Unexpectedly high RSSI samples

RE: Unexpectedly high RSSI samples

user79956 — Fri, 10 Jun 2022 18:04:58 GMT

Thanks for confirming that 👍

RE: Unexpectedly high RSSI samples

ketiljo — Fri, 10 Jun 2022 12:55:47 GMT

IF is 1 MHz, and fLO is at fRX - 1 MHz.

Sorry, I meant to say adjacent channel, not co-channel.

RE: Unexpectedly high RSSI samples

user79956 — Fri, 10 Jun 2022 11:37:50 GMT

Thanks for the reply.

So the IF is 1 MHz then, with fLO 1 MHz below the wanted? That would also explain the high level I see at f+1 MHz, which must be the response to LO leakage. Is the IF given in the datasheet? I couldn't see it. The figure for image rejection is a bit useless without knowing the IF, I think. Your comment about the co-channel rejection is a little confusing - co-channel is just that, an interferer on the same frequency as the wanted. It shouldn't be affected by the image response.

RE: Unexpectedly high RSSI samples

ketiljo — Thu, 09 Jun 2022 13:53:53 GMT

What you see at +2 MHz is the image frequency. This is at fRX - 2 MHz in 1 Mbps mode and fRF - 4 MHz in 2 Mbps mode. The reason you see it at +2 MHz is because fRX is moving and the carrier is static.

Co-channel rejection isn't as good at the image frequency as it's on the opposite side side of the carrier.

https://infocenter.nordicsemi.com/topic/ps_nrf52811/radio.html#unique_447008773

RE: Unexpectedly high RSSI samples

user79956 — Tue, 07 Jun 2022 15:32:52 GMT

Thanks for the reply,

Yep, I realised after I posted that the coded phy would still be 1 Mbps.

But the fact that I am seeing energy only ~30 dB down on the channel 2 MHz higher when I use an unmodulated calibrated source is still puzzling me. And it's not symmetrical. I've checked the signal on a spectrum analyser and it is very clean and within 120 Hz of 2441 MHz. Another plot at -40 dBm:

RE: Unexpectedly high RSSI samples

ketiljo — Thu, 02 Jun 2022 10:11:42 GMT

Remember that coded phy is running with 1 MBps so the carrier is ~1 MHz wide. The RSSI detector will measure the energy in the channel but some energy from the adjacent channel will also we part of this. The unmodulated carrier doesn't have a bandwidth so very little adjacent channel power.

RE: Unexpectedly high RSSI samples

user79956 — Wed, 01 Jun 2022 11:59:58 GMT

Thinking about this a bit more, comparing the levels that I'm seeing with the Tx 1st and 2nd adjacent channel spec numbers is a mistake as I am not taking into account the receiver channel bandwidth. If the Rx bandwidth is around 1 MHz* then that explains high levels at 1 and 2 MHz offsets, even with the unmodulated bench source. But it doesn't really explain the asymmetry on the plot obtained with the calibrated bench source. The 32MHz crystal on our board is 10ppm so I know it's not off channel by much.

*What will the receiver channel bandwidth be if I have configured the radio in Ble_1Mbit mode ?

Thanks

RE: Unexpectedly high RSSI samples

user79956 — Tue, 31 May 2022 14:27:44 GMT

I appreciate that the over-air symbol rate for 125 k coded is still 1 M/s but the 1st and 2nd adjacent levels still seem higher than the -25 dBc and -50 dBc given in the datasheet.

RE: Unexpectedly high RSSI samples

user79956 — Tue, 31 May 2022 14:05:19 GMT

Meant to add that the lower plot is with the signal generator output switched off but still connected.