Hi again Sverre,
Apologies for the delay in getting back to you with more info on this. We have finally had some interesting developments after resuming today.
We received samples of our new board spin and immediately sent them up for immunity testing. The problem was confirmed to also be present with the new PCBs. We then got the boards back and have set up an HP8640B signal generator and near-field probe in our lab to try to reproduce the issue.
Fortunately it looks like we can reproduce the issue with our limited setup (we can trigger the false carrier sense) and we've observed the following:
Aside: the symptoms below are for our 868MHz board using the latest ref design.
- false-carrier detection with unit subjected to a ~3V/m field at 433.7MHz effective up to about 434.2MHz
- false-carrier detection with unit subjected to a ~3V/m field at 216.9MHz effective up to about 217.1MHz
It is interesting that these look to be sub-harmonics of our 868MHz operating frequency -- also that they are quite narrow frequenecy-bands of influence.
More-over,
- The false-carrier detection seems to be most strongly felt at the 2nd sub-harmonic (434MHz), and less so at the 4th sub-harmonic (217MHz). We observe this through the probe-proximity required to trigger the problem. The probe needs to be much closer when applying a 217MHz field.
- The problem also seems to be most evident when presenting the near-field probe to the vicinity of our internal antenna compared to other areas
The other interesting thing is that
- we haven't been able to identify any other sensitive frequencies so far; nor any that are at multiples of 13MHz.
Hypothetically, if spurs were to cause the false-carrier detection problem, would we expect to see problem frequencies that are separated at 13MHz intervals?
We are also considering re-testing with:
- an rf-shield attached
- with the CC1190/CC1101 868 EMK boards
- and finally, with our 916MHz variant of the same design
We will continue our investigation tomorrow -- but thought you might be interested in the developments so far.
Based on this new information, are there any tests you might be able to suggest we can try to help point out where the susceptibility may lie? Thanks in advance.
Best regards,
Hayden
