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SlawekC
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2017/03/15 02:13:05 (permalink)
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RN4871 low transmition range problem

Hi,

I have project with two RN4871 connected with Transparent UART. In my example for testing purpose one PCB is using PIC MCU to transmit simple data via RN4871 to another RN4871 - connected via UART USB adapter to PC with putty terminal.

if the distance between modules is very short - say 5-10 cm everything works fine - transmission is stable for hours. But when distance increases to 25-30 cm just after few seconds the transmission is broken. This same problem I encountered trying to connect to modules using my Android SmartDiscover App or Windows 7 PC (on Windows 10 I gave up). Only 10-15 cm of transmission range.

BLE Modules RN4871 - came just from the box with newest firmware on board.

both I setup with:

SS,C0 - to expose Transparent UART

then I made connection from "PC" using
F
then
C,0,<MAC-of-second-module>

UART transmission speed is default - 112500 bps

I have no idea what's going wrong. Any help or suggestions will be appreciated.

best regards
Slawek
post edited by SlawekC - 2017/03/15 02:14:40

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#1
Jim Nickerson
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Jim Nickerson
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Re: RN4871 low transmition range problem 2017/03/15 06:16:14 (permalink)
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I wonder if reducing the baud rate helps ?
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SlawekC
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Re: RN4871 low transmition range problem 2017/03/15 06:49:48 (permalink)
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Hi Jim, thank you for the answer.
 
Spectrum analyser will be very large artillery for my problem I think. And I don't have one :-(

But please note that even discovering the BLE devices (RN4871) is limited to a dozen or so centimetres.
 
I have Windows 10 ASUS AIO computer on my desk in my lab room. From my saloon (7-8m) I can easily connect to my PC via BT with my HTC One9 Android phone. The RB4871 modules are "visible" to SmartDiscover App only if phone is 10-15 cm from them so I hink this is not related to UART baud rate speed. But any way I will try.
 
There is also one thin GND track under the antenna going. As I understand the manual this can limit effectiveness of antenna but I expected something like 70% not 3% of range.
 
I will try with module connected in the air just with wires soldered to it. Just to eliminate doubts associated with the PCB project.
 
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Jim Nickerson
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Re: RN4871 low transmition range problem 2017/03/15 07:17:04 (permalink)
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Though you may not have a spectrum analyzer if you are handy at working with small SMD parts this tutorial will guide you to modifying a SUP-2400 ( 10.00 or so on ebay ) to add to a SDR dongle (20.00 on amazon ) and using SDR# (Sharp) free software on your PC.
http://www.kd0cq.com/2016...-your-rtlsdr-for-5-00/
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SlawekC
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Re: RN4871 low transmition range problem 2017/03/15 09:52:13 (permalink)
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Yes I got idea to build pure man spectrum analyser. Very clever. But first of all I will try the module mounted just to goldpins on breadboard. I was told that RN4871 should work just right out from the box.
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SlawekC
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Re: RN4871 low transmition range problem 2017/03/16 10:42:59 (permalink)
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Just for test I soldered 31 mm wire as antenna to pin1 of modules and suddenly range jumped to 10m (full class2) and transmission works very good. I don't understand what's going on.
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Nikolay_Po
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Re: RN4871 low transmition range problem 2017/03/17 01:58:00 (permalink)
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May be the assembly site forgot to place a jumper from BT_RF output to ceramic chip antenna? Can you take a close-up shot of the module from antenna network side?
 
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SlawekC
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Re: RN4871 low transmition range problem 2017/03/17 02:42:46 (permalink)
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That was my first thought. But I always try to find problem on my side.
Please take a look at picture attached - there is open SMD jumper.
I bought 5 pcs and everyone has this jumper open. But I'm not sure if this jumper should be on or off.
Maybe someone from Microchip know this and can tell us.
 
BTW now I have only one working :-( - the rest four doesn't survive experiments - the modules are very fragile for hot-air de-soldering.
 

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Nikolay_Po
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Re: RN4871 low transmition range problem 2017/03/17 06:03:43 (permalink)
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Can you take a shot from different angle to see the BT_RF pin of the module and it's adjacent network? The spare place may be intended for antenna tuning capacitor.
The antenna may operate bad if you haven't placed the module on solid ground plane as shown in the datasheet. The impedance of the antenna system will significantly differ the nominal while the module is hanging on the wires instead of solid ground plane. That is why the performance may be bad even with good RF modules.
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SlawekC
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Re: RN4871 low transmition range problem 2017/03/17 06:53:04 (permalink)
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It's not easy take photo of the module with my HTC ;-) But seams that all parts are on place.
 
I'm testing modules not in the air. They are soldered to my mTouch PCB with plenty of ground plane.
 
I know that my design is rather "Acceptable" than "Best" but only 10cm of effective range id definitely to short even with very bad design...
 
BTW I can't fully understand the datasheet. On page 18 FIGURE 4-9: RN4871 RECOMMENDED PCB MOUNTING SUGGESTION shows that on Top layer all pins are surrounded with GND and pin BT_RF(1) is connected to this GND as well as pin GND(13) - maybe there is error and pin 2 should be connected.
 
On the FIGURE 5-4: RN4871 REFERENCE CIRCUIT - the pin BT_RF(1) is unconnected.
 
And how connect other pins if I follow recommendation on FIGURE 4-9 ?
 
Do you have your personal experience with this module?
 

