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From: comp.zrch.embedded on 24 Mar 2010 10:03
I am designing a PCB which will use a GPRS modem. The connection
between the Antenna SMA connector and the GSM modem is via a 20mm 50
ohm PCB track. This is a two layer 1.6mm thick 1 oz copper PCB and the
50 ohms impedance is achieved with a 260 thou track in a ground plane
with 8 thou clearance over a solid ground plane,. Do I really need to
get the PCB impedance checked by the pcb manufacturer as my current
supplier cannot do this for prototypes, will it have much effect for a
track of this length at 1900MHz.
Thanks
From: comp.zrch.embedded on 24 Mar 2010 11:33
On 24 Mar, 14:03, "comp.zrch.embedded" <sjo...(a)scannex.co.uk> wrote:
> I am designing a PCB which will use a GPRS modem. The connection
> between the Antenna SMA connector and the GSM modem is via a 20mm 50
> ohm PCB track. This is a two layer 1.6mm thick 1 oz copper PCB and the
> 50 ohms impedance is achieved with a 260 thou track in a ground plane
> with 8 thou clearance over a solid ground plane,. Do I really need to
> get the PCB impedance checked by the pcb manufacturer as my current
> supplier cannot do this for prototypes, will it have much effect for a
> track of this length at 1900MHz.
> Thanks

Sorry, got my dimensions wrong, Track width is 45 thou, ground
clearance 7 thou (coplanar) on a 63 thou pcb with bottom ground plane.
1 oz pcb copper!
From: Ian Shef on 24 Mar 2010 15:52
"comp.zrch.embedded" <sjones(a)scannex.co.uk> wrote in news:88ba0e9b-d70c-4425-
968d-994e12af01c0(a)19g2000yqu.googlegroups.com:

> On 24 Mar, 14:03, "comp.zrch.embedded" <sjo...(a)scannex.co.uk> wrote:
>> I am designing a PCB which will use a GPRS modem. The connection
>> between the Antenna SMA connector and the GSM modem is via a 20mm 50
>> ohm PCB track. This is a two layer 1.6mm thick 1 oz copper PCB and the
>> 50 ohms impedance is achieved with a 260 thou track in a ground plane
>> with 8 thou clearance over a solid ground plane,. Do I really need to
>> get the PCB impedance checked by the pcb manufacturer as my current
>> supplier cannot do this for prototypes, will it have much effect for a
>> track of this length at 1900MHz.
>> Thanks
>
> Sorry, got my dimensions wrong, Track width is 45 thou, ground
> clearance 7 thou (coplanar) on a 63 thou pcb with bottom ground plane.
> 1 oz pcb copper!
>

The answer depends on things like:
How close to 50 ohms do you need to be?
How closely can your vendor control track width? dielectric thickness?
What is the relative permittivity of the dielectric? How well controlled?

Your wavelength is about 158 mm. Your track length is is a fraction of this.
Is 50 ohms relevant? I don't know.

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