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Spurious signals when switching to/from transmit mode

Question asked by Lix N. Paulian on Apr 8, 2015
Latest reply on Jun 14, 2016 by Juan Carlos Pacheco



We have serious issues with a MK21D512 based transceiver: apparently the chip is generating spurious signals when switching to/from transmit mode. This leads to a substantial reduction in permissible power to achieve the FCC mandated adjacent channel attenuation as well as the Band-edge compliance. Our transceiver uses a FE + PA from Skyworks to achieve the 20 dBm output power. However, to eliminate other sources we measured the output of the bare-bone Freescale TWR-KW24D512 demo board, using the standard Freescale "Connectivity Test" software, with no amplifier (the board "as is"). Using continuous carrier modes (modulated or unmodulated) all parameters pass, however in PRBS9 mode where the transmitter switches on and off before and after each sequence sent, the issues are clearly visible. Attached are four snapshots that demonstrate the issue: first three are in PRBS mode at minimum (3) and paxiumum (31) power levels, and one image with a continuous modulated signal at maximum power. All images are taken with the spectrum analyser in Max Hold mode, after about 30 seconds of sampling the data.


Does anyone else found similar issues? Are there any solutions to this problem? Maybe with a different initialisation of one or more of the multitude modem registers in the chip?