Electronics > Beginners

Replace trace impedance with resistor?

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Illusionist:
@Dave, I know, and I wasn't really intending to have a go at you.

The trouble is, that $29 PCB is over our 'free' import limit of £15. It will get hit with tax and, worse, £10 to £15 ransom handling fee from the courier. So that $29 PCB lands here at £40+. And it does start out at 10x the cost of the two layer so I still don't think it's that cheap although it is certainly more affordable than it used to be.

A few people have asked JLCPCB to investigate adding the tax at their end. It's possible to do, and then we wouldn't have to pay the additional handling fee here. Just the tax.

Anyway, sorry for the thread drift FriedMule, now back to your regular scheduled programme :)

FriedMule:
Dave and Illusionist thanks for your great posts, I have visited the https://pcbshopper.com/ site where you can get prices from several places at once and it looks to me like the difference from 2 to 4 layer is about 3-4 times the price, while the thickness does not really matter.
The solution with using 0.8 instead of 1.6 sounds great but do make the board thinner, is that why some recommends a 4 x 0.4 layer board instead of a 2 x 0.4 layer, to make them less flimsy?

My 4 boards has to be mounted on the side (4.7" x 1.5" - / - 120mm x 39mm) with the same spacing as standard BNC's on a scope is mounted (is there a standard spacing, what is it?) I do not think that 0.8 thickness is a problem here?

The trace design uses Coplanar Waveguide, track width 1.27 mm, gap width 0.2 mm. I do not know the Dielectric Thickness or constant but is that posible just to add in the notes as something like "Please select a FR4 board where a coplanar waveguide, track width 1.27 mm, gap width 0.2 mm gives a 50 Ohm impedance +/- 1 Ohm"?

By the way, I have found a graph, showing losses when comparing a FR4 and a Rogers PCB. Am I right in that thay are nearly the same for an <1GHz 2 1/4 inch (57 mm) trace on a probe board?

ArthurDent:
Have you considered using a semi-rigid subminiature 50 ohm coax instead of a line built into the board? it would make the board simpler and cheaper. RG405 is only .086' outside diameter and there is also some that is .047" in diameter. Cut to length, form, and solder the ends of the shield to the ground plane on the board. Neat and simple. 

FriedMule:

--- Quote from: ArthurDent on September 01, 2019, 02:03:20 am ---Have you considered using a semi-rigid subminiature 50 ohm coax instead of a line built into the board? it would make the board simpler and cheaper. RG405 is only .086' outside diameter and there is also some that is .047" in diameter. Cut to length, form, and solder the ends of the shield to the ground plane on the board. Neat and simple.

--- End quote ---
Sounds interesting, I am planning to solder some RG400 directly to the end of that 50 Ohm trace but yes totally replacing the trace with a coax sounds logical.
The original designer did say that the 50 Ohm was important, and I think he sis solder some coax to the board after the track, makes me wonder why.

Illusionist:
0.8mm thickness is indeed flimsier. Whether it's a problem depends on how it will be treated. The problem is that much flexing is likely to crack solder joints of course, and possibly crack MLCC capacitors which tends to have very unfortunate results.

If the PCBs are fixed into the enclosure such that flexing cannot occur, as mine are, then there's no problem. If that can't be done then 0.8mm is a poor idea and you'll need 1.6mm. In that case, if you can't make the traces wide enough on 2-layer to get down to 50Ω, then the above solution of using some thin coax is a workable one.

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