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From:	"Evan Foss (evanfoss AT gmail DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Date:	Thu, 1 Sep 2016 12:42:32 -0400
Message-ID:	<CAM2RGhSB8uay4vNf37=wsetL-B3mBq-3TT8bq0mws8W2vPs4oQ@mail.gmail.com>
Subject:	Re: [geda-user] Microwave PCB layout simulation or How to eat all
	your processing power in 3 easy steps
To:	gEDA users mailing list <geda-user AT delorie DOT com>
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On Thu, Sep 1, 2016 at 3:51 AM, Gabriel Paubert (paubert AT iram DOT es) [via
geda-user AT delorie DOT com] <geda-user AT delorie DOT com> wrote:
> On Wed, Aug 31, 2016 at 09:25:23PM -0700, Larry Doolittle wrote:
>> Evan -
>>
>> On Thu, Sep 01, 2016 at 12:07:28AM +0000, Evan Foss (evanfoss AT gmail DOT com) [via geda-user AT delorie DOT com] wrote:
>> > On Wed, Aug 31, 2016 at 9:39 PM, Larry Doolittle <ldoolitt AT recycle DOT lbl DOT gov> wrote:
>> > > On Wed, Aug 31, 2016 at 03:53:25PM -0400, Evan Foss (evanfoss AT gmail DOT com) [via geda-user AT delorie DOT com] wrote:
>> > >> I thought soldermask issues were why most of the microwave and other
>> > >> RF hardware I have seen has no soldermask.
>> > > For the long lines, yes, depending on the designer, but eventually
>> > > those lines usually touch chips.  And high-speed serial lanes (with
>> > > the interesting frequency range now going up to 40 GHz) usually don't
>> > > have soldermask stripped off.
>> > At those frequencies I would actually think the chip would be supplied
>> > as bare die and wire bonded.
>>
>> These days Xilinx BGAs have I/O rated for 25 Gb/s, with pretty broad spectral
>> content up to 40 GHz.  I'm used to microwave circuits mostly ranging from
>> 1 - 3 GHz, and all the Mini-Circuits and Hittite chips that cover that range
>> (and higher) are in relatively conventional packages.
>
> Even to higher frequencies, we've been building here amplifiers with
> chips like Hittite's HMC462LP5 and using them up to 12GHz. They are
> specified to up 20GHz, but I can't test that high because we reach the
> limits of the connectors/launchers on the boards we've built.

Cool.

> At these frequencies, the choice of the substrate is important, we use
> 20mil thick Rogers RO4350 (or RO4003, can't remember) but the transition
> from coaxial cable to the controlled impedance track is the most critical
> part: I've had once an enclosure with a mistake of 0.5 or 0.6 mm in the
> depth of the hole in which a glass bead is inserted. The effect on the
> SWR was enormous, catastrophic at 10GHz and higher.

Glass bead?

> For the controlled impedance tracks, we typically use conductor backed
> coplanar waveguide, taking into account the effect of the cover of the
> shielded enclosure on the impedance.

Do you know of a publicly available design guide for those? I was
thinking about how pcb-rnd's new drc system should handle them last
night.

> And no these boards don't have solder masks, for other boards, where I
> also had to wire bond a diode, the solder mask was mostly removed along
> the tracks and the edges of the ground plane. Some was left as solder
> stops to limit solder flow along the track.

I knew it!

>     Gabriel

Thanks,
Evan

-- 
Home
http://evanfoss.googlepages.com/
Work
http://forge.abcd.harvard.edu/gf/project/epl_engineering/wiki/

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