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Nikolay_Po
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Re: RN4871 low transmition range problem 2017/03/17 07:18:33 (permalink)
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No, I have none experience with these or similar modules.
Good shot. But I didn't see the circuit because of small angle of sight of view to the board surface. Can you shot at close to 90 degrees angle (more perpendicular)?
The small ceramic ting at BT_RF pin should be an inductor. Have this inductor direct connection to an antenna? Or is it connected to first pin? In other words, is the ceramic antenna connected to the chip RF pin some way, probably through an inductor? Or the chip RF trace is feeding only the PCB edge pin?
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SlawekC
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Re: RN4871 low transmition range problem 2017/03/17 07:38:57 (permalink)
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Seams that everything is on place. There is connection between BT_RF pin and antenna checked with multimeter.

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Nikolay_Po
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Re: RN4871 low transmition range problem 2017/03/17 12:01:01 (permalink)
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OK. Thanks. Can you show board layout drawing around the module?
You may submit a "case" for Microchip support. May be you'll got some advices from an RF engineer.
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SlawekC
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Re: RN4871 low transmition range problem 2017/03/17 12:59:26 (permalink)
3 (1)
I'm in touch with Microchip Field Engineer from Poland now.
The board is working now with 31mm of wire as antenna - so I'm OK with this for now.
Anyway I have to redesign a little my PCB - remove all tracks from antenna area.
 

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Nikolay_Po
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Re: RN4871 low transmition range problem 2017/03/18 00:50:20 (permalink)
4 (1)
Thank you for sharing. IMHO this design is far from "acceptable". It seems you haven't read the module datasheet before PCB designing. You violated both requirements:
  • The module should rely on a big solid ground plane around it's body as shown in examples.
  • The antenna area should be completely free. Not only the tracks but even PCB base should be eliminated from antenna area. Even clear PCB base will disturb the antenna tuning due to it's dielectric properties which are different from the air.
Making new board ensure solid ground base around the body of the module like on examples from the datasheet. And of course make a cut of PCB material or shift the module toward the edge to ensure the antenna area completely clear.
When antenna tuning is corrupted the transceiver protection may be involved and the transmitter may be shut down due to high standing-wave ratio.
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SlawekC
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Re: RN4871 low transmition range problem 2017/03/19 05:22:15 (permalink)
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Hi Nikolay,

You are 100% right. I was so focused on mTouch part of my PCB design (it's quite large - 19 touch buttons) so I left the BT part "on side". I'm going to draw new version soon. But any way:

- why with simple 31 mm wire antenna the transmission is so good? Full 10m range.

- I received also the RN4870 modules on Friday. This module just soldered on air (four wires - power, God, TX, RX) works perfectly having 10m range.

- and when I place the module RN4870 on my PCB (with working MCU) I can't see any big transmission disturbance generated by the tracks under antena. But I know that this test is very "amateur".

But there is interesting question. The modules RN4870/71 are "brothers". Both exist in version "regulatory approved" with antena and "U" version without.
Bigger module with better antenna don't have BT_RF pin in "noneU" version. But both versions of RN4871 have this pin. Why?
And figure 4-9 from Datasheet suggest that pin BT_RF(1) should be soldered to the top copper layer track - kind of simple PCB antenna in my opinion ....
With design new PCB we will see.

And I would like to thank you for your help and suggestions. This is very helpful to me. The BT modules are completely new topic to me. I feel myself like child in dark forest.
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Nikolay_Po
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Re: RN4871 low transmition range problem 2017/03/19 09:22:29 (permalink)
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SlawekC
- I received also the RN4870 modules on Friday. This module just soldered on air (four wires - power, God, TX, RX) works perfectly having 10m range.

 
These modules are larger by size. Theirs antenna systems are different at least due to the sizes. That is why different modules may behave different in same conditions.
The such small module with ceramic antenna is pushed onto the limits. Else way they will design it even smaller. A little shift of the parameters may override the transmitter capabilities sharply. Also the receiver. Bad antenna coupling will decrease both the transmitter and the receiver performance making the link budget worse in square.
 

But there is interesting question. The modules RN4870/71 are "brothers". Both exist in version "regulatory approved" with antena and "U" version without.
Bigger module with better antenna don't have BT_RF pin in "noneU" version. But both versions of RN4871 have this pin. Why?

 
The approved modules may have not the BT_RF pins to not disturb the tuning by external BT_RF pin and it's possible connection. The absence of this pin helps decrease possible source of the errors and helps to maintain RF parameters stability on different layouts. The presence of BT_RF pins allows the redefinition the module use just by different jumper or inductor placement.


And figure 4-9 from Datasheet suggest that pin BT_RF(1) should be soldered to the top copper layer track - kind of simple PCB antenna in my opinion....

 
I have none spare time to look into the datasheet. Sure the Microchip FAE should be asked for all that questions before making a number of host PCBs.
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SlawekC
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Re: RN4871 low transmition range problem 2017/03/19 09:36:08 (permalink)
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Please note that RN4871 is the approved version of module, but still have the BT_RF module exposed. I curious why?

My FAE promised to share with me the PCB layout of PICtail (RN4871 version) so I'll be well supported designing my new PCB.
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JuliaDee
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Re: RN4871 low transmition range problem 2017/03/25 16:53:47 (permalink)
3 (1)
The RN4870 and RN4871 are shielded and pre-certified. The RN4870U and RN4871U do not have shields, and are not pre-certified.
 
If you follow the PCB layout recommendations in the data sheet closely, paying attention to all the keep-outs etc, it is no problem to get 7-10m from these modules with the on-board chip antenna. I'm getting 7m+ even with the board inside a metal box.

"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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