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Helpful ReplyRN4871 low transmition range problem

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SlawekC
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Re: RN4871 low transmition range problem 2017/03/26 13:02:04 (permalink)
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Dear Julia,
 
thanks for your comment. I know the difference between modules, and my idea was to use the pre-certified version. My device will be partially metal box - it's Metal Over Cap mTouch device so top layer is thin stainless steel plate. sides and bottom are from plastic.
 
I'm very new to BT modules at al,l and as I mention before I'm looking for the problem on my side first.
 
Maybe you could be so kind and share with us the part of your PCB in RN4871 area. This will be great help to me with second iteration of my PCB design. I'm meaning the print screen from your PCB editor. If it's not possible in public maybe you can send me the picture in private ?
 
thanks again
 
Slawek
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JuliaDee
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Re: RN4871 low transmition range problem 2017/03/26 13:25:32 (permalink) ☄ Helpfulby traversjames 2017/06/21 01:06:39
3 (1)
Sure, here's the layout:
 

 
It's a 4-layer board, following all the recommendations in the data sheet. RN4870 is on top (red). Layer 2 (not shown) is a solid ground plane, layer 3 (not shown) is mixed plane/signal but nothing under the BLE module, layer 4 is bottom signal traces.
 
I accidentally deleted all the little ground stitching vias from my most recent iteration, and it still works pretty well, lol. They will return in the next version, though. The keepout areas must extend through all layers.
 
If you plan on using fixed-length timeouts between sending commands, be very careful. The module does not like to be sent a command before having acknowledged the previous one, and the acknowledgement time can vary tremendously. It's not enough to look at a couple of responses and say "Oh, it looks like 150ms will be enough", because at some point it will take 600ms for the same response. Much safer I think to take the trouble of actually listening for the responses "CMD> ", "AOK", "%REBOOT%", etc. Don't forget to include the delimiters also, e.g. "CMD>" has a trailing space, "AOK" is followed by <CR>,<LF>, and so forth. The individual characters can be separated by relatively long time intervals - if you stop looking when you get just "AOK", the <CR><LF> might come 50ms later. The module firmware does not seem very robust (i.e. it's WAY too easy to brick), so better to play safe. Also note that hardware flow control is recommended; I'm not sure if you can get away without that for short messages or not.

"Given one hour in which to chop down a tree, I'd spend the first 30 minutes sharpening my axe" - Abraham Lincoln
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SlawekC
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Re: RN4871 low transmition range problem 2017/03/27 02:41:19 (permalink)
4 (1)
Hi Julia,
 
thank you very much. I will follow your recommendation of hardware and software part. You are using module RN4870. On my desk this module works perfectly even soldered to four wires. With full range of 10 meters. The low range problems I have are with RN4871 module. But now I have only one living module left he he he - working with 31mm simple wire antenna. Tomorrow I expect new bunch of 5 modules from Farnell so I will continue my journey and let you know the results.
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JuliaDee
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Re: RN4871 low transmition range problem 2017/06/20 18:33:54 (permalink)
4 (1)
UPDATE: I have a new design (different product) for which I'd like to use the RN4871 instead of RN4870. I used almost identical layout practices and target circuit, and followed the layout guidelines in the data sheet exactly. The RN4871 has much, much less range than the RN4870. The attached screen shot shows the RSSI for three modules at about 10m: BLE-xxx are both RN4871s and "Ferd Burferd" is an RN4870.

Attached Image(s)


"Given one hour in which to chop down a tree, I'd spend the first 30 minutes sharpening my axe" - Abraham Lincoln
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SlawekC
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Re: RN4871 low transmition range problem 2017/06/20 23:26:16 (permalink)
4.5 (2)
Hi Julia,
 
finally I gave up with RN4871 and I'm using it with 31mm simple wire antenna. The range is similar to RN4870 alone. If my product will be prepared for market I will change RN4871 to RN4870. Fortunately I have space on my PCB. Parallelly I'm in touch with Microchip Field Engineer to finally find the answer at the source. But this contact is quite slow. Four months till now sad: sad IMHO no one at Microchip take serious care on this modules.
 
best regards
 
Slawek
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