Effect of packet length on DTM waveform

[Maxwell1864]

I have a question regarding DTM using nRF Connect for Desktop. It is my understanding that the signal generated does not have a BLE packet structure but it is a continuous signal modulated according to the “Paket Type” selection. Thus, if I select 10101010 octet as modulating message, I’d expect something like the snapshot below repeated for the duration of the transmission. If that is the right behavior, how does “Packet length” (a characteristic of BLE packet structure) affect  the waveform?

I am seeing difference in the spectrum when “Packet length” (BLE protocol settings parameter) changes (see attached SA screenshots). So, I am trying to understand what “Packet length” affects in the modulation.  Should I expect the modulator to behave like in the picture below (i.e., if Packet length = 37 the modulator output consists of 37 sequences of 11110000 repeated until timeout)? Is “Packet length” the same as “Payload” (see snapshot below) or it does not have anything to do with it?

[Maxwell1864]

Clarification

10101010_37bytes

10101010_255bytes

11110000_37bytes

11110000_255bytes

PRBS9_37bytes

PRBS9_255bytes

[ketiljo]

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

[Maxwell1864]

Hi ketiljo,

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

[ketiljo]

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.

[Maxwell1864]

Thank you!