If there is no ground plane, you would have to be more creative.
Thanks for the reply Biff. There is a solid ground plane underneath. I was worried that the inductance of the long trace might cause problems getting power into the transmitter module, I have planned for a 100pF at VDD, I will try both I think.

- - - Updated - - -

I have 4 on off switch signals that cross the path of VDD. Where the signals cross I have used vias to jump under the VDD trace, therefore splitting the ground plane a little. Would this be ok? Or would it be better to use 0 Ohm links over the top of VDD, therefore keeping the GND plane intact?

I bet I know what is going to happen here, I am so concerned with getting it right that I will overthink it and it won't work. Them PCB's that you do whilst supping a nice cold beer usually work just fine, for years!! ahhhh lol.

you can do 100 pF right at the Vdd pin, but I would also parallel it another cap of at least 0.1 uF to keep the chip from oscillating at a few MHz. After the chip cap, there will be very little rf energy so even if the trace has inductance of phase length, your RF chip will never see it. (I am assuming we are talking about ceramic chip capacitors surface mounted).

About the only problem would be if you are really concerned about RF leakage outside of the enclosure, a long trace will act like an antenna and pick up RF energy, which will then leak outside the box to the power supply. They make ferrite beads (that you put in series at the enclosure wall) to knock that down somewhat.
Thanks biff your answers have been really helpful! I will add the caps and hopefully all is well.

Hi Everyone,

Thanks for all your help. I have received the PCB and everythings works great. I made the traces 1mm thick and added a bulk cap and a small de-coupling cap. No problems with the Tx and the range is great.

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