PCB connection to SMA, 2.4GHz

Hi,

I have posted this question before, long ago, and I've since put that project on hold...I'm back to it now

My problem is fairly simple, and I hope someone with RF design experience can help.

I have two 2.4GHz A/V sender modules, very small, both having half-pitch headers (being 1.27mm) for all signals, including the RF out. I have some SMD connectors for all of the signals, but I am now worried about any 'matching' between the 3-pin
module connector for 'RF out' and a PCB mount SMA connector.

The module:
**broken link removed**

The datasheet, with mechanical specs:
http://www.robot-italy.net/downloads/awm620.pdf

As you can see, the RF connector, is simply a 3-pin header, 1.27mm (0.05") so, if I use a SMD socket:

**broken link removed**

Using 'TXline' and a couple of other web scripts to work out the 'stripline' for connection to an SMA connector, it needs to be 3.2mm wide. Does length matter at all? I can make the trace only a few mm long as the SMA connector is as close to the module as possible. However, I cannot make a 3.2mm trace connect to the connector without narrowing it down (otherwise it'll overlap and connecto ro ground). Oh and I'm using FR4, 1.6mm copper clad, with ground planes on both sides (both GND).

So, am I over complicating this? Or would it be wise to connect the 1.27mm socket to the SMA via a specific sized trace? I'm afraid I'm not good at RF design, completely inexperienced, but I realise that at 2.4Ghz, things get very tricky, and I don't want to design a nice board, only to find out I've got insertion loos that halfs the RF range. Also, both modules have identical connectors, so this goes for Tx as well as Rx. Hope someone can help..

BuriedCode.

BuriedCode

from your post seems that the FR4 board is a 2 layers board with ground plane in the bottom layer under the track you need to design. The possibility are two:
1) you can change topology: you can design the track in CPW instead of Microstrip. So you can reduce the track wide and improve the isolation.
2) you can "not give a damn" of the track impedance and change it reducing the track wide: you will see that the performance of your transmitter/receiver not will be affect so much (if the track lenght is short).

By

Hi,

dinone, thanks for your reply!

I do indeed have a 2-layer board, there are some tracks one both sides, but both are filled with a ground plane. So, if I clear the ground plane from the top layer around the microstrip, and leave the bottom layer, it should be ok to use a microstrip?

It seems to be a bit of a 'black art', although I have been readingup on this subject for a while now, and I uderstand most of the theory.

1) you can change topology: you can design the track in CPW instead of Microstrip. So you can reduce the track wide and improve the isolation.

Click to expand...

Interesting, I've been having trouble finding a clear definition of 'CPW', although the software utility I have 'TXLine' shows that the trackwidth must be absolutely tiny 0.09mm, and the gap between the track and ground plane, even smaller, 0.01mm. Alas, I don't think my PCB manufactuer can cope with this. Still, its introduced me to something new.

2) you can "not give a damn" of the track impedance and change it reducing the track wide: you will see that the performance of your transmitter/receiver not will be affect so much (if the track lenght is short).

Click to expand...

This is what I was thinking, after all, the distance between the module's RF out connector and an SMA connector should be no more than 5mm, possibly 3-4mm.
After searchinng google for PCB designs involving 2.4GHz, it seems they get around the 'wide trackwidth' problem, by using a thinner board 0.8mm. This of course, halves the width that the microstrip has to be, down to rougly 1.5mm. Then again, I have seen designs that don't seem to obey any sort of rules. The have several 'impedence matching' tracks, but the actual connection to the SMA connector vary in width a great deal.

Anyway, thanks for your input, and if anyone else would like to add their two cents, I'd appreciate it.

BuriedCode.