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From: "Michael A. Terrell"
Subject: Re: Terminate/launch microstrip??
Date: Fri, 29 Nov 2002 10:57:03 -0500
Organization: Have you seen my bench? No, really! Where is it?
X-Mailer: Mozilla 4.61 [en]C-CCK-MCD (Win95; U)
> Terminate/launch microstrip??
> 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
> on the
> top and built up the groundplane on the backside
> with solder
> 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
> best thing
> 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,
> so kludging
> is ok.
> 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??
> Thanks, mike
> Bunch of stuff For Sale and Wanted at the link
> Tek 2465, TFT projector, ham radio, 30pS pulser
> Tektronix Concept Books...
If you can reduce the length of the trace slightly, you can put a
piece of copper foil over the end of the board and solder it to the
ground side, then solder the resistor to the top side.
What kind of connector are you using? There are "End launch" types
available in most families where you solder it to the end of the board.
Michael A. Terrell
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