NRF52840 RF Wireless Tuning

[AGeegee]

Hi,

I have a design which mostly follows the reference designs.

I've added a Resistor Network R16/17/24 to allow a UFL connector to connect to the ANT and isolate it from the NRF RF Output

I would like it if you can confirm if the process of tuning is correct.

• R16,R17,R24,C22,C23 all removed to isolate
• I have a VNA and set it to sweep from 1.5GHz to 3.0GHz.
• Open - R16/R17/R24 all removed
• Short - R24=0R, C23=0R, R15=Open
• 50ohm Load - R24=0R, C23=50ohm R15=Open

I do an initial sweep and see that the lowest loss needs to move up higher in frequency, so I cut some ANT track.

After cutting I see that the frequency has moved such that 2.44GHz is the lowest.

I then recalibrate over a Freq range of 2.35G -> 2.55G.

Then I populate some Matching Network values C23=3.0nH, R15=1.0pF, R24=0R, C22=Open

However it seems that I have cut too much ANT and then the frequency is too high.

So I then get a fresh board without ANT cuts, populate the same C23=3.0nH, R15=1.0pF, R24=0R, C22=Open and then cut again to minimise the return loss and confirm ANT length and matching values.

Could you please comment on this tuning process?

Thanks!

[AGeegee]

Hi Bendik,

Thanks for the reply, I was beginning to think older tickets don't get looked at.

Yes, we've received the new boards, and for some reason only 1 of the 7 units I have here don't seem to operate the RADIO.

The other boards have RSSIs close to -30dB when close to the RSSI Dongle. However the one with the issue has -90dBm or there abouts. It is transmitting because it has the same firmware as the other boards and I've reloaded it many times.

As mentioned, I reheated the radio side of the board and the NRF52 so ensure it wasn't a solder joint issue. The thing is, when I touch the board with the RSSI dongle, the RSSI does improve to -80dBm, so it definitely is transmitting something which seems to be the correct frequency.

With this board as well, the HF clock should have started as we're able to bring the board up and there is usually a check to ensure the HFXO is started before continuing. The RF frequency also matches that of the RSSI dongle.

One thing I did note was that we are not using the "F" rev3 IC, but the "D" Rev2 IC. In addition to that the C54 is not loaded on these boards. But the other boards seem fine without it with the "D" rev2 IC.

Many thanks,

Adrian

[Bendik Heiskel]

Hi Adrian,

AGeegee said:

As mentioned, I reheated the radio side of the board and the NRF52 so ensure it wasn't a solder joint issue. The thing is, when I touch the board with the RSSI dongle, the RSSI does improve to -80dBm, so it definitely is transmitting something which seems to be the correct frequency.

Ok ,then we can be sure that the radio is at least functioning.

 

AGeegee said:

One thing I did note was that we are not using the "F" rev3 IC, but the "D" Rev2 IC. In addition to that the C54 is not loaded on these boards. But the other boards seem fine without it with the "D" rev2 IC.

It shouldn't be a issue in this case, but you will see more noise on the transmitted signal without this capacitor on Rev 2

 

AGeegee said:

The problematic board shows the same resistance to ground at different points along the antenna as the other working boards. So far I tested 7 boards, and only 1 of them seems to have an issue with getting a strong output. I guess it might go down as a manufacturing defect, but I can't understand how or what the issue could be.

Just measuring the resistance won't realy tell us anything other than how the components behave at DC.

Could you try swapping out the components on the failing board, with either new components or the once on one of the functioning boards?

 

Best regards,

Bendik