Slight Picket Fencing


#1

I have designed pcb using the BC127 module. All is working but I am experiencing slight background noise. I am sure it is a problem related to ground plain issues but can find nothing in the tech data about ground plains and antenna swr. Does anyone have any ideas or suggestions? The modules are being used in HFP mode so I am dealing with analogue signals using mic input as per Sparkfun’s data and sch’s which work but for the noise I am experiencing on occation which I would like to solve. Any input would be greatly appreciated.
Thank you.
Laurie


#2

@laurie54,

For the antenna VSWR it needs to be 50ohm impedance to keep it in spec, this is stated on page 18 of the below document. However, given BT is all digital this will not produce background noise.

https://source.sierrawireless.com/~/media/support_downloads/airprime/hardware_specs_user_guides/41110693%20bc127%20pts_r1.ashx

For the noise it is almost always around the analog circuitry and if you obey generic good practice then you should be fine.

  • Avoid routing analog audio signals anywhere near power and/or any high frequency lines.
  • Ground everywhere around them, ideally having ground layers above and below.

Analog audio design is discussed in the above document between pages 20 and 24.

Regards

Matt


#3

Thank you for your reply Matt. I have been experimenting with these two boards one as master and the other as slave. I am sending you a drawing of the board’s top and bottom and you can see I have followed proper layout procedures for power parts and gnd plains. The modules antenna is not close to any other ccts and the board has a seperat gnd plain for audio, RF, CPU, and power supply. They are connected by star ground at one point only on the top of the board behind the module.
I am still getting a lot of interference in the audio transmission and if the two units are 10’ away from each other the interference is so great that the audio signal is lost altogether and if I stand still for 1 minuts the connection between boards also fails.
I have followed the board and parts layout of the Sparkfun’s board on purpose as the Purpletooth Jamboree board they where selling works very well and has non of the problems I am having. My parts placement is identical to Sparkfun board with audio in/ out at one end, power management at the other end and the module in the middle at the side of the board. All parts are behind the module with the antenna at boards edge. It makes no sense to meBottom to Matt.pdf (96.6 KB)
Top to Matt.pdf (118.7 KB)
there would be so much difference in boards.
Also I have shaped and cut brass sheet to make a shield for both sides of the board to isolate the audio in a “Faraday cage” which has not helped at all.


#4

That’s a bit beyond what we can do here.

Regards

Matt


#5

I understand it looks like a complex cct. But from what you said that it is a digital transmission and therefore not the antenna. If that where true the audio would not change at all within the trans/rec area. However the audio is ferfect when the two units are close together and get increasingly worse as the distance between them increases. This does show more than just a problem with the audio stages. It does indicate interaction between the antenna and signal strength rather than just an audio problem. I sent you a copy of the boards so you could see the audio is at the top 1/2 of the board identically to the Purpletooth Jambouree board. My differential lines from mic and the ones going to the headphone amp are longer but that should not have much if any effect as that sig in and out are changed to unbalanced lines at the plug and if anywhere that would be the problem area and having that point further away from the module would be better / less susceptible to interference than having that point closer to the module.

If what you say is true then the audio (good or bad) shoudl be the same threwout the usable rang of two units. This is not the case.


#6

I have determined without a doute it is the module which is loosing it’s signal. That is in turn causing the audio to produce the picket fencing noise as the signal strength drops.
The slave and master connect as they should. Two way communication is established and works very well as long as the units are close together. However the connection is completely lost when the units are 10 ft away from each other. This is less than 1/10th the advertised distance. I had thought it might just be my boards so I then did the same experiment with two of the Sparkfun evaluation boards under the same conditions. The results are the same. 10ft and the signal is lost. I can find no info about the ground plain for the antenna in any of the white sheets so I am asking you. What can be done to increase the range of these BC127 modules to get more than the 10ft I currently get? And that range is in an unobstructed room clear of any RF or other influences. 10ft is not even far enough to get from audio source to speakers in the corner of teh room.