CC3220: CC3220 Antenna SMA Connector Mounting

Andrew5821

Part Number: CC3220

We are using an SMA connector to connect the CC3220 to an external antenna. The enclosure is metal and thus we need an external dipole whip antenna. The issue is the SMA connector shell makes a very low impedance DC ground path which I do not prefer. Can I insert a 10pF between the shell (ground) of the SMA connector and the PCB ground to create isolation or will this just trash the antenna performance?

over 7 years ago

0 Vincent Rodriguez over 7 years ago

TI__Mastermind 28285 points

Andrew,

2.4 GHz doesn't have a DC component, and so you don't necessarily need a DC block in your RF path. You can add a 10 pF in series on your RF trace if you are worried about the DC path in your RF. Using 10 pF, you shouldn't effect your RF to much, but i would double check with either a measurement using a Network Analyzer or simulating it using a RF sim tool (I use ADS) to ensure you still have a 50 ohm match.

Regards,

VR

0 Andrew5821 over 7 years ago in reply to Vincent Rodriguez

Genius 4565 points

Vincent,

I don't need a DC block in the RF path, just in the ground (return path) or is that what you mean? My concern is if a customer disconnects the hot terminal of the battery and accidentally touches the metal chassis which is given a DC return path through the shell of the SMA jack.

0 Keith Barkley over 7 years ago in reply to Andrew5821

Guru 42085 points

You can, but it might be better to get an isolating SMA connector. RF guys like good solid grounds. Does your antenna require a ground plane?

0 Vincent Rodriguez over 7 years ago in reply to Keith Barkley

TI__Mastermind 28285 points

Andrew,

If your customer touches a live wire to your ground plane, it is possible for bad things to happen in any circumstance.

VR

0 Keith Barkley over 7 years ago in reply to Vincent Rodriguez

Guru 42085 points

Which is the point, he wants to not have a ground plane, i.e., grounded case.

0 Andrew5821 over 7 years ago in reply to Keith Barkley

Genius 4565 points

Keith,

It is a 1/4 wave dipole configuration. It appears the coax runs through a sleeve which then the shield gets terminated to. The sleeve is 1/4 wavelength and then the unshielded part of the of the coax sticking out the sleeve is 1/4 wavelength. So I would assume this configuration requires no ground.

It looks a lot like this.

martybugs.net/.../ducky_element.jpg

Unfortunately they don't make many isolated connectors. My only other brute force options is a plastic shoulder washer. The 10pF cap would be a really nice solution if it works.

0 Andrew5821 over 7 years ago in reply to Vincent Rodriguez

Genius 4565 points

They don't have access to the ground plane of the PCB, but they do have access to the metal shell which has a dc path to the ground plane thanks to the shell of the SMA connector. I want to put a dc blocking cap between the shell of the SMA and the ground plane. I just don't know if this impacts the antenna performance.

0 Vincent Rodriguez over 7 years ago in reply to Andrew5821

TI__Mastermind 28285 points

Andrew, I don't believe that should effect antenna performance, but this would be more of antenna matching question, and you might need to simulate this to make sure. You could also put a provision for this cap, and if it does effect it, you could either remove the cap and place a zero ohm resistor, or optimize the cap value to not effect RF performance.

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CC3220: CC3220 Antenna SMA Connector Mounting