Lpc2000

----- Original Message ----- 
From: "Guillermo Prandi" <yahoo.messenger@...>
To: <lpc2000@yahoogroups.com>
Sent: Wednesday, May 03, 2006 2:34 PM
Subject: [lpc2000] Re: OFFTOPIC: PCB impedance calculation without GND plane


> --- In lpc2000@yahoogroups.com, "Leon Heller" <leon.heller@...> wrote:
>>
>> ----- Original Message ----- 
>> From: "Guillermo Prandi" <yahoo.messenger@...>
>> To: <lpc2000@yahoogroups.com>
>> Sent: Wednesday, May 03, 2006 12:50 PM
>> Subject: [lpc2000] OFFTOPIC: PCB impedance calculation without GND
> plane
>>
>>
>> > Hi. Sorry for the offtopic, guys. If anybody can point me out
> some good
>> > forum/group to post this question, I'll be glad.
>> >
>> > I need to calculate trace witdh and cleareance to achive an
> impedance
>> > of nearly 50 ohm for an RF trace surrounded by a GND polygon. My
> board
>> > is SINGLE layered, so I have no power or GND planes. There's
> plenty of
>> > online calculators for this *if* I have a GND plane, but I
> couldn't
>> > find any for surface impedance calculation (i.e., between
> tracks). Any
>> > hints?
>>
>> You will find it impossible to get a 50 ohm impedance the way you
> describe.
>> Why not just put some copper pour on the other side of the track,
> making it
>> a microstrip section. A track width of 0.1" will give you about 50
> ohms with
>> FR4 material. If you are stuck with a single-sided board you could
> glue a
>> piece of copper foil to the other side of the board or use some 50
> ohm coax
>> cable. If the connection is short and the frequency not all that
> high, I
>> wouldn't bother with impedance matching anyway.
>>
>> Leon
>> --
>> Leon Heller, G1HSM
>> leon.heller@...
>> http://www.geocities.com/leon_heller
>>
>> Leon
>>
>> ---
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>>
>
> Thanks, Leon. I'll keep in mind your suggestions if I bump into this
> problem again, but you're right, a second layer is way easier. The
> line is relatively short (1 inch), but frequency is GSM (1.8/1.9
> GHz), so I want to be careful to keep signal integrity in good shape
> as much as I can within reason.

Yes, for GSM you do need to take some care with the track. I'm using a 
Siemens MC55 module which has a tiny Hirose coax socket (about 2.5 mm wide) 
for the antenna as well as an optional pad for connection to a PCB. I'm 
using the former as it makes things easier, although I had to buy an SMA 
adapter cable. I'm actually using a home-made single-sided PCB with it.