Effect of packet length on DTM waveform

RE: Effect of packet length on DTM waveform

Maxwell1864 — Wed, 11 Dec 2024 16:34:57 GMT

Thank you!

RE: Effect of packet length on DTM waveform

ketiljo — Mon, 09 Dec 2024 09:12:40 GMT

That's a special case where the carrier is not modulated and constant (100% duty cycle). So only a single tone at the center of the selected channel.

RE: Effect of packet length on DTM waveform

Maxwell1864 — Fri, 06 Dec 2024 15:05:33 GMT

Hi ketiljo,

Thank you for your clarification. DTM also allows to generate a constant unmodulated carrier: how is the packet structure in this case?

RE: Effect of packet length on DTM waveform

ketiljo — Fri, 06 Dec 2024 09:43:58 GMT

A DTM packet concists of a preamble (010101...), sync word) andd CRC in addition to the payload.

(From Bluetooth core spec, vol 6, part F)

So when you change the length of the packet, it's only the paylod that changes. The other data to payload ratio is less for a shorter packet. Hence more of the packet will be the expected payload for a longer packet. So when you look at the spectum like you do here, the longer packet will have more payload and less of other, more or less random, data.

DTM can't setup a constantly modulated carrier at 100% duty cycle. If you want that, you can use "radio test" instead: https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/samples/peripheral/radio_test/README.html

RE: Effect of packet length on DTM waveform

Maxwell1864 — Fri, 06 Dec 2024 00:21:18 GMT

Clarification

10101010_37bytes

10101010_255bytes

11110000_37bytes

11110000_255bytes

PRBS9_37bytes

PRBS9_255bytes