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Spurious tone in Tx with AD9361

Question asked by marcovalerioa on Feb 15, 2017
Latest reply on Feb 20, 2017 by rgetz

I am working with an FMCOMMS3 board connected to an Avnet Microzed Board.

When testing the Tx function of the board I have found a spurious and unwanted tone in the RF output.
The board is initialized using the No-OS software.
The default_init_param structure has been modified with the following rows:
    /* Base Configuration */
    0,        //two_rx_two_tx_mode_enable *** adi,2rx-2tx-mode-enable
    /* LO Control */
    1000000000UL,    //rx_synthesizer_frequency_hz *** adi,rx-synthesizer-frequency-hz
    1000000000UL,    //tx_synthesizer_frequency_hz *** adi,tx-synthesizer-frequency-hz



i.e., 1 Tx-1Rx configuration and RF LO at 1 GHz.


After initialization I activate the BIST function by writing the binary value 11000011 to the register 0x3F4, which should correspond to a tone with Full Scale amplitude and frequency equal to 3.84 MHz (the sampling frequency is 30.72 MHz, as returned by the function ad9361_get_rx_sampling_freq).


The output RF signal is shown in the attached picture:


At the center of the screen there is a residual carrier tone (1 GHz), on the right side of the carrier there is the modulation tone (1 GHz + 3.84 MHz), on the left side of the carrier tone there is the residual sideband  tone(1 GHz - 3.84 MHz).

The spurious tone is nearly -40 dB w.r.t. the modulation tone and has frequency close to 1 GHz - 3*3.84 MHz.

The spurious tone moves with the modulation frequency, e.g. the following picture is obtained with the 0x3F4 register set to the binary value 10000011, which corresponds to a frequency equal to 2.88 Mhz.



A similar behavior is found when using data generated by myself instead of BIST. However with my data I found an additional problem.

I start the transmission by providing to the FMCOMMS3 board TX_DATA, TX_FRAME and FB_CLK signals. When I stop the transmission all these signals are set to 0, however an RF emission is still active as shown in the following picture:



What is the correct method for effectively stopping the RF emission?


Many thanks

Marco V. Arbolino