nRF24L01+ PA LNA - Seems that the PA (power amplification) portion of the module is failing?

[Mahonroy]

Hey guys,

I have designed several circuits all of which use a nRF24L01+ PA LNA module (picture of it below).
Occasionally we would come across a problem where the module will only transmit when you touch the module or antenna with your finger. This generally means that the transmission is actually occurring, its just extremely weak (and possibly gets amplified by your body slightly) which makes it temporarily work when touching it. Sometimes we could just replace the module and everything would be fine. Other times we would reduce the power output slightly and then it would be fine. A couple times I experimented by replacing the 220uF capacitor with a 1000uF capacitor and then it would be fine... definitely a very frustrating problem.

The modules have always received transmissions perfectly fine.

Well, as time goes on, and more designs are made, I now have an extremely high reject rate with these modules (5 out of 6 exhibit this problem now). And now, even replacing the modules or reducing the power doesn't solve the problem. Some of my designs seem to exploit this problem easier than others.

The problem looks to be that the PA portion of the module is malfunctioning/overloaded which causes this extremely weak transmission to occur (this is only speculation). My different designs seem to play a role in causing this malfunction (also still speculation).

For all I know, touching the chip seems to remove noise somehow which allows the transmission to work... but the transmission is still extremely weak in this state, so it still seems its the PA.

I am at a complete loss on how to troubleshoot this thing, so figured I would see if any of you guys could lend some advice, or if you know what might be the problem? I have an oscilloscope and am willing to do whatever it takes to fix the problem, I just don't know what to do exactly, or what else to try.

Here are a couple things that I am aware of and have tried:

• Supposedly these modules are very susceptible to noise on power. In all of my designs, I generally use a MCP1700 LDO voltage regulator (3.3V), a bulk capacitor between 220uF and 1000uF close to the nRF24L01+, and decoupling capacitors close to the nRF24L01+ as well. After speaking with the engineers who designed the module, they recommended I try this power filter below. I got a batch of boards made to try it out and it didn't change a thing.

• There are a lot of counterfeit Nordic nRF24L01+ modules floating around that can have a number of problems. The supplier of my modules assured me they are genuine Nordic modules, but just to be sure, I desoldered the nRF24L01 chip, and solder a real Nordic chip in its place. I get the exact same problem still.
• These modules don't have a shield covering the electronics so I was wondering if that could be a problem. I rigged up one of these modules in its place and I get the exact same problem... only transmits when I touch the antenna or the shield.

• I took an old design that had a working nRF24L01+ module on-board, and a design that currently was not working. I desoldered both modules and swapped them, and the design that originally was working continued to work. The other device barely worked (had extremely low successful transmissions). This sort of reinforces the idea that different designs exploit this problem - whatever it may be.
• I also experimented with soldering the nRF24L01+ module upright and this didn't change a thing. The modules already overhand the board anyways, but I was out of ideas to try.

Here is a picture of the module I have been using:


The designs generally consist of a circuit like this:


And the RF modules always overhand the board like this for better reception:


Thanks for taking the time to check it out, and any help or advice would be greatly appreciated!

 

[CDRIVE]

I generally use a MCP1700 LDO voltage regulator (3.3V), a bulk capacitor between 220uF and 1000uF close to the nRF24L01+, and decoupling capacitors close to the nRF24L01+ as well.

That's much to much capacitance on the Output pin. You should be following the Datasheet recommendations which is only 1uF. Heavily loading a regulator output with capacitance can cause instability including breaking into oscillation.

Chris

 

[CDRIVE]

Looking at this schematic you have far too much capacitance on the output of the 5V regulator too.

Also, sourcing the 3V regulator with 5V doesn't provide much input overhead. My 'guess' is that if I checked the datasheet lower limit, 5V would be considered borderline.

Chris

 

[Mahonroy]

That's much to much capacitance on the Output pin. You should be following the Datasheet recommendations which is only 1uF. Heavily loading a regulator output with capacitance can cause instability including breaking into oscillation.

Chris

Hi Chris thanks for the response!
Here is another design that uses that recommended 1uF output capacitor on the MCP1700, and then a 220uF bulk capacitor. I think the concern is for power down protection correct? At which point a protection diode can be placed across IN & OUT pin of the regulator. The MCP1700 does not have a maximum capacitance rating for the output.

Looking at this schematic you have far too much capacitance on the output of the 5V regulator too.

Also, sourcing the 3V regulator with 5V doesn't provide much input overhead. My 'guess' is that if I checked the datasheet lower limit, 5V would be considered borderline.

Chris

The MCP1700 is a LDO voltage regulator so it doesn't need that much overhead. From looking at the datasheet it can supposedly operate with an input to output differential of only 178mV.

 

[CDRIVE]

The MCP1700 does not have a maximum capacitance rating for the output.

The MCP1700 is a LDO voltage regulator so it doesn't need that much overhead. From looking at the datasheet it can supposedly operate with an input to output differential of only 178mV.

Yes, I left the house, headed for my watering hole (don't do smart phones) and while driving there questioned myself about the low overhead statement I made. Especially since the regulator contained the "LDO" designation. No argument there but I still think that large capacitance on the output of those regulators is detrimental to their operation. After all, the fact that you didn't read a maximum output cap spec doesn't translate to hanging a 1F cap on the output is OK.

That said, I'm not committed to indict this issue as being the source of your problem. Fortunately for you we have a large cadre of minds that have kept them honed much sharper than mine. Sit tight. They'll be here.

Chris

 

[Mahonroy]

I have some good news.

I decided to test the RF modules grounding, so I scraped off a patch of the RF modules black solder mask near the PA and connected an additional bridge to the circuit boards ground. Sure enough this fixes it. I’ve done this to two of them so far… both didn’t work before and now they transmit just fine.

I am trying to completely understand what this means. Does this mean that the modules ground pin is too far away from something, and this causes a loss of power during transmission? Was the ground plane interfering/blocking the transmission, and the bridge makes that not happen somehow? Is there a bad ground loop I am not aware of? So it seems related to my layout and how I did the ground, I am just trying to understand it.

What do you guys think?

Here is the schematic:


Here is the layout:


Here is the solution:


What can I do differently with the layout so I don't need that ground bridge?

Thanks again, any help/advice is greatly appreciated!

 

[(*steve*)]

Do you have multiple ground pins on your module? Have you connected all of them to ground?

Or maybe it is as simple as them having very dodgy soldering. Are they used in an environment that has lots of vibration?

 

[Mahonroy]

Do you have multiple ground pins on your module? Have you connected all of them to ground?

Or maybe it is as simple as them having very dodgy soldering. Are they used in an environment that has lots of vibration?

There is only a single ground pin on the module (far left pin in the image) and that is it.

These are failing at QC... they have not even had the chance to be used yet.

 

[davenn]

There is only a single ground pin on the module (far left pin in the image) and that is it.

RF applications need LOTS of grounds, LOTS and LOTS ... the more the merrier, can never have too many

everything in the 2 shaded areas should also be continuously grounded between the module pcb and the main pcb





Dave

 

[CDRIVE]

Are all Grounds created equal? Definitely not in the realm of RF. Especially in the microwave region. I'm with Dave. The more the better. The broader and more direct is better yet.

Chris