TX Carrier frequency offset

RE: TX Carrier frequency offset

Frederic — Tue, 14 Apr 2020 18:39:23 GMT

I have changed the components and using 15pF capacitors fixes the issue. A quickest test that can be done is probing the crystal with an oscilloscope. It will add capacitance to the circuit and the carrier can be seen moving.

RE: TX Carrier frequency offset

Frederic — Tue, 07 Apr 2020 02:28:51 GMT

In the 19.4.2 64 MHz crystal oscillator (HFXO) table in the nrf52832 datasheet what exactly the C0 (7,5,3 pF) in the RS_HFXO_7PF, RS_HFXO_5PF, RS_HFXO_3PF values means? Is it the load capacitance of the crystal?

RE: TX Carrier frequency offset

Andreas — Fri, 03 Apr 2020 11:44:00 GMT

Hi,

The total offset is 24.1ppm, contributing to this is both the component variation and a base offset from incorrect capacitive loading. If we assume that your specific crystal is quite accurate, near the center of the normal distribution of this part, the majority of your 24.1ppm comes from incorrect loading. You should then expect up to ~20ppm on top of this, from crystal variation, temperature, aging etc. It's impossible to get the crystal loaded such that it is spot on (for a perfect crystal), but closer than 10ppm is generally possible with E12 capacitors and gives good enough overall accuracy for most crystals.

Best regards,

Andreas

RE: TX Carrier frequency offset

Frederic — Fri, 03 Apr 2020 01:29:34 GMT

in fact it's (2 402 058 [KHz] - 2 402 000 [KHz]) / 2 402 000 [KHz] = 24.1 e-6

RE: TX Carrier frequency offset

Andreas — Thu, 02 Apr 2020 08:20:22 GMT

Hi,

It's in the Bluetooth Core Spec, Vol 6, Part A, but it seems I was slightly incorrect. The BLE spec is 50ppm, 40ppm is the IC spec to have some margin for drift.

Best regards,

Andreas

RE: TX Carrier frequency offset

Frederic — Wed, 01 Apr 2020 13:31:52 GMT

I'm trying to find a link to the 40pm BLE requirement but I find only requirement about the offset with modulation but nothing with single-carrier. Could you share a link?

Thank again!

RE: TX Carrier frequency offset

Andreas — Wed, 01 Apr 2020 06:40:24 GMT

Hi,

This level of inaccuracy is very possible if the crystal is not correctly loaded. As Jay says, increasing the load capacitors to 15pF should bring it closer to center.

Either way, 24ppm is well inside the 40ppm BLE requirement with 10ppm tolerance from the crystal, 20 ppm will be outside though. nRF52 still works well outside this, but it will not be compliant and there is no guarantee the other end of the link will work with higher offsets.

Best regards,

Andreas

RE: TX Carrier frequency offset

Frederic — Wed, 01 Apr 2020 01:09:38 GMT

Problem is that I'm using a third-party mini-PCB so I don't really have access to change part but I think I will have to try. But is such level of accuracy of the carrier is possible?

RE: TX Carrier frequency offset

Jay Tyzzer — Wed, 01 Apr 2020 00:08:29 GMT

Hi Frederic, Have you tried using either a 32MHz crystal with a CL value of 8pF or.. the current Crystal but with 15pF load caps?

Load cap value = (CL*2) - (PinCap + Stray Cap) So figure (10*2) - (4+1) = 15.

Pin capacitance is 4pF and maybe 1pF for stray capacitance.

Which ever is easier to change out...

RE: TX Carrier frequency offset

Frederic — Tue, 31 Mar 2020 16:52:33 GMT

I'm using the nrf_radio_test_example firmware to run this test.