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Electronics => Projects, Designs, and Technical Stuff => Topic started by: peterr on April 20, 2022, 03:28:51 pm

Title: A little RF help needed ( bluetooth transmitter )
Post by: peterr on April 20, 2022, 03:28:51 pm
Hi,

I'd really appreciate a little help with this as I know very little about RF stuff.

I have a Bluetooth transmitter that has a PCB trace antenna.  I'd like to adapt it to using an external antenna.  Fortunately the PCB has unpopulated pads for a U.Fl surface mount antenna socket so much of the work is done after I solder on a suitable socket.

The output path from the Bluetooth chipset ( CSR8670 ) goes through a bluetooth bandpass filter and then splits two ways via three pads.  Two of the pads have an inductor in place which then goes to the printed antenna and a capacitor.  The other path ( currently disconnected ) seems to be designed to use an inductor via the currently unpopulated pad and the other pad shared with the common path,  The attached photo shows the arrangement with the broken path illustrated with a red line.

I looked at the CSR8670 datasheet for assistance with values for a suitable LC network on the alternative path but the datasheet doesn't give any help other than indicating that an LC network is not required.

So I could just solder a U.Fl socket, remove the existing inductor ( L4 ), bridge the pads across the red line and it should work if I'm reading the datasheet correctly.

Or should I populate the path to the U.Fl socket with an inductor and capacitor.  And if so, what values would I be looking at?

Thanks.
Title: Re: A little RF help needed ( bluetooth transmitter )
Post by: Terry Bites on April 20, 2022, 04:04:12 pm
I cant claim to have the Voodoo, but how about a scalpel and phone app that measures the signal strength to get started?
https://www.lairdconnect.com/resources/white-papers/antenna-matching-within-enclosure (https://www.lairdconnect.com/resources/white-papers/antenna-matching-within-enclosure)


Title: Re: A little RF help needed ( bluetooth transmitter )
Post by: virtualparticles on April 25, 2022, 03:48:23 pm
The datasheet claims that the PA output is 50 ohms, so you should be able to remove the connection to the left and bridge the connection to the connector with a piece of wire. Keep the cable from the UFL connector to the antenna short. The kinds of cables which fit this size connector are quite lossy.