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Erratic DC carrier offset on TRF372017

Matthias Ihmig
Prodigy 20 points
Other Parts Discussed in Thread: TRF372017, TRF370417, TRF3765

We're currently testing the TRF372017 for a potential design on an evaluation module.

Unfortunately, I'm currently stuck on trying to get a predictive DC offset behavior for a manual DC carrier offset calibration.


After minimizing the DC carrier output of the TRF3720 by using the AFE TX_QMC_OFFSET* compensation,
I'm experiencing the case, that for each setup/power up of the TRF3720,
the DC carrier is either OK, or it again appears very strongly.

In the OK state, the DC carrier is at about -80dBm,
in the BAD state, the carrier is quite strong with about -43dBm.


I discovered, the states change, i.e. the DC carrier appears/disappears only in these cases:

- TRF3720 power up/down

- Doing VCO calibration: (setting EN_CAL= 1)

- Switching on/off divider in LO output chain (setting PWD_LO_DIV=1 and =0)

- Switching on/off divider in modulator chain (PWD_TX_DIV=1 and =0)


Switching on/off PWD_OUT_BUF=1 (=0) does not change the current DC offset.
Also, not using EN_CAL (i.e. CAL_BYPASS=1 and VCO_TRIM=52 and VCO_SEL=0)
does _not_ fix the problem.


My register settings are as follows:
reg[1] = 0x79500029
reg[2] = 0x008009CA
reg[3] = 0x2100000B
reg[4] = 0xCB1C100C
reg[5] = 0x0D2A968D
reg[6] = 0xA590100E
reg[7] = 0xD071000F


Any ideas are greatly appreciated!


Thanks and best regards,

Matthias

  • 0
    TI__Expert 3300 points

    When the VCO output TX divider is reset, the optimal carrier cancellation setting may change. The LO cancellation generally remains better than wholly uncancelled feedthrough.

    To minimize this effect, minimize external LO feedthrough. Some feedthrough occurs through the integration of synthesizer and modulator inside the TRF372017 and cannot be eliminated. The effect can be eliminated entirely only by using separate synthesizer and modulator device, such as TRF3765 and TRF370417.

     

  • 0 in reply to Pete Hanish
    Prodigy 20 points

    Thanks for the quick reply!

    The extern LO Input buffer was already disabled (EN_EXTVCO = 0).

    The hint on the feedthrough still helped - after keeping the LO Output and divider switched off, and after finding a more robust DC calibration point, the DC carrier is still there and varies in power for each new calibration run, but all under -70dBm, which is fine in our case.