From: John Larkin
Subject: Re: Terminate/launch microstrip??
Date: Fri, 29 Nov 2002 10:56:32 -0800
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On Fri, 29 Nov 2002 04:51:23 -0800, mike wrote:
>I've been trying (unsuccessfully) to get signals
>of 50 ohm microstrip transmission lines on .062"
>For terminations, I've been trying two 100 ohm or
>50 ohm 1206 SMT resistors in various mechanical
>configurations. The only thing that seems to work
>to hang one 50 ohm resistor off the end of the
>normal to the board. I soldered it to the trace
>top and built up the groundplane on the backside
>to meet the end of the resistor.
>Works, but ain't rugged at all.
>This is a directional probe, so will be exposed to
>(un)nimble fingers in use.
>Is there some black magic I can apply to get the
>to properly terminate when placed on top of the
>Smaller resistors would probably help, but 1206 is
>what I got.
>The other problem is launching into the
>I need the input to be normal to the board. The
>I've come up with so far is to drill mostly thru
>backside and stuff .095 rigid coax thru the hole
>and solder to the trace. The TDR shows a nasty
>reflection about 50pS wide of 3% or so. By
>how far I shove the coax into the board I can
>the dip to a ring, but can't make it smaller.
>doesn't sound like much, but it's 10% of the
>length of the
>More black magic??
>A long time ago, I read an article about diddling
>trace geometry to compensate for launcher
>but can't find it. I'm only going to build one,
>I thought about trying to spice model it, but it's
>related to geometry than to parasitics in the
>themselves, yes? no? I'm coming to the conclusion
>there is no way I can get a resistor on the top to
>have a low inductance connection to the
>There's gotta be a way. RF guys do it every day??
Termination: assuming the backside of the board is ground, try making
a fair-sized topside ground island, nailed to the bottom with many
vias. Then have your termination resistor between the microstrip trace
and this ground island. Two 0805 or 0603 resistors would be better
than one 1206; play with the mounting *angles* of the resistors. If
you have the room, run a *lot* of microstrip trace before the
termination; the trace loss will inherently terminate the really fast
stuff, sort of like using a spool of lously coax as a terminator.
A good microstrip launcher is a gold-plated (ie, solderable)
square-flange SMA connector. Mount it on the edge of the board, flange
perpendictular, and solder the center pin to the microstrip and the
flange to the ground plane. This is at least as good as an official
edge-launch connector. If there's a capacitive bump, start hacking the
microstrip and solder blob with an x-acto knife to reduce width near
the solder joint.
If you really need your coax to come in at right angles to the board,
The problem with TDR is that it shows every tiny blemish, sort of
looking at a beauty queen's face with a microscope. If you're
concerned about a 50 ps, 3% bump, you should realize that that's
actually very good in this business.
What are you working on?